By using the Thomas-Fermi electron density distribution for quantum degenerate electrons, the hydrodynamic equations for ions, and the Poisson equation, planar and nonplanar ion-acoustic solitary waves in an unmagnetized collisionless plasma are investigated. The reductive perturbation method is used to derive cylindrical and spherical Korteweg-de Vries equations. Numerical solutions of the latter are presented. The present results can be useful in understanding the features of small but finite amplitude localized ion-acoustic solitary pulses in a degenerate plasma.
The nonlinear propagation of small but finite amplitude dust ion-acoustic waves (DIAWs) in an ion-beam driven plasma consisting of Boltzmannian electrons, positive ions, and stationary negatively charged dust grains is studied by using the standard reductive perturbation technique. It is shown that there exist two critical values (γc1) and (γc2) of ion beam to ion phase velocity ratio (γ), above and below which the beam generated solitons are not possible. The effects of the parameters, namely, γ, the ratio of the ion beam to plasma ion density (μi), the dust to ion density ratio (μd), and the ion beam to plasma ion mass ratio (μ) on both the amplitude and width of the stationary DIAWs, are analyzed numerically, and applications of the results to laboratory ion beam as well as space plasmas (e.g., auroral plasmas) are explained.
By employing one-dimensional quantum hydrodynamic (QHD) model for a three species quantum plasma, nonlinear properties of dust acoustic solitary waves are studied. For this purpose a Korteweg-de Vries (KdV) equation is derived, incorporating quantum corrections. The quantum mechanical effects are also examined numerically both on the profiles of the amplitude and the width of dust acoustic solitary waves. It is found that the amplitude remains constant but the width shrinks for different values of a dimensionless electron quantum parameter H-e=root(Z(d0)h(2)omega(2)(pd))/m(e)m(d)C(d)(4), where Z(d0) is the dust charge state, h is the Planck constant divided by 2 pi, omega(pd) is the dust plasma frequency, m(e) (m(d)) is the electron (dust) mass, and C-d is the dust acoustic speed.
A new dispersion relation for low-frequency compressional electromagnetic waves is derived by employing quantum magnetohydrodynamic model and Maxwell equations in cold quantum dusty magnetoplasmas. The latter is composed of inertialess electrons, mobile ions, and immobile charged dust particulates. The dispersion relation for the low-frequency compressional electromagnetic modes is further analyzed for the waves propagating parallel, perpendicular, and oblique to the external magnetic field direction. It is found theoretically and numerically that the quantum parameter alpha(q)=(n(i0)/n(e0))h(2)/(4m(e)m(i)) affects the real angular frequencies and the phase speeds of the compressional electromagnetic modes. Here, n(i0) (n(e0)) is the equilibrium number density of the ions (electrons), m(e) (m(i)) is the electron (ion) mass, and h is the Plank constant divided by 2 pi.
We have studied the linear dispersion relation for Langmuir waves in plasmas of very high density, based on the Dirac-Heisenberg-Wigner formalism. The vacuum contribution to the physical observables leads to ultraviolet divergences, which are removed by a charge renormalization. The remaining vacuum contribution is small and is in agreement with previously derived expressions for the time-dependent vacuum polarization. The main new feature of the theory is a damping mechanism similar to Landau damping, but where the plasmon energy gives rise to creation of electron-positron pairs. The dependence of the damping rate (pair-creation rate) on the wavenumber, temperature, and density is analyzed. Finally, the analytical results of linearized theory are compared with numerical solutions.
We study the contribution from the electron spin to the ponderomotive force, using a quantum kinetic model, including the spin-orbit correction. Specifically, we derive an analytical expression for the ponderomotive force, applicable for electrostatic waves propagating parallel to an external magnetic field. To evaluate the expression, we focus on the case of Langmuir waves and on the case of the spin resonance wave mode, where the classical and spin contributions to the ponderomotive force are compared. Somewhat surprisingly, depending on the parameter regime, we find that the spin contribution to the ponderomotive force may dominate for the Langmuir wave, whereas the classical contribution can dominate for the spin resonance mode.
We report on six dipolarization fronts (DFs) embedded in fast earthward flows detected by the Magnetospheric Multiscale mission during a substorm event on 23 July 2017. We analyzed Ohm's law for each event and found that ions are mostly decoupled from the magnetic field by Hall fields. However, the electron pressure gradient term is also contributing to the ion decoupling and likely responsible for an electron decoupling at DF. We also analyzed the energy conversion process and found that the energy in the spacecraft frame is transferred from the electromagnetic field to the plasma (J & BULL; E > 0) ahead or at the DF, whereas it is the opposite (J & BULL; E < 0) behind the front. This reversal is mainly due to a local reversal of the cross-tail current indicating a substructure of the DF. In the fluid frame, we found that the energy is mostly transferred from the plasma to the electromagnetic field (J & BULL; E & PRIME; < 0) and should contribute to the deceleration of the fast flow. However, we show that the energy conversion process is not homogeneous at the electron scales due to electric field fluctuations likely related to lower-hybrid drift waves. Our results suggest that the role of DF in the global energy cycle of the magnetosphere still deserves more investigation. In particular, statistical studies on DF are required to be carried out with caution due to these electron scale substructures.
One of the goals of synthesizing and trapping antihydrogen is to study the validity of charge-parity-time symmetry through precision spectroscopy on the anti-atoms, but the trapping yield achieved in recent experiments must be significantly improved before this can be realized. Antihydrogen atoms are commonly produced by mixing antiprotons and positrons stored in a nested Penning-Malmberg trap, which was achieved in ALPHA by an autoresonant excitation of the antiprotons, injecting them into the positron plasma. In this work, a hybrid numerical model is developed to simulate antiproton and positron dynamics during the mixing process. The simulation is benchmarked against other numerical and analytic models, as well as experimental measurements. The autoresonant injection scheme and an alternative scheme are compared numerically over a range of plasma parameters which can be reached in current and upcoming antihydrogen experiments, and the latter scheme is seen to offer significant improvement in trapping yield as the number of available antiprotons increases.
Recent progress in the understanding and prediction of the tungsten behaviour in the core of JET H-mode plasmas with ITER-like wall is presented. Particular emphasis is given to the impact of poloidal asymmetries of the impurity density. In particular, it is shown that the predicted reduction of temperature screening induced by the presence of low field side localization of the tungsten density produced by the centrifugal force is consistent with the observed tungsten behaviour in a JET discharge in H-mode baseline scenario. This provides first evidence of the role of poloidal asymmetries in reducing the strength of temperature screening. The main differences between plasma parameters in JET baseline and hybrid scenario discharges which affect the impact of poloidally asymmetric density on the tungsten radial transport are identified. This allows the conditions by which tungsten accumulation can be avoided to be more precisely defined.
The impact of the change of the mass of hydrogen isotopes on the turbulent particle flux is studied. The trapped electron component of the turbulent particle convection induced by collisionality, which is outward in ion temperature gradient turbulence, increases with decreasing thermal velocity of the isotope. Thereby, the lighter is the isotope, the stronger is the turbulent pinch, and the larger is the predicted density gradient at the null of the particle flux. The passing particle component of the flux increases with decreasing mass of the isotope and can also affect the predicted density gradient. This effect is however subdominant for usual core plasma parameters. The analytical results are confirmed by means of both quasi-linear and nonlinear gyrokinetic simulations, and an estimate of the difference in local density gradient produced by this effect as a function of collisionality has been obtained for typical plasma parameters at mid-radius. Analysis of currently available experimental data from the JET and the ASDEX Upgrade tokamaks does not show any clear and general evidence of inconsistency with this theoretically predicted effect outside the errorbars and also allows the identification of cases providing weak evidence of qualitative consistency.
An explanation of the strong heating observed in the core of a reversed-field pinch in the quasi-single-helicity state is presented. A magnetic island is formed, in which the heat transport coefficient is much smaller than in the surrounding chaotic sea, because of the formation of well defined magnetic surfaces. The values of the thermal conductivity obtained with the M1TEV [F. Porcelli , Phys. Rev. Lett 82, 1458 (1999)] two-dimensional transport code are in very good agreement with the estimates of the ion diffusion coefficient inside the island, given by a Hamiltonian guiding center code. Moreover, the values of thermal conductivity are in the tokamak range, and are consistent with the peak temperatures measured in the Reversed Field eXperiment [P. Sonato , Fusion Eng. Des. 66-68, 161 (2003)] at Consorzio RFX, Padova, Italy. The effect of the island width and the different powers deposited inside the island on the final temperature peak are also investigated.
A scheme of generation of ultraintense proton beams relevant for proton fast ignition (PFI) which employs multi-ps, circularly polarized laser pulse irradiating a thick (≥ 10 μm) H-rich target is proposed and examined using one-dimensional particle-in cell-simulations. It is shown that a 5-ps laser pulse of intensity ∼ (2–5) × 1020W/cm2 irradiating the target of the areal proton density ∼ 2 × 1020cm−2 can produce – with a high energetic efficiency – a proton beam (plasma block) of parameters (intensity, energy fluence, pulse duration, proton energy spectrum) close to those required for PFI. At a fixed total laser energy, the proton beam parameters can be controlled and fitted to the PFI requirements by changing the laser intensity (energy fluence) and/or the target thickness as well as by using a shaped (curved) target inserted into a guiding cone.
The amplitude modulation of quantum ion-acoustic waves (QIAWs) along an external magnetic field is studied in a quantum electron-positron-ion (e-p-i) magnetoplasma. Reductive perturbation technique is used to derive the three-dimensional nonlinear Schroumldinger equation which governs the slow modulation of QIAW packets. Accounting for the effects of the electron to ion number density ratio (mu), the normalized ion-cyclotron frequency (omega(c)) as well as the ratio (H) of the "plasmonic energy density" to the Fermi energy, new regimes for the modulational instability of QIAWs are obtained and analyzed. In contrast to one-dimensional unmagnetized e-p-i plasmas, the instability growth rate is shown to suppress with increasing mu or decreasing the values of H. The predicted results could be important for understanding the salient features of modulated QIAW packets in dense astrophysical plasmas as well as to the next generation intense laser solid density plasma experiments.
Dusty plasmas are electrically quasi-neutral media that, along with electrons, ions, neutral gas, radiation, and electric and/or magnetic fields, also contain solid or liquid particles with sizes ranging from a few nanometers to a few micrometers. These media can be found in many natural environments as well as in various laboratory setups and industrial applications. As a separate branch of plasma physics, the field of dusty plasma physics was born in the beginning of 1990s at the intersection of the interests of the communities investigating astrophysical and technological plasmas. An additional boost to the development of the field was given by the discovery of plasma crystals leading to a series of microgravity experiments of which the purpose was to investigate generic phenomena in condensed matter physics using strongly coupled complex (dusty) plasmas as model systems. Finally, the field has gained an increasing amount of attention due to its inevitable connection to the development of novel applications ranging from the synthesis of functional nanoparticles to nuclear fusion and from particle sensing and diagnostics to nano-contamination control. The purpose of the present perspectives paper is to identify promising new developments and research directions for the field. As such, dusty plasmas are considered in their entire variety: from classical low-pressure noble-gas dusty discharges to atmospheric pressure plasmas with aerosols and from rarefied astrophysical plasmas to dense plasmas in nuclear fusion devices. Both fundamental and application aspects are covered.
Multidevice pedestal scaling experiments in the DIII-D, ASDEX Upgrade (AUG), and JET tokamaks are presented in order to test two plasma physics pedestal width models. The first model proposes a scaling of the pedestal width Delta/a proportional to rho*(1/2) to rho* based on the radial extent of the pedestal being set by the point where the linear turbulence growth rate exceeds the E x B velocity. In the multidevice experiment where rho* at the pedestal top was varied by a factor of four while other dimensionless parameters where kept fixed, it has been observed that the temperature pedestal width in real space coordinates scales with machine size, and that therefore the gyroradius scaling suggested by the model is not supported by the experiments. The density pedestal width is not invariant with rho* which after comparison with a simple neutral fuelling model may be attributed to variations in the neutral fuelling patterns. The second model, EPED1, is based on kinetic ballooning modes setting the limit of the radial extent of the pedestal region and leads to Delta(psi) proportional to beta p(1/2). All three devices show a scaling of the pedestal width in normalised poloidal flux as Delta(psi) proportional to beta p(1/2), as described by the kinetic ballooning model; however, on JET and AUG, this could not be distinguished from an interpretation where the pedestal is fixed in real space. Pedestal data from all three devices have been compared with the predictive pedestal model EPED1 and the model produces pedestal height values that match the experimental data well.
The penetration of plasma clouds, or plasmoids, across abrupt magnetic barriers (of the scale less than a few ion gyro radii, using the plasmoid directed velocity) is studied. The insight gained earlier, from detailed experimental and computer simulation investigations of a case study, is generalized into other parameter regimes. It is concluded for what parameters a plasi-noid should be expected to penetrate the magnetic barrier through self-polarization, penetrate through magnetic expulsion, or be rejected from the barrier. The scaling parameters are n(e), upsilon(o), B-perpendicular to, m(i), T-i, and the width w of the plasmoid. The scaling is based on a model for strongly driven, nonlinear magnetic field diffusion into a plasma which is a generalization of the earlier laboratory findings. The results are applied to experiments earlier reported in the literature, and also to the proposed application of impulsive penetration of plasmoids from the solar wind into the Earth's magnetosphere.
Azimuthally rotating dense plasma structures, spokes, have recently been detected in several high power impulse magnetron sputtering (HiPIMS) devices used for thin film deposition and surface treatment, and are thought to be important for plasma buildup, energizing of electrons, as well as cross-B transport of charged particles. In this work, the drift velocities of these spokes are shown to be strongly correlated with the critical ionization velocity, CIV, proposed by Alfven. It is proposed as the most promising approach in combining the CIV and HiPIMS research fields is to focus on the role of spokes in the process of electron energization.
The hierarchy of unstable modes when two counter-streaming pair plasmas interact over a flow-aligned magnetic field has been recently investigated [Phys. Plasmas 23, 062122 (2016)]. The analysis is here extended to the case of an arbitrarily tilted magnetic field. The two plasma shells are initially cold and identical. For any angle θ ∈ [0, π/2] between the field and the initial flow, the hierarchy of unstable modes is numerically determined in terms of the initial Lorentz factor of the shells γ0, and the field strength as measured by a parameter denoted σ. For θ = 0, four different kinds of mode are likely to lead the linear phase. The hierarchy simplifies for larger θ's, partly because the Weibel instability can no longer be cancelled in this regime. For θ > 0.78 (44°) and in the relativistic regime, the Weibel instability always govern the interaction. In the non-relativistic regime, the hierarchy becomes θ-independent because the interaction turns to be field-independent. As a result, the two-stream instability becomes the dominant one, regardless of the field obliquity.
A relativistic fluid model is implemented to assess the role of the ions motion in the linear phase of relativistic beam plasma electromagnetic instabilities. The all unstable wave vector spectrum is investigated, allowing us to assess how ion motions modify the competition between every possible instability. Beam densities up to the plasma one are considered. Due to the fluid approach, the temperatures must remain small, i.e., nonrelativistic. In the cold limit, ions motion affect the most unstable mode when the beam gamma factor bM/mi, being the beam to plasma density ratio, i the ion charge, M their mass, and m the electrons. The return current plays an important role by prompting Buneman-type instabilities which remain in the nonrelativistic regime up to high beam densities. Nonrelativistic temperatures only slightly affect these conclusions, except in the diluted beam regime where they can stabilize the Buneman modes.
The electromagnetic instabilities driven by a relativistic electron beam, which moves through a magnetized plasma, are analyzed with a cold two-fluid model. It allows for any angle B between the beam velocity vector and the magnetic field vector and considers any orientation of the wavevector in the two-dimensional plane spanned by these two vectors. If the magnetic field is strong, the two-stream instability dominates if B=0 and the oblique modes grow faster at larger B. A weaker magnetic field replaces the two-stream modes with oblique modes as the fastest-growing waves. The threshold value separating both magnetic regimes is estimated. A further dimensionless parameter is identified, which determines whether or not the wavevector of the most unstable wave is changed continuously, as B is varied from 0 to /2. The fastest growing modes are always found for a transverse propagation of the beam with B=/2, irrespective of the magnetic field strength. ©2008 American Institute of Physics
The temperature-dependent fluid model from Phys. Plasmas 13, 042106 (2006) is expanded in order to explore the oblique electromagnetic instabilities, which are driven by a hot relativistic electron beam that is interpenetrating a hot and magnetized plasma. The beam velocity vector is parallel to the magnetic-field direction. The results are restricted to nonrelativistic temperatures. The growth rates of all instabilities but the two-stream instability can be reduced by a strong magnetic field so that the distribution of unstable waves becomes almost one dimensional. For high beam densities, highly unstable oblique modes dominate the spectrum of unstable waves as long as omega(c)/omega(p)less than or similar to 2 gamma(3/2)(b), where omega(c) is the electron gyrofrequency, omega(p) is the electron plasma frequency, and gamma(b) is the relativistic factor of the beam. A uniform stabilization over the entire k space cannot be achieved.
Particle-in-cell simulations are widely used as a tool to investigate instabilities that develop between a collisionless plasma and beams of charged particles. However, even on contemporary supercomputers, it is not always possible to resolve the ion dynamics in more than one spatial dimension with such simulations. The ion mass is thus reduced below 1836 electron masses, which can affect the plasma dynamics during the initial exponential growth phase of the instability and during the subsequent nonlinear saturation. The goal of this article is to assess how far the electron to ion mass ratio can be increased, without changing qualitatively the physics. It is first demonstrated that there can be no exact similarity law, which balances a change in the mass ratio with that of another plasma parameter, leaving the physics unchanged. Restricting then the analysis to the linear phase, a criterion allowing to define a maximum ratio is explicated in terms of the hierarchy of the linear unstable modes. The criterion is applied to the case of a relativistic electron beam crossing an unmagnetized electron-ion plasma.
The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.
Heavy impurities, such as tungsten (W), can exhibit strongly poloidally asymmetric density profiles in rotating or radio frequency heated plasmas. In the metallic environment of JET, the poloidal asymmetry of tungsten enhances its neoclassical transport up to an order of magnitude, so that neoclassical convection dominates over turbulent transport in the core. Accounting for asymmetries in neoclassical transport is hence necessary in the integrated modeling framework. The neoclassical drift kinetic code, NEO [E. Belli and J. Candy, Plasma Phys. Controlled Fusion P50, 095010 (2008)], includes the impact of poloidal asymmetries on W transport. However, the computational cost required to run NEO slows down significantly integrated modeling. A previous analytical formulation to describe heavy impurity neoclassical transport in the presence of poloidal asymmetries in specific collisional regimes [C. Angioni and P. Helander, Plasma Phys. Controlled Fusion 56, 124001 (2014)] is compared in this work to numerical results from NEO. Within the domain of validity of the formula, the factor for reducing the temperature screening due to poloidal asymmetries had to be empirically adjusted. After adjustment, the modified formula can reproduce NEO results outside of its definition domain, with some limitations: When main ions are in the banana regime, the formula reproduces NEO results whatever the collisionality regime of impurities, provided that the poloidal asymmetry is not too large. However, for very strong poloidal asymmetries, agreement requires impurities in the Pfirsch-Schluter regime. Within the JETTO integrated transport code, the analytical formula combined with the poloidally symmetric neoclassical code NCLASS [W. A. Houlberg et al., Phys. Plasmas 4, 3230 (1997)] predicts the same tungsten profile as NEO in certain cases, while saving a factor of one thousand in computer time, which can be useful in scoping studies. The parametric dependencies of the temperature screening reduction due to poloidal asymmetries would need to be better characterised for this faster model to be extended to a more general applicability.
We study the influence of collisions on the dynamics of a cold non-relativistic plasma. It is shown that even a comparatively small collision frequency can significantly change the large amplitude wave solution. Published by AIP Publishing.
We derive two equations describing the coupling between electromagnetic and electrostaticoscillations in one-dimensional geometry in a magnetized cold and non-relativistic plasma. The nonlinear interaction between the wave modes is studied numerically. The effects of the external magnetic field strength and the initial electromagneticpolarization are of particular interest here. New results can, thus, be identified.
The resonant interaction between three waves in a uniform magnetized plasma is reconsidered. Starting from previous kinetic expressions, we limit our investigation to waves propagating perpendicularly to the external magnetic field. It is shown that reliable results can only be obtained in the two-dimensional case, i.e., when the wave vectors have both x and y components. (C) 2015 AIP Publishing LLC.
We have extended previous quantum kinetic results to compute the exchange correction to the electrostatic electron susceptibility for arbitrary frequencies and wavenumbers in the low temperature limit. This has allowed us to make a general comparison with a much used hydrodynamic expression, based on density functional theory, for exchange effects. For low phase velocities, as for ion-acoustic waves, wave-particle interaction leads to a strong enhancement of the exchange correction and the hydrodynamic result is smaller by an order of magnitude. The hydrodynamic expression gives a useful approximation when the phase velocity is 2.5 times the Fermi velocity. If this condition is not fulfilled, the hydrodynamical theory gives misleading results. We discuss the implications of our results for the model choice for quantum plasmas, especially regarding particle dispersive effects.
A set of fluid equations, taking into account the spin properties of the electrons and positrons in a magnetoplasma, are derived. The magnetohydrodynamic limit of the pair plasma is investigated. It is shown that the microscopic spin properties of the electrons and positrons can lead to interesting macroscopic and collective effects in strongly magnetized plasmas. In particular, it is found that new Alfvénic solitary structures, governed by a modified Korteweg–de Vries equation, are allowed in such plasmas. These solitary structures vanish if the quantum spin effects are neglected. Our results should be of relevance for astrophysical plasmas, e.g., in pulsar magnetospheres, as well as for low-temperature laboratory plasmas.
We study the influence of collisions on the dynamics of a cold non-relativistic plasma. It is shown that even a comparatively small collision frequency can significantly change the large amplitude wave solution.
We derive two equations describing the coupling between electromagnetic and electrostatic oscillations in one-dimensional geometry in a magnetized cold and non-relativistic plasma. The nonlinear interaction between the wave modes is studied numerically. The effects of the external magnetic field strength and the initial electromagnetic polarization are of particular interest here. New results can, thus, be identified.
The resonant interaction between three waves in a uniform magnetized plasma is reconsidered. Starting from previous kinetic expressions, we limit our investigation to waves propagating perpendicularly to the external magnetic field. It is shown that reliable results can only be obtained in the two-dimensional case, i.e., when the wave vectors have both x and y components.
Resistive wall modes (RWM) in the reversed field pinch are studied and a detailed comparison of experimental growth rates and linear magnetohydrodynamic (MHD) theory is made. RWM growth rates are experimentally measured in the thin shell device EXTRAP T2R [P. R. Brunsell , Plasma Phys. Controlled Fusion 43, 1 (2001)]. Linear MHD calculations of RWM growth rates are based on experimental equilibria. Experimental and linear MHD RWM growth rate dependency on the equilibrium profiles is investigated experimentally by varying the pinch parameter Theta=B-theta(a)/<B-phi> in the range Theta=1.5-1.8. Quantitative agreement between experimental and linear MHD growth rates is seen. The dominating RWMs are the internal on-axis modes (having the same helicity as the central equilibrium field). At high Theta, external nonresonant modes are also observed. For internal modes experimental growth rates decrease with Theta while for external modes, growth rates increase with Theta. The effect of RWMs on the reversed-field pinch plasma performance is discussed.
An array of saddle coils having Nc =16 equally spaced positions along the toroidal direction has been installed for feedback control of resistive wall modes (RWMs) on the EXTRAP T2R reversed-field pinch [P. R. Brunsell, H. Bergsaker, M. Cecconello, Plasma Phys. Controlled Fusion 43, 1457 (2001)]. Using feedback, multiple nonresonant RWMs are simultaneously suppressed for three to four wall times. Feedback stabilization of RWMs results in a significant prolongation of the discharge duration. This is linked to a better sustainment of the plasma and tearing mode toroidal rotation with feedback. Due to the limited number of coils in the toroidal direction, pairs of modes with toroidal mode numbers n, n′ that fulfill the condition ∫n- n′ ∫ = Nc are coupled by the feedback action from the discrete coil array. With only one unstable mode in a pair of coupled modes, the suppression of the unstable mode is successful. If two modes are unstable in a coupled pair, two possibilities exist: partial suppression of both modes or, alternatively, complete stabilization of one target mode while the other is left unstable.
From a magnetofluid description with intrinsic magnetization, a new plasma instability is obtained. The plasma magnetization is produced by the collective electron spin. The instability develops in a nonuniform plasma when the electron concentration and temperature vary along an externally applied magnetic field. Alfvén waves play an important role in the instability. The instability properties are numerically investigated for a particular example of an ultrarelativistic degenerate plasma in exploding white dwarfs.
The problem of the Darrieus-Landau instability at a discontinuous deflagration front in a compressible flow is solved. Numerous previous attempts to solve this problem suffered from the deficit of boundary conditions. Here, the required additional boundary condition is derived rigorously taking into account the internal structure of the front. The derived condition implies a constant mass flux at the front; it reduces to the classical Darrieus-Landau condition in the limit of an incompressible flow. It is demonstrated that in general the solution to the problem depends on the type of energy source in the flow. In the common case of a strongly localized source, compression effects make the Darrieus-Landau instability considerably weaker. Particularly, the instability growth rate is reduced for laser ablation in comparison to the classical incompressible case. The instability disappears completely in the Chapman-Jouguet regime of ultimately fast deflagration.
The structure of a weak shock in a quantum plasma is studied, taking into account both dissipation terms due to thermal conduction and dispersive quantum terms due to the Bohm potential. Unlike quantum systems without dissipations, even a small thermal conduction may lead to a stationary shock structure. In the limit of zero quantum effects, the monotonic Burgers solution for the weak shock is recovered. Still, even small quantum terms make the structure nonmonotonic with the shock driving a train of oscillations into the initial plasma. The oscillations propagate together with the shock. The oscillations become stronger as the role of Bohm potential increases in comparison with thermal conduction. The results could be of importance for laser-plasma interactions, such as inertial confinement fusion plasmas, and in astrophysical environments, as well as in condensed matter systems.
Magnetic helicity fluxes are investigated in a family of gauges in which the contribution from ideal magnetohydrodynamics takes the form of a purely advective flux. Numerical simulations of magnetohydrodynamic turbulence in this advective gauge family exhibit instabilities triggered by the build-up of unphysical irrotational contributions to the magnetic vector potential. As a remedy, the vector potential is evolved in a numerically well behaved gauge, from which the advective vector potential is obtained by a gauge transformation. In the kinematic regime, the magnetic helicity density evolves similarly to a passive scalar when resistivity is small and turbulent mixing is mild, i.e., when the fluid Reynolds number is not too large. In the dynamical regime, resistive contributions to the magnetic helicity flux in the advective gauge are found to be significant owing to the development of small length scales in the irrotational part of the magnetic vector potential.
The internal structure of the toroidicity-induced Alfven eigenmode (TAE) is studied by comparing soft x-ray profile and beam ion loss data taken during TAE activity in the DIII-D tokamak [W. W. Heidbrink , Nucl. Fusion 37, 1411 (1997)] with predictions from theories based on ideal magnetohydrodynamic (MHD), gyrofluid, and gyrokinetic models. The soft x-ray measurements indicate a centrally peaked eigenfunction, a feature which is closest to the gyrokinetic model's prediction. The beam ion losses are simulated using a guiding center code. In the simulations, the TAE eigenfunction calculated using the ideal MHD model acts as a perturbation to the equilibrium field. The predicted beam ion losses are an order of magnitude less than the observed similar to6%-8% losses at the peak experimental amplitude of deltaB(r)/B(0)similar or equal to2-5x10(-4).
This work presents an analysis of the ion outflow from magnetic reconnection throughout fully kinetic simulations with typical laboratory plasma values. A symmetric initial configuration for the density and magnetic field is considered across the current sheet. After analyzing the behavior of a set of nine simulations with a reduced mass ratio and with a permuted value of three initial electron temperatures and magnetic field intensity, the best ion acceleration scenario is further studied with a realistic mass ratio in terms of the ion dynamics and energy budget. Interestingly, a series of shock wave structures are observed in the outflow, resembling the shock discontinuities found in recent magnetohydrodynamic simulations. An analysis of the ion outflow at several distances from the reconnection point is presented, in light of possible laboratory applications. The analysis suggests that magnetic reconnection could be used as a tool for plasma acceleration, with applications ranging from electric propulsion to production of ion thermal beams.
We present an analysis of the electron dynamics during rapid island merging in asymmetric magnetic reconnection. We consider a doubly periodic system with two asymmetric transitions. The upper layer is an asymmetric Harris sheet of finite width perturbed initially to promote a single reconnection site. The lower layer is a tangential discontinuity that promotes the formation of many X-points, separated by rapidly merging islands. Across both layers, the magnetic field and the density have a strong jump, but the pressure is held constant. Our analysis focuses on the consequences of electron energization during island coalescence. We focus first on the parallel and perpendicular components of the electron temperature to establish the presence of possible anisotropies and non-gyrotropies. Thanks to the direct comparison between the two different layers simulated, we can distinguish three main types of behavior characteristic of three different regions of interest. The first type represents the regions where traditional asymmetric reconnections take place without involving island merging. The second type of regions instead shows reconnection events between two merging islands. Finally, the third regions identify the regions between two diverging island and where typical signature of reconnection is not observed. Electrons in these latter regions additionally show a flat-top distribution resulting from the saturation of a two-stream instability generated by the two interacting electron beams from the two nearest reconnection points. Finally, the analysis of agyrotropy shows the presence of a distinct double structure laying all over the lower side facing the higher magnetic field region. This structure becomes quadrupolar in the proximity of the regions of the third type. The distinguishing features found for the three types of regions investigated provide clear indicators to the recently launched Magnetospheric Multiscale NASA mission for investigating magnetopause reconnection involving multiple islands.
Full drift kinetic modelling including finite orbit width effects has been used to assess the passive stabilisation of the resistive wall mode (RWM) that can be expected in the ITER advanced scenario. At realistic plasma rotation frequency, the thermal ions have a stabilising effect on the RWM, but the stability limit remains below the target plasma pressure to achieve Q = 5. However, the inclusion of damping arising from the fusion-born alpha particles, the NBI ions, and ICRH fast ions extends the RWM stability limit above the target beta for the advanced scenario. The fast ion damping arises primarily from finite orbit width effects and is not due to resonance between the particle frequencies and the instability.