Table of contents

This volume contains the proceedings of the International Workshop on Sources of Polarized Leptons and High Brightness Electron Beams (PESP2010). The workshop was hosted by the University of Bonn from 21–24 September 2010. PESP2010 has been conducted as satellite workshop preceding the International SPIN Physics Symposium (SPIN2010). In view of the expanding and challenging field of the production of high brightness lepton beams, the selection of workshop topics was extended to cover the demands of modern accelerator developments and applications. As novel light sources such as FELs and ERLs place new requirements on the properties of unpolarized beams in terms of brightness and intensity, the generation of DC and pulsed beams was included as a major topic in the scientific program. In keeping with tradition, polarized beams formed another key topic. The discussion was however extended to the generation of polarized positron beams, as e.g. required for future linear colliders. Besides the presentation of recent developments in these fields and status reports from laboratories worldwide, additional emphasis was put on the applications of polarized electron beams.

PESP2010 was attended by 61 registered participants from all over the world. The scientific program comprised 30 talks in plenary sessions and 7 posters. An overview of the scientific results presented at PESP2010 was given in a summary talk at the SPIN2010 symposium. In total 24 contributions to the proceedings were submitted and reviewed by members of the scientific advisory committee before publication in this volume.

Many people contributed to the success of the workshop. Among them are members of the Local Organizing Committee as well as the scientific and technical staff of the Physics Institute and the in-house accelerator facility ELSA, who patiently and efficiently took care of the numerous organizational aspects related to the event. The environment they created was highly conducive to a productive working atmosphere and fruitful discussions. The dedication of all who contributed is gratefully acknowledged. We would like to extend our warmest thanks to all of them.

Bonn, May 10, 2011

Wolfgang Hillert and Kurt Aulenbacher Editors

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

PESP2010 was a satellite workshop preceding SPIN2010, published as Volume 295 of Journal of Physics: Conference Series.

Polarization of both beams at a future Linear Collider would be ideal for facing both expected and unforeseen challenges in searches for new physics: fixing the chirality of the couplings and enabling the higher precision for the polarization measurement itself as well as for polarization-dependent observables, it provides a powerful tool for studying new physics at the future Linear Collider, such as discovering new particles, analyzing signals model-independently and resolving precisely the underlying model. Techniques and engineering designs for a polarized–positron source are well advanced. Potential constraints concerning luminosity, commissioning and operating issues appear to be under control. This article mainly treats with the impact of polarized beams on physics beyond the Standard Model, whereas the fundamentals in polarization as well as the gain in electroweak precision physics are summarized in the corresponding part I [1].

At the superconducting Darmstadt electron linac a 100 keV source of polarized electrons has been installed. Major components had been tested prior to installation at an offline teststand. Commissioning of the new source at the S-DALINAC will take place early in 2011. We report on the performance of the teststand, simulations, developments on the laser systems, new radio-frequency components for the S-DALINAC injector, and the status of the implementation of the source.

Since 2000 an inverted source of polarized electrons at the electron stretcher accelerator ELSA routinely provides a pulsed beam with a polarization degree of about 80%. One micro-second long pulses with 100 nC charge are produced by irradiating a strained-layer superlattice photocathode with laser light from a flashlamp-pumped Ti:Sa laser. A rectangular pulse shape is achieved by operating the source in space charge limitation.

The proposed hadron physics program requires an intensity upgrade to 200 mA which can be achieved by enlarging the emission area or by improving the quantum efficiency (QE). The resulting changes of the beam parameters (like emittance and space charge) and of the optics of the transfer line were investigated in numerical simulations.

In order to enhance the source performance a new load lock system with crystal storage and atomic hydrogen cleaning will be installed in the near future.

Development of two different polarized electron guns is ongoing at BNL. One aims at extremely high brightness at a moderate beam current. This design uses a superconducting RF gun and a test setup is built to show that a Gallium-Arsenide cathode with negative affinity has a sufficiently long quantum efficiency lifetime in such an environment. An electron injector using this technology may eliminate the need of the electron damping ring and a long transport line at the International Linear Collider. The other project aims at producing a high beam current with moderate emittance requirements, dubbed the "Gatling gun". In this DC gun, bunches are extracted from 20 separate cathodes and merged into a single beam using a rotating magnetic field. Such an electron gun could serve as an injector for the electron-ion collider eRHIC, which is planned at BNL. We will report on the status of these projects.

We have developed a 500-kV, 10-mA photocathode DC gun for energy recovery linac (ERL) light sources. A segmented ceramic insulator with guard rings is employed to improve robustness at high voltage operation, because this structure can prevent field emission electrons from directly striking the ceramic surface. We have recently succeeded in applying 500 kV on the ceramics for eight hours without any discharge. This high voltage testing was performed with a simple configuration without NEG pumps, cathode and anode electrodes to mainly study the field emission from a tube supporting the cathode electrode. The same high voltage testing with a full configuration necessary for beam generation was carried out up to 380 kV where some increase of radiation was observed. Up-to-date status of our gun development is presented in detail.

Accelerator drivers for Energy Recovery Linac (ERL) and Free-Electron Laser (FEL) based light sources demand electron injectors which deliver high brightness bunches on the several hundreds of picocoulomb scale, at repetition rates between 1 MHz and 1 GHz (or higher), corresponding to an average current between 0.1 and 100 mA. Simultaneous satisfaction of these injector requirements is considerably beyond the current state-of-the-art. Daresbury Laboratory is concentrating efforts on the development of high average current III-V and X_nY_3−nSb photocathode-based DC and SRF photocathode guns for ERL applications. The ultimate goal of this research is their integration with the ALICE ERL.

The ALICE accelerator (Accelerators and Lasers In Combined Experiments) at Daresbury Laboratory is a 35 MeV ERL. The electron beam drives an infra-red free-electron laser (FEL) and THz light sources, but can also be used to generate X-rays through Compton back-scattering (CBS). ALICE also acts as the injector for the EMMA NS-FFAG machine. This paper will outline the project status and milestones achieved thus far, focussing on the status and performance of the photoinjector gun and the injection line.

As the first superconducting RF photo-injector (SRF gun) in practical operation, the SRF gun has been successfully connected to the superconducting linac ELBE at Forschungzentrum Dresden-Rossendorf. The injection with this new gun will improve the beam quality for the users of the radiation source. The SRF gun contains a 3½ cell superconducting accelerating cavity with a frequency of 1.3 GHz. The design is for use of normal conducting photocathodes. At present, caesium telluride photocathodes are applied which are illuminated by an ultraviolet laser beam. The kinetic energy of the produced electron beam is 3 MeV which belongs to a peak electric field of 16 MV/m in the cavity. The maximum bunch charge which is obtained and measured in a Faraday cup is about 400 pC (20 μA average current at a repetition rate of 50 kHz). The SRF gun injector is connected to the ELBE accelerator via a dogleg with two 45° deflection magnets. This connection beam line was commissioned in January 2010. A first beam injection into the ELBE accelerator has been carried out with a bunch charge of 120 pC (6 μA at 50 kHz). Detailed measurements showed that beam loss occurred in the dogleg above 60 pC due to the correlated energy spread. In order to find the optimal operation conditions, energy spread was measured in dependence of bunch charge, laser phase and further gun parameters. The Cs₂Te photocathode shows an excellent life time. It is in the gun since May 2010 with about 300 h beam time and about 7 C extracted charge. In the present cavity, the limit for the acceleration gradient is field emission due to some defect on the cavity surface and problems during cleaning. Therefore a modified 3½ niobium cavity has been fabricated, which will increase the RF gradient in the gun and thus improve the beam parameters further.

As part of the BERLinPro study, HZB is developing an SRF photoelectron injector. The R&D will be carried out in three stages, the first of which is currently being installed at HZB's HoBiCaT facility. It consists of an SRF cavity with SC solenoid, electron beam diagnosics and drivelaser systems.

We have suggested using a superlattice semiconductor instead of a bulk semiconductor as an NEA-photocathode for small emittance and high quantum yield because the mini-band energy width and high joint density of states at the bottom of the conduction band can bring small photoelectron's momentum spread and high quantum yield at excitation energy of the band gap. We developed the test sample of the AlGaAs-GaAs superlattice semiconductor designed by using Klonig-Penny-Bastard model. The obtained quantum yield spectrum from the AlGaAs-GaAs superlattice semiconductor after NEA-surface activation was a step function of excitation photon energy most likely caused by the quantum confinement effect.

We successfully developed a transmission-type photocathode, and a high spin-polarization (90%) with a super-high brightness (1.3×10⁷ Acm⁻²sr⁻¹) of electron beam was achieved. In this research, the elimination of thickness modulation of GaAs/GaAsP superlattice by introduction of a GaAs inter-layer on a GaP substrate is the key point to realize the high spin-polarization. The thickness modulation of the superlattice is related with the surface roughness of the buffer layer. The compressive strain introduced in the GaAsP buffer layer on the GaP substrate causes hillock formation, where several degree off-angle surfaces are formed. The GaAs inter-layer deposited on the GaP substrate introduced a tensile strain in the GaAsP buffer layer instead of the compressive strain and relatively smooth GaAsP buffer layer was achieved. The smooth GaAsP buffer layer was attributed to the periodic GaAs/GaAsP superlattice layer growth.

Although NEA (negative electron affinity) surface has been attracting great interest, atomic level structure of the NEA surface has not been elucidated. In this work, Cs/GaAs NEA surfaces were analyzed by fluorescence XAFS (x-ray adsorption fine structure) method. Cs-LIII XAFS measurement was conducted on the Cs/GaAs surfaces obtained by Yo-Yo method. By analyzing the local structure around Cs from the XAFS spectra, it was shown that Cs-O-As (or Ga) or O-Cs-As (or Ga) structure might be formed on Cs/GaAs surface. The results suggested that single atomic layer of Cs was adsorbed on GaAs surface intermediated by O on the Cs/GaAs NEA surface.

The results of experimental and theoretical studies of spin polarized electron transport in semiconductor SL used for photoemitter application are presented. The experimental study is based on the time resolved measurements of electron emission from the cathode after its photoexcitation by fs laser pulse. The response and spin relaxation times have been determined by means of measured time dependent intensity and polarization of electron emission. We also performed theoretical calculations of photocathode pulse response and compared the obtained results with experimental data. Our analysis testifies the presence of partial electron localization in SL. Electron capture by localized states leads to unavoidable losses of quantum efficiency and polarization.

The photoemission spectroscopy using synchrotron radiation has been carried out to study the electron affinity of cesium telluride thin films. We observed a formation of NEA surface on p-type gallium arsenide deposited with cesium telluride. An ultra thin film of cesium telluride is discussed as a possible candidate forming an NEA surface. The electron affinity of cesium telluride thin film was found to increase with time after fabrication. An increase of the affinity resulted in deterioration of a quantum efficiency under laser light exposure with constant wavelength. The surface of photocathode in RF gun can suffer enormous damage easily at the conditioning process of RF gun before electron extraction. A method to refresh the photocathode surface after RF conditioning has been required. A photodetachment process is a candidate to refresh aged/damaged photocathode with X-ray exposure. A recovery of quantum efficiency for the aged photocathode film by this refreshing process is shown.

The superconducting RF test facility (STF) in KEK is a facility to promote R&D of the International Linear Collider (ILC) cavities and cryomodule. L-band photocathode RF gun has been developed at KEK-STF as an electron beam source for cryomodule test scheduled in autumn of 2011. The RF cavity of the gun will be operated with a 1.3 GHz RF frequency, 1 msec RF pulse width, 5 Hz repetition rate at normal conductivity. The cavity was prepared by collaborative work with DESY and FNAL, and fabricated by FNAL. The RF conditioning of the cavity has been started since April 2010. A cesium telluride thin film as a photocathode material has been adopted, and the preparation equipment for cesium telluride has been newly designed and constructed. By using this new system, a fabrication and a performance estimation of the cesium telluride thin film as a photocathode are the next step of the research.

In order to study spin related phenomena in nano-size materials, spin-polarized electron source (PES) has been employed for the incident beam in transmission electron microscope (TEM). The PES has been designed and constructed with optimizing for spin-polarized TEM. The illuminating system of TEM is also designed to focus the spin-polarized electron beam emitted from a semiconductor photocathode with a negative electron affinity (NEA) surface. The beam energy is set to below 40 keV which is lower energy type as a TEM, because the spin interaction with condensed matters is very small corresponding with a Coulomb interaction. The polarized electron gun has realized in an extra high vacuum (XHV) condition and high field gradient of 4 MV/m on a surface of photocathode. Furthermore, it demonstrated that 40-keV polarized electron beam was operated with a sub-milli second pulse mode by using the backside excitation type photocathode. This high performance PES will make it possible to observe dynamically a magnetic field images with high contrast and highspeed temporal imaging in TEM.

Recently, we have developed a transmission-type polarized electron sources (PES)s for the generation of a high brightness beam. The developed PES can applied the extraction voltage of 20 kV at 4 mm electrode-gap and enables to realize the source beam radius of a few micro meter. As the results of prototype gun experiments, the brightness of ∼2 × 10⁷ A.cm⁻².sr⁻¹ and a charge density lifetime of 1.8 × 10⁸ C.cm⁻² were obtained. A maximum polarization of ~90 % and quantum efficiency of 0.09 % was achieved simultaneously using the transmission photocathode with GaAs-GaAsP strained superlattice layers. Up to now, another two electron gun was manufactured, and those mechanical designs are almost same as that of the prototype. The prototype gun and the second gun have been already operated for the photocathode R&D and for the Spin-LEEM application, respectively. In the near future, further experiments are prepared by using the third gun.

This paper describes measurements of the beam polarization and quantum efficiency for photoemission using two-photon excitation from unstrained bulk GaAs illuminated with pulsed, high intensity 1560nm laser light. Quantum efficiency is linearly proportional to 1560nm peak laser intensity, which was varied in three independent ways, indicating that the emitted electrons are promoted from the valence to the conduction band via two-photon absorption. Beam polarization was measured using a microMott polarimeter, with a value of 16.8(4)% polarization at 1560nm, which is roughly half the measured value of 33.4(8)% using 778 nm light.

We have measured the extractable charge during one lifetime (Charge lifetime, Q_τ) of a NEA-GaAs-photocathode for two beam diameters. A limitation of Q_τ to 1100 Coulomb due to ion-back-bombardment was observed for a 1.5 mm diameter emission site. When increasing the laser diameter from .3 to 1.5 mm we observe an increase of Q_τ by a factor ≈ 5, which is in disagreement with the assumed proportionality of the charge lifetime with emission area. Possible reasons for this discrepancy are analyzed.

The high luminosity requirements and the option of a polarized beam present a great challenge for the design of the positron source for future linear colliders. This contribution provides an overview of the proposed designs for the polarized positron sources of the International Linear Collider (ILC) and the Compact Linear Collider (CLIC).

It is a challenge to provide an intense polarized positron source for future linear colliders. The positron yield, polarization and spin transport have to be optimized taking into account the energy deposition in the positron target and the activation of the source area. The Geant4-based simulation tool PPS-Sim has been developed for this purpose. Different production schemes, target options and matching devices can be configured and simulated. All source components and field parameters can be chosen easily. The main features of PPS-Sim as well as some selected simulation results are presented.

For new experiments with polarized electron and positron beams the precise and quick measurement of the beam polarization is required. The relative electron polarization has been monitored on-line using a compact transmission Compton polarimeter for a wide range of average electron currents up to 100/μA and electron beam energy of 3.5 MeV. The asymmetry induced by the beam polarization can be monitored precisely with ΔA/A < 0.2 %. Even small relative variations of the beam polarization during a beam time may be detected using this polarimeter, such as an increase ∼ 1 % and subsequent decrease of the beam polarization.

A Fabry-Perot cavity polarimeter installed in 2003 at HERA was designed to measure the longitudinal polarisation of the electron beam with high precision for each electron bunch spaced with a time interval of 96 ns. By interacting a high intensity laser (enhanced up to a few kW within the cavity) with the HERA electron beam it is possible to measure its polarisation with a relative statistical precision of 2% per bunch per minute. Detailed systematics studies have been performed and in particular a complete theoretical model has been developed in order to control at the per mill level the degree of circular polarisation of the laser beam. The result is a total systematic uncertainty of 1%. This is the first time that such a precision is achieved in the difficult, hostile and noisy environment of a particle collider.

Electron/photon tensor-correlation coefficients may allow to design a polarimeter that can measure all components of beam polarization simultaneously—a so-called vector polarimeter. Besides its purpose as a beam diagnostic device this vector polarimeter would also allow to test theoretical predictions for the electron-photon polarization correlations at energies between 1 MeV and 3.5 MeV. As a first step we have set up a measurement of the helicity transfer to the photon as a function of energy which is based on the Compton absorption method.