Table of contents

In magnetically confined fusion devices, the use of millimeter waves (mmw) at the electron cyclotron (EC) frequencies ranges from plasma diagnostics to plasma heating, current drive and core confinement preservation. For large tokamaks such as ITER, numerical simulations and analytical estimates suggest that plasma edge turbulence could significantly broaden the EC-beam, possibly preventing tearing modes stabilization at the designed power levels. We report measurements of mmw-beam scattering by plasma turbulence in the TCV tokamak. A mmw-Gaussian beam is injected from the top of the device and the transmitted power is measured at the bottom. We show that the measured plasma density fluctuations in the upper part of the scrape-off layer (SOL) are the cause of fluctuations of the transmitted mmw-power. A full-wave model based on COMSOL multiphysics is presented and compared against the wave-kinetic-equation solver WKBeam in a TCV case. Using the SOL turbulence simulations from the GBS code, comparison between the scattering effect on the mmw-beam with both the full-wave simulations and the experiments are ongoing. We also present experimental observations of rapid changes in the transmitted power caused by ELMs in ELMy H-mode plasma.

High external gas injection rates are foreseen for future devices to reduce divertor heat loads and this can influence pedestal stability. Fusion yield has been estimated to vary as strongly as ${T}_{e,{\rm{ped}}}^{2}$ so an understanding of the underlying pedestal physics in the presence of additional fuelling and seeding is required. To address this, a database scanning plasma triangularity, fuelling and nitrogen seeding rates in neutral beam (NBH) heated ELM-y H-mode plasmas was constructed on TCV. Low nitrogen seeding was observed to increase pedestal top pressure but all other gas injection rates led to a decrease. Lower triangularity discharges were found to be less sensitive to variations in gas injection rates. No clear trend was measured between plasma top P_e and stored energy which is attributed to the non-stiffness of core plasma pressure profiles. Peeling ballooning stability analysis put these discharges close to the ideal MHD stability boundary. A constant for D in the relation pedestal width $w=D\sqrt{{\beta }_{\theta }^{{\rm{Ped}}}}$, was not found. Experimentally inferred values of D were used in EPED1 simulations and gave good agreement for pedestal width. Pedestal height agreed well for high triangularity but was overestimated for low triangularity. IPED simulations showed that relative shifts in pedestal position were contributing significantly to the pedestal height and were able to reproduce the measured profiles more accurately.

Poloidal asymmetries of the E × B plasma flow are known to play a role in neoclassical transport. One obvious reason is that an asymmetrical potential can produce a flux across the magnetic field. Also the associated distribution function may correlate with the magnetic drift velocity to enhance the neoclassical flux. Finally, poloidal variations of the electric potential can produce poloidal asymmetries of an impurity density, which in turn may modify the neoclassical transport coefficients. According to conventional neoclassical theory, the level of poloidal asymmetry of the electric potential is expected to be very small. Poloidal flow asymmetries can be driven by small scale turbulence via nonlinear coupling, and therefore change this result. In the present work, a general framework for the generation of axisymmetric structures of potential by turbulence is presented. Zonal flows, geodesic acoustic modes and convective cells are described by a single model. This is done by solving the gyrokinetic equation coupled to the quasi-neutrality equation. This calculation provides a predictive calculation of the frequency spectrum of flows given a specified forcing due to turbulence. It also shows that the dominant mechanism comes from zonal flow compression at intermediate frequencies, while ballooning of the turbulence Reynolds stress appears to be the main drive at low frequency.

Complex plasmas are plasmas containing solid particles typically in the micrometer range. These microparticles are highly charged and become an additional, dominating component of the plasma. Complex plasmas are model systems to study strong coupling phenomena in classical condensed matter. They offer the unique opportunity to go beyond the limits of continuous media down to the fundamental length scale of classical systems—the interparticle distance—and thus to investigate all relevant dynamic and structural processes using the fully resolved motion of individual particles, from the onset of cooperative phenomena to large strongly coupled systems. Unlike 'regular' plasma species the charged microparticles are strongly affected by gravity. An electric field in the sheath or a temperature gradient are usually employed to compensate for gravity, which provides favorable conditions to study two-dimensional or stressed three-dimensional (3D) systems on ground. However, in order to perform precision measurements with large isotropic 3D systems in the bulk plasma, microgravity conditions are absolutely necessary. Since 2001, this research under microgravity conditions has continuously been performed on board the International Space Station ISS within the Russian/German(European) Plasmakristall(PK)-Program. In the long-term research laboratories PKE-Nefedov (2001–2005), PK-3 Plus (2006–2013) and PK-4 (2014-ongoing), fundamental processes in liquid or crystalline complex plasmas as well as basic complex plasma issues were addressed. Highlights are: refinement of the theories of particle charging and ion drag, electrorheological plasmas, lane formation and phase separation in binary mixtures, crystallization and melting, wave propagation, shear flow and transition to turbulent motion. In this review, we will address results from microgravity research and discuss the perspectives for future studies.

An analysis of ASDEX Upgrade plasma is performed to understand the near SOL transport and develop the predictive basis for the electron temperature gradient, ${\lambda }_{{T}_{e,u}}.$ All of the unseeded L- and H-mode attached and N seeded H-mode discharges studied are shown that the analyzed ASDEX Upgrade dataset is in the conduction-limited regime, i.e. the parallel transport in the near SOL is dominated by Spitzer–Harm conduction. By studying a H–L back transition, it is shown that the 'bifurcation' in the core plasma between H- and L-mode regimes also exists in the perpendicular transport in the near SOL region. Through power balance and the Spitzer–Harm condution, the SOL perpendicular transport can be derived as ${\chi }_{\perp }\propto {C}_{{\rm{H}},{\rm{L}}}^{\chi }{n}_{e}^{-1}{T}_{e}^{3/2},$ with ${C}_{{\rm{L}}}^{\chi }/{C}_{{\rm{H}}}^{\chi }\approx 2.$ For detached plasmas, the SOL upstream electron profile is found to be broader than an equivalent attached plasma under certain conditions. By comparing ${\lambda }_{{T}_{e,u}}$ with global energy confinement, it is found that the discharges with broadened profiles also have degraded confinement, while those with unchanged profiles have similar confinement to attached plasma. The widening of the SOL is also found to coincide with the dropping of upstream temperature. Finally, comparisons of a N seeded H-mode with high pedestal top pressure and an I-mode plasma with an L-mode reference are both found to break the generally observed correlation between the T_e SOL decay length and the pedestal top pressure. Thus, the relationship is shown to be non-causal and must instead be due to similar dependences on other plasma parameters. This means that higher global energy confinement is not necessarily imply larger heat flux in the divertor and motivates the search for regimes that optimize both. The implications of these results for SOL transport are discussed.

Direct-drive is one of the key approaches in the study of inertial confinement fusion, but the laser imprinting caused by laser intensity inhomogeneities is one of the main obstacles to achieving ignition in direct-drive. It has previously been demonstrated that a thin high-Z overcoat on the laser side of the target can significantly mitigate laser imprinting (S P Obenschain et al 2002 Phys. Plasmas9 2234). In the current work, the 1D multi-group radiation hydrodynamic code RDMG, coupled with the detailed configuration accounting non-LTE atomic physics package MBDCA (RDMG−MBDCA) was used to study a Au-coated ignition target and its implosion performances under laser direct-drive, and a bare CH target was also simulated for comparison. Our study shows that the shell compressibility in the Au-coated target is enhanced with a smaller in-flight adiabat α_if and a higher neutron yield Y_id than in the bare CH target. This is because the Au coating helps to maintain a hotter CH plasma, which can ablate a wider electron conduction region with lower density leading to a weaker second shock, creating a more compressed shell and a higher yield than the bare CH target. We also compared the simulations from RDMG−MBDCA with those from RDMG−AA which is coupled with an averaged-atom (AA) non-LTE model. As a result, the shell from the AA model is less compressed with a higher α_if and a lower Y_id because the AA model gives a higher inward x-ray emission during the pre-pulse than the DCA model does, which therefore drives a stronger shock and leads to a higher fuel entropy.

In quiescent runaway electron plasmas in the DIII-D tokamak, whistler waves with frequencies between 90 and 200 MHz are driven unstable in plasmas with appreciable hard x-ray and non-thermal electron cyclotron emission (ECE). Narrow (δf < 50 kHz) discrete modes with erratically spaced frequencies are observed. Unstable modes often extend over a range Δf ≃ 50 MHz but lower frequency unstable modes are usually most intense. The dependency of the frequency on field and density implies a wavenumber k ≃ 150 m⁻¹ with parallel wavenumber k_∥ ≪ k. Reducing the gap between the plasma and the wall increases the number of detected modes. Lowering the magnetic field promotes instability. Nonlinear limit-cycle-like oscillations in the whistler amplitude occur on a 10 ms timescale. The ECE signals often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways. Sawteeth cause transient stabilization of the whistlers.

Magnetic perturbation (MP) fields are currently studied in ASDEX Upgrade and many other tokamaks in terms of edge localized mode control and the implication onto steady state divertor power load and access to detachment. Previous studies at ASDEX Upgrade in low density, attached L-mode (Faitsch et al 2017 Plasma Phys. Control. Fusion59 095006) are combined with high density L- and H-mode studies (Brida et al 2017 Nucl. Fusion57 116006). A consistent interpretation is presented for the steady state power load variation due to the 3D MP. The deviation from an axisymmetric power load is reducing with increasing density, being an effect of the increasing broadening in the divertor region. No deleterious effect on the access to a detached divertor regime is found. Experimental results from ASDEX Upgrade reveal similar power load profiles for plasmas without and a toroidal averaged profile in presence of an external MP. EMC3-Eirene modeling is showing agreement only if screening currents in the plasma as a response to the external magnetic field are added. In plasma scenarios with field penetration and locking of internal modes a fundamentally different power load pattern is observed. The pattern is in agreement with edge ergodization and the extent can be used to determine the magnitude of current needed to describe the internal mode.

The influence of the internal energy content, either vibrational or electronic, on the probability of CO elementary processes induced by electron-impact is investigated. In particular the vibronic excitations of the first singlet state of the CO spectrum, A¹Π, are derived in the framework of the similarity approach, characterizing also the radiative properties to the ground electronic state. Moreover the ionization of the ground state to the first three states of the molecular ion are calculated with the binary-encounter-dipole model, investigating the branching ratio of the three channels and their vibrational dependence. Finally the total ionization of the CO metastable a³Π state is considered in the binary-encounter-Bethe approach.

The role of the COMPASS tokamak in research of generation, confinement and losses of runaway electron (RE) population is presented. Recently, two major groups of experiments aimed at improved understanding and control of the REs have been pursued. First, the effects of the massive gas injection ($\sim {10}^{21}$ Ar/Ne particles) and impurity seeding ($\sim {10}^{18}$ particles) were studied systematically. The observed phenomena include generation of the post-disruption RE beam and current conversion from plasma to RE. Zero loop voltage control was implemented in order to study the decay in simplified conditions. A distinctive drop of background plasma temperature and electron density was observed following an additional deuterium injection into the RE beam. With the loop voltage control the parametric dependence of the current decay rate dI/dt can be studied systematically and possibly extrapolated to larger facilities. Second, recent results of experiments focused on the role of the magnetic field in physics of RE were analysed. In this contribution, special attention is given to the observed effects of the resonant magnetic perturbation on the RE population. The benefits of the RE experiments on COMPASS was reinforced by diagnostic enhancements (fast cameras, Cherenkov detector, vertical ECE etc) and modelling efforts (in particular, coupling of the METIS and LUKE codes).

Solar coronal mass ejections (CMEs) are large-scale eruptive events in which large amounts of plasma (up to 10¹³—10¹⁶ g) and magnetic fields are expelled into interplanetary space at very high velocities (typically 450 km/s, but up to 3000 km/s). When sampled in situ by a spacecraft in the interplanetary medium, they are termed interplanetary CMEs. They are nowadays considered to be the major drivers of space weather and the associated geomagnetic activity. The detectable space weather effects on Earth appear in a broad spectrum of time and length scales and have various harmful effects for human health and for our technologies on which we are ever more dependent. Severe conditions in space can hinder or damage satellite operations as well as communication and navigation systems and can even cause power grid outages leading to a variety of socio-economic losses. Therefore, the International Space Environment Service has set up a collaborative network of space weather service-providing warning centres around the globe, delivering coordinated operational space weather services for the benefit of the extensive user community. In order to improve the forecasts and predictions, NASA, ESA and other agencies have set-up space weather modelling frameworks. Here, we discuss how such frameworks enable to run and couple different space weather models, and to validate their results by comparing them with those of other similar models and, where possible, to in situ data. Examples of such frameworks are the Community Coordinated Modeling Center (CCMC, NASA GSFC), the Space Weather Modeling Framework at the Center for Space Environment Modeling at the University of Michigan, and ESAs novel Virtual Space Weather Modeling Centre that is being developed. The latter one includes space weather models that are geographically distributed and is the focus of this paper.

We report on recent progress on laser-plasma acceleration using a low energy and high-repetition rate laser system. Using only few milliJoule laser energy, in conjunction with extremely short pulses composed of a single optical cycle, we demonstrate that the laser-plasma accelerator (LPA) can be operated close to the resonant blowout regime. This results in the production of high charge electron beams (>10 pC) with peaked energy distributions in the few MeV range and relatively narrow divergence angles. We highlight the importance of the plasma density profile and gas jet design for the performance of the LPA. In this extreme regime of relativistic laser-plasma interaction with near-single-cycle laser pulses, we find that the effect of group velocity dispersion and carrier envelope phase can no longer be neglected. These advances bring LPAs closer to real scientific applications in ultrafast probing.

Efficient plasma core fuelling is a critical issue for achieving steady-state scenarios in magnetically confined fusion devices. The current preferred method to achieve this goal is cryogenic pellet injection (PI). PI's are now installed in most medium- and large-sized fusion devices. In this paper, recent results from PI experiments in the stellarator TJ-II are reported. Of particular interest is the influence of fast electrons. Whilst edge populations of such electrons do not affect PI in the TJ-II, fast electrons residing in its hot core have significant influence on both pellet ablation and fuelling efficiency. If present in the core, their radial location can be determined from the light emission profile produced by an ablating pellet as it traverses the plasma and confirmed from the power spectral distribution spectrograph of the total secondary beam current collected by a scanning heavy ion beam probe system. Moreover, it is found that if a pellet is subject to excess ablation due to core fast electron impacts, the resultant pellet efficiency (deposited particles/delivered particles) increases by up to 50% with respect to injections into similar, fast-electron free, plasmas. This is consistently found for microwave-heated plasmas, where pellet penetration is shallower than the magnetic axis in TJ-II, when such a population is present, as well as for neutral beam created and heated plasmas in which fast-electrons, generated during magnetic field ramp-up, persist in the core along a discharge. Similarly, when polystyrene pellets (TESPEL) are injected into the same machine sector, an analogous increase in post-injection particle deposition is seen. It is postulated that vaporization of the pellet through heating by fast electrons modifies normal neutral cloud and plasmoid development and hence affects outwards drifting that is inherent to magnetic fusion devices.

The properties of an Ar/C₂H₂ dusty plasma (ion, electron and neutral particle densities, effective electron temperature and dust charge) in glow and afterglow regimes are studied using a volume-averaged model and the results for the glow plasma are compared with mass spectrometry measurements. It is shown that dust particles affect essentially the properties of glow and afterglow plasmas. Due to collection of electrons and ions by dust particles, the effective electron temperature, the densities of argon ions and metastable atoms are larger in the dusty glow plasma comparing with the dust-free case, while the densities of most hydrocarbon ions and acetylene molecules are smaller. Because of a larger density of metastable argon atoms and, as a result, of the enhancement of electron generation in their collisions with acetylene molecules, the electron density in the afterglow dusty plasma can have a peak in its time-dependence. The results of numerical calculations are in a good qualitative agreement with experimental results.

We discuss the possibility of obtaining highly precise measurements of the ionization potential depression in dense plasmas with spectrally resolved x-ray scattering, while simultaneously determining the electron temperature and the free electron density. A proof-of-principle experiment at the Linac Coherent Light Source, probing isochorically heated carbon samples, demonstrates the capabilities of this method and motivates future experiments at x-ray free electron laser facilities.

Transport and confinement characteristics, and microinstabilities for high-${T}_{{\rm{i}}}/{T}_{{\rm{e}}}$ and high-${T}_{{\rm{e}}}/{T}_{{\rm{i}}}$ isotope plasmas in Large Helical Device are explored by using the gyrokinetic Vlasov simulation GKV with hydrogen isotope ions and real-mass kinetic electrons. The experimental data indicate that the thermal diffusivity is reduced in the deuterium-dominated plasmas, where the deviation from the gyro-Bohm scaling in the overall tendency and the strong anomaly of the electron heat transport are identified. Linear gyrokinetic analyses identify that the growth rates of the ion temperature gradient and trapped electron mode (TEM) instabilities in the deuterium plasmas are reduced due to the change in the profile gradients and/or the isotope ion mass, and the radial dependence of the mixing-length diffusivity is qualitatively consistent with the experimental tendency. Also, TEM-like turbulent fluctuations are examined by the using the phase contrast imaging measurement and the linear gyrokinetic calculation for the high-${T}_{{\rm{e}}}/{T}_{{\rm{i}}}$ deuterium plasma.

Helicon plasma sources using radio frequency (rf) waves are very useful, as they can produce high-density (∼10¹³ cm⁻³) plasma easily with a broad range of external operating parameters. Various kinds of featured helicon sources in a wide range of geometrical scales have been developed and characterized to control plasmas as required. Furthermore, particle production efficiency has demonstrated excellent performance over a very wide range of plasma sizes. High-beta (∼1) plasmas can also be easily achieved, showing the importance of the neutrals effect. As one of the many applications in areas ranging from fundamental to application fields, a space propulsion system with an advanced concept of an electrodeless condition (no direct contact between the plasma and electrodes/antennas) has been developed, owing to the expectation of a longer life operation. Among many proposals, two trials of electrodeless, additional acceleration methods are introduced, emphasizing the importance of some diagnostics. Here, we will review and describe the present status of our high-density rf plasma production with additional acceleration in an advanced propulsion field.

The acceleration of hydrogen ions up to 35 MeV is observed in the z-pinch experiments on the GIT-12 generator at a 3 MA current and 0.6 MV driving voltage. High ion energies are obtained with a novel configuration of a deuterium gas-puff z-pinch. In this configuration, a hollow cylindrical plasma shell is injected around an inner deuterium gas puff to form a homogeneous, uniformly conducting layer between electrodes at the initial phase of z-pinch implosion. The stable implosion at the velocity up to 650 km s⁻¹ is important to deliver more current onto the z-pinch axis. Magnetohydrodynamic instabilities become apparent first at stagnation. After the disruptive development of m = 0 instabilities, ∼20 ns pulses of high-energy photons, neutrons, electrons, and ions are observed. The average neutron yield is 2 × 10¹². The ion emission is characterized by various diagnostic techniques including those based on the usage of neutron-producing samples. When a large neutron-producing sample is placed onto the axis below a cathode mesh, the neutron yield is increased up to (1.1 ± 0.3) × 10¹³. Considering a ∼130 kJ energy input into z-pinch plasmas and magnetic field, this implies the neutron production efficiency of ∼10⁸ neutrons per one Joule of the z-pinch energy.

The impact of three-dimensional (3D) tokamak geometry from external magnetic perturbations on edge instabilities has been examined in high confinement mode plasmas with edge localised modes (ELMs) in ASDEX Upgrade. The 3D geometry has been probed using rigidly rotating MP fields. The measured distortions of the plasma boundary are compared to single-fluid ideal magnetohydrodynamic (MHD) equilibria using VMEC and MARS-F applying ideal and resistive MHD, whereas VMEC uses only ideal MHD. Both codes accurately reproduce the measured radial displacements of the edge density and temperature profiles in amplitude, toroidal phase and their dependence on the applied poloidal mode spectrum.

The induced 3D geometry distorts the local magnetic shear, which locally reduces the stabilising effect from field-line bending at certain most unstable field lines. Around these field lines, we observe additional stable ideal MHD modes with clear ballooning structure in-between ELMs. After their immediate appearance, they saturate and then grow on timescales of the pedestal pressure recovery. The subsequent ELMs show strongly localised magnetic perturbations of the initial crash and accompanied energetic electrons around the same most unstable field lines. These are strong signatures that filaments at the ELM onset preferentially erupt on these most unstable ('bad') field lines with their unfavourable 3D geometry where preceding ballooning modes are observed.

Turbulence, magnetic reconnection, and shocks can be present in explosively unstable plasmas, forming a new electromagnetic environment, which we call here turbulent reconnection, and where spontaneous formation of current sheets takes place. We will show that the heating and the acceleration of particles is the result of the synergy of stochastic (second order Fermi) and systematic (first order Fermi) acceleration inside fully developed turbulence. The solar atmosphere is magnetically coupled to a turbulent driver (the convection zone), therefore the appearance of turbulent reconnection in the solar atmosphere is externally driven. Turbulent reconnection, once it is established in the solar corona, drives the coronal heating and particle acceleration.

The effects of plasma shaping, in particular of the triangularity δ, on plasma turbulence, in terms of relative density fluctuations, have been studied in the TCV tokamak. It has been found that for inner wall limited L-mode plasmas, negative triangularity leads to a substantial reduction of turbulence amplitude, as well as of the spectral index and correlation length, consistent with the beneficial effect on energy confinement. Crucially, this reduction extends deep in the core, where the local triangularity becomes vanishingly small. A stabilizing effect of effective collisionality on trapped electron mode turbulence was also observed. These observations are consistent with previous experimental results on the effects of triangularity and collisionality on electron heat transport, as well as with global gyrokinetic GENE simulation results.

Combining a chirped laser pulse with a chromatic lens yields a flying focus—a laser focus that moves dynamically in time. This provides control over the propagation of the peak laser intensity within an extended focal region that can be many times larger than the system's Rayleigh length. Any velocity is achievable, including backward relative to the laser propagation direction. Previous simulation results have shown that a laser beam with a flying focus can create a counter-propagating ionization wave and subsequently pump a frequency-downshifted laser via the stimulated Raman scattering instability. Compared to a conventional Raman amplification scheme, several advantages were highlighted, including improved temperature control, plasma uniformity, and precursor growth mitigation. Here, we extend those results to demonstrate additional benefits: (1) the flying focus makes it possible for an unseeded Raman amplifier to produce a short, high-intensity beam; and (2) the flying focus minimizes collisional absorption of the pump, facilitating amplifier operation at higher plasma densities. Preliminary experiments have laid the groundwork for a high-performance plasma-based laser amplifier. The focal spot dynamics were initially confirmed at low intensity. It was subsequently demonstrated that ionization waves of arbitrary velocity can be produced at higher intensity. Here, we show a counter-propagating ionization front moving at approximately the speed of light—the optimal result for a Raman amplifier.

Producing a burning plasma in the laboratory has been a long-standing milestone for the plasma physics community. A burning plasma is a state where alpha particle deposition from deuterium–tritium (DT) fusion reactions is the leading source of energy input to the DT plasma. Achieving these high thermonuclear yields in an inertial confinement fusion (ICF) implosion requires an efficient transfer of energy from the driving source, e.g., lasers, to the DT fuel. In indirect-drive ICF, the fuel is loaded into a spherical capsule which is placed at the center of a cylindrical radiation enclosure, the hohlraum. Lasers enter through each end of the hohlraum, depositing their energy in the walls where it is converted to x-rays that drive the capsule implosion. Maintaining a spherically symmetric, stable, and efficient drive is a critical challenge and focus of ICF research effort. Our program at the National Ignition Facility has steadily resolved challenges that began with controlling ablative Rayleigh–Taylor instability in implosions, followed by improving hohlraum-capsule x-ray coupling using low gas-fill hohlraums, improving control of time-dependent implosion symmetry, and reducing target engineering feature-generated perturbations. As a result of this program of work, our team is now poised to enter the burning plasma regime.

Often the enhanced electromagnetic radiation generated in solar and stellar flares shows a pronounced (quasi)-oscillatory pattern—quasi-periodic pulsations (QPP), with characteristic periods ranging from a fraction of a second to several tens of minutes. We review recent advances in the empirical study of QPP in solar and stellar flares, addressing the intrinsic non-stationarity of the signal, i.e. the variation of its amplitude, period or phase with time. This non-stationarity could form a basis for a classification of QPP, necessary for revealing specific physical mechanisms responsible for their appearance. We could identify two possible classes of QPP, decaying harmonic oscillations, and trains of symmetric triangular pulsations. Apparent similarities between QPP and irregular geomagnetic pulsations Pi offer a promising avenue for the knowledge transfer in both analytical techniques and theory. Attention is also paid to the effect of the flare trend on the detection and analysis of QPP.

We report results of global resistive magnetohydrodynamic (MHD) simulations, which predict that wide-spread reconnecting instabilities may cause an internal disruption when 5 MW off-axis negative-ion-based neutral beam (N-NB) injection is used during the current ramp-up phase of a JT-60SA plasma. The simulations include fast ions as passive test particles and were initialized with plasma profiles obtained from an integrated transport simulation. The beam-driven current produces a nonmonotonic safety factor profile with three q = 2 rational surfaces. The resulting double or multiple kink-tearing instabilities redistribute both the bulk and fast particles in the inner half of the plasma. While the radial profiles appear to be flattened, the actual field topology and particle distributions are found to saturate in helical states. Moreover, because magnetic drifts are still large during the current ramp-up phase, there is a large difference between the topology of the magnetic field and 500 keV deuteron orbits. Fast ion phase space islands may thus appear and overlap at different times and in different regions than their magnetic counterparts.

A self-consistent kinetic model is developed to study air (N₂–20%O₂) plasma discharges in single pulses, their afterglows, and in repetitively pulsed discharges produced at low-pressures (133, 210 and 400 Pa) in a 1 cm inner radius tube. Since the pulse duration ranges from ∼0.3 up to ∼10 ms, the model includes the interplay of the vibrational kinetics of N₂ and chemical kinetics involving neutral, excited and charged species, which are coupled to the electron Boltzmann equation during the active plasma phase. A satisfactory agreement is found between modelling simulations and different experimental results. A comprehensive and detailed analysis of the modelling calculations is carried out, providing a general overview and insight of the most important mechanisms that take place in air plasmas at low-pressures.

Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfvén eigenmodes (TAEs) and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.

This article introduces an updated focus of plasma nanoscience after a dozen years of successful international research efforts. The concept of plasma-nano-interface involves two fundamental options: plasmas in contact with nanoscale features and plasmas which have nanoscale dimensions on their own. Non-equilibrium and transient features pertinent to the both cases are discussed in view of the recent progress in the field. Plasma-like phenomena in nanometer-sized materials, the third dimension of plasma nanoscience, also benefit from the insights based on fundamentals of plasma physics. Opportunities for future science discoveries, cross-disciplinary collaborations and translational research and development are highlighted.

Particle-containing (complex or dusty) plasmas offer and require the development of sophisticated diagnostic techniques to measure their properties such as particle motion, particle density, particle size and demixing properties. Here, an overview of selected optical diagnostic techniques for dusty plasmas and some recent applications are given. As examples, angle-resolved optical tomography reveals the spatially resolved particle density in dense dust systems as well as the size distribution of particles trapped in the plasma. Phase separation of binary mixtures of dust particles can be studied by exploiting the fluorescence of dyed particles. There, the dynamics of the demixing process can be measured in terms of an effective diffusion coefficient. Finally, three-dimensional trajectories of particles are determined from stereoscopy allowing to study the motion of hundreds of particles in a restricted volume.

This paper reports on the effect of on- and off-axis heating power deposition on the impurity confinement in purely electron cyclotron resonance heated He plasmas on the stellarator Wendelstein 7-X. Therefore, impurity transport times τ_I have been determined after Fe impurity injections by laser ablations and monitoring the temporal impurity emissivities by the x-ray imaging spectrometer HR-XIS. A significant increase of τ_I has been observed when changing the power deposition from on- to off-axis heating with energy confinement times τ_E being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power P_ECRH and electron density n_e has been investigated by keeping the heating power deposition on-axis. The observed τ_I scaling compares well to known τ_I scaling laws observed in other machines. A comparison of τ_I and τ_E yields an averaged ratio of τ_E/τ_I = 1.3 and transport times in the range of τ_I = 40–130 ms and τ_E = 40–190 ms. Comparing those absolute values to neoclassical predictions supports the recently observed nature of anomalous transport in Wendelstein 7-X, given within the up to now investigated operational parameters.

In this study we investigate extraction and acceleration of positive and negative ions from a pulsed inductively coupled plasma (ICP) in oxygen and hydrogen. Experiments were performed in a directed ribbon beam system where ions can be produced at an incident angle non-normal to the target. Positive and negative ions were transported to the wafer. The RF plasma source, extraction electrodes and wafer were biased synchronously. The energy of extracted ions closely follows the amplitude of the applied bias voltage and it ranges from few hundreds of volts to 10 kV. The peak beam current density can reach 100 A m⁻². The ion beam angles, ion current, and ion composition are reported. The injection physics requires correlating the positive and negative ion and electron densities near the extraction opening with the extracted currents. The plasma was modeled using CRTRS, a plasma fluid code that self-consistently solves for ICP power deposition, electrostatic potential, and plasma dynamics. A new perspective on the possible production of angled ion beams on surfaces is discussed.

Generation of the defect particles during the plasma-assisted metal dry etching process is induced by the various mechanisms. Most of these mechanisms are caused by the non-volatility of metal-halide compounds generated during the etching process. Degeneration of the metal etching process chamber condition is observed as the frequent process fault caused by the defects, but the worse condition is not recovered by itself. Because of this property of the metal etching process, proper work of the preventive maintenance (PM) to restore the process chamber or the addition of a discharge cleaning step is required periodically. However, inadequate PM or discharge cleaning by the uncertain cause analysis of the defect generation should be a just temporary remedy, and might lead the repetition of similar problems. To solve this problem, the virtual metrology (VM) model based on the plasma information (PI) parameters, known as PI-VM, was developed and applied for the defect control of the metal layer dry etching processes in organic light emitting diode (OLED) display manufacturing. To obtain the information about the generation rates of non-volatile compounds and their removal rates by the exhaustion system, PI parameters are designed with the consideration of the reaction kinetics in the metal etching plasma volume, sheath, and reacting surface using the big data of equipment engineering system (EES) and optical emission spectroscopy (OES) accumulated during the mass production process. The developed PI-VM index could be applied to a 2–3 h earlier alarm system for the defect occurrence, and had succeeded over 90% of alarm rate. This PI-VM alarm was applied to predictive control of the process by the early substitution of the discharge cleaning step or by the repair of the proper parts in the process chamber. By the control of processes based on the predicted results of the PI-VM, management of the mass production line with about 30% decreased defect was possible in OLED display manufacturing.

Herein, recent progress on indirectly-driven inertial confinement fusion (ICF) work at the National Ignition Facility (NIF) is briefly reviewed. An analytic criteria for an ICF burning plasma is given and compared to recent ICF implosion data from the NIF. Scaling of key hot-spot performance metrics is derived from simple physics considerations, including some speculative impacts of asymmetry on the assembly and disassembly of an ICF implosion. A steepest descent solution for the nonlinear equation for hot-spot pressure at peak compression, with the full effects of alpha-heating, is also given. To test if the scalings derived in this paper have some merit, they are compared to data from a variety of recent implosion campaigns on NIF and good agreement is observed. Given the implications of the scalings and existing data, a strategy for injecting more energy into the hot-spot of NIF indirectly driven ICF implosions is defined and the principles of the strategy is discussed. The importance of implosion velocity, late-time ablation pressure, and implosion scale with good symmetry in obtaining the goal of ∼50% more hot-spot energy are highlighted along with the limitations of trying to leverage low fuel-adiabat.

Shock waves are one of the most common plasma phenomena. They play a significant role in astrophysical and magnetospheric environments, as well as in laboratory plasmas. The paper focuses on two issues that underline important similarities and dissimilarities between collisional and collisionless shocks. The two types of shocks are similar in that they can be formed by the overtaking mechanism. We demonstrate that geometrical structures appearing early in the process of shock formation from the generic smooth initial state are quite universal and similar in both cases. Characterization of these structures (dubbed here the 'mussel-shell') is presented. Alongside with these similarities, there exist also significant differences between the two types of shocks. The classical collisional shocks usually connect two well-defined equilibrium states (those before and after the shock transition). Each of these equilibrium states is characterized by the thermodynamic parameters of density, temperature, and pressure whose upstream and downstream values are related by the continuity of mass, momentum, and energy flux. In the collisionless plasmas, however, this description does not work: the final state can be any of the much broader class of states constrained only by the requirement of being stable with respect to collisionless plasma instabilities. To find a final state (which does not have to be Maxwellian) one now has to follow the evolution of the system through the whole transition, and a lot of universality is lost. In some situations even the separation of scales between the global flow and shock transition may be lost. Taken together, these phenomena reveal a fascinating interplay of hydrodynamics, statistical mechanics, and plasma physics.

Wendelstein 7-X is a highly optimized stellarator that went into operation in 2015. With a 30 cubic meter volume, a superconducting coil system operating at 2.5 T, and steady-state heating capability of eventually up to 10 MW, it was built to demonstrate the benefits of optimized stellarators at parameters approaching those of a fusion power plant. We report here on the first results with the test divertor installed, during the second operation phase, which was performed in the second half of 2017. Operation with a divertor, and the addition of several new fueling systems, allowed higher density operation in hydrogen as well as helium. The effects that higher density operation had on both divertor operation and global confinement will be described. In particular, at high densities detachment was observed, and the highest fusion triple product for a stellarator was achieved.

Post-disruption runaway electron (RE) beams in tokamaks with large current can cause deep melting of the vessel and are one of the major concerns for ITER operations. Consequently, a considerable effort is provided by the scientific community in order to test RE mitigation strategies. We present an overview of the results obtained at FTU and TCV controlling the current and position of RE beams to improve safety and repeatability of mitigation studies such as massive gas (MGI) and shattered pellet injections (SPI). We show that the proposed RE beam controller (REB-C) implemented at FTU and TCV is effective and that current reduction of the beam can be performed via the central solenoid reducing the energy of REs, providing an alternative/parallel mitigation strategy to MGI/SPI. Experimental results show that, meanwhile deuterium pellets injected on a fully formed RE beam are ablated but do not improve RE energy dissipation rate, heavy metals injected by a laser blow off system on low-density flat-top discharges with a high level of RE seeding seem to induce disruptions expelling REs. Instabilities during the RE beam plateau phase have shown to enhance losses of REs, expelled from the beam core. Then, with the aim of triggering instabilities to increase RE losses, an oscillating loop voltage has been tested on RE beam plateau phase at TCV revealing, for the first time, what seems to be a full conversion from runaway to ohmic current. We finally report progresses in the design of control strategies at JET in view of the incoming SPI mitigation experiments.

For stellarators, which need no or only small amounts of current drive, electron-cyclotron-resonance heating (ECRH) is a promising heating method even for the envisaged application in a fusion power plant. Wendelstein 7-X (W7-X) is equipped with a steady-state capable ECRH system, operating at 140 GHz, which corresponds to the 2nd cyclotron harmonic of the electrons at a magnetic field of 2.5 T. Ten gyrotrons are operational and already delivered 7 MW to W7-X plasmas. Combined with pellet injection, the highest triple product (0.68 × 10²⁰ keV m⁻³ s), observed up to now in stellarators, was achieved (Sunn Pedersen et al 2018 Plasma Phys. Control. Fusion61 014035). For the first time, W7-X plasmas were sustained by 2nd harmonic O-mode heating, approaching the collisionality regime for which W7-X was optimized. Power deposition scans did not show any indication of electron temperature profile resilience. In low-density, low-power plasmas a compensation of the bootstrap current with electron-cyclotron current drive (ECCD) was demonstrated. Sufficiently strong ECCD close to the plasma centre produced periodic internal plasma-crash events, which coincide with the appearance of low order rationals of the rotational transform.

In recent experiments at the ASDEX Upgrade tokamak the existence of an Edge Resonant Transport Layer (ERTL) was revealed as the main transport mechanism responsible for the measured fast-ion losses in the presence of externally applied 3D fields. The Monte Carlo orbit-following code ASCOT was used to study the fast-ion transport including the plasma response calculated with MARS-F, reproducing a strong correlation of fast-ion losses with the poloidal mode spectra of the 3D fields. In this work, a description of the physics underlying the ERTL is presented by means of numerical simulations together with an analytical model and experimental measurements to validate the results. The degradation of fast-ion confinement is calculated in terms of the variation of the toroidal canonical momentum (δP_ϕ). This analysis reveals resonant patterns at the plasma edge activated by 3D perturbations and emphasizes the relevance of nonlinear resonances. The impact of collisions and the radial electric field on the ERTL is analysed.

The most investigated ion acceleration mechanisms in solid targets driven by intense laser pulses are revisited, including the target normal sheath acceleration (TNSA), radiation pressure acceleration (RPA), hybrid scheme of radiation pressure-sheath acceleration and Coulomb explosion (CE). In these acceleration mechanisms, the competition, between thermal pressure, radiation pressure and electrostatic pressure, decides the acceleration dynamics. Thermal pressure induced by hot electrons dominates TNSA, while RPA is ideally driven by steady radiation pressure and CE is mainly governed by the electrostatic pressure stored in the target. Two-dimensional particle-in-cell simulations are performed to display and elucidate this dynamical evolution among different mechanisms by changing the target thickness.