Although the interaction of a flat foil with currently available laser intensities is now considered a routine process, during the last decade, emphasis has been given to targets with complex geometries aiming at increasing the ion energy. This work presents a target geometry where two symmetric side holes and a central hole are drilled into the foil. A study of the various side-hole and central-hole length combinations is performed with two-dimensional particle-in-cell simulations for polyethylene targets and a laser intensity of $5.2\times 10^{21}~\text{W}~\text{cm}^{-2}$. The holed targets show a remarkable increase of the conversion efficiency, which corresponds to a different target configuration for electrons, protons and carbon ions. Furthermore, diffraction of the laser pulse leads to a directional high energy electron beam, with a temperature of ${\sim}40~\text{MeV}$, or seven times higher than in the case of a flat foil. The higher conversion efficiency consequently leads to a significant enhancement of the maximum proton energy from holed targets.

When two collisionless plasma shells collide, they interpenetrate and the overlapping region may turn Weibel unstable for some values of the collision parameters. This instability grows magnetic filaments which, at saturation, have to block the incoming flow if a Weibel shock is to form. In a recent paper (Bret, J. Plasma Phys., vol. 82, 2016b, 905820403), it was found by implementing a toy model for the incoming particle trajectories in the filaments, that a strong enough external magnetic field $\unicode[STIX]{x1D63D}_{0}$ can prevent the filaments blocking the flow if it is aligned with them. Denoting by $B_{f}$ the peak value of the field in the magnetic filaments, all test particles stream through them if $\unicode[STIX]{x1D6FC}=B_{0}/B_{f}>1/2$. Here, this result is extended to the case of an oblique external field $B_{0}$ making an angle $\unicode[STIX]{x1D703}$ with the flow. The result, numerically found, is simply $\unicode[STIX]{x1D6FC}>\unicode[STIX]{x1D705}(\unicode[STIX]{x1D703})/\cos \unicode[STIX]{x1D703}$, where $\unicode[STIX]{x1D705}(\unicode[STIX]{x1D703})$ is of order unity. Noteworthily, test particles exhibit chaotic trajectories.

The possibility of performing electron density and temperature measurements in a high power helicon plasma is a crucial issue in the framework of the AWAKE (Advanced WAKefield Experiment) project, which demonstrates acceleration of particles using $\text{GeV}~\text{m}^{-1}$ electric fields in plasmas. For AWAKE, a helicon is currently envisaged as a candidate plasma source due to its capability for low electron and ion temperature, high electron density and production of an elongated plasma column. A plasma diagnostic to accurately determine the electron density in AWAKE regimes would be a valuable supporting tool. A demonstration Thomson scattering (TS) diagnostic was installed and successfully tested on the resonant antenna ion device (RAID) at the Swiss Plasma Center of Ecole Polytechnique Fédérale de Lausanne. RAID produces a helicon plasma column with characteristics similar to those of the AWAKE helicon source, and is therefore an optimal testbed for application to the AWAKE device. The spectrometer employed in RAID is based on polychromators which collect the light scattered by plasma electrons in spectrally filtered wavelength regions. Results from TS on RAID demonstrate conditions of electron density and temperature respectively of $n_{e}=1.10\,(\pm 0.19)\times 10^{19}~\text{m}^{-3}$ and $T_{e}=2.3\,(\pm 0.6)~\text{eV}$ in a steady-state discharge in an Ar plasma with 5 kW of RF power. If the same polychromator system is used for AWAKE, where the electron density attained is $2\times 10^{20}~\text{m}^{-3}$, the contribution to measurement error due to coherent scattering is ${\sim}2.5\,\%$. Presented here are details of the TS diagnostic and the first tests in RAID, and the expectations for the system when employed on the AWAKE device.

The plasma channel formation in the focus of a knife-like nanosecond laser pulse irradiating a gas target is studied theoretically, and in gas-dynamics computer simulations. The distribution of the electromagnetic field in the focus region, obtained analytically, is used to calculate the energy deposition in the plasma, which then is implemented in the magnetohydrodynamic computer code. The modelling of the channel evolution shows that the plasma profile, which can guide the laser pulse, is formed by the tightly focused short knife-like lasers. The results of the simulations show that a proper choice of the convergence angle of a knife-like laser beam (determined by the focal length of the last cylindrical lens), and laser pulse duration may provide a sufficient degree of azimuthal symmetry of the formed plasma channel.

This paper reports the recent development of a full-scale particle-in-cell (PIC) simulation package highlighting an efficient electro-statics (ES)-PIC implementation for spacecraft charging problems. Numerical simulations are crucial in studying plasma flows because analytical solutions are rare, and experiments are expensive. There are many types of plasma flow that need various numerical methods for efficient and accurate simulations; as such, how to implement and organize those different methods into one comprehensive simulation package is challenging. This work adopted several modern software design patterns and developed a versatile package that includes various PIC schemes. This package has an open architecture, clean interfaces with both serial and parallel simulation capabilities. Two benchmark test cases are included to demonstrate the capabilities of this package. Then two plasma flows around a positively charged probe are simulated, and the results are discussed. The simulation results are consistent with past simulation results, and new insights are obtained. This work can lead to the development and organization of more sophisticated plasma simulation solvers in the future.

We have investigated the influence of a strong magnetic field on various aspects of a quantum Fermi plasma. Due to the strong magnetic field, the distribution function becomes anisotropic. First, we consider non-degenerate quantum, Landau and Kelly distribution function. It was found that the adiabatic equation is similar to the adiabatic equation for a Maxwell distribution function, when we include the magnetic field in the energy expression. Using the Kelly distribution for a degenerate, quantum Fermi gas, parallel and perpendicular components of the pressure were derived. It was found that perpendicular component of pressure never becomes zero and three-dimensional system always stay three-dimensional. Lastly, we investigated electron emission from metals and have shown the influence of the magnetic field. We calculated thermionic emission, the so-called Richardson effect. In addition, we investigate the influence of external electromagnetic radiation on the electron current density (Hallwachs effect) from metals.

The transient transmission of an electromagnetic wave through cold, unmagnetized and collisionless plasmas is described both analytically and numerically for its normal incidence from vacuum upon a plasma half-space. Exact formulas for the electromagnetic field are written in integral forms, which are convenient for approximate analysis and comparison with the results of direct numerical simulations. The time when the plasma particle oscillations become self-consistent with the electromagnetic field can be calculated from the simplified formulas for an arbitrary distance from the plasma–vacuum interface. Special attention is paid to the formation of the electrostatic oscillation in the case when the frequency of the incident wave is equal to the plasma frequency. The amplitudes of the vanishing magnetic field and the forming electrostatic oscillation are calculated as functions of time and the distance from the plasma–vacuum interface. The formation of the electrostatic oscillation is a slow process because the electromagnetic power penetrating into the plasma tends to zero with time. The transmitted plasma electromagnetic field is also simulated by the a finite-difference time-domain (FDTD) code. The difficulties of the numerical simulation of the quasi-electrostatic field are discussed. The analytical results can be used for the validation of the FDTD codes for plasma waves.

We study the propagation of nonlinear waves in non-relativistic electron–positron plasmas. The waves are assumed to propagate at small angles with respect to the equilibrium magnetic field. We derive the equation describing the wave propagation under the assumption that the waves are weakly dispersive and also can weakly depend on spatial variables orthogonal to the equilibrium magnetic field. We obtain solutions of the derived equation describing solitons. Then we study the stability of solitons with respect to transverse perturbations.

We derive a sufficient condition for the linear stability of plasma equilibria with incompressible flow parallel to the magnetic field, $\boldsymbol{B}$, constant mass density and anisotropic pressure such that the quantity $\unicode[STIX]{x1D70E}_{d}=\unicode[STIX]{x1D707}_{0}(P_{\Vert }-P_{\bot })/B^{2}$, where $P_{\Vert }$ ($P_{\bot }$) is the pressure tensor element parallel (perpendicular) to $\boldsymbol{B}$, remains constant. This condition is applicable to any steady state without geometrical restriction. The condition, generalising the respective condition for magnetohydrodynamic equilibria with isotropic pressure and constant density derived in Throumoulopoulos & Tasso (Phys. Plasmas, vol. 14, 2007, 122104), involves physically interpretable terms related to the magnetic shear, the flow shear and the variation of total pressure perpendicular to the magnetic surfaces. On the basis of this condition we prove that, if a given equilibrium is linearly stable, then the ones resulting from the application of Bogoyavlenskij symmetry transformations are linearly stable too, provided that a parameter involved in those transformations is positive. In addition, we examine the impact of pressure anisotropy, flow and torsion of a helical magnetic axis, for a specific class of analytic equilibria. In this case, we find that the pressure anisotropy and the flow may have either stabilising or destabilising effects. Also, helical configurations with small torsion and large pitch seem to have more favourable stability properties.

The turbulence-induced quasi-linear particle flux of a highly charged, collisional impurity species is calculated from the electrostatic gyrokinetic equation including collisions with the bulk ions and the impurities themselves. The equation is solved by an expansion in powers of the impurity charge number $Z$. In this formalism, the collision operator only affects the impurity flux through the dynamics of the impurities in the direction parallel to the magnetic field. At reactor-relevant collisionality, the parallel dynamics is dominated by the parallel electric field, and collisions have a minor effect on the turbulent particle flux of highly charged, collisional impurities.

The island divertor topology of the Wendelstein 7-X (W7-X) scrape-off-layer exhibits regions of highly varying connection length. Here, we present drift-plane simulations of seeded filaments in regions of sharp transitions in parallel connection length – a parameter which dictates the propagation regime for plasma blobs. It is determined that a transition in parallel connection length alters the trajectory of filaments; filaments which enter regions of lower connection length are decelerated, and vice versa. It is also determined that if the lobes of a potential dipole created by diamagnetic drifts within the filament exist in two regions of distinct parallel connection length, the filament is then steered towards the region of higher connection length. The extreme case of a narrow region of varying connection length can also alter the trajectory of a filament, depending on the extent of this region. Finally, simulations mimicking the view from the W7-X gas puff imaging (GPI) diagnostic view plane are presented. It is determined that filaments in the view of the W7-X GPI diagnostic exhibit a predominantly poloidal propagation due to the radial electric field, since the radial velocity is relatively small.

The edge region of quasi-adiabatic plasma is pedagogically simulated by the dynamics between the electric potential $\unicode[STIX]{x1D711}$ and the electron density $n$ whose equilibrium density gradient is negative and held fixed. The zonal flow (ZF) $V$ is either enforced sinusoidally or generated self-consistently from the turbulence. Cross-phase $\unicode[STIX]{x1D6FF}$ between $\unicode[STIX]{x1D711}$ and $n$, which is important in the determination of the turbulence level and the transport, is strongly influenced by the ZF. In the region near $V=0$, $\unicode[STIX]{x1D6FF}$ becomes negative due to the large gradient of the ZF. It is found that the instabilities are quenched there, and the fluctuations decay. The ZF thus works as a transport barrier in the region where the ZF changes direction with large gradient.

An error in Klimas & Viñas (J. Plasma Phys., vol. 85 (6), 2019, 905850610) is noted and explained. It is shown that the results in Klimas and Viñas were unaffected by the error. Further ramifications for future non-periodic spectral simulations are discussed.

Impurity temperature screening is a favourable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\unicode[STIX]{x1D702}^{-1}\equiv d\ln n/d\ln T$. In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here, we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry breaking in the magnetic field. For the majority of this work, neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with electrostatic potential $\unicode[STIX]{x1D6F7}=\unicode[STIX]{x1D6F7}(r)$, where $r$ is a flux-surface label. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes.

Recently, the validity range of the approximations commonly used in neoclassical calculation has been reconsidered. One of the primary motivations behind this trend is observation of an impurity hole in LHD (Large Helical Device), i.e. the formation of an extremely hollow density profile of an impurity ion species, such as carbon $\text{C}^{6+}$, in the plasma core region where a negative radial electric field ($E_{r}$) is expected to exist. Recent studies have shown that the variation of electrostatic potential on the flux surface, $\unicode[STIX]{x1D6F7}_{1}$, has significant impact on neoclassical impurity transport. Nevertheless, the effect of $\unicode[STIX]{x1D6F7}_{1}$ has been studied with radially local codes and the necessity of global calculation has been suggested. Thus, we have extended a global neoclassical code, FORTEC-3D, to simulate impurity transport in an impurity hole plasma including $\unicode[STIX]{x1D6F7}_{1}$ globally. Independently of the $\unicode[STIX]{x1D6F7}_{1}$ effect, an electron root of the ambipolar condition for the impurity hole plasma has been found by global simulation. Hence, we have considered two different cases, each with a positive (global) and a negative (local) solution of the ambipolar condition, respectively. Our result provides another support that $\unicode[STIX]{x1D6F7}_{1}$ has non-negligible impact on impurity transport. However, for the ion-root case, the radial $\text{C}^{6+}$ flux is driven further inwardly by $\unicode[STIX]{x1D6F7}_{1}$. For the electron-root case, on the other hand, the radial particle $\text{C}^{6+}$ flux is outwardly enhanced by $\unicode[STIX]{x1D6F7}_{1}$. These results indicate that how $\unicode[STIX]{x1D6F7}_{1}$ affects the radial particle transport crucially depends on the profile of the ambipolar-$E_{r}$, which is found to be susceptible to $\unicode[STIX]{x1D6F7}_{1}$ itself and the global effects.

We present the elements required to construct two devices used in an undergraduate plasma physics laboratory. The materials and construction costs of the sources, the vacuum systems and probe drives and electrical circuits are presented in detail in the text and the first appendix. We also provide the software for probe motion and data acquisition as well as the electrical schematics for key components. Experiments which have been performed are listed and two (resonance cones and whistler waves) are described in greater detail. The machines are flexible and original research is possible.