ICFA Beam Dynamics Panel Newsletters Special Issue

This is a general information article of the ICFA Beam Dynamics Newsletter No. 85. It contains foreword from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent Ph.D. thesis section, a forthcoming beam dynamics events section, and a section of announcements from the beam dynamics panel.

A new generation electron-positron collider — the Super Tau-Charm Facility (STCF) — has been proposed in China. The STCF will have a luminosity greater than 0.5 × 10³⁵ cm^-2 s^-1 and a center-of-mass energy range of 2 to 7 GeV. Injectors are an essential component of circular colliders, particularly positron generating systems. The proposed STCF injector, Linac, accelerates both electrons and positrons from 1.0 GeV up to 3.5 GeV with a high intensity of 1.5 nC per bunch for Tau-Charm research. An injector with a high-energy electron-target positron production system was designed. The design baseline of the STCF injector Linac, positron source, and damping ring have been carried out in this paper.

With the development of particle physics, higher requirements are put forward for the brightness and energy of the collider. A new collider — the Super Tau-Charm Facility (STCF) — is under study in China. To meet the high acquirement of the quality of the beam, a digital low-level RF (LLRF) system that achieves high stability plays a critical role as a part of the microwave system of Linacs. This paper introduces the design of a digital LLRF system that works in the S-band (2856 MHz) as required by Linacs of the STCF. The system architecture and hardware design as well as the control algorithms and software design are discussed and tested. The system includes a signal source, a frequency synthesis system and an IF signal processor. The phase noise of the signal source is as low as 20.3 fs, and the low-level IF signal processor adopts amplitude and phase control by a PI-type controller. They jointly realize ultra-low phase noise signal output. The table experiments show that the short-term amplitude and phase stability of the system is better than 0.01% and 0.01°, respectively. The beam experiments show that the closed-loop phase stability in the cavity is about 0.1°. The results prove that the LLRF system meets the requirement of the STCF.

For CEPC such big size machine survey and alignment, several key issues need to be considered: the measurement and data processing method, error accumulation control, the global and relative accuracy, the measurement efficiency, component alignment method and the deformation problem. To these issues an overall survey and alignment scheme was made. The global datums will be established for the measurement and data reduction. A multi-level control network will be built for the global and relative position control, the establish method and measurement method are introduced and the accuracies are estimated. To the component alignment, the schemes of fiducialization, pre-alignment and tunnel alignment are introduced, the deformation problem was analysed and monitor equipment will be applied. To improve the measurement efficiency a new kind of instrument was designed, the prototype has been fabricated and some experiments has been carried out.

In 2012, the Higgs boson was successfully observed at the LHC. Due to the relatively low mass of the Higgs, it is possible to build a Higgs factory using a circular e⁺e^-collider. Consequently, the Circular Electron-Positron Collider (CEPC) was proposed shortly after the observation of the Higgs. After several years of pre-studies, the CEPC study group completed the Conceptual Design Report (CDR) in 2018. Since then, a series of key technology R&D efforts have been carried out, and the accelerator design has been continuously optimized. The accelerator design can meet the scientific objectives by allowing operation at different energies for Higgs/W/Z, and ttbar, with high luminosities. Key technologies required for mass production have been developed, including the high-performance superconducting accelerating cavities and modules, high-efficiency RF power sources, magnets, and vacuum systems, etc,. This paper will present the selected key technology R&D accomplishments and updated accelerator key parameters.

In 2022, the FCC Feasibility Study management mandated a working group to analyse the best configuration of the FCC-ee tunnel in the arc regions, in view of the construction of a mock-up of the arc half-cell. One of the main and most challenging goals of the study, named FCC-ee Arc Half-Cell Mock-up Project, was to perform a preliminary investigation on the principles of supporting the Short-Straight Sections and dipoles of the half-cells, both for the booster and for the collider machines. This is an important input needed for the choice of the best configuration of the relative placement of the booster with respect to the collider. The structural stiffness, mass and stability of the supporting structures must be optimized to minimize the vibrations transmitted/transferred to the magnetic system of the accelerators by elements such as pumps, water cooling system, beam thermomechanical stresses, powering elements, etc. To perform the study, tools such as CAD software, FEM and analytical techniques were employed. This paper summarizes the preliminary design concepts and the results of the simulations performed.

As the study of the Future Circular Collider (FCC) project progresses, the choice of the exact circumference of the accelerator tunnel becomes essential. It is defined by RF-related constraints, mainly the RF frequency of the FCC-hh, as well as the revolution frequency ratios with the hadron injector. It is shown firstly how favourable circumference options assuming the present LHC RF frequency have been identified and ranked from the RF point of view. Secondly, the entire frequency range of interest has been systematically scanned, assuming the possibility of operating SPS or LHC as the potential injector of FCC-hh. Considerations for the RF synchronization between injector and collider have been taken into account. Several options are studied in detail, amongst them the particularly attractive circumferences of 90478.6 m and 90837.7 m, and compared to the present working hypothesis of 90658.2 m. The results are analysed and rated from the point of view of flexibility and longitudinal beam dynamics.

At CERN, each generation of particle accelerators led to the implementation of new geodetic, survey and alignment methodologies following the increasing accuracy requirements and the evolution of survey instruments. The Future Circular Collider (FCC), representing the next generation of particle accelerator, will in its turn require new development to ensure the accomplishment of the construction of the 91 km tunnel and the correct positioning of the machine components. Mainly because of larger size of the machine but also due to the design of its components, the existing geodetic infrastructure has to be extended and upgraded. The accelerator component alignment methodology currently used in not scalable, and the Machine Detector Interface alignment procedure must be revised including monitoring of the alignment. The article summarizes the different aspects of the geodetic, survey and alignment challenges that are currently investigated during the FCC feasibility studies.

The European Organisation for Nuclear Research (CERN) is planning a Future Circular Collider (FCC), to be the successor of the current Large Hadron Collider (LHC). Significant civil engineering is required to accommodate the physics experiments and associated infrastructure. The 90.6 km, 5.5 m diameter tunnel will be situated in the Geneva region, straddling the Swiss-French border. Civil engineering studies are to incorporate the needs of both the FCC lepton collider (FCC-ee) and the FCC hadron collider (FCC-hh), as the tunnel will host both machines successively.

The collimation system of the Future Circular Collider, operating with leptons (FCC-ee), must protect not only the experiments against backgrounds, but also the machine itself from beam losses. With a 17.8 MJ stored energy of the electron and positron beams, they are highly destructive, and beam losses risk to cause damage or a quench of superconducting elements. Accurate collimation simulation tools and models are needed to design the collimation system and optimize the collimation performance, including magnetic tracking, synchrotron radiation and optics tapering, as well as particle-matter interactions. As no existing code was found that incorporated all these features, a new simulation software tool has been developed. The tool is based on an interface between a particle tracking engine, pyAT or Xtrack, and a Monte-Carlo particle-matter interaction engine for collimator scattering, BDSIM, which is based on Geant4. Results from a simulation of edge scattering from a beam halo collimator in the FCC-ee are presented to demonstrate the capabilities of the tool.

At SuperKEKB, the maximum beam currents and the luminosity could be limited by the beam injection in the near future. We estimated requirements to the beam injection to achieve the next target luminosity of 1 × 10³⁵cm^-2s^-1. Two cases of β_y^* = 1 mm and 0.8 mm are considered. A simulation shows that we need to suppress the emittance growth of the injecting positron beam in the beam transport (BT) line down to its design value to meet the requirement with β_y^* = 1 mm. In the case of β_y^* = 0.8 mm, we need to improve dynamic aperture of LER in addition to the suppression of the emittance growth. Efforts to improve the beam quantity and quality in LINAC and in BT are summarized. The present most serious problem concerning quality of the injecting beams is emittance growth in the BT lines for both electron and positron beams. Simulations and experiments show that the effects of ISR and CSR in the BT lines cause a large horizontal emittance growth of the electron beam. Several measures to suppress the emittance growth are under considerations.

Since April 2020, the SuperKEKB has been operating with the crab waist scheme. The luminosity record achieved in June 2022 was 4.71 × 10³⁴ cm^-2s^-1, which overtook its predecessor KEKB by more than a factor of 2. The beam-beam interaction plays a key role in causing vertical blowup and consequently limiting the luminosity performance of SuperKEKB. In this paper, we examine luminosity tunings under the influence of beam-beam effects and review the luminosity performance of SuperKEKB with the crab-waist operation from 2020 to 2022.

Electron-positron collider Super Charm Tau factory is planned to be built in the National center for physics and mathematics, Sarov. It is a double ring collider with crab waist collision scheme operating in wide beam energy range from 1.5 GeV to 3.5 GeV with peak luminosity of 10³⁵ cm^-2s^-1. The polarized electron source and three Siberian Snakes provide 80% longitudinally polarized electron beam at 2 GeV. Superconducting wigglers decrease damping times, effects of intra-beam scattering and increase Touschek beam lifetime, particularly at low energy. The paper presents the status of collider design and optimization of luminosity and beam lifetime.

All modern electron-positron collider designs use Crab Waist collision scheme as a basic principle for substantial luminosity increase. However, this approach has strong demands on beam dynamics and technical collider design, which are very difficult to satisfy. This paper discusses a program to study fundamental features of Crab Waist collision scheme at Budker Institute of Nuclear Physics.

The Circular Electron Positron Collider is a double ring lepton collider covering beam energy from 45 GeV (Z) to 180 GeV (tt-bar). Beam coupling impedance and the collective effects that are triggered by the impedance are important subjects when targeting high machine performance of an electron positron collider. A robust impedance model is required for the instabilities evaluations as well as to investigate their possible mitigations. Meanwhile, a thorough investigation on the collective effects is essential to identify the critical issues on the beam instabilities. In this paper, a detailed impedance model has been built for the collider ring and dominant impedance contributors have been identified by evaluating the effective impedances. The potential collective instabilities driven by the impedance are also discussed.

One of the major upcoming challenges in particle physics is achieving precise measurements of the Z, W, and H bosons, as well as the top quark. To meet these targets, the next e⁺e^- collider complex, FCC-ee, will need to achieve unprecedented luminosities. The FCC-IS European Study is investigating the feasibility of these challenges, with a cornerstone of the study being the design and optimization of the high-energy booster (HEB). This paper provides an update on the status of the HEB of FCC-ee in light of recent developments in the injector and collider survey, as well as an overview of ongoing work on longitudinal stability and design robustness in relation to field, alignment, and diagnostics errors. Constraints and effects related to the design frequency of the accelerating cavities, as well as collective effects, are also highlighted. Lastly, the paper presents an investigation into an alternative arcs cell design.

The purpose of this paper is to calculate the longitudinal and transverse wakefields of the FCC collimators using the electromagnetic codes ECHO3D and IW2D. We cross-checked our results using CST particle studio for long bunches, and found them to be in good agreement. The obtained results show that the collimators give one of the highest contributions to the overall FCC-ee wake potentials. In particular, using the code PyHEADTAIL, we have found that the geometric contribution of the collimators' wakefield reduces significantly the transverse mode coupling instability threshold. Therefore, it is imperative to explore and implement solutions that effectively mitigate this wakefield source.

The SuperKEKB is an electron-positron collider consisting of two storage rings: the 4 GeV-positron low-energy ring and the 7 GeV-electron high-energy ring. The impedance of the rings has been modeled using electromagnetic simulation codes and used to study the single-bunch collective instabilities. The wake potentials are exported from the impedance model and included in a particle tracking simulation code, PyHEADTAIL. In this paper, we review the impedance modeling and the results of the collective instability simulations.

The SuperKEKB is an electron-positron double-ring asymmetric-energy collider at the High Energy Accelerator Research Organization (KEK) in Japan. It uses a novel collision method referred to as the nano-beam scheme to avoid the so-called hourglass effect. The highest peak luminosity achieved thus far is 4.65 × 10³⁴ cm^-2s^-1, which was reached on June 8^th, 2022. Although the SuperKEKB holds the record for the world's highest instantaneous luminosity, several unresolved issues prevent a further increase in luminosity. One of the major issues is inadequate stability and reproducibility of the beam orbit and optics. This paper discusses optics distortion caused by orbit deviation near strong sextupole magnets. Numerical estimations indicate that a horizontal orbit deviation of a few tens of micrometers induces a sizable distortion of the optical functions at the interaction point. Analyses of the amount of betatron tune feedback and orbit at sextupole magnets used for crab waist and vertical local chromaticity correction indicate that the beam-current dependence of the vertical tune shift is attributable to the change of the beam orbit at the sextupoles. The orbit fluctuation at the sextupoles causes beta-beating and makes stable operation at high beam currents more difficult. The results show that orbit tuning at the sextupoles improves both the injection efficiency and the detector background level. Optics deterioration within a few days is another urgent issue in high-beam-current operation. A vertical orbit change of a few tens of microns at sextupole magnets installed in arc cells is a matter of concern according to numerical estimations.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 84. It contains two forewords from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent Ph.D. thesis section, a forthcoming beam dynamics events section, and a section of announcements from the beam dynamics panel.

Negative ions compact cyclotrons serve many applications and are known to be effective due to their simple and efficient extraction as well as the large circulating currents without the requirement of separated turns. TR30s are compact cyclotrons that have continued to experience impressive technological advances over the last few decades, driven primarily by medical isotope production needs. Their space charge limits are in the range of 1 to 2 mA, and we explored the combination of our machines' age and the challenges of maintaining their performances. Certain subsystems are detailed with specific examples from more than 30 years of around-the-clock, quasi continuous operations with TR30 cyclotrons. The subsystems' failures and their part in the overall downtime will be presented.

A summary of numerical modeling capabilities regarding high power cyclotrons and fixed field alternating gradient machines is presented. This paper focuses on techniques made available by the OPAL simulation code.

For this issue, papers on the topic of cyclotron beam physics have been solicited and chosen to highlight the main areas both of historic interest and of active research. I take the opportunity to outline the differences and similarities between cyclotron dynamics as compared to other accelerator types. As well, I try to introduce the major areas of interest, referring to papers in this issue, as appropriate.

One of the decisive issues in the design and operation of cyclotrons is the choice of the beam extraction method. Typical methods are extraction by electrostatic extractors and by stripping. The former method requires DC high voltage electrodes which are notorious for high-voltage breakdowns. The latter method requires beams of atomic or molecular ions which are notorious for rest gas and Lorentz stripping. Here we discuss the conditions to be met such that a beam will leave the magnetic field of an isochronous cyclotron purely by fast acceleration.

The RIKEN RI (radioactive isotope) Beam Factory (RIBF) provides the world's most intense heavy ion beams exceeding 345 MeV/u and is the primary facility for in-flight RI beam generation. The RIBF has steadily improved its performance. In particular, the intensity of the uranium beam, which is critical for producing in-flight fission RI beams, has been dramatically increased by a factor of 240 compared to 2008. To further increase the intensity of the uranium beam, a new acceleration scheme using charge stripper rings (CSRs) has been proposed as a cost-effective way to increase charge stripping efficiency. In this paper, we discuss the key design issues of CSR and required upgrades of existing ring cyclotrons with the introduction of CSR and space-charge effects.

Stripping extraction of hydrogen molecular ions has gained interest in the cyclotron industry due to its high extraction efficiency. However, the magnetic field could result in undesired Lorentz dissociation of the hydrogen anion/molecular ions during acceleration. Studies of dissociation under electric fields comparable to that of a Lorentz-transformed magnetic field in a typical cyclotron (a few MV/cm) are sparse. Hence, in order to fill in the missing yet crucial information when designing a cyclotron, this work compiles and summarizes the study of Lorentz dissociation of H^-, H⁺₂ and H⁺₃ for stripping extraction in a cyclotron.

At IBA a high-intensity compact self-extracting cyclotron is being studied. There is no dedicated extraction device but instead, a special shaping of the magnetic iron and the use of harmonic coils to create large turn-separation. Proton currents up to 5 mA are aimed for. This would open new ways for large-scale production of medical radioisotopes. The main features of the cyclotron are presented. A major variable of the beam simulations is the space charge effect in the cyclotron centre. Using the SCALA-solver of Opera3D, we attempt to find the ion source plasma meniscus and the beam phase space and current extracted from it. With these properties known, we study the bunch formation and acceleration under high space charge condition with our in-house tracking code AOC. We also discuss a new tool that automates optimization of cyclotron settings for maximizing beam properties such as extraction efficiency.

Existing analytical models for transverse beam dynamics in isochronous cyclotrons are often not valid or not precise for relativistic energies. The main difficulty in developing such models lies in the fact that cross-terms between derivatives of the average magnetic field and the azimuthally varying components cannot be neglected at higher energies. Taking such cross-terms rigorously into account results in an even larger number of terms that need to be included in the equations. In this paper, a method is developed which is relativistically correct and which provides results that are practical and easy to use. We derive new formulas, graphs and tables for the radial and vertical tunes in terms of the flutter, its radial derivatives, the spiral angle and the relativistic gamma. Using this method, we study the 2ν_r = N structural resonance (N is number of sectors) and provide formulas and graphs for its stopband. Combining those equations with the new equation for the vertical tune, we find the stability zone and the energy limit of compact isochronous cyclotrons for any value of N. We confront the new analytical method with closed orbit simulations of the IBA C400 cyclotron for hadron therapy.

Compact H^- cyclotrons are used all across the globe to produce medical isotopes. Machines with external ion sources have demonstrated average extracted currents on the order of a few mA, although reported operational numbers are typically around 1 mA or below. To explore the possibility of extracting even more current from such cyclotrons, it is important to understand the mechanisms that drive intensity limits and how they scale. In this paper we review some of the key aspects of the beam dynamics in the central region of compact cyclotrons, including rf electric focusing and space charge effects. We derive the scaling of the phase acceptance with the rf gap voltage, harmonic number, etc. We also explore the scaling with different types of ions such as H^-, H⁺₂ and H⁺₃. We discuss the impact of mechanical erosion of the central region electrodes. Thoughout the paper, we use examples and experimental data from two compact H^- cyclotrons for reference: the TR-30 series and the TRIUMF 500 MeV machine.

We're concerned with future accelerators of high intensity protons for isotope production. To this end, we initiate a proposal to design an innovative superconducting H⁺₃ cyclotron TR150, aiming at proton energy of 70–150 MeV and proton current of ∼ 1.0 mA. Cyclotrons in this energy range are not developed world-wide; moreover numerous highly interesting and increasingly demanded radio­nuclides are in this energy range. Our machine shall be designed to accelerate H⁺₃ ion, the simplest and stable triatomic molecular ion, by injection from an external ion source and extraction by stripping. This has potential to extract proton beam of variable energies with very high extraction efficiency, and thus enables to simultaneously provide multiple proton beams for multiple external production targets. A baseline design of our machine and the beam dynamics studies are presented in this paper.

The MW class proton accelerators are expected to play important roles in many fields, attracting institutions to continue researching and tackling key problems. The continuous wave (CW) isochronous accelerator obtains a high-power beam with higher energy efficiency, which is very attractive to many applications. Scholars generally believe that the energy limitation of the isochronous cyclotron is ∼1 GeV. To get higher beam power by the isochronous machine, enhancing the beam focusing become the most important issue.

Adjusting the radial gradient of the average magnetic field makes the field distribution match the isochronism. When we adjust the radial gradient of the peak field, the first-order gradient is equivalent to the quadrupole field, the second-order, the hexapole field, and so on. Just like the synchrotron, there are quadrupoles, hexapole magnets, and so on, along the orbits to get higher energy, as all we know.

If we adjust the radial gradient for the peak field of an FFA's FDF lattice and cooperate with the angular width (azimuth flutter) and spiral angle (edge focusing) of the traditional cyclotron pole, we can manipulate the working path in the tune diagram very flexibly. During enhancing the axial focusing, both the beam intensity and the energy of the isochronous accelerator are significantly increased. Here a 2 GeV CW FFA with 3 mA of average beam intensity design is presented. It is essentially an isochronous cyclotron although we use 10 FDF lattices. The key difficulty is that the magnetic field and each order of gradient should be accurately adjusted in a large radius range.

As a high-power proton accelerator with high energy efficiency, we adopt high-temperature superconducting (HTS) technology for the magnets. 15 RF cavities with a Q value of 90000 provide energy gain per turn of ∼15 MeV to ensure the CW beam intensity reaches 3 mA. A 1:4 scale, 15-ton HTS magnet, and a 1:4 scale, 177 MHz cavity have been completed. The results of such R&D will also be presented in this paper.

An accurate and detailed field map is important for cyclotron beam dynamics studies. During the long history of cyclotron studies, many techniques have been developed by cyclotron pioneers for the treatment of median plane field map. In this paper, we take the TRIUMF 500 MeV cyclotron as an example to study the asymmetric field resulting from imperfect median plane symmetry. The "Gordon approach" and a highly accurate compact finite differentiation method are used to investigate the historical field survey data. The redundancy in the survey data is revealed by the expansion method, which also makes it possible to correct the error in the measurement. Finally, both the azimuthal field B_θ and the axial gradient of the axial field dB_z/dz in the median plane are corrected using the radial field map B_r. The influence of the correction is examined by recalculating the equilibrium orbit properties of the TRIUMF cyclotron. The result shows significantly increased vertical centering errors of the closed orbits. A further simulation study suggests that these centering errors can be reduced to below 1.5 cm by adjusting the trim coils' B_r field within the output limits of our trim coils' power supplies. The error in the measurement field data may explain why the calculated trim coils' settings during the cyclotron commissioning in 1974 encountered difficulty.

The linear coupling resonance ν_r - ν_z = 1 in a cyclotron is driven by the first harmonic in the radial gradient of the radial magnetic field. In the TRIUMF 500 MeV cyclotron, this resonance is encountered multiple times. When the circulating beam is off-centred radially passing through the resonance, the radial betatron oscillation can be converted into vertical oscillation, which can cause beam losses and radio-activation. We investigated this resonance with goal to correct it by using the available harmonic correction coils. Moreover, we improved the cyclotron vertical tune measurement by using trim coils to create a flat-top radial field, and thus confirmed an extra ν_r - ν_z = 1 coupling resonance passage as this is unexpected from the historical tune diagram. To avoid this passage, the local vertical tune is adjusted to stay farther away from the resonance line by using the trim coils axial field, but at the cost of a local excursion in isochronism. After the correction and the avoidance of this resonance, both the coherent and incoherent vertical oscillations are decreased, thus helping to reduce the machine tank spills under high intensity operation. In this paper, we present the results of calculations and simulations as well as measurements that we undertook.

In this paper we demonstrate that cyclotrons can be made to have precisely constant betatron tunes over wide energy ranges. In particular, we show that the horizontal tune can be made constant and does not have to follow the Lorentz factor γ, while still perfectly satisfying the isochronous condition. To make this demonstration we developed a technique based on the calculation of the betatron tunes entirely from the geometry of realistic non-hard-edge closed orbits. We present two particular cyclotron designs, one compact cyclotron and one ring cyclotron. The compact cyclotron design is backed up by a 3-dimensional finite element magnet calculation, that we also present here.

The statistical σ-matrix approach is used to derive general envelope equations for ellipsoidal bunches with space charge in isochronous and non-isochronous fixed-field accelerators (FFAs) These accelerators couple the radial and longitudinal phase spaces due to momentum dispersion induced by the space charge effect. This generates a rotation of the bunch around the local vertical axis which is known in the field as the vortex effect. A special invariant of the vortex is found which represents its total angular momentum with respect to its center. For the isochronous cyclotron, a special solution of the envelope equations is found which describes a circular bunch with equal radial and longitudinal RMS sizes. The existence of this solution is a direct consequence of the circular symmetry of the single particle Hamiltonian in a co-moving coordinate frame. The stationary solution of the circular bunch fixes the relation between bunch sizes and accelerated current (or the total charge per bunch) for given emittances. The general envelope equations may easily be implemented in existing numerical envelope codes.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 83. It contains two forewords from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent doctoral theses section, along with a forthcoming beam dynamics events section.

In this paper we discuss design considerations and beam dynamics challenges associated with laser-driven plasma-based accelerators as applied to multi-TeV-scale linear colliders. Plasma accelerators provide ultra-high gradients and ultra-short bunches, offering the potential for compact linacs and reduced power requirements. We show that stable, efficient acceleration with beam quality preservation is possible in the nonlinear bubble regime of laser-plasma accelerators using beam shaping. Ion motion, naturally occuring for dense beams (i.e., low emittance and high energy) severely damps transverse beam instabilities. Coulomb scattering by the background ions is considered and it is shown that the strong focusing in the plasma strongly suppresses scattering-induced emittance growth. Betatron radiation emission from the transverse motion of the beam in the plasma will result in beam power loss and energy spread growth; however for sub-100 nm emittances, the beam power loss and energy spread growth will be sub-percent for multi-TeV-class plasma linacs.

I discuss some key opportunities and challenges of a PWFA collider, and outline some objectives which I consider important to be able to assess the machine performance, assuming that numerous technical challenges can be solved. The highlighted topics are purely the choices of this author. Several other articles in this issue are relevant for a collider design, and discuss challenges for different sub-systems of a collider, including the articles on the beam delivery system [1], drive-beam generation [2], and emittance preservation [3]. A more complete overview of agreed challenges and objectives can be found in international research roadmaps [4,5]. Here, we highlight in particular the option of a PWFA γγ collider.

The combination of advantages in positron acceleration over plasma-based accelerators and high gradient over conventional accelerators puts the structure-based wakefield accelerator (SWFA) in a unique spot on the road to a multi-TeV linear collider. As a result of the significant advancements that have been made throughout the past several decades, the SWFA related research continues gaining special attention from the accelerator community. In this article, we will present a survey of the research on SWFAs, with a particular focus on the challenges in beam dynamics, and lay out a roadmap toward its ultimate goal of delivering a mature linear collider design.

Particle acceleration in dielectric microstructures powered by infrared lasers, or "dielectric laser acceleration" (DLA), is a promising area of advanced accelerator research with the potential to enable more affordable and higher-gradient accelerators for energy frontier science and a variety of other applications. DLA leverages well-established industrial fabrication capabilities and the commercial availability of tabletop lasers to reduce cost, with axial accelerating fields in the GV/m range. Desirable luminosities would be obtained by operating with very low charge per bunch but at extremely high repetition rates. And as a consequence of its unique operating parameter regime, coupling of the laser to the accelerator can potentially be in the 50% range and with low beamstrahlung energy loss due at the interaction point, making DLA a promising approach for a future multi-TeV linear collider.

Linear colliders are an attractive platform to explore high-precision physics of newly discovered particles. The recent significant progress in advanced accelerator technologies has motivated their applications to colliders which has been discussed in the alegro workshop. In this paper we discuss structure wakefield acceleration, namely collinear wakefield acceleration and two-beam acceleration. We especially discuss available drive and witness beam sources based on L and S-band radiofrequency technology, and also summarize available and forthcoming longitudinal shaping techniques to improve the overall acceleration efficiency via the transformer ratio.

In a collinear beam-driven wakefield accelerator, a bunch of charged particles is accelerated by a strong electric field that is generated in a medium by a preceding high-charge drive bunch. Multiple beam-driven acceleration concepts have been proposed and demonstrated in proof-of-principle experiments. In some concepts, the medium is plasma where very strong electric fields are created due to the motion of ions and electrons with respect to each other. In other configurations, the medium is a slow-wave electromagnetic structure made of dielectric and/or metal, and high gradients are achieved due to the very short duration of the electromagnetic pulse excited in the structure by the drive bunch. Because of the high charge, and consequently long length of the drive bunch, wakefields excited by the leading particles of the drive bunch affect the trailing particles in the same bunch and result in beam-driven instabilities obstructing the drive bunch's stable propagation and extended interactions with the witness bunch, ultimately terminating the energy transfer process. This paper presents an overview of the drive-bunch beam dynamics in beam-driven structure- and plasma-based accelerators with a focus on beam instabilities that limit stable propagation of the drive bunch, such as the beam break-up instability and transverse defocusing and deflection in cases of cylindrical and planar structures and plasma waveguides. Possible mitigation techniques are discussed.

We discuss recent developments and challenges of beam dynamics in Dielectric Laser Acceleration (DLA), for both high and low energy electron beams. Starting from ultra-low emittance nanotip sources the paper follows the beam path of a tentative DLA light source concept. Acceleration in conjuction with focusing is discussed in the framework of Alternating Phase Focusing (APF) and spatial harmonic ponderomotive focusing. The paper concludes with an outlook to the beam dynamics in laser driven nanophotonic undulators, based on tilted DLA grating structures.

Positron sources are the key elements for the future and current lepton collider projects such as ILC, CLIC, SuperKEKB, FCC-ee, Muon Collider/LEMMA, etc., introducing challenging critical requirements for high intensity and low emittance beams in order to achieve high luminosity. In fact, due to their large production emittance and constraints given by the target thermal load, the main collider parameters such as the peak and average current, the emittances, the damping time, the repetition frequency and consequently the luminosity are determined by the positron beam characteristics. In this paper, the conventional positron sources and their main properties are explored for giving an indication to the challenges that apply during the design of the advanced accelerator concepts. The photon-driven positron sources as the novel approach proposed, primarily for the future linear colliders, are described highlighting their variety and problematic.

Emittance is a beam quality that is vital for many future applications of advanced accelerators, such as compact free-electron lasers and linear colliders. In this paper, we review the challenges of preserving the transverse emittance during acceleration, both inside and outside accelerator stages. Sources of emittance growth range from space charge and instabilities caused by transverse wakefields, which can occur in any advanced accelerator scheme regardless of medium or driver type, to sources more specific to plasma accelerators, such as mismatching, misalignment, ion motion, Coulomb scattering, chromaticity between stages, and more.

The Beam Delivery System (BDS) is a critical component of a high-energy linear collider. It transports the beam from the accelerator and brings it to a focus at the Interaction Point. The BDS system includes diagnostic sections for measuring the beam energy, emittance, and polarization, as well as collimators for machine protection. The length of the BDS increases with collision energy. Higher collision energies also require higher luminosities, and this is a significant constraint on the design for energy-frontier machines. Here, we review BDS designs based on traditional quadrupole magnets and examine the challenges involved in extending these to the Multi-TeV regime consistent with requirements for advanced accelerator concepts.

We introduce and review available and active research facilities that involve novel acceleration concepts in Asia. Most of the facilities equip with high-peak-power (>10 TW) lasers with tens of femtosecond duration for laser wakefield acceleration. The activities in Asia are growing and several problems on the realization of high energy frontier accelerators would be accessed through the existing facilities.

Research on the application of advanced and novel accelerator schemes to high-energy physics requires facilities capable of producing multi-GeV particle beams. We briefly review the challenges faced by advanced accelerators in reaching collider-relevant parameters and give a concise description of relevant European facilities and large scale installations, either in operation or in a state of advanced design, with their main goals. We also emphasize contributions from smaller, mostly university groups or laboratories. These facilities and groups advance the field considerably and address some of the challenges arising in the translation of advanced accelerator concepts to a future high-energy physics machine. We highlight the fact that there is in addition the strong need for a dedicated European facility with a scientific and R&D program specific to the research questions exclusive to a plasma-based e^-e⁺ linear collider.

Demonstrating the viability of Advanced Accelerator Concepts (AAC) relies on experimental validation. Over the last three decades, the U.S. has maintained a portfolio of advanced and novel accelerator test facilities to support research critical to AAC. The facilities have enabled pioneering developments in a wide variety of beam and accelerator physics, including plasma-wakefield and structure-wakefield acceleration. This paper provides an overview of the current portfolio of U.S. facilities possessing charged particle drive beams with high energies, on the order of tens of joules per pulse, or drive lasers with high peak powers, on the order of a petawatt, and are actively conducting AAC research.

Plasma wakefield acceleration (PWA) channels are characterized by very high accelerating gradients and very strong focusing fields. We propose to employ these properties for effective production of low emittance high energy muon beams, consider muon beam dynamics in the PWFA cell and analyze various options and potential of the PWA-based muon sources.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 82. It contains two forewords from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent Ph.D. thesis section, a forthcoming beam dynamics events section, and a section of announcements from the beam dynamics panel.

Machine learning (ML) tools such as encoder-decoder convolutional neural networks (CNN) can represent incredibly complex nonlinear functions which map between combinations of images and scalars. For example, CNNs can be used to map combinations of accelerator parameters and images which are 2D projections of the 6D phase space distributions of charged particle beams as they are transported between various particle accelerator locations. Despite their strengths, applying ML to time-varying systems, or systems with shifting distributions, is an open problem, especially for large systems for which collecting new data for re-training is impractical or interrupts operations. Particle accelerators are one example of large time-varying systems for which collecting detailed training data requires lengthy dedicated beam measurements which may no longer be available during regular operations. We present a novel method of adaptive ML for time-varying systems. Our approach is to map very high (N ≈ 100k) dimensional inputs (a combination of scalar parameters and images) into the low dimensional (N ≈ 2) latent space at the output of the encoder section of an encoder-decoder CNN. We then actively tune the low dimensional latent space-based representation of complex system dynamics by the addition of an adaptively tuned feedback vector directly before the decoder sections builds back up to our image-based high-dimensional phase space density representations. This method allows us to learn correlations within and to quickly tune the characteristics of incredibly large parameter space systems and to track their evolution in real time based on feedback without massive new data sets for re-training. We demonstrate that our method can accurately predict and track the phase space of charged particle beams at various locations in a particle accelerator by adaptively adjusting in real-time while the unknown input beam distribution of the accelerator is changing in shape, charge, and offset and while the RF system of the accelerator itself is also changing in an unpredictable way. For FACET-II we demonstrate that such an approach has the potential to use transverse deflecting cavity and energy spread spectrum beam measurements to accurately predict 2D projections of the 6D phase space of the electron beam at the plasma wakefield acceleration interaction point where such diagnostics are unavailable.

In contrast to a "single particle table-top trap", an essential feature of a storage ring "trap" is that 10¹⁰ or more particles can have their spins aligned in a polarized beam. This is a nunber of polarized particles large enough for the beam polarization to be detected externally, and fed back to permit external control of the beam polarization. Though the table large enough for any such "storage ring trap" is quite large, the level of achievable spin control, though classical, not quantum mechanical, can be comparable to the control of one or a small number of polarized particles in a low energy trap. Motivated to investigate time reversal invariance, especially the detection of non-zero electric dipole moments (EDMs) this paper describes the design of a low energy storage ring having the superimposed electric and magnetic bending needed to "freeze" the spins of polarized beams. For electrons (of either sign) and protons the spins can be frozen with all-electric bending but, in general, superimposed electric/magnetic bending is required. Since constructive bending superposition in one direction implies destructive superposition in the other direction, counter-circulating beams must differ, either in particle type or momentum, in order for their orbits to be identical. For globally frozen spin operation the bunch polarizations remain constant relative to the momenta, for example remaining parallel to the circuating beam momentum vectors. With superimposed electric and magnetic bending, the globally frozen spin condition can be met over a continua (specific to particle type) of E/B ratios. When this condition is met, the out-of-plane, EDM-induced precession accumulates monitonically, which is obligatory for producing a measurably large EDM signal. As Koop has explained, the EDM signal will still accumulate if the polarization is allowed to "roll like a wheel" around a radial axis.

The first mention of the impedance concept appeared on November 1966 in the CERN internal report Longitudinal instability of a coasting beam above transition, due to the action of lumped discontinuities by V.G. Vaccaro. Then, a more general treatment of it appeared in February 1967 in the CERN yellow reportLongitudinal instabilities of azimuthally uniform beams in circular vacuum chambers of arbitrary electrical properties by A.M. Sessler and V.G. Vaccaro. The concept of wake field came two years later, in 1969, in the paper The wake field of an oscillating particle in the presence of conducting plates with resistive terminations at both ends by A.G. Ruggiero and V.G. Vaccaro. This was the beginning of many studies, which took place over the last five decades, and today, impedances and wake fields continue to be an important field of activity, as concerns theory, simulation, bench and beam-based measurements. Building a reliable impedance or wake field model of a machine is the first necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics nonlinearities, transverse damper, noise, space charge, electron cloud, beam-beam (in a collider), etc. Beam collective instabilities, and their mitigation, cover a wide range of effects in particle accelerators and they have been the subjects of intense research. As the machines performance was pushed new mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. With the increasing power of our computers this becomes easier but the need to continue and develop theories remains, to have a better understanding of the interplays between all these effects: the subject of impedance and beam instabilities in particle accelerators is far from being exhausted, as testified by the many new instability and stabilizing mechanisms which have been recently explained or discovered. Furthermore, in the context of the studies for possible future accelerators, some uncharted territories remain such as, for instance, the collective instabilities during the necessary ionization cooling for a muon collider.

Coherent Synchrotron Radiation (CSR) is an important and often detrimental effect in particle accelerators. While one-dimensional models have been successfully used to design and explain the behavior of modern machines, questions remain about their domain of validity. In recent years, two- and three-dimensional models have been developed that are amenable to efficient numerical computation. This article gives an overview of CSR computation from its discovery through the present state of the art.

Computer modeling is essential to research on Advanced Accelerator Concepts (AAC), as well as to their design and operation. This paper summarizes the current status and future needs of AAC systems and reports on several key aspects of (i) high-performance computing (including performance, portability, scalability, advanced algorithms, scalable I/Os and In-Situ analysis), (ii) the benefits of ecosystems with integrated workflows based on standardized input and output and with integrated frameworks developed as a community, and (iii) sustainability and reliability (including code robustness and usability).

The ever increasing demands placed upon machine performance have resulted in the need for more comprehensive particle accelerator modeling. Computer simulations are key to the success of particle accelerators. Many aspects of particle accelerators rely on computer modeling at some point, sometimes requiring complex simulation tools and massively parallel supercomputing. Examples include the modeling of beams at extreme intensities and densities (toward the quantum degeneracy limit), and with ultra-fine control (down to the level of individual particles). In the future, adaptively tuned models might also be relied upon to provide beam measurements beyond the resolution of existing diagnostics. Much time and effort has been put into creating accelerator software tools, some of which are highly successful. However, there are also shortcomings such as the general inability of existing software to be easily modified to meet changing simulation needs. In this paper possible mitigating strategies are discussed for issues faced by the accelerator community as it endeavors to produce better and more comprehensive modeling tools. This includes lack of coordination between code developers, lack of standards to make codes portable and/or reusable, lack of documentation, among others.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 81. It contains two forewords from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent Ph.D. thesis section, a forthcoming beam dynamics events section, and a section of announcements from the beam dynamics panel.

The first electron lenses — understood as "lenses made of electrons" rather than "lenses to focus electrons" — were envisioned in the mid-1990s and built in the early 2000s for compensation of beam-beam effects in the Tevatron proton-antiproton collider. Since then, the lenses — a novel instrument for high-energy particle accelerators — have been added to the toolbox of modern beam facilities, being particularly useful for the energy frontier superconducting hadron colliders ("supercolliders"). In this article we briefly present the history of ideas and developments toward effective use of low-energy high-current bright electron beams in high energy accelerators and discuss the promise of their future applications.

Two electron lenses are installed in Relativistic Heavy Ion Collider (RHIC). They were used as operational head-on beam-beam compensators in proton-proton collisions, with Gaussian transverse electron beam profiles. One of the lenses was also used with a hollow transverse profile to test hadron beam halo removal under various conditions. Although presently not in the design, the lenses may find applications in the Electron-ion Collider (EIC) for either collimation or beam-beam mitigation.

The proposed electron lens for Fermilab's Integrable Optics Test Accelerator (IOTA) will broaden its capabilities by enabling new research in nonlinear integrable optics, space-charge compensation, proton beam cooling, and more. The electron lens is based on a 5–10 keV, 1–2 A electron beam, shaped using a 0.7 m long, 0.8 T solenoidal magnetic field. A cryogen-free superconducting solenoid has been designed to provide this solenoidal field, taking into consideration the constraints on space, utilities, and infrastructure in the IOTA experimental hall. The solenoid is made of copper stabilized niobium-titanium conductor, conduction-cooled using 4 K closed-cycle cryocoolers. This paper describes the overall design of the solenoid encompassing its mechanical construction, current leads optimization, cryogenic thermal modeling that provides estimates of cooldown time and static/dynamic heat loads, and quench analysis.

The McMillan electron lens will be one of the experimental implementations of nonlinear integrable lattices in the Fermilab Integrable Optics Test Accelerator (IOTA). We describe the physics of the nonlinear McMillan lens and calculate the tune dependence with amplitude from the experimental parameters. Some of the implications for the design of experiments in IOTA are discussed.

Electrons lenses produce a high-intensity electron beam and have a variety of applications to circular hadron accelerators. Electron beams of different transverse cross sections and distributions may be designed, depending on the desired application, and they are produced and steered along the orbit of the hadron beam, overlapping with it for typical distances of a few meters before being deflected away and disposed of. Hollow electron beams find applications to high-intensity beam collimation for machines like the CERN Large Hadron Collider (LHC). Such devices can be integrated in a collimation system to improve the halo-cleaning performance through an active control of the halo dynamics: the annular distribution of the electrons excites resonantly the beam tails surrounding the beam core, while the core itself remains unperturbed, as ideally it only "sees" the field-free "hole" in the electron distribution. Hollow electron lenses are part of the upgrade baseline of the High-Luminosity project of the LHC (HL-LHC) and will be installed in the machine during a long shutdown in 2025–2027 to mitigate effects from beam losses so to improve the collimation system performance. This paper describes the hollow electron lens project within the HL-LHC collimation upgrade.

Hollow Electron Lenses (HEL) will be installed at the High Luminosity Large Hadron Collider to provide a continuous and controlled depletion of beam halo particles by interaction with a superimposed hollow electron beam, of intensity as high as 5 A, and radii 1.1–2.2 mm for 7 TeV LHC operations. In this paper, issues related to the propagation of high intensity hollow electron beams are discussed and the simulations of the electron beam dynamics with feedback to the HEL design are presented. The main results are the rise of the electron beam accelerating voltage from 10 kV, as in the initial proposal, to 15 kV and the validation of the 5 T magnetic field at the main solenoids as being sufficient to guarantee a stable electron beam.

The High Luminosity LHC project (HL-LHC) foresees the construction and installation of important new equipment to increase the performance of the LHC machine. The Hollow Electron Lens (HEL) is a promising system to control the beam halo. It improves the beam collimation system of the HL-LHC and mitigates possible equipment damage in case of failure scenarios from halo losses. The halo can store up to 30 MJ energy. The specifications for this new device are quite demanding. The source, an electron gun with an annular shaped cathode, must deliver a current up to 5 A. This is five times higher than the current in the existing electron lenses in Fermi and Brookhaven national laboratories. This note describes the programme carried out to design and test high-perveance guns equipped with two types of high-performance scandate cathodes. The size of the final gun for the HL-LHC lenses is now considerably smaller than the one of the first prototype, allowing a reduction of diameter and cost of the superconducting magnet system used to steer the electron beam. The tests carried out at FNAL, BVERI and BJUT demonstrated that the developed cathodes fulfil the specifications and can supply a 5 A fully Space Charge Limited (SCL) current.

The FAIR heavy-ion synchrotrons SIS18 and SIS100 as part of the new FAIR accelerator facility at GSI will be operated at the "space charge limit" for light and heavy-ion beams. In SIS100 beam loss due to space charge induced resonance crossing should not exceed a few percent during the 1 s injection plateau. In order to further increase the beam intensities beyond the FAIR reference parameters, a new concept of (partial) space charge compensation by pulsed electron lenses is considered. We describe the general space charge compensation concept together with detailed simulations results. A prototype lens for SIS18 is presently under development and will be used to validate the concept. This new approach, aiming for mitigating the most prominent intensity limitation, should be applicable to many of the existing large scale proton and heavy-ion synchrotrons world wide.

Further progress of fundamental particle physics requires high intensity and high brightness of accelerated proton and ion beams. This goal is essential for the FAIR hadron beams at GSI, for the neutrino production at the facilities such as Fermilab and JPARC, and for the Large Hadron Collider luminosity at CERN. One of the most formidable obstacles toward that goal is the beam's own space charge, whose forces cause beam emittance growth, losses and lifetime degradation. Typically, such effects become intolerable when the space charge tune-shift parameter Δ Q_SC exceeds ∼ - (0.25–0.5). To reduce these detrimental effects, it was suggested to use electron lenses to compensate the space charge forces. This paper reports on detailed particle-in-cell space charge and electron lens compensation simulations for extremely intense proton bunches whose space charge tune-shift parameter exceeds -1.0. Different scenarios were evaluated based on reduction in the emittance growth and particle loss at a 4σ aperture. We investigate phenomena and issues related to the focusing lattice errors, importance of the transverse and longitudinal matching of the electron beam profiles to the proton ones, and vary the strength and the number of the electron lenses distributed around a circular machine to optimize the reduction of harmful space charge effects.

Further progress of fundamental physics requires accelerated beams of high intensity. The intensities, however, are limited by many factors, including coherent beam instabilities. Usual methods to control the instabilities, such as octupole magnets, beam feedback dampers and employment of chromatic effects, may be ineffective or insufficient. Electron lenses were proposed as a means to provide stabilizing spread in the beam betatron frequencies. It was shown that electron lenses are uniquely effective for Landau damping of transverse beam instabilities in high energy particle accelerators, and that their employment does not compromise incoherent (single particle) stability, dynamic aperture and the beam lifetime. Here we consider effectiveness of the Landau damping with electron lenses when the space charge tune shift cannot be neglected. We demonstrate that the desired stability can be assured with proper choice of the electron beam parameters and current distributions.

Space charge (SC) forces of a circulating beam in a ring have both linear (defocusing) and nonlinear components, due to a nonuniform beam distribution. The linear component of SC forces produces a betatron tune shift, which is the largest for a zero-amplitude particle, while the nonlinear component produces an amplitude-dependent betatron tune spread. These SC effects are responsible for several undesirable phenomena in accelerators: emittance growth, particle losses, beam halo, etc. In this paper, we investigate the possibility to mitigate the distributed SC forces by a thin McMillan lens, providing an axially-symmetric kick, which is qualitatively opposite to the accumulated effect of beam's own SC. Experimentally, the proposed concept can be tested in Fermilab's IOTA ring. A thin McMillan lens can be implemented by a short (70) insertion of an electron beam with a specifically chosen density distribution in transverse directions. In this article, to test if McMillan lenses can reduce the tune spread induced by SC, we make several simulations with a 6-D particle tracking code, Synergia. We choose such beam and lattice parameters that the SC tune spread is roughly 0.5 and the emittance growth due to the half-integer resonance is clearly observed without the SC compensation. Then, we focus on reducing the emittance growth by adjusting the bare betatron tunes using the ring quadrupoles, and reducing the tune spread by the McMillan lenses. The results of reducing a large tune spread (≈ 0.5), reported here, are not perfect, but substantial. There is still room for further investigation. The simulations performed so far indicate that McMillan lenses can cope with an SC tune spread of ≈ 0.1 per lens.

Space charge compensation allows for increased beam current in linacs and RFQs. A novel application of space charge compensation, the electron column, offers the opportunity to realize more intense beams for high energy physics in circular accelerators. The concept relies on ionization of residual gas by the primary beam, with electromagnetic fields used to confine and shape the space charge neutralizing plasma. Prior experimental efforts and simulation studies are reviewed. They indicate that electron columns could be successfully deployed in accelerator rings. The experimental demonstration of an electron column is underway at the Integrable Optics Test Accelerator at Fermilab. The experiment, instrumentation, and physics program are discussed.

The electron lens in the Fermilab Integrable Optics Test Accelerator (IOTA) will enable new research in nonlinear integrable optics, space-charge compensation, electron cooling, and the stability of intense beams. This research addresses scientific questions on high-brightness beams and operational challenges of high-power accelerators for nuclear and particle physics. We review the roles that electron lenses play in this field and the physical principles behind their applications. The design criteria and specifications for the IOTA storage ring and electron lens are then discussed. We conclude with a description of the components of the apparatus.

In this paper we study the possibility of the space-charge compensation in the Fermilab Booster rapid cycling synchrotron with the use of several electron columns and show the significant promise of such technique.

Electromagnetic interaction of colliding beams along with other nonlinear fields often limits the beams' lifetimes and luminosities. Nonlinearities result in the spread of betatron frequencies (footprint) and, thus, may enhance dynamic diffusion of particles due to high order resonances. One of the possible ways to eliminate nonlinearities and overcome the corresponding difficulties is compensation of nonlinear forces, but, in practice, it is hardly possible to obtain exact linearity of the system. The compensation with a single nonlinear lens cannot cope with distributed nonlinearities, nonlinearities due to parasitic crossings, etc. In the article, we present a method to compute parameters of nonlinear element (lens) that eliminates both the footprint and resonance strength without achieving full compensation.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 80. It contains two forewords from the editor-in-chief and the issue editor, a recent Ph.D. thesis section, and a section of announcements from the beam dynamics panel.

The single bunch selector of the SPIRAL2 accelerator allows reducing the bunch rate to the target of the Neutron For Science (NFS) facility in view of time of flight experiments. It involves two processes: a static dipole magnet deviates the beam on a 7.3 kW scraper, while the electric field of two high voltage travelling wave pulses maintains the selected bunch on axis at the required repetition rate. This original principle, named "inverted duty cycle", was chosen to provide reasonable power requirement for the pulse generators, high rejection of the side bunches, and safe operation for the experimental target. This paper gives an overview of the whole system, recalls the main component design issues and performances and reports the successful commissioning results with the first 0.2 and 5 mA proton beams.

The first cavity of a standalone superconducting (sc) continuous wave (cw) heavy ion Linac as a demonstration of the capability of 217 MHz multi gap Crossbar H-mode structures (CH) has been already commissioned at high acceleration gain. The worldwide first beam test with a superconducting multi gap CH-cavity was a milestone of the R&D work of HIM and GSI in collaboration with Goethe University Frankfurt (GUF) in preparation of the sc cw heavy ion Linac project, substituting GSI-UNILAC as a heavy ion high duty factor Linac. Recently the first two of four fully equipped cw-Linac cryomodules are in procurement. To meet the future FAIR science requirements higher beam intensity has to be achieved in the present GSI-accelerator complex. In the last years ion source developments, in particular for the high current Vacuum Arc Ion Sources (VARIS), were concentrated on heavy elements, as Bi and Pb, aiming for stable routine ion source operation at a sufficient rep. rate and high production efficiency. Stripping is a key technology for all heavy ion accelerators. After upgrade of the supersonic N ₂ -gas jet implementation of high current foil stripping, recently a new H ₂ gas cell, using a pulsed gas regime synchronized with arrival of the beam pulse has been developed. An enhanced stripper gas density as well as a simultaneously reduced gas load results in an increased stripping efficiency, while the beam emittance remains the same. A new record beam intensity (11.1 emA) for ²³⁸ U ²⁸⁺ beams at 1.4 MeV/u has been achieved, applying the pulsed high density H ₂ -stripper target to a high intensity ²³⁸ U ⁴⁺ beam from the VARIS ion source. Further ion source developments have been accomplished recently providing for sufficient heavy ion beam intensities at the High Current Injector Linac. A machine investigation program has been performed in 2020. The focus was to optimize the entire FAIR injector chain for high intensity heavy ion beam after the successful implementation of different upgrade measures. Besides a dedicated operation mode applying UNILAC, as a heavy ion Linac, at a synchronous phase significantly lower than 30 degrees for high intensity proton beam, could be established. Thus, UNILAC is able to deliver a sufficient proton beam intensity for the FAIR commissioning phase, when the FAIR-proton Linac is not yet available.

The superconducting, heavy ion synchrotron SIS100 is the core of the new FAIR facility at GSI, Darmstadt, Germany. Its unique design is dedicated to the acceleration of intermediate charge state heavy ions. Several new technical approaches assure the stabilization of the vacuum dynamics and the minimization of charge related beam loss. Beside high intensity heavy ions, SIS100 will accelerate all ions from Protons to Uranium, and in spite of the fact that superconducting magnets are used, SIS100 shall be as flexible in ramping and cycling as a normal conducting synchrotron.

The Facility for Rare Isotope Beams (FRIB) will be the world's premier rare-isotope beam facility. Experiments with the majority (∼80%) of the isotope predicted to exist will become available. The FRIB facility is based on a superconducting (SC) heavy ion linac with output energy above 200 MeV/u for any ions at beam power of 400 kW. FRIB includes a target facility for in-flight production of rare isotopes. A three-stage fragment separator will be used to prepare fast rare isotope beams with high-purity for nuclear physics experiments. The installation work of the accelerator and experimental systems is approaching completion and multi-stage beam commissioning activities started in summer 2017 with expected project completion in early 2022. The commencement of operation for users' experiments is planned immediately following the project completion.

The RAON (Rare Isotope Accelerator complex for ON-line experiments) accelerator that can accelerate various kinds of beams from proton to uranium using superconducting radio-frequency (RF) cavities is currently under construction with the goal of completion in 2021 by the Rare Isotope Science Project in Daejeon, Korea. At Sindong site in Daejeon, the accelerator equipment of the low energy linac section consisting of the quarter-wave resonator (QWR) and half-wave resonator (HWR) cavities is being installed in 2020, and the installation of accelerator equipment including single-spoke resonators (SSR1 and SSR2) cavities in the high energy linac section will be completed by the end of 2021. According to the installation schedule, we are preparing the beam commissioning to verify the performance of the accelerator equipment and to achieve the desired beam parameters. For the successful beam commissioning, several physics application tools to be used at the beam commissioning are being developed, and the beam dynamics studies have been also continued. Here we will present ongoing beam dynamics studies and beam commissioning scenarios under planning. In addition, we will introduce the application tools being developed for successful beam commissioning in the RAON accelerator.

The Heavy Ion Research Facility in Lanzhou (HIRFL) is a multi-disciplinary research facility that provides heavy ion beams for physical, biomedical and material sciences. It is a major academic facility of China and one of the world's important centers in nuclear physics and accelerators. The facility was built step by step at Institute of Modern Physics (IMP) over a half century. The first cyclotron was built with great assistance from the former Soviet Union in 1960s, and the newest linear accelerator was tested successfully in 2019. The HIRFL accelerator can provide beams from proton to Uranium with energies of hundreds MeV/u, and hence diverse fundamental sciences and applied researches were carried out at IMP. In this paper, an introduction of the HIRFL accelerator complex was presented. Details of the HIRFL components including ion sources, cyclotrons, synchrotrons, linac and experimental terminals were described. The current operation status and upgrade plans were reported.

The High Intensity heavy-ion Accelerator Facility (HIAF) is under construction. The lattice of HIAF/BRing is the same for both heavy ion beams and proton beam. Since the proton beam will cross the transition energy during acceleration, a γ_t jump scheme with maximum Δ γ_t of 2 is designed to reduce the growth of momentum spread and influence of collective effects near transition. The broadband impedance of the ceramic-lined rings in the vacuum chamber has been significantly reduced by a 2 μm-copper coating. Thus, the impedance model has been updated for analyzing collective effects in the BRing. The longitudinal collective effects can be compensated by the multi-harmonic feedforward system. The transition energy crossing point should be optimized to reduce the bunch oscillations after transitions. In both two transverse planes, the normalized emittance growth given by the Simulation Platform for Collective Instabilities (CISP) are stopped by the broadband impedance near transition and there is no obvious transverse collective instability, which is quite different from the Vlasov results. More researches will be performed to find out the reason.

The use of charge strippers is almost inevitable for the efficient acceleration of particularly heavy ions such as uranium in heavy-ion accelerator complexes. At the RIKEN RI beam factory (RIBF), the total charge stripping efficiency of two strippers, He gas and rotating graphite sheet disk strippers, used for uranium acceleration is less than 5%, which creates a serious bottleneck for potential intensity upgrades in the near future. We have proposed using charge stripper rings (CSRs) as a cost-effective way to enhance the charge stripping efficiency in a multi-stage accelerator complex involving cyclotrons such as the RIBF. In this paper, we present some calculation results on the key design issues of a CSR.

Main scientific goal of the NICA/MPD project (Nuclotron-based Ion Collider fAcility/ Multi Purpose Detector) under construction at JINR (Dubna) is exploration of the phase diagram of strongly interacting matter in the region of high compression. The planned experimental program includes studies of possible signs of the phase transitions and critical phenomena in heavy ion (up to Au) collisions at the center-of-mass energies up to 11 GeV/u. The collider experiment provides optimum conditions for efficient energy scan measurements. Attainment of the required average luminosity of the order of 10²⁷ cm^-2·s^-1for such physics experiment faces several accelerator physics and technology challenges. Contrary to high energy colliders, the luminosity of NICA case will be mostly limited by the Laslett tune shift, while the beam-beam tune shift parameter will be negligibly small. Flexible operational procedures for the beam storage and short bunch formation have been developed to provide maximum peak luminosity over wide energy range. Beam cooling is critical to counteract luminosity decay due to intra-beam scattering. NICA collider design addresses these issues and satisfies all the requirements for the colliding beams experiments.

This is a general information article of the ICFA Beam Dynamics Newsletter No. 79. It contains two forewords from the Editor-in-Chief and the issue editor, a workshop and conference report section, a recent Ph.D. thesis section, a forthcoming beam dynamics events section, and a section of announcements from the beam dynamics panel.

The question of interplay of coherent and incoherent space charge driven resonances and of their Landau damping has found some interest in beam dynamics of modern high-intensity synchrotrons. We revisit the theoretical and simulation models describing coherent half-integer parametric resonances, analyze their Landau damping in 2D beams on the basis of simulated tune spectra and conclude that above second order (envelope modes) they play no role in realistic, Gaussian-like beam models. We also analyze incoherent resonance effects in the beam core regions and find that their role has been underestimated in part of the literature, in particular with regard to the very long-term beam evolution as in synchrotrons. We conclude that for such time scales more careful analysis of realistic simulation models—including the incoherent frozen space charge approximations—is needed to support synchrotron design and evaluation of experiments.

A brief overview is given of the recent work done by the beam-physics group of Hiroshima University on resonant beam instabilities in high-intensity hadron rings. Emphasis is placed upon two essentially different instability mechanisms, namely, coherent and incoherent resonances that occur respectively in the beam core and in the beam tail. On the basis of a semi-empirical coherent resonance condition, a simple rule is introduced to construct a new type of stability map. The proposed diagram enables one to find the optimum operating region of any circular machine in the betatron tune space very easily and quickly.

The effect of space charge in bunches stored for many synchrotron oscillations combined with the nonlinear resonances in a storage ring may create a particle diffusion to large amplitudes and eventually create unwanted beam loss. We review and discuss the mechanism which creates beam loss with the focus on how to estimate the number of particles that will be crossed by the stable fixed lines generated by a coupled resonance and space charge.

Accelerator storage ring designs based on nonlinear integrable Hamiltonian systems provide a novel test-bed for studying the interplay between nonlinear dynamics and space charge at high intensity. In this work, the structure of beam Vlasov equilibria is explored for a constant focusing channel based on the nonlinear focusing potential of the Integrable Optics Test Accelerator. The dynamics of the single-particle orbits is explored in the combined space charge and external focusing fields as a function of beam current, and the self-consistent relaxation of a mismatched beam to equilibrium is characterized.

The SIS100 synchrotron as a part of the new FAIR accelerator facility at GSI should be operated at the "space charge limit" for light and heavy-ion beams. Losses due to space charge induced resonance crossing should not exceed a few percent during a full cycle. Detailed magnet field measurements are now available for 72 out of the total 108 main SIS100 dipole magnets. Particle tracking studies including nonlinear field errors up to 7^th order in the main magnets together with different space charge models are performed. Because of the long time scales reduced space charge models are employed for tune scans. First comparisons with simulations using a self-consistent space charge solver are discussed as well as potential measures to further improve the options in tune space for the reference intensities and beyond.

Space charge effects in high intensity and high brightness synchrotrons can lead to undesired beam emittance growth, beam halo formation and particle loss. A series of dedicated machine experiments has been performed over the past decade in order to study these effects in the particular regime of long-term beam storage as required for certain applications. This paper gives an overview of the present understanding of the underlying beam dynamics mechanisms, with particular emphasis on space charge induced losses and the experience gained at the CERN injector complex. The focus is on the space charge induced periodic resonance crossing, which has been identified as the main mechanism causing beam degradation for long storage times. Examples of space charge driven and error driven resonances are presented, including possible mitigation strategies. Furthermore, an outlook for possible future directions of studies is presented.

The J-PARC 3-GeV rapid cycling synchrotron (RCS) has recently achieved a 1-MW beam operation with considerably low fractional beam loss of a couple of 10⁻³ as a result of continuous efforts iterating experiments and numerical simulations. This success of the 1-MW beam operation opened a door to further beam power ramp-up beyond 1 MW; we are now promoting 1.2∼1.5-MW-equivalent high-intensity beam tests looking ahead to future upgrades at J-PARC. In this article, we first review the current status of beam loss in the 1-MW beam operation, then presenting the recent results of the 1.2-MW beam tests with particular emphasis on our approaches to beam loss issues. The beam intensity limit of the RCS is also discussed with well-established numerical simulations.

The China Spallation Neutron Source (CSNS) accelerators consist of an 80 MeV H⁻ linac and a 1.6 GeV proton rapid cycling synchrotron (RCS), which provides 1.6 GeV proton beam to the target at a repetition rate of 25 Hz. The beam commissioning of CSNS accelerators had been started since April 2017, and to the end of February 2020, the CSNS reached the design beam power of 100 kW. The most important issue in high-power beam commissioning is the beam loss control, as well as the control of induced activities, to meet the requirement of manual maintenance. A series of beam loss optimization work had been done to reduce the uncontrolled beam loss. Transverse and longitudinal matching in the MEBT was performed. For RCS commissioning, the orbit and lattice correction, the matching between the rf frequency and the dipole field, the tuning of the injection beam were performed. Many efforts have beam made to depress the effects of space charge and collective instability for beam loss control, such as injection optimization, longitudinal beam dynamics optimization, tune optimization. The beam commissioning procedure and results for CSNS accelerators are introduced in this paper.

In high-intensity linear accelerators, the tune spreads induced by the space-charge forces in the radial and longitudinal planes are key parameters for halo formation and beam losses. For matched beams, they are the parameters governing the number of resonances (including coupling resonances), which affect the beam and determine the respective sizes of the stable and halo areas in phase space. The number and strength of the resonances excited in mismatched beams leading to even higher amplitude halos are also directly linked to the tune spreads. In this paper, the equations making the link between the basic linac parameters (rf frequency, zero-current phase advances, beam intensity and emittances) and the tune spreads are given. The analysis of the way these linac parameters can be chosen to minimize the tune spreads is presented. The ESS linac parameters are used as an example for this study.

Space charge forces are a significant contribution to the beam dynamics in high intensity hadron linacs, often causing beam halo that results in beam loss. Currently, there is limited success in modeling beam distributions in high intensity hadron linacs at the RMS level, and no demonstrated capability for measuring or modeling distributions at the level of beam halo. This article discusses the current state of the art capability in this area, and explores next steps in both simulation and beam instrumentation that will enable measurement and modeling of beam halo in high intensity hadron linacs.

The LANSCE accelerator has been in operation for nearly five decades. During this period, it was transformed from a 0.8 MW average proton beam power machine used primarily for the study of meson physics, to a multi-user high-power facility for fundamental research and national security applications, concurrently delivering beams to five distinct experimental areas. Minimization of beam losses in multi-beam application is a key issue for effective operation of accelerator facility. This paper summarizes recent experimental results in understanding and lowering of beam losses in LANSCE linear accelerator and high-energy beam transport lines.

The proton linac of the European Spallation Source, under construction in Lund, Sweden, had beam commissioning of its ion source (IS) and the following low energy beam transport (LEBT) at their final locations from September 2018 to July 2019. This was first of several beam commissioning stages for the linac of ESS, towards the start of the user program in 2023. This paper presents highlights of characterizations of the IS and LEBT from the aforementioned beam commissioning period, including behavioral change of the IS against its parameters, error source identifications of the beam trajectory in the LEBT, and preliminary characterization of the LEBT output beam against solenoid strengths in LEBT.

The nonlinear space-charge effects are an important topic in high intensity accelerator beam dynamics and have been extensively studied using macroparticle tracking simulations. In this paper, we report on recent advances in the simulation of space-charge effects using a symplectic multiparticle space-charge model. The transverse space-charge limit was explored for a periodic focusing and defocusing lattice. The artificial numerical emittance growth in the macroparticle space-charge simulation was analyzed using a one-dimensional model.