Table of contents

Plasma-based positron sources are attracting significant attention from the research community, thanks to their rather unique characteristics, which include broad energy tuneability and ultra-short duration, obtainable in a compact and relatively inexpensive setup. Here, we show a detailed numerical study of the positron beam characteristics obtainable at the dedicated user target areas proposed for the EuPRAXIA facility, the first plasma-based particle accelerator to be built as a user facility for applications. It will be shown that MeV-scale positron beams with unique properties for industrial and material science applications can be produced, alongside with GeV-scale positron beams suitable for fundamental science and accelerator physics.

A newly developed tool to simulate a tokamak full discharge is presented. The tokamak 'flight simulator' Fenix couples the tokamak control system with a fast and reduced plasma model, which is realistic enough to take into account several of the plasma non-linearities. A distinguishing feature of this modeling tool is that it only requires the pulse schedule (PS) as input to the simulator. The output is a virtual realization of the full discharge, whose time traces can then be used to judge if the PS satisfies control/physics goals or needs to be revised. This tool is envisioned for routine use in the control room before each pulse is performed, but can also be used off-line to correct PS in advance, or to develop and validate reduced models, control schemes for future machines like a commercial reactor, simulating realistic actuators and sensors behavior.

The transport and losses of fusion-born alpha particles are studied in the presence of a single-helicity tearing mode, characterized by ($m = 2,n = 1$). The analysis is performed by means of the recently developed Toroidal Accelerated Particle Simulator (TAPaS). Although such modes have usually been believed to result only in a local flattening of the radial profiles, it is shown that the density profile can exhibit a global modification leading to significant losses of alpha particles. This is due to the fact that, although the magnetic field does not exhibit any chaotic behaviour, the trajectories of alpha particles do, as revealed by their Poincaré maps. Such results are in qualitative agreement with past observations and simulations of energetic particles generated by neutral beam injection in TFTR, DIII-D and AUG tokamaks. In-depth analysis is carried out to characterize the impact of the tearing mode on the transport and losses of fusion-born alpha-particles with a realistic density profile. The impact of the amplitude is evidenced. Moreover, the effect of the island rotation frequency is assessed based on a detailed analysis of the linear resonances in phase-space, in agreement with the simulation results. Finally, the probability density function of the exit time has been computed and the transport of alpha particles has been found to be anomalous.

The design, licensing and operation of magnetic confinement fusion reactors impose various limitations on the amount of metallic dust particles residing inside the plasma chamber. In this context, predictive studies of dust production and migration constitute one of the main sources of relevant data. These are mainly conducted using dust transport codes, which rely on coupled dust-plasma and dust-wall interaction models, and require external input on the dust and droplet initial conditions. Some particularities of dust modelling in reactor-relevant conditions are analyzed with an emphasis on dust generation mechanisms relevant for disruption scenarios and on dust remobilization mechanisms relevant for ramp-up scenarios. Emerging topics such as dust production by runaway electron impact and pre-plasma remobilization of magnetic dust are also discussed.

An analysis of edge localised mode-free (quiescent) H-mode discharges exhibiting edge harmonic magnetoydrodynamic activity in the JET-carbon wall machine is presented. It is observed that the otherwise quiescent pulses with multiple-n harmonic oscillations are sustained until a threshold in pedestal electron density and collisionality is crossed. The macroscopic pedestal parameters associated with the quiescent phase are compared with those of a database of JET-ELMy discharges with both carbon and ITER-like wall (ILW). This comparison provides the identification of the existence regions in the relevant pedestal and global plasma parameters for edge harmonic oscillations (EHOs) in JET plasmas. Although the ELMy database scans pedestal collisionality and β values typical of ET-carbon quiescent operation, shaping and current are not simultaneously compatible with EHO existence. Nevertheless, ILW operation with JET-carbon quiescent-like parameters could in principle be achieved, and improved pedestal performance could be observed in more recent JET-ILW pulses.

A recent characterization of core turbulence carried out with a Doppler reflectometer in the optimized stellarator Wendelstein 7-X (W7-X) found that discharges achieving high ion temperatures at the core featured an ITG-like suppression of density fluctuations driven by a reduction of the gradient ratio $\eta_i = L_n/L_{T_i}$ (Carralero et al 2021 Nucl. Fusion61 096015). In order to confirm the role of ITG turbulence in this process, we set out to establish experimentally the relation between core density fluctuations, turbulent heat flux and global confinement. With this aim, we consider the scenarios found in the previous work and carry out power balance analysis for a number of representative ones, including some featuring high ion temperature. We also evaluate the global energy confinement time and discuss it in the context of the ISS04 inter-stellarator scaling. We find that when turbulence is suppressed as a result of a reduction of η_i, there is a reduction of ion turbulent transport, and global performance is improved as a result. This is consistent with ITG turbulence limiting the ion temperature at the core of W7-X. In contrast, when turbulence is reduced following a decrease in collisionality, no changes are observed in transport or confinement. This could be explained by ITG modes being combined with TEM turbulence when the latter is destabilized at low collisionalities.

A relativistic electron source based on high power laser interaction with gas jet targets has been developed at the Institute of Plasma Physics and Lasers of the Hellenic Mediterranean University. Initial measurements were conducted using the 'Zeus' 45 TW laser with peak intensities in the range of 10¹⁸–10¹⁹ W cm⁻² interacting with a He pulsed gas jet having a 0.8 mm diameter nozzle. A significant improvement of the electron signal was measured after using an absorber to improve the laser pulse contrast from 10⁻¹⁰ to 10⁻¹¹. A high stability quasi-mono-energetic electron beam of about 50 MeV was achieved and measured using a magnetic spectrometer for pulsed gas jet backing pressure of 12 bar. Supplementary studies using a 3 mm diameter nozzle for backing pressures in the range of 35–40 bar showed electron beam production with energies spread in the range from 50 to 150 MeV. The pulsed jet density profile was determined using interferometric techniques. Particle-in-cell simulations, at the above experimentally determined conditions, support our experimental findings.

This paper presents recent progress in studies of neoclassical tearing modes (NTMs) on TCV, concerning the new physics learned and how this physics contributes to a better real-time (RT) control of NTMs. A simple technique that adds a small (sinusoidal) sweeping to the target electron cyclotron (EC) beam deposition location has proven effective both for the stabilization and prevention of $2/1$ NTMs. This relaxes the strict requirement on beam-mode alignment for NTM control, which is difficult to ensure in RT. In terms of the EC power for NTM stabilization, a control scheme making use of RT island width measurements has been tested on TCV. NTM seeding through sawtooth (ST) crashes or unstable current density profiles (triggerless NTMs) has been studied in detail. A new NTM prevention strategy utilizing only transient EC beams near the relevant rational surface has been developed and proven effective for preventing ST-seeded NTMs. With a comprehensive modified Rutherford equation (co-MRE) that considers the classical stability both at zero and finite island width, the prevention of triggerless NTMs with EC beams has been simulated for the first time. The prevention effects are found to result from the local effects of the EC beams (as opposed to global current profile changes), as observed in a group of TCV experiments scanning the deposition location of the preemptive EC beam. The co-MRE has also proven able to reproduce well the island width evolution in distinct plasma scenarios on TCV, ASDEX Upgrade and MAST, with very similar constant coefficients. The co-MRE has the potential to be applied in RT to provide valuable information, such as the EC power required for NTM control with RT-adapted coefficients, contributing to both NTM control and integrated control with a limited set of actuators.

High-energy bremsstrahlung emission can occur owing to electron scattering in the nuclei or ions Coulomb field following the relativistic-electron generation in high-intensity laser interaction with plasmas. Such emission of photons in the keV–MeV energy range is of interest to characterize the relativistic-electron populations and develop laser-based photons sources. Even if it is a well-established and widely studied emission process, its modeling in laser-plasma scenarios needs further investigation. Moreover, advanced near-critical double-layer targets (DLTs), consisting in a low-density foam deposited on a thin solid substrate, have never been explored extensively for bremsstrahlung photon emission. Therefore, in this paper, we show the rationale, advantages, limitations, application regime, and complementarity of different modeling approaches and apply them to the unconventional configuration based on DLTs. We use multi-dimensional particle-in-cell (PIC) simulations coupled with a Monte Carlo strategy to simulate bremsstrahlung in two ways: integrated into the PIC loop itself or after the simulation with two separate codes. We also use simplified semi-analytical relations to retrieve the photon properties starting only from information on the relativistic electrons. With these tools, we investigate bremsstrahlung emission when an ultra-intense laser (0.8 µm wavelength, 30 fs duration, $a_0 = 20$ and 3 µm waist) interacts with DLTs having different properties. Despite some limitations of the numerical tools, we find that all approaches significantly agree on the characteristics of ~1–100 MeV photon emission. This points to the possibility of adopting the different modeling approaches in a complementary way while at the same time identifying the best suited for a specific scenario. Regardless, DLTs appear to overall boost the high energy photon emission while at the same time enabling control of the emission itself.

An effort to achieve high-energetic ion beams from the interaction of ultrashort laser pulses with a plasma, volumetric acceleration mechanisms beyond target normal sheath acceleration have gained attention. A relativistically intense laser can turn a near critical density plasma slowly transparent, facilitating a synchronized acceleration of ions at the moving relativistic critical density front. While simulations promise extremely high ion energies in this regime, the challenge resides in the realization of a synchronized movement of the ultra-relativistic laser pulse ($a_0\gtrsim$ 30) driven reflective relativistic electron front and the fastest ions, which imposes a narrow parameter range on the laser and plasma parameters. We present an analytic model for the relevant processes, confirmed by a broad parameter simulation study in 1D- and 3D-geometry. By tailoring the pulse length and plasma density profiles at the front side, we can optimize the proton acceleration performance and extend the regions in parameter space of efficient ion acceleration at the relativistic density surface.

Recent simulations show that very large electric and magnetic fields near the kilo tesla strength will likely be generated by ultra-intense lasers at existing facilities over distances of hundreds of microns in underdense plasmas. Even stronger ones are expected in the future although some technical difficulties must be overcome. In addition, it has been shown that vacuum exhibits a peculiar non-linear behaviour in the presence of high magnetic and electric field strengths. In this work, we are interested in the analysis of the thermodynamical contributions of vacuum to the energy density and pressure when radiation interacts with it in the presence of an external magnetic field. Using the Euler–Heisenberg formalism in the regime of weak fields i.e. smaller than critical quantum electrodynamics field strength values, we evaluate these magnitudes and analyse the highly anisotropic behaviour we find. Our work has implications for photon–photon scattering with lasers and astrophysically magnetized underdense systems far outside their surface where matter effects are increasingly negligible.

One of the most important features of plasma-based accelerators is their compactness because plasma modules can have dimensions of the order of mm $\textrm{cm}^{-1}$, providing very high-accelerating fields up to hundreds of GV $\textrm{m}^{-1}$. The main challenge regarding this type of acceleration lies in controlling and characterising the plasma itself, which then determines its synchronisation with the particle beam to be accelerated in an external injection stage in the laser wakefield acceleration (LWFA) scheme. This issue has a major influence on the quality of the accelerated bunches. In this work, a complete characterisation and optimisation of plasma targets available at the SPARC_LAB laboratories is presented. Two plasma-based devices are considered: supersonic nozzles for experiments adopting the self-injection scheme of laser wakefield acceleration and plasma capillary discharge for both particle and laser-driven experiments. In the second case, a wide range of plasma channels, gas injection geometries and discharge voltages were extensively investigated as well as studies of the plasma plumes exiting the channels, to control the plasma density ramps. Plasma density measurements were carried out for all the different designed plasma channels using interferometric methods in the case of gas jets, spectroscopic methods in the case of capillaries.

During the past decennia, progress in the area of high-energy astroparticle physics was exceptional, mainly due to the great success of the bridging of particle- and astrophysics both in theory and in the instrumentation of astroparticle physics observatories. Multimessenger data coming from charged cosmic-ray-, gamma-ray- and neutrino-observatories start to shed more and more light on the nature and origin of cosmic rays. At the same time, the development of methods for the investigation of cosmic-ray transport, acceleration and interaction has advanced to the true potential of tying these different pieces of multimessenger data together, this way closing in on the origin of cosmic rays. In recent years, this rapid interplay between modeling and observations has made it clear that it is essential to add the ingredient of plasma physics to the problem. It has been shown that even the interpretation of data of highly relativistic cosmic rays at TeV energies and above is in need of a proper modeling of the plasma physics involved. One of the most important examples is the understanding of wave-particle interactions. In simulations of cosmic-ray transport in the Galaxy, the cosmic-ray diffusion coefficient is typically approximated with a Kolmogorov-type cascade model, resulting in an energy-dependent parallel diffusion coefficient $\kappa_{\parallel}\propto E^{\gamma}$ with $\gamma = 1/3$. Here, we show how the energy dependence of the diffusion coefficient can be investigated systematically as a function of $\delta B/B$. The complex energy behavior that goes well beyond a simple powerlaw interpretation will be presented together with a formal definition of an energy range that indeed can be approximated as a powerlaw. These results are applied to cosmic-ray transport in the Milky Way. Finally, the transition between the ballistic and diffusive regime will be investigated for astrophysical sources with special focus on relativistic plasmoids of active galaxies.

A 3D two-fluid simulation, using plasma parameters as measured by MMS on 8 September 2015, shows the nonlinear development of the Kelvin–Helmholtz instability at the Earth's magnetopause. It shows extremely rich dynamics, including the development of a complex magnetic topology, vortex merging and secondary instabilities. Vortex induced and mid-latitude magnetic reconnection coexist and produce an asymmetric distribution of magnetic reconnection events. Off-equator reconnection exhibits a predominance of events in the Southern Hemisphere during the early nonlinear phase, as observed by satellites at the dayside magnetopause. The late nonlinear phase shows the development of vortex pairing for all latitudes while secondary Kelvin–Helmholtz instability develops only in the Northern Hemisphere, leading to an enhancement of the occurrence of off-equator reconnection there. Since vortices move tailward while evolving, this suggests that reconnection events in the Northern Hemisphere should dominate at the nightside magnetopause.

The effect of the presence or absence of a grounded substrate beneath dielectric targets, including cancer cells, during exposure to the cold atmospheric plasma jet is studied in the experiments and in fluid model simulations for the discharge parameters typical for the medical applications. It is shown that the dynamics of streamers generated in each positive cycle of ac voltage depends on the grounded substrate position. The streamers approach the target more often if the grounded substrate is beneath the target, that provides more intensive plasma-target interaction. In this case, the measured spectrum of plasma jet emission near the target demonstrates much higher intensity compared to an electrically isolated target case. The calculated and measured discharge currents with time demonstrate a mismatch of frequencies of the ac voltage and current over the target. The viability of A431 human skin carcinoma and MX7 mouse rhabdomyosarcoma cells treated by cold atmospheric plasma jet with/without the grounded substrate is measured with MTT assay 24 h after. The results show an enhanced suppression of the cell viability when using the grounded substrate for both cell lines. Achieving effective death of tumor cells with a shorter irradiation time can be considered an advantage of using a grounded electrode beneath the bio target.

Theoretical and computational results for the generation of a powerful shock wave with pressure behind the front exceeding a gigabar level in the half-space of a solid when the boundary layer is heated by a flux of laser-accelerated electrons are presented. The influence of the energy flux density of the heating stream, the characteristic initial energy and the electron spectrum on the characteristics of the shock wave is investigated. The main attention is paid to the generation of an extremely powerful shockwave, which can be applied in experiments to study the equation of state of matter. For this, the requirements for the parameters of a laser pulse that can ensure the propagation of a plane shock wave with a gigabar pressure when a substance is heated by a beam of laser-accelerated fast electrons, taking into account its divergence, are established. It is shown that one of the features of the propagation of a shock wave under the impact of a thermal piston heated by fast electrons consists in the radiation cooling of the peripheral region of the substance covered by the shock wave. An increase in the compression of matter due to radiation cooling leads to a multiple increase in the density of matter in the peripheral region of the shock wave compared to the density at its front. The final result of this work is to substantiate the use of shock waves driven by a beam of laser-accelerated electrons in a laboratory experiment to study the properties of matter, in particular, metals compressed to a density of several tens of g cc⁻¹ under the action of gigabar pressure.

Recent experience with metallic devices operating in ITER relevant regions of the operational space, has shown that the disruptivity of these plasmas is unacceptably high. The main causes of the disruptions are linked to impurity accumulation in the core and edge cooling, resulting in unstable current profiles. Avoidance and prevention of the consequent instabilities require the early detection of anomalous electron temperature profiles. A series of indicators have been developed and their performances compared, to find the most suitable inputs for disruption predictors. Their properties are assessed on the basis of information content, reliability and real-time availability. The best performing ones provide much better results than the ones reported in the literature, as shown by both numerical tests with synthetic data and the analysis of experimental signals from JET with the ITER-like wall. They provide better accuracy, lower false alarms and earlier detection. The improved discriminatory capability of the developed indicators is expected to significantly improve the performance of the most advanced predictors recently reported in the literature.

In plasma wakefield accelerators, the wave excited in the plasma eventually breaks and leaves behind slowly changing fields and currents that perturb the ion density background. We study this process numerically using the example of a Facility for Advanced aCcelerator Experimental Tests (FACET) experiment where the wave is excited by an electron bunch in the bubble regime in a radially bounded plasma. Four physical effects underlie the dynamics of ions: (1) attraction of ions toward the axis by the fields of the driver and the wave, resulting in formation of a density peak, (2) generation of ion-acoustic solitons following the decay of the density peak, (3) positive plasma charging after wave breaking, leading to acceleration of some ions in the radial direction, and (4) plasma pinching by the current generated during the wave-breaking. The interplay of these effects results in the formation of various radial density profiles, which are difficult to produce in any other way.

The thermal helium beam diagnostic at ASDEX Upgrade is used to infer the electron density n_e and temperature T_e in the scrape-off layer and the pedestal region from the emission of visible lines of the locally injected helium. The link between n_e and T_e and the emission is provided by a collisional radiative model, which delivers the evolution of the populations of the relevant excited states as the He atoms travel through the plasma. A computationally efficient method with just three effective states is shown to provide a good approximation of the population dynamics. It removes an artificial rise of T_e at the plasma edge when using a simple static model. Furthermore, the re-absorption of the vacuum ultra-violet resonance lines has been introduced as an additional excitation mechanism being mainly important in the region close to the injection point. This extra excitation leads to a much better fit of the measured line ratios in this region for larger puff rates.

The derivation of the multi-temperature generalized Zhdanov closure is provided, starting from the most general form of the left-hand side of the moment-averaged kinetic equation with the Sonine–Hermite polynomial ansatz for an arbitrary number of moments. The process of arriving at the reduced higher-order moment equations, with its assumptions and approximations, is explicitly outlined. The generalized multi-species multi-temperature coefficients from our previous article are used to compute values of higher-order moments such as heat flux in terms of the lower-order moments. Transport coefficients and the friction and thermal forces for magnetic confinement fusion relevant cases with the generalized coefficients are compared to the scheme with the single-temperature coefficients previously provided by Zhdanov et al. It is found that the 21 N-moment multi-temperature coefficients are adequate for most cases relevant to fusion. Furthermore, the 21 N-moment scheme is also tested against the trace approximation to determine the range of validity of the trace approximation with respect to fusion-relevant plasmas. Possible refinements to the closure scheme are also illustrated to account for quantities which might be significant in certain schemes, such as the drift approximation.

In this work, we have investigated the influence of magnetic island (MI) on electrostatic toroidal ion temperature gradient (ITG) mode, where the ions are described by gyro-kinetic equations including MI, and adiabatic approximation is used for electrons. The eigen-equation for short-wavelength toroidal ITG mode in Fourier-ballooning representation is derived, and the corresponding eigenvalue as well as mode structure are solved. Both the flattening effects of MI on plasma pressure and MI-scale ${\vec E} \times {\vec B}$ shear flow are considered. It is found that when only considering the flattening effects of MI, the ITG mode can be stabilized compared to the case without MI. Meanwhile, the effective toroidal ITG mode drive can be enhanced by including MI-scale ${\vec E} \times {\vec B}$ flow, which indicates the dominant destabilizing by MI-scale ${\vec E} \times {\vec B}$ flow over the stabilizing by flattening profile, which results in a higher growth rate than the case without MI. It is also found that the total flow shearing may prevent the ITG turbulence spreading from the X-point of MI but is not strong enough to prevent spreading from the separatrix across the O-point of larger MI via comparison between the flow shearing rate and the linear growth rate. Furthermore, the corresponding width of the lowest-order mode structure in the ballooning angle is slightly widened (narrowed) for the case without (with) MI-scale ${\vec E} \times {\vec B}$ flow, compared to the case without MI. Moreover, the shifted even symmetry in the ballooning angle is not qualitatively influenced by the presence of MI. The mode structure is radially asymmetric but is symmetric with respect to the phase of MI at the O-point.

The results of experiments on the study of plasma compression of nested wire arrays of mixed composition and the generation of powerful pulses of soft x-ray radiation (SXR), carried out on a pulse power facility Angara-5-1 at a current level of up to 3 MA, are presented. Based on the latest experimental data on the intensity of plasma formation of various substances $\dot m$ (in μg(cm² ns)⁻¹) (Mitrofanov et al 2020 Plasma Phys. Rep.46 1150–80) and on the features of the dynamics of plasma compression in nested arrays (Mitrofanov et al 2018 Plasma Phys. Rep.44 203–35), a nested wire array design has been developed which makes it possible to obtain a high peak SXR power in comparison with the known designs of single and nested tungsten wire arrays. During the implosion of nested arrays of mixed composition, consisting of plastic fibers and tungsten wires, shorter and more powerful SXR pulses were obtained with a maximum peak power P_SXR^max∼ 10 TW with a full width at half maximum (FWHM) duration of ∼5 ns compared to the parameters of SXR pulses upon compression of single tungsten arrays: P_SXR^max∼ 5 TW and FWHM ∼ 10 ns. Thus, under the conditions of our experiments, we have shown the possibility of a twofold increase in the peak SXR power during compression of nested arrays by optimizing their design.

With the forthcoming 10–100PW laser facilities, laser-driven electron–positron-pair production has gained particular interest. Here, a scheme to enhance the generation of dense electron–positron-pairs is proposed and numerically demonstrated, employing double laser pulses at the intensity level of ${10^{23}}\,\,{\text{W c}}{{\text{m}}^{ - 2}}$. The first laser accelerates a thin foil to a relativistic speed via the radiation-pressure-acceleration mechanism and a counter-propagating laser irradiates this flying plasma layer. The simulation results indicate that a high-yield and well-collimated positron beam ($\sim 5.5 \times {10^{10}}$ positrons/pulse, 8.8 nC/pulse) is generated with a large peak density $\left( {1.1 \times {{10}^{21}}{\text{ c}}{{\text{m}}^{ - 3}}} \right){ }$ by using tens-of-PW laser pulses.

The kinetic stimulated Raman scattering (SRS) is found to result in significant Weibel-generated magnetic fields via 2D particle-in-cell simulations. During the high-intensity laser pulse, the daughter electron plasma waves of SRS heat the electrons effectively and lead to anisotropy in the velocity space. This anisotropy results in the development of a quasi-static magnetic field near the laser speckle, and the growth rate has been discussed. The results show that the kinetic SRS can lead to an averaged magnetic field of more than 10 T, which can be an important magnetic field source in laser-plasma experiments. Besides, the energy of the Weibel-field undergoes an oscillatory rise with the SRS bursts and can be stable after cutting off the laser. Moreover, in the magnetized plasmas, the application of a longitudinal magnetic field enhances the SRS, but interestingly, it significantly reduces the growth rate of Weibel instability. Simulation results also indicate that a small transverse magnetic field can evidently change the motion of the hot electrons, which dramatically destroys the symmetry of the SRS and the Weibel-generated magnetic fields.

Artificial neural networks (NNs) are trained, based on the numerical database, to predict the no-wall and ideal-wall β_N limits, due to onset of the n = 1 (n is the toroidal mode number) ideal external kink instability, for the HL-2M tokamak. The database is constructed by toroidal computations utilizing both the equilibrium code CHEASE (Lütjens et al 1992 Comput. Phys. Commun.69 287) and the stability code MARS-F (Liu et al 2000 Phys. Plasmas7 3681). The stability results show that (1) the plasma elongation generally enhances both β_N limits, for either positive or negative triangularity plasmas; (2) the effect is more pronounced for positive triangularity plasmas; (3) the computed no-wall β_N limit linearly scales with the plasma internal inductance, with the proportionality coefficient ranging between 1 and 5 for HL-2M; (4) the no-wall limit substantially decreases with increasing pressure peaking factor. Furthermore, both the NN model and the convolutional neural network (CNN) model are trained and tested, producing consistent results. The trained NNs predict both the no-wall and ideal-wall limits with as high as 95% accuracy, compared to those directly computed by the stability code. Additional test cases, produced by the Tokamak Simulation Code (Jardin et al 1993 Nucl. Fusion33 371), also show reasonable performance of the trained NNs, with the relative error being within 10%. The constructed database provides effective references for the future HL-2M operations. The trained NNs can be used as a real-time monitor for disruption prevention in the HL-2M experiments, or serve as part of the integrated modeling tools for ideal kink stability analysis.

The effect of energy transfer by laser-accelerated fast electrons on thermonuclear gain of a shock-ignited ICF target at different powers and durations of the high-intensity part of the laser pulse (spike) responsible for igniting shock wave generation has been investigated on the basis of hydro-kinetic numerical simulations. The key result of these studies is that the fast-electron energy transfer is able to provide a great contribution to igniting shock wave pressure to maintain a high thermonuclear gain with a significant decrease in the energy of the igniting part of the laser pulse. Calculations were performed for the 2nd harmonic Nd-laser pulse in order to justify shock-ignition experiments at the Megajoule-class facility, which is currently under construction in Russia. Spike energy conversion to fast electron energy and its temperature were selected in the ranges, which are discussed in the literature. It has been found that fast electrons with a temperature of 50–70 keV, whose energy contains 20%–40% of spike energy, make such a large contribution to the pressure of the igniting shock wave that the gain factor retains its value of 70–80 with spike energy decrease by 1.5–2 times.

We propose to use a frequency-doubled pulse colliding with the driving pulse at an acute angle to trigger ionization injection in a laser wakefield accelerator. This scheme effectively reduces the duration of the injection; thus, high injection quality is obtained. Three-dimensional particle-in-cell simulations show that electron beams with energy of ${\sim} 500\ \textrm{MeV}$, a charge of ${\sim} 40\ \textrm{pC}$, energy spread of ${\sim}1\%$ and normalized emittance of a few millimeter milliradian can be produced by ${\sim} 100\ \textrm{TW}$ laser pulses. By adjusting the angle between the two pulses, the intensity of the trigger pulse and the gas doping ratio, the charge and energy spread of the electron beam can be controlled.

Bolometric tomography is a widely applied technique to infer important indirect quantities in magnetically confined plasmas, such as the total radiated power. However, being an inverse and ill-posed problem, the tomographic algorithms have to be carefully steered to converge on the most appropriate solutions, and often specialists have to balance the quality of the obtained reconstructions between the core and the edge of the plasma. Given the topology of the emission and the layout of the diagnostics in practically all devices, the tomographic inversions of bolometry are often affected by artefacts, which can influence derived quantities and specific studies based on the reproduced tomograms, such as power balance studies and the benchmarking of gyrokinetic simulations. This article deals with the introduction of a simple, but very efficient methodology. It is based on constraining the solution of the tomographic inversions by using a specific estimate of the initial solution, built with the data from specific combinations of detectors (called 'masks'). It has been tested with phantom and with real data, using the Maximum Likelihood approach at JET. Results show how the obtained tomograms improve sensibly both in the core and at the edge of the device, when compared with those obtained without the use of masks as the initial guess. The correction for the main artefacts can have a significant impact on the interpretation of both the core (electron transport, alpha heating) and the edge physics (detachment, SOL). The method is completely general and can be applied by any iterative algorithm starting from an initial guess for the emission profile to be reconstructed.

This paper presents a linear dynamic model for the current and position of plasma in Damavand tokamak (DT) based on the RZIP methodology. Experimental data shots of DT are employed to validate the model and to show its accuracy. Since there are uncertainties in the model parameters, a robust decentralized control system is designed by considering the interaction between the input-output channels. Robust proportional and proportional-integral controllers are designed to obtain robust closed-loop stability and performance based on the Kharitonov theorem. The simulation results of the closed-loop system in two scenarios are presented to show the effectiveness of the introduced control system.

Self-consistent core-scrape-off layer numerical simulations of an ASDEX-Upgrade discharge where the nitrogen (N) seeding is gradually replaced with the krypton (Kr) seeding during the plasma current flat-top phase are presented. These simulations are performed with the COREDIV code focusing on the prediction of the impurity evolution (W, Kr, N) with matched global plasma parameters: total and core radiation, temperature at the target plate and W concentration. The numerical results are compared with experimental measurements for shot #30503 at three different time points: 2.5 s (only N seeding), 4.2 s (N + Kr seeding) and 5.2 s (only Kr seeding). The calculated electron temperature at the divertor plate can be reduced to 3 eV with the highest Kr seeding. A good agreement between modelling results and experimental observations is reported.

Achieving a successful plasma current ramp-up in a full tungsten tokamak can be challenging due to the large core radiation (and resulting low core temperature) that can be faced with this heavy metallic impurity if its relative concentration is too high. Nitrogen injection during the plasma current ramp-up of WEST discharges greatly improves the core temperature and magnetohydrodynamic (MHD) stability. Experimental measurements and integrated simulations with the RAPTOR code, complemented with the QuaLiKiz neural network for computing turbulent transport, allow a detailed understanding of the mechanisms at play. Increased edge radiation during this transient phase is shown to improve confinement properties, driving higher core temperature and better MHD stability. This also leads to increased operation margins with respect to tungsten contamination.

High poloidal beta scenarios with a favorable energy confinement (${\beta _{\text{p}}}\sim 1.9$, ${H_{98{\text{y}}2}}\sim 1.4)$ have been achieved on the Experimental Advanced Superconducting Tokamak using only radio frequency wave heating. Gyrokinetic simulations are carried out with experimental plasma parameters and tokamak equilibrium data of a typical high ${\beta _{\text{p}}}$ discharge using the gyrokinetic toroidal code. Linear simulations show that electron-temperature scale length and electron-density scale length destabilize the turbulence, collision effects stabilize the turbulence, and the instability propagates in the electron diamagnetic direction. These properties indicate that the dominant instability in the core of high ${\beta _{\text{p}}}$ plasma is a collisionless trapped electron mode. Ion thermal diffusivity, calculated by nonlinear gyrokinetic simulations, is consistent with the experimental value, in which the electron collision effects play an important role. Further analyses show that instabilities with ${k_\theta }{\rho _{\text{s}}} > 0.38$ are suppressed by collision effects and collision effects reduce the radial correlation length of turbulence, resulting in the suppression of the turbulence.

Gyro-average is a crucial operation to capturing the essential finite Larmor radius (FLR) effect in gyrokinetic simulation. In order to simulate strongly shaped plasmas, an innovative multi-point average method based on non-orthogonal coordinates has been developed to improve the accuracy of the original multi-point average method in gyrokinetic particle simulation. This new gyro-average method has been implemented in the gyrokinetic toroidal code (GTC). Benchmarks have been carried out to prove the accuracy of this new method. In the limit of concircular tokamak, ion temperature gradient (ITG) instability is accurately recovered for this new method and consistency is achieved. The new gyro-average method is also used to solve the gyrokinetic Poisson equation, and its correctness is confirmed in the long-wavelength limit for realistically shaped plasmas. The improved GTC code with the new gyro-average method is used to investigate the ITG instability with EAST magnetic geometry. The simulation results show that the correction induced by this new method in the linear growth rate is more significant for short-wavelength modes where the FLR effect becomes important. Due to its simplicity and accuracy, this new gyro-average method can find broader applications in simulating shaped plasmas in realistic tokamaks.

We present a novel approach to analyzing phase-space distributions of electrons ponderomotively scattered off an ultra-intense laser pulse and comment on the implications for the thus conceivable in-situ laser-characterization schemes. To this end, we present fully relativistic test particle simulations of electrons scattered from an ultra-intense, counter-propagating laser pulse. The simulations unveil non-trivial scalings of the scattered electron distribution with the laser intensity, pulse duration, beam waist, and energy of the electron bunch. We quantify the found scalings by means of an analytical expression for the scattering angle of an electron bunch ponderomotively scattered from a counter-propagating, ultra-intense laser pulse, also accounting for radiation reaction (RR) through the Landau–Lifshitz (LL) model. For various laser and bunch parameters, the derived formula is in excellent quantitative agreement with the simulations. We also demonstrate how, in the radiation-dominated regime, a simple re-scaling of our model's input parameter yields quantitative agreement with numerical simulations based on the LL model.

In plasmas of lower collisionality, such as the scrape-off layer (SOL) of a fusion tokamak device, the parallel heat conductivity of ions becomes smaller than the classical Spitzer–Harm model due to a nonlocal kinetic effect. We have assessed, by simulation, the impact and role of the kinetic effect of ion heat conductivity (abbreviated as ion KE in this paper) on DEMO-relevant tokamak SOL plasma, using the Japanese demonstration tokamak reactor concept JA DEMO. A series of test simulations, where the ion KE is modeled by a widely used free-streaming energy (FSE)-limited model, has demonstrated: (a) the ion KE decreases the ion parallel heat flux density in JA DEMO SOL (at the baseline operation scenario) around the X-point and further upstream of the low-field side (LFS) area along the separatrix, where the parallel collisionality is smaller. Up to 40%–60% decrease is observed, compared to the case without (w/o) ion KE, among the test cases where the ion KE level is scanned over the possible range (i.e. parameter α_i of the FSE-limited model is varied over $0.2 \leqslant \alpha _{\textrm{i}} \leqslant 2.0$); (b) the ion KE leads to significant increase in the ion temperature T_i (up to 600% increase) and significant decrease in the ion density n_i (up to −80%) over wide area of the SOL upstream. Energy balance analysis has suggested that the ion KE affects the upstream n_i and T_i by the power of 0.4 and −0.4, respectively, of the flux limiting factor, as long as spatial changes in plasma parameters are moderate. The results of this study serve as a fundamental assessment of the ion KE for DEMO- relevant SOL plasma, clarifying the need for further sophistication of the model toward more quantitative prediction.

A new method was developed to model the neutral population produced by the gas puff based charge exchange recombination spectroscopy systems at ASDEX Upgrade (AUG). With this method, the edge impurity density on the high field side (HFS) and low field side (LFS) can be obtained without the need to apply a neutral beam injection system. The neutral penetration needed for the calculation of the impurity density is obtained with a new gas puff module implemented in the FIDASIM code. The LFS impurity density profile evaluated with the new gas puff module matches the impurity density calculated with standard beam-based charge exchange diagnostics. Impurity temperature, rotation and density profiles at the HFS and LFS of an AUG H-mode discharge are presented. Edge impurity toroidal and poloidal flows show asymmetric structures. The impurity density asymmetries obtained with the new gas puff module are consistent with the observed flow structure.

The unstable spectra of plane Poiseuille flow (PF) in the presence of a longitudinal magnetic field are numerically investigated using an eigenvalue solver of incompressible magnetohydrodynamic equations. It is found that the strength of the magnetic field and the dissipative effect of the magnetic perturbation have played different roles in different parameter regions. The magnetic field has a strong suppression effect on the classical plane PF instability with a large Reynolds number $\mathcal R_e$ in the region with the magnetic Prandtl number $\mathcal P_m = [0.1, 1]$ or the magnetic Reynolds number $\mathcal R_m = [10^3, 10^6]$. Here, the Reynolds number and the magnetic Reynolds number are defined as $\mathcal R_e = a V_0/\nu$ and $\mathcal R_m = aV_0\mu/\eta$, where a, V₀, ν and η are the typical length, velocity, viscosity and resistivity, respectively. The magnetic Prandtl number is defined as $\mathcal P_m = \mathcal R_m/\mathcal R_e \propto \nu/\eta$, which is proportional to the ratio of the viscosity and the resistivity of the fluid medium. As the strength of the magnetic field increases, the PF instability can be completely stabilized in the limit of $\mathcal P_m \to \infty$ or/and $\mathcal R_m \to \infty$. It is interestingly found that a new instability branch is excited in the small magnetic Prandtl number ($\mathcal P_m \to 0$) or moderate magnetic Reynolds number ($\mathcal R_m = 10^4 \sim 10^6$) and large Reynolds number ($\mathcal R_e \to \infty$) regions. This new type of instability is verified to be driven by the magnetic Reynolds stress and modulated by the dissipative effect of the magnetic perturbation. The wavelength of the original PF instability gradually shifts to the long wavelength region, but the wavelength of the new branch is almost unchanged, as $\mathcal R_e$ increases with fixed $\mathcal R_m$. However, the wavelength of the original instability branch is almost unchanged, but the wavelength of the new instability branch shifts to the long wavelength region, as $\mathcal R_m$ increases with fixed $\mathcal R_e$.

The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 to $20 \ \textrm{kHz}$, being well below the toroidal gap in the Alfvén continuum and of the same order as the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered in the observations characterized by very low additional heating. The calculation of the coupled Alfvén–acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfvén eigenmodes (BAEs) rather than beta-induced Alfvén–acoustic eigenmodes. As a main novelty compared to previous work, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfvén continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating magnetic islands, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAMs), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction between TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

Mega Ampere Spherical Tokamak (MAST) pedestal data has been analysed, where a pedestal database of 892 shots was obtained, using the upgraded MAST Thomson scattering (TS) diagnostic. Various edge localised mode (ELM) types are discussed, where characteristics and trends of MAST pedestals are shown. The data from the upgraded TS diagnostic confirms pedestal characteristics found in earlier analysis, using previous TS systems. Using the database, calculations of the bootstrap current are obtained using the different analytical formulae (Sauter and Redl), and comparisons are performed. The upgraded MAST TS system now spans the full plasma mid-plane, such that direct comparisons between inboard and outboard pedestals can be obtained, and asymmetries in the density pedestal width were found. To increase confidence in spherical tokamak pedestal predictions, the assumptions of Europed have been validated from the MAST pedestal data, and a value for the kinetic ballooning mode constraint ($C \sim 0.145$) has been obtained. The first spherical tokamak pedestal predictions were performed in Europed and compared to experimental values. Using C = 0.145 the temperature pedestal height was predicted to within 10% of the experimental value. In addition type II ELMs on MAST are analysed, and stability analysis and parameter scans have been performed using ESSIVE. Similar magneto-hydrodynamic (MHD) stability properties are observed for type I and type II ELMs, originating from the mixed ELM regimes; it is therefore difficult to distinguish these ELMs using the ideal MHD codes.

Nonlinear structures such as shock waves and vortices widely exist in nature and the Universe. They have also been separately observed in laser–plasma interactions. We show for the first time that two perpendicularly propagated collisionless electrostatic shock waves (CESs) can be excited by a moving electron vortex (EV). The latter is driven by an ultrashort intense laser pulse propagating through a sandwich nonuniform underdense plasma slab and is found to move perpendicularly to the density gradient. Two CESs are observed on both sides of the passing route of the EVs. The left-side CES is induced by a high-density electron layer, which originates from the vortex front and is compressed and accelerated during the EV motion. The right-side CES is induced by supersonic ions accelerated by the EVs directly. Ion acceleration by such CESs along the directions perpendicular to the vortex propagation is also observed. This study reveals the transformation of nonlinear structures and provides new routes for laser energy dissipation in plasmas.