Volume 19, Issue 5, May 2012

Directdriveimplosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have showed discrepancies between simulations of the scattered (nonabsorbed) light levels and measured ones that indicate the presence of a mechanism that reduces laser coupling efficiency by 10%–20%. This appears to be due to crossedbeam energy transfer (CBET) that involves electromagneticseeded, lowgain stimulated Brillouin scattering. CBET scattersenergy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. Onedimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higherZ ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.
 LETTERS


Pure electron plasmas confined for 90 ms in a stellarator without electron sources or internal objects
View Description Hide DescriptionWe report on the creation and up to 90 ms sustainment of pure electron plasmas confined in a stellarator without internal objects. Injection of positrons into such plasmas is expected to lead to the creation of the first electronpositron plasma experiments. These newly created plasmas will also allow a study of pure electron plasmas without the perturbing presence of internal objects. The plasmas were created by thermionic emission of electrons from a heated, biased filament that was retracted in 20 ms. The confinement of these transient plasmas is different from that of steady state plasmas with internal objects and emissive filaments, and is generally shorter, limited by ion buildup. The decay time is increased by lowering the neutral pressure, lowering the electron plasma temperature, or operating with neutrals with high ionization energies (helium). These findings are all consistent with ion accumulation being the cause for the shorter than expected confinement times. The magnetic field strength also moderately increases the decay times. The deleterious effect of ions is not expected to imply a similar deleterious effect when introducing positrons, but it implies that ion accumulation must be avoided also in an electronpositron experiment.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Magnetohydrodynamic spin waves in degenerate electronpositronion plasmas
View Description Hide DescriptionLow frequency magnetosonic waves are studied in magnetized degenerate electronpositronion plasmas with spin effects. Using the fluid equations of magnetoplasma with quantum corrections due to the Bohm potential, temperature degeneracy, and spin magnetization energy, a generalized dispersion relation for oblique magnetosonic waves is derived. Spin effects are incorporated via spin force and macroscopic spin magnetization current. For three different values of angle θ, the generalized dispersion relation is reduced to three different relations under the low frequency magnetohydrodynamic assumptions. It is found that the effect of quantum corrections in the presence of positron concentration significantly modifies the dispersive properties of these modes. The importance of the work relevant to compact astrophysical bodies is pointed out.

Hamiltonian magnetohydrodynamics: Helically symmetric formulation, Casimir invariants, and equilibrium variational principles
View Description Hide DescriptionThe noncanonical Hamiltonian formulation of magnetohydrodynamics(MHD) is used to construct variational principles for continuously symmetric equilibrium configurations of magnetized plasma, including flow. In particular, helical symmetry is considered, and results on axial and translational symmetries are retrieved as special cases of the helical configurations. The symmetry condition, which allows the description in terms of a magnetic flux function, is exploited to deduce a symmetric form of the noncanonical Poisson bracket of MHD. Casimir invariants are then obtained directly from the Poisson bracket. Equilibria are obtained from an energyCasimir principle and reduced forms of this variational principle are obtained by the elimination of algebraic constraints.

Coupling between whistler waves and slowmode solitary waves
View Description Hide DescriptionThe interplay between electron and ionscale phenomena is of general interest for both laboratory and space plasma physics. In this paper, we investigate the linear coupling between whistler waves and slow magnetosonic solitons through twofluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ionscales. This raises the interesting question of how ionscale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons.

Modeling the propagation of whistlermode waves in the presence of fieldaligned density irregularities
View Description Hide DescriptionWe present a numerical study of propagation of VLF whistlermode waves in a laboratory plasma. Our goal is to understand whistler propagation in magnetic fieldaligned irregularities (also called channels or ducts). Two cases are examined, that of a highfrequency () whistler in a density depletion duct and that of a lowfrequency () whistler in a density enhancement. Results from a numerical simulation of whistler wave propagation are compared to data from the UCLA Los Angeles Physics Teachers Alliance Group plasma device and whistler propagation in preexisting density depletion and density enhancement ducts is demonstrated.

Perturbative analysis of sheared flow Kelvin–Helmholtz instability in a weakly relativistic magnetized electron fluid
View Description Hide DescriptionIn the interaction of intense lasers with matter/plasma, energetic electrons having relativistic energies get created. These energetic electrons can often have sheared flow profiles as they propagate through the plasma medium. In an earlier study [Phys. Plasmas17, 022101 (2010)], it was shown that a relativistic sheared electron flow modifies the growth rate and threshold condition of the conventional Kelvin—Helmholtz instability. A perturbative analytic treatment for the case of weakly relativistic regime has been provided here. It provides good agreement with the numerical results obtained earlier.

Gyrosymmetry: Global considerations
View Description Hide DescriptionIn the guiding center theory, smooth unit vectors perpendicular to the magnetic field are required to define the gyrophase. The question of global existence of these vectors is addressed using a general result from the theory of characteristic classes. It is found that there is, in certain cases, an obstruction to global existence. In these cases, the gyrophase cannot be defined globally. The implications of this fact on the basic structure of the guiding center theory are discussed. In particular, it is demonstrated that the guiding center asymptotic expansion of the equations of motion can still be performed in a globally consistent manner when a single global convention for measuring gyrophase is unavailable. The latter fact is demonstrated directly by deriving a new expression for the guidingcenter PoincaréCartan form exhibiting no dependence on the choice of perpendicular unit vectors.

Magnetothermal instability in laser plasmas including hydrodynamic effects
View Description Hide DescriptionThe impact of both density gradients and hydrodynamics on the evolution of the field compressing magnetothermal instability is considered [J. J. Bissell et al., Phys. Rev. Lett. 105, 175001 (2010)]. Hydrodynamic motion is found to have a limited effect on overall growthrates; however, density gradients are shown to introduce an additional source term corresponding to a generalised description of the field generating thermal instability [D. Tidman and R. Shanny, Phys. Fluids 17, 1207 (1974)]. The field compressing and field generating source terms are contrasted, and the former is found to represent either the primary or sole instability mechanism for a range of conditions, especially those with Hall parameter . The generalised theory is compared to numerical simulation in the context of a recent nanosecond gasjet experiment [D. H. Froula et al., Phys. Rev. Lett. 98, 135001 (2007)] and shown to be in good agreement: exhibiting peak growthrates and wavelengths of order and , respectively. The instability’s relevance to other experimental conditions, including those in inertial confinement fusion (I.C.F.) hohlraums, is also discussed.
 Nonlinear Phenomena, Turbulence, Transport

Electron kappa distribution and steadystate Langmuir turbulence
View Description Hide DescriptionIn a recent pair of papers, the present author discussed a selfconsistent theory of asymptotically steadystate electron distribution function and Langmuir turbulence intensity in one [P. H. Yoon, Phys. Plasmas 18, 122303 (2011)] and three [P. H. Yoon, Phys. Plasmas 19, 012304 (2012)] dimensions. The resulting electron distribution function is a type of kappa distribution that features a nonMaxwellian energetic tail component. However, while the onedimensional solution is rigorously correct, the threedimensional solution, which was obtained using the cylindrical coordinate representation, contains two features that may be inconsistent for fieldfree plasmas. One is the assumption that the resonance condition can be approximated by . Needless to say, this is not the most general condition. The second inconsistency is that while the electron distribution is isotropic in velocity, the Langmuir turbulence intensity depends on the wave propagation direction. While these features may not be too unrealistic in the presence of an implicit ambient magnetic field, they certainly cannot be correct if the plasma is genuinely unmagnetized. In the present paper, we rectify such shortcomings by properly reformulating the problem using a spherical coordinate system in a truly freefield plasma.

Effect of nonthermal electrons on oblique electrostatic excitations in a magnetized electronpositronion plasma
View Description Hide DescriptionThe linear and nonlinear propagation of ionacoustic waves are investigated in a magnetized electronpositronion (epi) plasma with nonthermal electrons. In the linear regime, the propagation of two possible modes and their evolution are studied via a dispersion relation. In the cases of parallel and perpendicular propagation, it is shown that these two possible modes are always stable. Then, the Kortewegde Vries equation describing the dynamics of ionacoustic solitary waves is derived from a weakly nonlinear analysis. The influence on the solitary wave characteristics of relevant physical parameters such as nonthermal electrons,magnetic field, obliqueness, positron concentration, and temperature ratio is examined. It is observed that the increasing nonthermal electrons parameter makes the solitary structures much taller and narrower. Also, it is revealed that the magnetic field strength makes the solitary waves more spiky. The present investigation contributes to the physics of the nonlinear electrostaticionacoustic waves in space and laboratory epi plasmas in which wave damping produces an electron tail.

Tripolar vortex formation in dense quantum plasma with iontemperaturegradients
View Description Hide DescriptionWe have derived system of nonlinear equations governing the dynamics of lowfrequency electrostatictoroidaliontemperaturegradient mode for dense quantum magnetoplasma. For some specific profiles of the equilibrium density, temperature, and ion velocity gradients, the nonlinear equations admit a stationary solution in the form of a tripolar vortex. These results are relevant to understand nonlinear structure formation in dense quantum plasmas in the presence of equilibrium iontemperature and density gradients.

Hexagonal superlattice pattern consisting of colliding filament pairs in a dielectric barrier discharge
View Description Hide DescriptionCollidingpairs hexagonal superlattice pattern (CPHSP) is studied in a dielectric barrier dischargesystem. The evolution of CPHSP bifurcating from a hexagonal pattern to chaos is shown. The phase diagrams of CPHSP as a function of discharge parameters are given. From a series of pictures taken by a high speed video camera, collisions between two spots are observed and the superposition of many collisions results in each big spot presenting four small spots on long time scales. Measurements of the correlation between filaments indicate that the pattern is an interleaving of four different transient hexagonal sublattices. Depending on the discharging sequence, the forces exerted on one colliding spot are discussed briefly.

Energy spectrum, dissipation, and spatial structures in reduced Hall magnetohydrodynamic
View Description Hide DescriptionWe analyze the effect of the Hall term in the magnetohydrodynamicturbulence under a strong externally supported magnetic field, seeing how this changes the energy cascade, the characteristic scales of the flow, and the dynamics of global magnitudes, with particular interest in the dissipation. Numerical simulations of freely evolving threedimensional reduced magnetohydrodynamics are performed, for different values of the Hall parameter (the ratio of the ion skin depth to the macroscopic scale of the turbulence) controlling the impact of the Hall term. The Hall effect modifies the transfer of energy across scales, slowing down the transfer of energy from the large scales up to the Hall scale (ion skin depth) and carrying faster the energy from the Hall scale to smaller scales. The final outcome is an effective shift of the dissipation scale to larger scales but also a development of smaller scales. Current sheets (fundamental structures for energy dissipation) are affected in two ways by increasing the Hall effect, with a widening but at the same time generating an internal structure within them. In the case where the Hall term is sufficiently intense, the current sheet is fully delocalized. The effect appears to reduce impulsive effects in the flow, making it less intermittent.

Quasiperiodic behavior of ion acoustic solitary waves in electronion quantum plasma
View Description Hide DescriptionThe ion acoustic solitary waves are investigated in an unmagnetized electronion quantum plasmas. The one dimensional quantum hydrodynamic model is used to study small as well as arbitrary amplitude ion acoustic waves in quantum plasmas. It is shown that ion temperature plays a critical role in the dynamics of quantum electron ion plasma, especially for arbitrary amplitude nonlinear waves. In the small amplitude region Kortewegde Vries equation describes the solitonic nature of the waves. However, for arbitrary amplitude waves, in the fully nonlinear regime, the system exhibits possible existence of quasiperiodic behavior for small values of ion temperature.

Effect of poloidal asymmetries on impurity peaking in tokamaks
View Description Hide DescriptionPoloidal impurity asymmetries are frequently observed in tokamaks. In this paper, the effect of poloidal asymmetry on electrostaticturbulenttransport is studied, including the effect of the drift. Collisions are modeled by a Lorentz operator, and the gyrokinetic equation is solved with a variational approach. The impurity transport is shown to be sensitive to the magnetic shear and changes sign for in the presence of inboard accumulation. The zeroflux impurity density gradient (peaking factor) is shown to be rather insensitive to collisions in both ion temperature gradient and trapped electron mode driven cases. Our results suggest that the asymmetry (both the location of its maximum and its strength) and the magnetic shear are the two most important parameters that affect the impurity peaking.
 Magnetically Confined Plasmas, Heating, Confinement

Gauge properties of the guiding center variational symplectic integrator
View Description Hide DescriptionVariational symplectic algorithms have recently been developed for carrying out longtime simulation of charged particles in magnetic fields [H. Qin and X. Guan, Phys. Rev. Lett. 100, 035006 (2008); H. Qin, X. Guan, and W. Tang, Phys. Plasmas (2009); J. Li, H. Qin, Z. Pu, L. Xie, and S. Fu, Phys. Plasmas 18, 052902 (2011)]. As a direct consequence of their derivation from a discrete variational principle, these algorithms have very good longtime energy conservation, as well as exactly preserving discrete momenta. We present stability results for these algorithms, focusing on understanding how explicit variational integrators can be designed for this type of system. It is found that for explicit algorithms, an instability arises because the discrete symplectic structure does not become the continuous structure in the limit. We examine how a generalized gauge transformation can be used to put the Lagrangian in the “antisymmetric discretization gauge,” in which the discrete symplectic structure has the correct form, thus eliminating the numerical instability. Finally, it is noted that the variational guiding center algorithms are not electromagnetically gauge invariant. By designing a model discrete Lagrangian, we show that the algorithms are approximately gauge invariant as long as A and are relatively smooth. A gauge invariant discrete Lagrangian is very important in a variational particleincell algorithm where it ensures current continuity and preservation of Gauss’s law [J. Squire, H. Qin, and W. Tang (to be published)].

Kinetic damping of resistive wall modes in ITER
View Description Hide DescriptionFull 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 fusionborn alpha particles, the NBI ions, and ICRH fast ions extends the RWM stability limit above the target 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.

Auxiliary ECR heating system for the gas dynamic trap
View Description Hide DescriptionPhysics aspects of a new system for electron cyclotron resonance heating (ECRH) at the magnetic mirror device Gas Dynamic Trap (GDT, Budker Institute, Novosibirsk) are discussed. This system based on two 400 kW/54.5 GHz gyrotrons is aimed at increasing the electron temperature up to the range 250–350 eV for improved energy confinement of hot ions. The key physical issue of the GDT magnetic field topology is that conventional ECRH geometries are not accessible. The proposed solution is based on a peculiar effect of radiation trapping in inhomogeneous magnetized plasma. Under specific conditions, oblique launch of gyrotron radiation results in generation of righthandpolarized (R) electromagnetic waves propagating with high N _{} in the vicinity of the cyclotron resonance layer, which leads to effective singlepass absorption of the injected microwave power. In the present paper, we investigate numerically an optimized ECRH scenario based on the proposed mechanism of wave propagation and discuss the design of the ECRH system, which is currently under construction at the Budker Institute.

Drift wave dispersion relation for arbitrarily collisional plasma
View Description Hide DescriptionThe standard local linear analysis of drift waves in a plasma slab is generalized to be valid for arbitrarily collisional electrons by considering the electrons to be governed by the driftkinetic equation with a BGKlike (BhatnagarGrossKrook) collision operator. The obtained dispersion relation reduces to that found from collisionless kinetic theory when the collision frequency is zero. Electrontemperature fluctuations must be retained in the standard fluid analysis in order to obtain good quantitative agreement with our general solution in the highly collisional limit. Any discrepancies between the fluid solution and our general solution in this limit are attributed to the limitations of the BGK collision operator. The maximum growth rates in both the collisional and collisionless limits are comparable and are both on the order of the fundamental drift wave frequency. The main role of the destabilizing mechanism is found to be in determining the parallel wave number at which the maximum growth rate will occur. The parallel wave number corresponding to the maximum growth rate is set by the waveparticle resonance condition in the collisionless limit and transitions to being set by the real frequency being on the order of the rate for electrons to diffuse a parallel wavelength in the collisional limit.

On the delayed gas breakdown in a ringing thetapinch with bias magnetic field
View Description Hide DescriptionA single particle model and particleincell simulations are used to elucidate the breakdown physics in a ringing thetapinch with a bias magnetic field. Previous experimental results show that gas breakdown occurs when the bias magnetic field is nullified by the thetapinchmagnetic field. The analyses presented here agree with the experimental results and show that electron kinetic energy does not exceed the ionization threshold of deuterium until the net magnetic field is approximately zero. Despite the presence of a strong electric field, the gyromotion of electrons within the bias magnetic field prevents them from gaining energy necessary to ionize the gas. Parametric analysis of the peak electron energy as a function of the bias and preionization magnetic fields reveals that: (1) when the bias magnetic field is ≈97% of the preionization magnetic field, peak electron energies are highly erratic resulting in poor overall ionization, and (2) full ionization with repeatable behavior requires a preionization to bias magnetic field ratio of approximately 2 to 1 or higher.