We describe here efforts to create and study magnetized electron–positron pair plasmas, the existence of which in astrophysical environments is well-established. Laboratory incarnations of such systems are becoming ever more possible due to novel approaches and techniques in plasma, beam and laser physics. Traditional magnetized plasmas studied to date, both in nature and in the laboratory, exhibit a host of different wave types, many of which are generically unstable and evolve into turbulence or violent instabilities. This complexity and the instability of these waves stem to a large degree from the difference in mass between the positively and the negatively charged species: the ions and the electrons. The mass symmetry of pair plasmas, on the other hand, results in unique behaviour, a topic that has been intensively studied theoretically and numerically for decades, but experimental studies are still in the early stages of development. A levitated dipole device is now under construction to study magnetized low-energy, short-Debye-length electron–positron plasmas; this experiment, as well as a stellarator device that is in the planning stage, will be fuelled by a reactor-based positron source and make use of state-of-the-art positron cooling and storage techniques. Relativistic pair plasmas with very different parameters will be created using pair production resulting from intense laser–matter interactions and will be confined in a high-field mirror configuration. We highlight the differences between and similarities among these approaches, and discuss the unique physics insights that can be gained by these studies.

The onset of magnetic reconnection in space, astrophysical and laboratory plasmas is reviewed discussing results from theory, numerical simulations and observations. After a brief introduction on magnetic reconnection and approach to the question of onset, we first discuss recent theoretical models and numerical simulations, followed by observations of reconnection and its effects in space and astrophysical plasmas from satellites and ground-based detectors, as well as measurements of reconnection in laboratory plasma experiments. Mechanisms allowing reconnection spanning from collisional resistivity to kinetic effects as well as partial ionization are described, providing a description valid over a wide range of plasma parameters, and therefore applicable in principle to many different astrophysical and laboratory environments. Finally, we summarize the implications of reconnection onset physics for plasma dynamics throughout the Universe and illustrate how capturing the dynamics correctly is important to understanding particle acceleration. The goal of this review is to give a view on the present status of this topic and future interesting investigations, offering a unified approach.

While it is well known that every nearly periodic Hamiltonian system possesses an adiabatic invariant, extant methods for computing terms in the adiabatic invariant series are inefficient. The most popular method involves the heavy intermediate calculation of a non-unique near-identity coordinate transformation, even though the adiabatic invariant itself is a uniquely defined scalar. A less well-known method, developed by S. Omohundro, avoids calculating intermediate sequences of coordinate transformations but is also inefficient as it involves its own sequence of complex intermediate calculations. In order to improve the efficiency of future calculations of adiabatic invariants, we derive generally applicable, readily computable formulas for the first several terms in the adiabatic invariant series. To demonstrate the utility of these formulas, we apply them to charged-particle dynamics in a strong magnetic field and magnetic field-line dynamics when the field lines are nearly closed.

Self-amplified spontaneous emission (SASE) free electron laser (FEL) devices have disclosed an unexpected interplay between the laser intensity growth and regions of the electron bunch of the order of the coherence length. They are usually identified with the bunch slice and contribute to the laser dynamics with their own characteristics. The dynamical effects inducing geometrical and phase space misalignment of bunch slice in X-ray operating FELs can be traced back to a plethora of phenomena, both in the Linac accelerating section or inside the beam transport optic magnet. They are responsible for spoiling of the beam projected qualities and, if not corrected properly, induce an increase of the saturation length and a decreasing of the output power. We discuss the inclusion of these effects in models employing scaling formulae.

Analytical solutions of the Grad–Shafranov equilibrium equation in simply connected plasma configurations, comprised of toroidal magnetic surfaces and open surfaces connected to electrodes, are reviewed and generalised. The Grad–Shafranov equation is linearised introducing assumptions on plasma current and pressure, which preserve regularity of solutions on the symmetry axis, as required for a simply connected geometry. Particular solutions are found by separation of variables both in cylindrical coordinates and in spherical ones. Equilibria that model local or global features of PROTO-SPHERA plasmas are constructed by combining a few particular solutions.

An ion cyclotron resonance heating (ICRH) antenna system must launch radio frequency (RF) power with a wavenumber spectrum which maximizes the coupling to the plasma. It should also ensure good absorption while minimizing the wave interaction with the plasma edge. Such interactions lead to impurity release, whose effect has been measured far from the antenna location (Klepper et al. 2013; Wukitch et al. 2017; Perkins et al. 2019) and can involve the entire scrape-off layer. In the normal heating scenario, for which the frequency of the waves launched by the antenna is larger than the ion cyclotron frequency of the majority ions \omega > \omega _{\textrm {ci},\textrm {maj}}$\omega > \omega _{\textrm {ci},\textrm {maj}}$, release of impurities due to ICRH can be affected by minimizing the low |k_{\parallel }| < k_0$|k_{\parallel }| < k_0$ power spectrum components of the antenna. Impurity release can be the result of low central absorption of the waves or power transfer from the fast to the slow wave due to the presence of a confluence in the plasma edge. In ASDEX Upgrade (AUG), a reduction of heavy impurity release by ICRH in the plasma was qualitatively well correlated to the parallel electric field and RF currents flowing around the antenna (Bobkov et al. 2017). In this article, we first show a correlation between the reduction in impurity release by ICRH in AUG and the rejection of the low |k_{\parallel }| < k_0$|k_{\parallel }| < k_0$ region of the antenna power spectrum. We show that the same correlation holds for results obtained in the Alcator C-Mod tokamak. Finally, using this idea, we reproduce ICRH induced impurity release behaviour in a not yet published experiments of JET, and make predictions for ITER and DEMO.

Magnetised plasma turbulence can have a multiscale character: instabilities driven by mean temperature gradients drive turbulence at the disparate scales of the ion and the electron gyroradii. Simulations of multiscale turbulence, using equations valid in the limit of infinite scale separation, reveal novel cross-scale interaction mechanisms in these plasmas. In the case that both long-wavelength (ion-gyroradius-scale) and short-wavelength (electron-gyroradius-scale) linear instabilities are driven far from marginal stability, we show that the short-wavelength instabilities are suppressed by interactions with long-wavelength turbulence. Two novel effects contributed to the suppression: parallel-to-the-field-line shearing by the long-wavelength {{\boldsymbol {E}} \times \boldsymbol {B}}${{\boldsymbol {E}} \times \boldsymbol {B}}$ flows, and the modification of the background density gradient by the piece of the long-wavelength electron adiabatic response with parallel-to-the-field-line variation. In contrast, simulations of multiscale turbulence where instabilities at both scales are driven near marginal stability demonstrate that when the long-wavelength turbulence is sufficiently collisional and zonally dominated the effect of cross-scale interaction can be parameterised solely in terms of the local modifications to the mean density and temperature gradients. We discuss physical arguments that qualitatively explain how a change in equilibrium drive leads to the observed transition in the impact of the cross-scale interactions.

When mass falls on the polar regions of a neutron star in a binary X-ray source system, it tends to spread out over the entire surface. A long-standing question in research on this problem is: will the mass be anchored on the magnetic field lines and drag the field with it or is there a special mechanism that allows the mass to slip through the magnetic field lines, leading to much less distortion? As the amount of mass falling on the neutron star can actually be comparable with the neutron star mass, the question of which alternative holds is very important. We suggest an efficient mechanism that will allow the mass to slip through the lines. The mechanism is based on a strong ideal Schwarzschild (Structure and Evolution of the Stars. Princeton University Press, 1958) instability. As the instability itself is ideal, it cannot directly force the mass to slip though the lines. However, it can create a cascade of eddies whose scale extends down to a resistive scale, at the same time mixing the field lines up without breaking them. On this scale the mass can cross the lines. This instability is efficient enough that it can produce a mass flow in the plasma without growing to a large amplitude but saturates at a small one. The instability determines the mass per flux distribution of the accumulated material on different lines so that the equilibrium is marginal to the instability on every line. This makes the equilibrium unique. Thus, as the extra mass on the neutron star grows, the state of the outer shell proceeds through a sequence of unique critically unstable equilibria. In an appendix, an attempt is made to track the critical equilibria over long times.

Using analytical theory and hybrid-kinetic numerical simulations, we demonstrate that, in a collisionless plasma, long-wavelength ion-acoustic waves (IAWs) with amplitudes \delta n/n_0 \gtrsim 2/\beta$\delta n/n_0 \gtrsim 2/\beta$ (where \beta \gg {1}$\beta \gg {1}$ is the ratio of thermal to magnetic pressure) generate sufficient pressure anisotropy to destabilize the plasma to firehose and mirror instabilities. These kinetic instabilities grow rapidly to reduce the pressure anisotropy by pitch-angle scattering and trapping particles, respectively, thereby impeding the maintenance of Landau resonances that enable such waves’ otherwise potent collisionless damping. The result is wave dynamics that evince a weakly collisional plasma: the ion distribution function is near-Maxwellian, the field-parallel flow of heat resembles its Braginskii form (except in regions where large-amplitude magnetic mirrors strongly suppress particle transport), and the relations between various thermodynamic quantities are more ‘fluid-like’ than kinetic. A nonlinear fluctuation–dissipation relation for self-sustaining IAWs is obtained by solving a plasma-kinetic Langevin problem, which demonstrates suppressed damping, enhanced fluctuation levels and weakly collisional thermodynamics when IAWs with \delta n/n_0 \gtrsim 2/\beta$\delta n/n_0 \gtrsim 2/\beta$ are stochastically driven. We investigate how our results depend upon the scale separation between the wavelength of the IAW and the Larmor radius of the ions, and discuss briefly their implications for our understanding of turbulence and transport in the solar wind and the intracluster medium of galaxy clusters.

The dynamical behaviour of weakly nonlinear, low-frequency sound waves are investigated in a plasma composed of only positive and negative ions incorporating the effects of a weak external uniform magnetic field. In the plasma model the mass (temperature) of the positive ions is smaller (larger) than that of the negative ions. The dynamics of the nonlinear wave is shown to be governed by a novel nonlinear equation. The stationary plane wave (analytical and numerical) nonlinear analysis on the basis of experimental parameters reveals that the nonlinear wave does have quasi-periodic and chaotic solutions. The Poincarè return map analysis confirms these observed complex structures.

The excitation of low frequency dust acoustic (or dust density) waves in a dusty plasma can be driven by the flow of ions relative to dust. We consider the nonlinear development of the ion–dust streaming instability in a highly collisional plasma, where the ion and dust collision frequencies are a significant fraction of their corresponding plasma frequencies. This collisional parameter regime may be relevant to dusty plasma experiments under microgravity or ground-based conditions with high gas pressure. One-dimensional particle-in-cell simulations are presented, which take into account collisions of ions and dust with neutrals, and a background electric field that drives the ion flow. Ion flow speeds of the order of a few times thermal are considered. Waveforms of the dust density are found to have broad troughs and sharp crests in the nonlinear phase. The results are compared with the nonlinear development of the ion–dust streaming instability in a plasma with low collisionality.

The formation and the dynamics of coherent magnetic field structures in the context of laser plasma interaction has attracted considerable attention. In the literature the formation of these structures has, however, mostly been reported in the wake of a laser pulse propagating in an underdense plasma medium (Bulanov et al., Phys. Rev. Lett., vol. 76, 1996, pp. 3562–3565; Nakamura & Mima Phys. Rev. Lett., vol. 100, 2008, 205006; Bulanov et al., Plasma Phys. Rep., vol. 31, no. 5, 2005, pp. 369–381; Naumova et al., Phys. Plasmas, vol. 8, no. 9, 2001, pp. 4149–4155; Nakamura et al., Phys. Rev. Lett., vol. 105, no. 13, 2010, 135002). The study here focuses on the formation of coherent structures by an intense laser pulse when it interacts with an overdense plasma medium. The laser in this case gets reflected and partially dumps its energy to the lighter electrons species. Particle-in-cell simulation studies have been carried out in two dimensions to show that the energetic electrons (generated at the critical layer and having relativistic energies), together with the background plasma electrons often self-organize to form distinct electron current vortices. These electron vortices have associated magnetic fields with monopolar or dipolar symmetries depending on the rotation profile of the electron current. The formation, stability and dynamics of these structures in the context of overdense plasma is of special importance as they provide a possibility of energy transport into those regions of plasma which are inaccessible to lasers. For such applications, higher energy content (involvement of relativistic electrons in their formation) of these structures is desirable. It is shown that their salient propagation characteristics even at relativistic energies follow the rules of electron magnetohydrodynamics (EMHD) (Isichenko & Marnachev, Sov. Phys. JETP, vol. 66, 1987, p. 702; Biskamp et al., Phys. Rev. Lett., vol. 76, 1996, p. 1264) (Generalized - EMHD Yadav et al., Phys. Plasmas, vol. 15, no. 6, 2008, 062308; Yadav et al., Phys. Plasmas, vol. 16, no. 4, 2009, 040701) for homogeneous (inhomogeneous) plasma medium, respectively.

This paper presents the results of studies of plasma flow parameters on the PF-1000U facility. A distinctive feature of this facility is the ability to create profiled initial gas distributions using gas puffs. In the experiments described, a combined system for filling the vacuum chamber with a working gas was used, in which an additional injection of various gases (deuterium, helium, neon and their mixtures) into the axial region of the chamber prefilled with deuterium was performed using a pulse valve. Thus, both the pinching processes and, accordingly, the generation of axial plasma flows and the conditions of their propagation in the background gas of the facility chamber were affected. Regimes with the generation of compact stable plasma formations propagating over long distances were found. The results obtained can be used in laboratory modelling of astrophysical jets from young stellar objects.

We study numerically the mechanisms of proton acceleration in gas–foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a 2\ \mathrm {\mu }$2\ \mathrm {\mu }$m-thick solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of \sim$\sim$4 compared with a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons’ positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analysed recently by Debayle et al. (New J. Phys., vol. 19, 2017, 123013). Our analysis also reveals the important role of the neighbouring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer.

The paper analyses the penetration of a magnetic field into the plasma bridge of nanosecond and microsecond opening switches. For switches with a conduction time of ~100 ns, simple formulae are derived to estimate the magnetic field velocity in collisionless and collisional plasmas. It is shown that in both cases this velocity is determined by the magnetic field rise rate to plasma density ratio raised to the power of 1/2. As the conduction time is increased to ~1 {\rm \mu}${\rm \mu}$s, the field velocity starts to depend on the plasma aggregation by a magnetic piston. At the same time, irrespective of the conduction time, the electron flow velocity is limited by the radial drift velocity in crossed magnetic and polarization electric fields. Such a limitation suppresses the current channel conductivity with respect to the Spitzer value by a factor equal to the electron magnetization parameter raised to one or another power. On completion of the conduction phase, the rate of rise of the switch resistance is proportional to the electron drift velocity. The peak switch voltage obtained in calculations is compared with its values recorded in experiments on mega-ampere current switching. A procedure is also presented for calculating the switch parameters to obtain the maximum possible voltage in the phase of current cutoff.

The dependence of fishbone cycle on energetic particle intensity has been investigated in EAST low-magnetic-shear plasmas. It is observed that the fishbone mode growth rate, saturation amplitude as well as fishbone cycle frequency clearly increase with increasing neutral beam injection (NBI) power. Moreover, enhanced electron density and temperature perturbations as well as energetic particle loss were observed with greater injected NBI power. Simulation results using M3D-K code show that as the NBI power increases, the resonant frequency and the energy of the resonant particles become higher, and the saturation amplitude of the mode also changes, due to the non-perturbative energetic particle contribution. The relationship between the calculated energetic particle pressure ratio and fishbone cycle frequency is obtained as {f_{\textrm{FC}}} = 2.2{(1000{\beta _{\textrm{ep,calc}}} - 0.1)^{5.9 \pm 0.5}}${f_{\textrm{FC}}} = 2.2{(1000{\beta _{\textrm{ep,calc}}} - 0.1)^{5.9 \pm 0.5}}$. Results consistent with the experimental observations have been achieved based on a predator–prey model.

The dynamical behaviours of electromagnetic (EM) solitons formed due to nonlinear interaction of linearly polarized intense laser light and relativistic degenerate plasmas are studied. In the slow-motion approximation of relativistic dynamics, the evolution of weakly nonlinear EM envelope is described by the generalized nonlinear Schrödinger (GNLS) equation with local and nonlocal nonlinearities. Using the Vakhitov–Kolokolov criterion, the stability of an EM soliton solution of the GNLS equation is studied. Different stable and unstable regions are demonstrated with the effects of soliton velocity, soliton eigenfrequency, as well as the degeneracy parameter R=p_{Fe}/m_ec$R=p_{Fe}/m_ec$, where p_{Fe}$p_{Fe}$ is the Fermi momentum and m_e$m_e$ the electron mass and c$c$ is the speed of light in vacuum. It is found that the stability region shifts to an unstable one and is significantly reduced as one enters from the regimes of weakly relativistic (R\ll 1)$(R\ll 1)$ to ultrarelativistic (R\gg 1)$(R\gg 1)$ degeneracy of electrons. The analytically predicted results are in good agreement with the simulation results of the GNLS equation. It is shown that the standing EM soliton solutions are stable. However, the moving solitons can be stable or unstable depending on the values of soliton velocity, the eigenfrequency or the degeneracy parameter. The latter with strong degeneracy (R>1)$(R>1)$ can eventually lead to soliton collapse.

In this paper, we propose an efficient diagnostic technique for determining spatially resolved measurements of the ion density ratio in a magnetized two-ion species plasma. Shear Alfvén waves were injected into a mixed helium–neon plasma using a magnetic loop antenna, for frequencies spanning the ion cyclotron regime. Two distinct propagation bands are observed, bounded by \omega < \varOmega _\textrm {Ne}$\omega < \varOmega _\textrm {Ne}$ and \omega _{ii} < \omega < \varOmega _\textrm {He}$\omega _{ii} < \omega < \varOmega _\textrm {He}$, where \omega _{ii}$\omega _{ii}$ is the ion–ion hybrid cutoff frequency and \varOmega _\textrm {He}$\varOmega _\textrm {He}$ and \varOmega _\textrm {Ne}$\varOmega _\textrm {Ne}$ are the helium and neon cyclotron frequencies, respectively. A theoretical analysis of the cutoff frequency was performed and shows it to be largely unaffected by kinetic electron effects and collisionality, although it can deviate significantly from \omega _{{ii}}$\omega _{{ii}}$ in the presence of warm ions due to ion finite Larmor radius effects. A new diagnostic technique and accompanying algorithm was developed in which the measured parallel wavenumber k_\parallel$k_\parallel$ is numerically fit to the predicted inertial Alfvén wave dispersion in order to resolve the local ion density ratio. A major advantage of this algorithm is that it only requires a measurement of k_\parallel$k_\parallel$ and the background magnetic field in order to be employed. This diagnostic was tested on the Large Plasma Device at UCLA and was successful in yielding radially localized measurements of the ion density ratio.

A new concept for the generation of thrust for space propulsion is introduced. Energetic thrust is generated in the form of plasmoids (confined plasma in closed magnetic loops) when magnetic helicity (linked magnetic field lines) is injected into an annular channel. Using a novel configuration of static electric and magnetic fields, the concept utilizes a current-sheet instability to spontaneously and continuously create plasmoids via magnetic reconnection. The generated low-temperature plasma is simulated in a global annular geometry using the extended magnetohydrodynamic model. Because the system-size plasmoid is an Alfvenic outflow from the reconnection site, its thrust is proportional to the square of the magnetic field strength and does not ideally depend on the mass of the ion species of the plasma. Exhaust velocities in the range of 20 to 500\ \textrm {km}\ \textrm {s}^{-1}$500\ \textrm {km}\ \textrm {s}^{-1}$, controllable by the coil currents, are observed in the simulations.