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The equilibrium and trapping of dust particles in a plasma sheath are investigated, both experimentally and theoretically. A self-consistent sheath model including input power as one of the model parameters is proposed, to predict the equilibrium position of particle trap. The electron temperature and density are estimated from the observed current and power (I-P) characteristics using the sheath model developed. Direct comparisons are made between the measured equilibrium position and the predicted equilibrium position. The equilibrium position moves closer to the electrode with increasing rf power and particle size. The position is apparently related to the sheath thickness, which decreases with increasing rf power. The model can correctly predict the experimentally observed trend in the equilibrium position of particle trap. It is found that the particle charge becomes positive when the particle gets closer to the electrode, due to the dominant influence of ion currents to the particle surface. [ABSTRACT FROM AUTHOR]

Several interrelated phenomena near the surface ablated into a discharge plasma, such as ablation and ionization in accelerated plasma are studied. Two characteristic ablation modes are identified, namely, ablation mode with a velocity at the Knudsen layer edge smaller than the local sound speed and a velocity at the Knudsen layer edge close to the sound speed. The existence of these two ablation modes is determined by the current density in the acceleration region. The nonequilibrium ionization region in the presence of strong electromagnetic plasma acceleration is studied. In the subsonic regime, the ionization region thickness is proportional to the ionization rate and inversely proportional to the magnetic field. Conditions for ionization equilibrium in the accelerating plasma are determined. The specific example of a micropulsed plasma thruster is considered. It is concluded that both the equilibrium and nonequilibrium ionization regimes occur in this device. [ABSTRACT FROM AUTHOR]

Studies a model for the sedimentation equilibrium of a charged colloidal suspension. Two-dimensional two-component plasma in a gravitational field; Computation of density profiles; Heavy particles' accumulation at the bottom of the container; Excess of light counterions' formation of a cloud floating at some altitude.

Presents a study that described a model for predicting equilibrium ion charge state distributions and soft x-ray emissions for highly charged ion plasmas confined in magnetic mirrors. Description of the model and simulation code; Investigation of typical code inputs and outputs and parameter space; Simulation results and discussion.

Two floating sheath solutions with strong electron emission in planar geometry have been proposed, a "space-charge limited" (SCL) sheath and an "inverse" sheath. SCL and inverse models contain different assumptions about conditions outside the sheath (e.g., the velocity of ions entering the sheath). So it is not yet clear whether both sheaths are possible in practice, or only one. Here we treat the global presheath-sheath problem for a plasma produced volumetrically between two planar walls. We show that all equilibrium requirements (a) floating condition, (b) plasma shielding, and (c) presheath force balance, can indeed be satisfied in two different ways when the emission coefficient γ > 1. There is one solution with SCL sheaths and one with inverse sheaths, each with sharply different presheath distributions. As we show for the first time in 1D-1V simulations, a SCL and inverse equilibrium are both possible in plasmas with the same upstream properties (e.g., same N and Te). However, maintaining a true SCL equilibrium requires no ionization or charge exchange collisions in the sheath, or else cold ion accumulation in the SCL's "dip" forces a transition to the inverse. This suggests that only a monotonic inverse type sheath potential should exist at any plasma-facing surface with strong emission, whether be a divertor plate, emissive probe, dust grain, Hall thruster channel wall, sunlit object in space, etc. Nevertheless, SCL sheaths might still be possible if the ions in the dip can escape. Our simulations demonstrate ways in which SCL and inverse regimes might be distinguished experimentally based on large-scale presheath effects, without having to probe inside the sheath. [ABSTRACT FROM AUTHOR]

An exactly solvable model of equilibrium and stability of a gravitationally bound low dust density cloud embedded in background plasma is described. The dust is distributed almost homogeneously within the cloud. The corresponding eigen value equation is solved using the Frobenius method to calculate the complete set of radial eigen modes and corresponding frequencies. The low dust density cloud is shown to be stable. This model can be useful in benchmarking simulation codes and modelling dust density distribution in the interstellar cloud in the HII region. [ABSTRACT FROM AUTHOR]

Drive particle beams in linear or weakly nonlinear regimes of the plasma wakefield accelerator quickly reach a radial equilibrium with the wakefield, which is described in detail for the first time. The equilibrium beam state and self-consistent wakefields are obtained by combining analytical relationships, numerical integration, and first-principles simulations. In the equilibrium state, the beam density is strongly peaked near the axis, the beam radius is constant along most of the beam, and longitudinal variation of the focusing strength is balanced by varying beam emittance. The transverse momentum distribution of beam particles depends on the observation radius and is neither separable nor Gaussian. [ABSTRACT FROM AUTHOR]

"Supersolitons," the structures associated with the stationary solitary solutions with the Mach number greater than those associated with the double layers, were introduced in 2012. Later, many researchers have reported the existence domain of the supersolitons in different plasma constituents. However, their evolutionary dynamical behavior and stability were main concerns and were not yet explored. We performed fluid simulation of ion acoustic supersolitons in a plasma containing two-temperature electrons having kappa distributions in the presence of cold fluid ions. Our simulation shows that a specific form of the initial perturbation in the equilibrium electron and ion densities can evolve into ion acoustic supersolitons, which maintain their shape and size during their propagation. This is first-ever simulation to confirm the stability of the supersolitons that opens a new era in the field of solitary wave structures in space and laboratory plasmas. [ABSTRACT FROM AUTHOR]

Investigates the equilibria of a rigidly royating fully ionized plasma column. Assumptions of the single-fluid equations for plasma column; Details on the single-fluid equations for singly charged ions and electrons; Information on the Gaussian particle density distribution.

This investigation is about plasma modeling for the control of vertical instabilities in Damavand tokamak. This model is based on online magnetic measurement. The algebraic equation defining the vertical position in this model is based on instantaneous force-balance. Two parameters in this equation, including decay index, n, and lambda, Λ, have been considered as functions of time-varying poloidal field coil currents and plasma current. Then these functions have been used in a code generated for modeling the open loop response of plasma. The main restriction of the suitability analysis of the model is that the experiments always have to be performed in the presence of a control loop for stabilizing vertical position. As a result, open loop response of the system has been identified from closed loop experimental data by nonlinear neural network identification method. The results of comparison of physical model with identified open loop response from closed loop experiments show root mean square error percentage less than 10%. The results are satisfying that the physical model is useful as a Damavand tokamak vertical movement simulator. [ABSTRACT FROM AUTHOR]

The dispersion relation of geodesic acoustic modes in the tokamak plasma with an equilibrium radial electric field is derived and analyzed. Multiple branches of eigenmodes have been found, similar to the result given by the fluid model with a poloidal mass flow. Frequencies and damping rates of both the geodesic acoustic mode and the sound wave increase with respect to the strength of radial electric field, while the frequency and the damping rate of the lower frequency branch slightly decrease. Possible connection to the experimental observation is discussed. [ABSTRACT FROM AUTHOR]

The fast termination phase of a vertical displacement event (VDE) in a tokamak is modeled as a sequence of shrinking equilibria, where the core current profile remains constant so that the safety-factor at the axis, qaxis, remains fixed and the qedge systematically decreases. At some point, the n = 1 kink mode is destabilized. Kink modes distort the magnetic field lines outside the plasma, and surface currents are required to nullify the normal component of the B-field at the plasma boundary and maintain equilibrium at finite pressure. If the plasma touches a conductor, the current can be transferred to the conductor, and may be measurable by the halo current monitors. This report describes a practical method to model the plasma as it evolves during a VDE, and determine the surface currents, needed to maintain equilibrium. The main results are that the onset conditions for the disruption are that the growth-rate of the n = 1 kink exceeds half the Alfven time and the associated surface current needed to maintain equilibrium exceeds one half of the core plasma current. This occurs when qedge drops below a low integer, usually 2. Application to NSTX provides favorable comparison with non-axisymmetric halo-current measurements. The model is also applied to ITER and shows that the 2/1 mode is projected to be the most likely cause of the final disruption. [ABSTRACT FROM AUTHOR]

Two reconnection problems are considered. The first problem concerns global physics. The plasma in the global reconnection region is in magnetostatic equilibrium. It is shown that this equilibrium can be uniquely characterized by a set of constraints. During reconnection and independently of the local reconnection physics, these constraints can be uniquely evolved from any initial state. The second problem concerns Petschek reconnection. Petschek's model for fast reconnection, which is governed by resistive MHD equations with constant resistivity is not validated by numerical simulations. Malyshkin et al. [Phys. Plasmas 12, 102920 (2005)], showed that the reason for the discrepancy is that Petschek did not employ Ohm's law throughout the local diffusion region, but only at the X-point. A derivation of Petschek reconnection, including Ohm's law throughout the entire diffusion region, removes the discrepancy. This derivation is based largely on Petschek's original 1964 calculation [in AAS-NASA Symposium on Solar Flares (National Aeronautics and Space Administration, Washington, D.C., 1964), NASA SP50, p. 425]. A useful physical interpretation of the role which Ohm's law plays in the diffusion region is presented. [ABSTRACT FROM AUTHOR]

The linear stability of the tearing mode (TM) in a plasma column is investigated in the presence of viscosity and finite equilibrium current density gradients (i.e., asymmetries). It is shown that for low β, both effects are essential in order to properly describe the mode behaviour close to marginality. In particular, the theory introduces a critical threshold for the destabilization, such that the perturbation grows only if Δ′>Δ′cr. The value of Δ′cr depends on the equilibrium configuration and on the plasma parameters. Most importantly, Δ′cr can take negative values, thus allowing unstable tearing modes for Δ′ < 0 (even in the absence of bootstrap current). [ABSTRACT FROM AUTHOR]

Coaxial helicity injection in the HIT-II [A. J. Redd et al., Phys. Plasmas 9, 2006 (2002)] spherical torus is modeled with time-dependent resistive magnetohydrodynamics computations run to steady state for conditions without strong relaxation. Laboratory and computed results on injector current and plasma current agree reasonably well as toroidal magnetic field and injector flux are scaled. The scalings are consistent with a dimensional estimate from the Grad-Shafranov equation that provides a new perspective on a previously published model based on a current-sheet equilibrium and the magnetic pressure required for the 'bubble-burst' criterion. Numerical solutions of the Grad-Shafranov equation with an assumed current profile also indicate large qualitative changes as the predicted criterion is crossed. [ABSTRACT FROM AUTHOR]

Poloidal rotation is routinely observed in present-day tokamak experiments, in particular near the plasma edge and in the high-confinement mode of operation. According to the magnetohydrodynamic (MHD) equilibrium theory [R. Betti and J. P. Freidberg, Phys. Plasmas 7, 2439 (2000)], radial discontinuities form when the poloidal velocity exceeds the poloidal sound speed (or rather, more correctly, the poloidal magneto-slow speed). Two-dimensional compressible magnetohydrodynamic simulations show that the transonic discontinuities develop on a time scale of a plasma poloidal revolution to form an edge density pedestal and a localized velocity shear layer at the pedestal location. While such an MHD pedestal surrounds the entire core, the outboard side of the pedestal is driven by the transonic discontinuity while the inboard side is caused by a poloidal redistribution of the mass. The MHD simulations use a smooth momentum source to drive the poloidal flow. Soon after the flow exceeds the poloidal sound speed, the density pedestal and the velocity shear layer form and persist into a quasi steady state. These results may be relevant to the L-H transition, the early stages of the pedestal and edge transport barrier formation. [ABSTRACT FROM AUTHOR]

The equilibrium velocity distributions in parallel propagating, circularly polarized low-frequency Alfvénic turbulence are presented. The equilibrium solutions of the monochromatic Alfvén waves are extended to those of incompressible Alfvénic turbulence with the broadband spectrum in low beta plasmas. It is shown that the 'apparent temperature' due to the nonresonant ion heating is explained on the basis of the equilibrium velocity distribution. The anisotropic equilibrium velocity distribution corresponding to Alfvénic turbulence in the Chew-Goldberger-Low system is also demonstrated. It is suggested that the distribution can be the back ground structure in the asymmetric velocity distributions observed in the solar wind. [ABSTRACT FROM AUTHOR]

The existence of localized structure of electromagnetic waves in relativistic electron-positron plasmas is investigated based on the pseudo-potential theory, without making any assumptions on the magnitudes of the flow velocity and temperature of the medium. The conditions for the localization of electromagnetic wave in the form of dark (dip type) soliton are found. In the small amplitude approximation, it is found that the dip becomes deeper and narrower as the temperature is raised. In low temperature T < mc2, localized solution exists only if the equilibrium longitudinal fluid velocity (parallel to the direction of propagation) in the wave frame is larger than the classical thermal velocity T/m of the plasma. For ultra-relativistically high temperature T >> mc2, it is shown that dark soliton can exist if the equilibrium longitudinal velocity is larger than c/3. [ABSTRACT FROM AUTHOR]

The equilibria bifurcations of the biparametric version of the classical Pierce diode, a one-dimensional plasma-filled device, are analyzed in detail. Our investigation reveals that this spatiotemporal model is not structurally stable in relation to a second control parameter, the ratio of the plasma ion density to the injected electron beam density. For the first time, we relate the existence of one-fluid chaotic regions with specific biparametric equilibria bifurcations, identifying the restricted regions in the parametric plane where they occur. We show that the system presents several biparametric scenarios involving codimension-two transcritical bifurcations. Finally, we provide the spatial profile of the stable and unstable one-fluid equilibria in order to describe their metamorphoses. [ABSTRACT FROM AUTHOR]

Continuous operation of a tokamak for fusion energy has clear engineering advantages but requires conditions beyond those sufficient for a burning plasma. The fusion reactions and external sources must support both the pressure and the current equilibrium without inductive current drive, leading to demands on stability, confinement, current drive, and plasma-wall interactions that exceed those for pulsed tokamaks. These conditions have been met individually, and significant progress has been made in the past decade to realize scenarios where the required conditions are obtained simultaneously. Tokamaks are operated routinely without disruptions near pressure limits, as needed for steady-state operation. Fully noninductive sustainment with more than half of the current from intrinsic currents has been obtained for a resistive time with normalized pressure and confinement approaching those needed for steady-state conditions. One remaining challenge is handling the heat and particle fluxes expected in a steady-state tokamak without compromising the core plasma performance. [ABSTRACT FROM AUTHOR]

The finite-element M3D code [W. Park et al., Phys. Plasmas 6, 1796 (1999)] has been modified to include a free-boundary equilibrium solver with arbitrary toroidal and poloidal flows. With this modification, the M3D code now has the capability to self-consistently model two essential ingredients necessary for equilibrium calculations in the edge region, namely, free-boundary and arbitrary flow. As a free-boundary code, M3D includes the separatrix and scrape-off layer regions in the equilibrium calculation. Poloidal flows in the subsonic, supersonic, and transonic regimes can be calculated with the new version of the M3D code. Calculation results show that the presence of equilibrium flows, in particular those next to the plasma boundary, can considerably influence the position of the X-point and magnetic separatrix shape/location and hence the position of the strike point on the divertor plates. Moreover, it is shown that poloidal flow is not a rigid-body rotation, with the fastest flows occurring on the inboard side of the plasma. A numerical confirmation of the 'de Laval nozzle' model of Betti and Freidberg [R. Betti and J. P. Freidberg, Phys. Plasmas 7, 2439 (2000)] for free-boundary equilibrium calculations is obtained, with the formation of the predicted discontinuities between regions of subsonic and supersonic flows (with respect to the poloidal sound speed). Finally, a detailed comparison between isentropic and isothermal equilibria is presented, showing qualitative analogies and quantitative differences. [ABSTRACT FROM AUTHOR]

In order to interpret the spectrum of a photoionized Si plasma, we calculate the level populations and emissions using a collisional-radiative equilibrium model with detailed atomic data. In our calculations, levels with principal quantum number up to 9 are included and the electron impact processes are also taken into account. The peak around 1855 eV, which is not reproduced in the previous simulation, is obvious in the present results. Spectra analyzing shows that this peak mainly comes from the intercombination line of He-like Si. Our calculations indicate that the electron impact processes enhance this intercombination line about once, although the electron temperature is pretty low. [ABSTRACT FROM AUTHOR]

A hybrid gyrokinetic ions/massless fluid electron model is used to study the stability of high-n toroidal Alfvén eigenmodes (TAEs) in ITER [M. Shimada et al., Nucl. Fusion 47, S1 (2007)]. The hybrid model has been implemented in the particle-in-cell turbulence simulation code GEM [Y. Chen and S. E. Parker, J. Comput. Phys. 220, 839 (2007)]. The adequacy of the hybrid model for simulating TAEs has been previously demonstrated [J. Lang et al., Phys. Plasmas 16, 102101 (2009)] by comparing the simulated TAE mode frequency and structure with an eigenmode analysis, and the thermal ion kinetic damping effect with analytic theory. By using a global particle-in-cell code the effects of large orbit width and nonlocal mode structures can be accurately included. Damping rate due to numerical filtering is carefully monitored, and convergence with respect to particle number, grid resolution, etc., is thoroughly tested. The simulations show that the most unstable modes in ITER lie in the rage of 10

We discuss a new family of solutions of the Grad-Shafranov (GS) equation that describes D-shaped toroidal plasma equilibria with sharp gradients at the plasma edge. These solutions have been derived by exploiting the continuous Lie symmetry properties of the GS equation and in particular a special type of 'weak' symmetries. In addition, we review the continuous Lie symmetry properties of the GS equation and present a short but exhaustive survey of the possible choices for the arbitrary flux functions that yield GS equations admitting some continuous Lie symmetry. Particular solutions related to these symmetries are also discussed. [ABSTRACT FROM AUTHOR]

Growth rates of edge localized modes for various benchmark equilibria, including a diverted equilibrium, are calculated using the nonideal fluid code M3D-C1. Growth rates calculated by M3D-C1 in the ideal limit are found to agree with those calculated by ideal magnetohydrodynamics codes. The effects of nonuniform density and resistivity profiles are explored, as well as the sensitivity of growth rates to the position of the ideal vacuum-plasma interface. Growth rates of the diverted equilibrium are found to be particularly sensitive to moving this interface inward from the separatrix, but less sensitive to extending the plasma region beyond the separatrix. The resistivity profile within the plasma is found not to affect growth rates significantly; however, growth rates may be greatly reduced by treating the outer region as a resistive plasma instead of an ideal vacuum. Indeed, it is found that for typical scrape-off layer (SOL) temperatures, the resistive SOL model behaves more like an ideal plasma than a vacuum. [ABSTRACT FROM AUTHOR]

Growth rates of edge localized modes for various benchmark equilibria, including a diverted equilibrium, are calculated using the nonideal fluid code M3D-C1. Growth rates calculated by M3D-C1 in the ideal limit are found to agree with those calculated by ideal magnetohydrodynamics codes. The effects of nonuniform density and resistivity profiles are explored, as well as the sensitivity of growth rates to the position of the ideal vacuum-plasma interface. Growth rates of the diverted equilibrium are found to be particularly sensitive to moving this interface inward from the separatrix, but less sensitive to extending the plasma region beyond the separatrix. The resistivity profile within the plasma is found not to affect growth rates significantly; however, growth rates may be greatly reduced by treating the outer region as a resistive plasma instead of an ideal vacuum. Indeed, it is found that for typical scrape-off layer (SOL) temperatures, the resistive SOL model behaves more like an ideal plasma than a vacuum. [ABSTRACT FROM AUTHOR]

This paper presents the first direct experimental measurements of the Salpeter enhancement of collisions due to particle correlations. The perpendicular-to-parallel collision rate ν⊥∥ is measured in laser-cooled pure ion plasmas, spanning the regimes from weak to strong magnetization, and from weak to strong particle correlations. The abrupt suppression of collisions in the strongly magnetized regime of T<=10-3 eV is observed, mitigated by the Salpeter enhancement when correlation effects become significant. This ν⊥∥ enhancement due to correlations is directly analogous to the enhancement of fusion collisions in hot dense stellar plasmas. The measured collisional enhancement is approximately exp(Γ), where Γ is the correlation parameter, and this is quantitatively consistent with analytical estimates based on thermal equilibrium shielding and correlations. [ABSTRACT FROM AUTHOR]

A cylindrical magnetohydrodynamic model including plasma pressure and longitudinal flow has been employed for the study of resistive wall mode (RWM) in reversed field pinch (RFP) plasmas. In order to validate the model, a careful comparison with the experimental measurements in RFX-mod [P. Sonato et al., Fusion Eng. Des. 66–68, 161 (2003)] on the mode growth rates has been made by well matching the equilibrium parameters F, Θ, and βp. The RWM instability spectrum, which varies with the equilibrium parameters, is also calculated for comparison. The sensitivity of the mode growth rate to the equilibrium parameters is studied in details. It is concluded that the model can provide consistent accuracy in studies of RWM in RFP plasmas. The analysis based on the balance of the potential energy components has been carried out in order to obtain the physical understanding on the mode behavior. [ABSTRACT FROM AUTHOR]

The nature of plasma equilibrium in a magnetic field with stochastic regions is examined. It is shown that the magnetic differential equation that determines the equilibrium Pfirsch–Schlüter currents can be cast in a form similar to various nonlinear equations for a turbulent plasma, allowing application of the mathematical methods of statistical turbulence theory. An analytically tractable model, previously studied in the context of resonance-broadening theory, is applied with particular attention paid to the periodicity constraints required in toroidal configurations. It is shown that even a very weak radial diffusion of the magnetic field lines can have a significant effect on the equilibrium in the neighborhood of the rational surfaces, strongly modifying the near-resonant Pfirsch–Schlüter currents. Implications for the numerical calculation of three-dimensional equilibria are discussed. [ABSTRACT FROM AUTHOR]

Stability properties of hydromagnetic waves (shear and compressional Alfven waves) in spatially homogeneous plasma are investigated when the equilibrium particle velocity distributions in both parallel and perpendicular directions (in reference to the ambient magnetic field) are modeled by kappa distributions. Analysis is presented for the limiting cases |ξα|<1 and |ξα|>1 for which solutions of the dispersion relations are analytically tractable. Here ξα(α=e,i) is the ratio of the wave phase speed and the electron (ion) thermal speed. Both low and high β (=plasma pressure/magnetic pressure) plasmas are considered. The distinguishing features of the hydromagnetic waves in kappa distribution plasma are (1) both Landau damping and transit-time damping rates are larger than those in Maxwellian plasma because of the enhanced high-energy tail of the kappa distribution and (2) density and temperature perturbations in response to the electromagnetic perturbations are different from those in Maxwellian plasma when |ξα|<1. Moreover, frequency of the oscillatory stable modes (e.g., kinetic shear Alfven wave) and excitation condition of the nonoscillatory (zero frequency) unstable modes (e.g., mirror instability) in kappa distribution plasma are also different from those in Maxwellian plasma. Quantitative estimates of the differences depend on the specific choice of the kappa distribution. For simplicity of notations, same spectral indices κ∥ and κ⊥ have been assumed for both electron and ion population. However, the analysis can be easily generalized to allow for different values of the spectral indices for the two charged populations. [ABSTRACT FROM AUTHOR]

In tokamak experiments, equilibrium reconstruction codes are used to calculate the location of the last closed flux surface, to map diagnostic information, and to derive important properties like current density and safety factor. At JET, the equilibrium code EFIT is automatically executed after each discharge. For speed and robustness, intershot EFIT is based on magnetic probe measurements only. As a consequence, the intershot profiles of the safety factor can be wrong for a variety of plasma scenarios. Internal diagnostic information, the pitch angle as measured with the motional stark effect, Faraday rotation angles, as well as pressure profile information can increase the accuracy of the EFIT equilibrium. In this paper, the accuracy of the internal diagnostics at JET and their impact on the EFIT results are discussed in detail. The influence of control parameters like the form of the test functions for ff′ and p′ on the equilibrium is investigated. The qmin from this analysis agrees with information from magnetohydrodynamics analysis (e.g., Alfvén cascades and sawtooth analysis) to within 10%–15%. [ABSTRACT FROM AUTHOR]

Convective blob propagation in the scrape-off-layer and/or limiter shadow region is analyzed analytically and numerically with emphasis on effects of a gradient in the equilibrium plasma density. The gradient of the equilibrium plasma density is taken into account beyond the Boussineque approximation. It is shown that the vorticity modification due to the plasma density gradient leads to the acceleration for the blobs propagating into the region of lower density and de-acceleration for the blobs propagating toward the regions of higher density. Analytical estimates are corroborated by direct numerical simulations. [ABSTRACT FROM AUTHOR]

The theory of variances of equilibrium reconstruction is presented. It complements existing practices with information regarding what kind of plasma characteristics can be reconstructed, how accurately, and what remains beyond the abilities of diagnostic systems. The σ-curves, introduced by the present theory, give a quantitative assessment of quality of effectiveness of diagnostic systems in constraining equilibrium reconstructions. The theory also suggests a method for aligning the accuracy of measurements of different physical nature and for improvements of numerical algorithms used in reconstruction. [ABSTRACT FROM AUTHOR]

A previous analysis of the nonlinear dissipative equilibrium of a beam-penetrated plasma with nonthermal electron “tails” [D. V. Rose, J. Guillory, and J. H. Beall, Phys. Plasmas 9, 1000 (2002)] is extended to the case of a relativistic, momentum-angle-scattered electron beam (with or without accompanying ions) penetrating a fully ionized low-density nearly collisionless plasma, and to include the energy balance of the nonthermal plasma tail electron population on electron collisional timescales long compared with the primary instability growth time. Quasistationary nonlinear “dissipative equilibrium” states are quantified for various ranges of relativistic beam parameters and various tail-enhanced Landau damping rates for shorter-wavelength space-charge waves. Conditions for quasisteady wave populations are found, and for energy balance between beam energy input to and dynamic friction cooling of the nonthermal “tail electrons.” Finally, some potentially incorrect inferences based on a thermal interpretation of bremsstrahlung from such a plasma are quantified. All of these microphysical processes evolve on timescales inaccessible to conventional magnetohydrodynamic modeling of astrophysical jets, and may lead to energetics corrections to such fluid models. [ABSTRACT FROM AUTHOR]

An analysis of radiation induced instabilities in an expanding plasma with considerable presence of dust particles and equilibrium cooling is presented. It is shown that the equilibrium expansion and cooling destabilize the radiation condensation modes and the presence of dust particles enhances this effect. The results obtained are examined in the context of ionized, dusty-plasma environment such as that of a planetary nebula. It is shown that due to nonstatic equilibrium and finite equilibrium cooling, small-scale localized structures formed as a result of thermal instability, become transient, which agrees with the observational results. The dust-charge fluctuation is found to heavily suppress these instabilities. [ABSTRACT FROM AUTHOR]

This is a mathematical study of the formation of tangential discontinuities, or current sheets, in a magnetic field evolving in an electrically perfectly conducting fluid in response to deformation of its domain, an effect first treated by Hahm and Kulsrud [Phys. Fluids 28, 2412 (1985)]. Explicit examples are presented of three-dimensional, untwisted fields, anchored to the boundary, that cannot assume a force-free state in the absence of current sheets. The underlying physics of this process is as described by the Parker theory of spontaneous current sheets, namely, that for most continuous magnetic fields in complex three-dimensional geometry, there is an incompatibility between the preservation of field topology and point-by-point force balance to achieve equilibrium. This incompatibility is removed through discontinuous plasma displacements that produce magnetic tangential discontinuities. In contrast to the twisted magnetic fields central to the Parker theory, fixing the connectivity between the anchored magnetic footpoints alone is sufficient to lead to current-sheet formation in untwisted fields. This mode of sheet formation may produce spatially dense multitudes of sheets to heat a plasma throughout its macroscopic volume such as implied by several phenomena in the solar corona. [ABSTRACT FROM AUTHOR]

The evolution of high current electron beams in dense plasmas has been investigated by using two-dimensional particle-in-cell (PIC) simulations with immobile ions. It is shown that electron beams are split into many filaments at the beginning due to the Weibel instability, and then different filamentation beams attract each other and coalesce. The profile of the filaments can be described by formulas. Hammer et al. [Phys. Fluids 13, 1831 (1970)] developed a self-consistent relativistic electron beam model that allows the propagation of relativistic electron fluxes in excess of the Alfvén-Lawson critical-current limit for a fully neutralized beam. The equilibrium solution has been observed in the simulation results, but the electron distribution function assumed by Hammer et al. is different from the simulation results. [ABSTRACT FROM AUTHOR]

The linear stability of a class of force-free equilibria in cylindrical geometry is investigated. The class consists of cylindrically symmetric force-free equilibria for which the ratio μ between the parallel current density and the magnetic field is a step function of the radius. It is suggested that plasmas in reversed field pinches could be roughly represented by such equilibria as a consequence of a small departure from an initial force-free state with constant μ, the latter being reached after a relaxation process according to the classical theory proposed by Taylor [Phys. Rev. Lett. 33, 1139 (1974)]. A fully analytical derivation of the tearing stability parameter Δ′ for such class of equilibria is given. It is then shown with one explicit example how the presence of a downward step of relatively small height can destabilize the innermost resonant mode, which would otherwise be stable if μ were constant. A possible implication of this mechanism for the formation of cyclic quasisingle helicity states observed in reversed field pinches is proposed. Considerations on the ideal stability of the class of equilibria under investigation are also given. [ABSTRACT FROM AUTHOR]

The evolution of many plasma systems is adiabatic, i.e., plasma entropy is conserved in each magnetic flux tube. An apparently surprising result found recently from simulations of a forced magnetic reconnection problem (the “Newton Challenge” [J. Birn et al., Phys. Plasmas 13, 092117 (2006)]) is that even in the presence of a dissipative process such as reconnection, the entropy within a flux tube can still be approximately conserved, due to the strong localization of the dissipation. To address plasma equilibrium with such adiabatic constraints, a novel code has been developed that computes equilibria with entropy profile as input, using the alternating dimension method [H. Grad et al., Proc. Natl. Acad. Sci. USA 72, 3789 (1975)]. The code alternates between solving the two–dimensional (2D) Grad-Shafranov equation to obtain the field configuration (flux function A) from the pressure profile P(A) and a 1D ordinary differential equation that uses the entropy conservation to derive the pressure function P(A) from a flux surface average. As a particular application, the code is used to compute equilibria relevant to the Newton Challenge, with a grid reflecting the reconnected state topology (with an X and an O point). The equilibria found agree very well with late stages of magnetohydrodynamic simulations of the Newton Challenge. This agreement not only validates the new code, but also proves that the final state approached by the reconnection simulations is indeed an equilibrium quasiadiabatically connected with the initial state. The results also show a significant release of magnetic energy through reconnection. Finally, other potential applications of the new code, especially to adiabatic evolution of space plasmas, are discussed. [ABSTRACT FROM AUTHOR]

The effect of sheared equilibrium plasma rotation on the stability of tearing modes in an Ohmic (low plasma β) regime is investigated. It is found, by means of numerical MHD simulations in a cylindrical geometry, that plasma rotation in the equivalent toroidal direction can result either in the increase or in the decrease of the instability growth rate. Perpendicular plasma viscosity and plasma rotation shear at the modes’ rational surface play a key role on assessing the effect of shear flow. While destabilizing for low viscosity plasmas (ratio of the resistive to viscous diffusion time scales τR/τV<1), for viscous plasmas (τR/τV>1) shear flow reduces the growth rate. Above a given threshold in the rotation shear (that depends on the ratio τR/τV) a tearing mode, unstable in the absence of rotation, can be stabilized. The effect of sheared toroidal flow on mode stability, in both viscosity regimes that are considered in the paper, is qualitatively independent of the aspect ratio. [ABSTRACT FROM AUTHOR]

The kinetic stability of the n=1, m=1 internal kink mode is analyzed for realistic equilibria typical of the standard operation scenario of ITER (the International Thermonuclear Experimental Reactor) [ITER Physics Basis Editors, Nucl. Fusion 39, 2137 (1999)]. The kinetic effects modify the inertia and the perturbed potential energy δW of the mode, the two key elements determining the mode stability. Numerical results are obtained for ITER-like equilibria with different q profiles. For moderate magnetic shear within the q=1 surface, the low frequency magnetohydrodynamic (MHD) branch is fully suppressed by the kinetic effects for the expected profiles and parameters up to twice the expected plasma β while the high frequency fishbone branch is found to be destabilized as the plasma β and the radius of the q=1 surface increase. The MHD branch can be destabilized at higher plasma β or larger radii of the q=1 surface only for q profiles with a low magnetic shear within the q=1 surface. [ABSTRACT FROM AUTHOR]

A fast scheme is proposed for determining the three-dimensional density distribution of a pure electron plasma under the thermal relaxation in a Penning trap on the basis of the observation of the axially integrated density distribution. The analysis that includes the contribution of the conducting wall reveals the appearance of a halo distribution surrounding the core distribution around the midplane. The core is confirmed to approach the Penning-type equilibrium distribution. Also proposed is a new scheme of the temperature determination of the electrons by analyzing a radial profile of the particles extracted with energy selection. This method is available on the basis of the self-consistent potential distribution associated with the equilibrium density distribution. The application of the two schemes of data analysis shows that the electron temperature decreases well below 0.1 eV with a 1/e folding time of ≈4× cyclotron-cooling time in the trap kept at room temperature. [ABSTRACT FROM AUTHOR]

The equilibrium and stability of oblate field-reversed configurations (FRCs) have been studied in the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. In the absence of a passive stabilization, tilt and shift instabilities often become unstable, with the tilt in particular limiting the plasma lifetime. The tilt instability can be mitigated by either including a passive stabilizing conductor, or by forming very oblate plasmas. Large perturbations (n=2 and 3) may still remain after passive stabilization is applied. These perturbations have the characteristics of co-interchange modes, which have never been observed, and can lead to the early termination of the plasma. The co-interchange modes can be minimized through the formation of plasmas with a very oblate shape, leading to the maximum FRC lifetime. A code has been developed to calculate equilibria for these plasmas. A rigid-body model explains the improved stability of oblate plasmas to n=1 tilt modes. Numerical calculations indicate improved stability to n>=2 co-interchange modes for the very oblate plasma shapes. [ABSTRACT FROM AUTHOR]