Volume 18, Issue 10, October 2011

The cycling dynamics of the internal kink mode, which drives sawtooth oscillations in tokamak plasmas, is studied using the three dimensional, nonlinear magnetohydrodynamic(MHD) code XTOR2F [H. Lütjens and J.F. Luciani, J. Comput. Phys. 229, 8130 (2010)]. It is found that sawtooth cycling, which is characterized by quiescent ramps and fast crashes in the experiment, can be recovered in twofluid MHD provided that a criterion of diamagnetic stabilization is fulfilled. The simulation results indicate that diamagnetic effects alone may be sufficient to drive sawteeth with complete magnetic reconnection in high temperature Ohmic plasmas.
 LETTERS


Secondary instability as cause of minor disruptions in density limit tokamak plasmas
View Description Hide DescriptionExperimental evidence was found in JET plasmas of a new instability at the onset of minor disruptions. This instability is observed during the growth of the well known m/n = 2/1 magnetic island and is localized close to it, behaving as a secondary instability to the island. The large heat fluxes towards the plasma edge, characteristic of minor disruptions, occur during the low rotation phase of the magnetic island at a time the amplitude of the secondary instability suffers a large increase. No poloidal or toroidal mode numbers could be assigned to the secondary instability.

Origin of higher temperatures in multidipolar plasma devices
View Description Hide DescriptionHotfilament discharge devices with multidipolar surface magnetic fields have densities and temperatures higher than in these devices without multidipolar fields. Probe data show a much higher density of secondary electrons from the wall with multidipolar fields that is best explained by the wall secondaries being confined by the magnetic mirror effect. A relatively simple mathematical model for energy balance shows that the heating of the bulk plasmaelectrons by collisions with the greater number of secondaries from the wall accounts quantitatively for the increased temperature.

Production of neutrons up to 18 MeV in highintensity, shortpulse laser matter interactions
View Description Hide DescriptionThe generation of highenergy neutrons using laseraccelerated ions is demonstrated experimentally using the Titan laser with 360 J of laser energy in a 9 ps pulse. In this technique, a shortpulse, highenergy laser accelerates deuterons from a CD_{2} foil. These are incident on a LiF foil and subsequently create high energyneutrons through the ^{7}Li(d,xn) nuclear reaction (Q = 15 MeV). Radiochromic film and a Thomson parabola ionspectrometer were used to diagnose the laser accelerated deuterons and protons. Conversion efficiency into protons was 0.5%, an order of magnitude greater than into deuterons. Maximum neutronenergy was shown to be angularly dependent with up to 18 MeV neutrons observed in the forward direction using neutron timeofflight spectrometry. Absolutely calibrated CR39 detected spectrally integrated neutron fluence of up to 8 × 10^{8} n sr^{−1} in the forward direction.

Observation of plasma array dynamics in 110 GHz millimeterwave air breakdown
View Description Hide DescriptionWe present dynamical measurements of selforganizing arrays of plasma structures in air induced by a 110 GHz millimeterwave beam with linear or circular polarization. The formation of the individual plasmas and the growth of the array pattern are studied using a fastgated (5–10 ns) intensified camera. We measure the timedependent speed at which the array pattern propagates in discrete steps toward the millimeterwave source, observing a peak speed greater than 100 km/s. We observe the expansion of an initially spherical plasma into a disk or an elongated filament, depending on the polarization of the incident beam. The results show good agreement with onedimensional ionizationdiffusion theory and twodimensional simulations.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Effects of heavy ion temperature on lowfrequency kinetic Alfvén waves
View Description Hide DescriptionHeavy ionelectron (or proton)temperature ratio varies in a wide range in the solar and space environment. In this paper, proton and heavy ion temperatures are included in a threefluid plasma model. For the specified parameters, lowfrequency (≪ heavy ion gyrofrequency) kinetic Alfvén waves (KAWs) with sub and superAlfvénic speeds are found to coexist in the same plasma environment. Our results show that the temperature ratio of heavy ions to electrons can considerably affect the dispersion, propagation, and electromagneticpolarizations of the KAWs. In particular, the temperature ratio can increase the ratio of parallel to perpendicular electric fields and the normalized electric to magnetic field ratio, the variations of which are greatly different in regions with a high heavy ion temperature and with a low one. The results may help to understand the physical mechanism of some energization processes of heavy ions in the solar and space plasma environment. Effects of the ratio of electron thermal to Alfvén speeds and the heavy ion abundance on these parameters are also discussed.

Current drive in recombining plasma
View Description Hide DescriptionThe Langevin equations describing the average collisional dynamics of suprathermal particles in nonstationary plasma remarkably admit an exact analytical solution in the case of recombining plasma. The current density produced by arbitrary particle fluxes is derived including the influence of charge recombination. Since recombination has the effect of lowering the charge density of the plasma, thus reducing the charged particle collisional frequencies, the evolution of the current density can be modified substantially compared to plasma with fixed charge density. The current drive efficiency is derived and optimized for discrete and continuous pulses of current, leading to the discovery of a nonzero “residual” current density that persists indefinitely under certain conditions, a feature not present in stationary plasmas.

Empirical model of whistler anisotropy instability
View Description Hide DescriptionEmpirical formulae for the real frequency and growth rate associated with the whistleranisotropy instability are obtained. The electrons are assumed to have an anisotropic distribution function, with Maxwellian parallel distribution. Under such an assumption complex roots of the dispersion relation depend only on two dimensionless parameters, namely, the temperatureanisotropy factor A = T _{⊥e }/T _{∥e } − 1, where T _{⊥e } and T _{∥e } are the perpendicular and parallel electron temperatures, respectively, and parallel electron beta, β _{∥} = (8πnT _{∥e }/B ^{2})^{1/2}, where n and B are the plasma density and magnetic field intensity, respectively. Comparison against exact numerical roots show that analytical formulae describe the whistler instability over a wide range of parallel electron beta and temperatureanisotropy factor. The present result may be useful for circumstances in which the use of exact numerical roots becomes impractical, such as in the radiation belt quasilinear transport coefficient calculation.

Plasma dynamics in microsecond megaampere plasma opening switches
View Description Hide DescriptionThe paper considers the transport of a magnetic field in highly ionized plasma of microsecond megaampere plasma opening switches. Selfsimilar solutions for plasmaaggregation by a linearly increasing magnetic field are derived. For these solutions, the magnetic field energy in the current channel is much lower than the energy of the accelerated plasmaflow. The effect of Joule heating of the plasma becomes profound only with a uniform current density. It is shown that the evolution of the magnetic field in the accelerated flow is reduced to diffusion with an effective electrical conductivity proportional to the harmonic average of the Spitzer conductivity and conductivity dependent on the magnetic field in the current channel. Thus, during about the first 100 ns of the current pulse the conductivity of the current channel increases due to the plasma heating and, as the plasma is accelerated, its conductivity decreases.

The ion kinetic D’Angelo mode
View Description Hide DescriptionAn extension of hydrodynamic D’Angelo mode of inhomogeneous sheared plasma flow along the magnetic field into the short wavelength range, where the hydrodynamic treatment is not valid, has been considered. We find that D’Angelo mode in this wavelength range is excited by inverse ion Landau damping and is a shear flow driven ionkinetic mode.

The splitted laser beam filamentation in interaction of laser and an exponential decay inhomogeneous underdense plasma
View Description Hide DescriptionThe splitted beam filamentation in interaction of laser and an exponential decay inhomogeneous underdense plasma is investigated. Based on WentzelKramersBrillouin (WKB) approximation and paraxial/nonparaxial ray theory, simulation results show that the steady beam width and single beam filamentation along the propagation distance in paraxial case is due to the influence of ponderomotive nonlinearity. In nonparaxial case, the influence of the offaxial of and (the departure of the beam from the Gaussian nature) and (the departure from the spherical nature) results in more complicated ponderomotive nonlinearity and changing of the channel density and refractive index, which led to the formation of two/three splitted beam filamentation and the selfdistortion of beam width. In addition, influence of several parameters on two/three splitted beam filamentation is discussed.

Experimental verification of the KruskalShafranov stability limit in linetied partialtoroidal plasmas
View Description Hide DescriptionThe stability properties of partialtoroidal flux ropes are studied in detail in the laboratory, motivated by ubiquitous arched magnetic structures found on the solar surface. The flux ropes studied here are magnetized arc discharges formed between two electrodes in the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)]. The three dimensional evolution of these flux ropes is monitored by a fast visible light framing camera, while their magnetic structure is measured by a variety of internal magnetic probes. The flux ropes are consistently observed to undergo largescale oscillations as a result of an external kink instability. Using detailed scans of the plasma current, the guide field strength, and the length of the flux rope, we show that the threshold for kink stability is governed by the KruskalShafranov limit for a flux rope that is held fixed at both ends (i.e., q _{ a } = 1).

Waves in a bounded quantum plasma with electron exchangecorrelation effects
View Description Hide DescriptionWithin a quantum hydrodynamic model, the collective excitations of the quantum plasma with electron exchangecorrelation effects in a nanocylindrical wave guide are studied both analytically and numerically. The influences of the electron exchangecorrelation potential, the radius of the wave guide, and the quantum effect on the dispersionproperties of the bounded quantum plasma are discussed. Significant frequencyshift induced by the electron exchangecorrelation effect, the radius of the wave guide and the quantum correction are observed. It is found that the influence of the electron exchangecorrelation, the radius of the wave guide and the quantum correction on the wave modes in a bounded nanowaveguide are strongly coupled.

Theory of coupled whistlerelectron temperature gradient mode in high beta plasma: Application to linear plasma device
View Description Hide DescriptionThis paper presents a theory of coupled whistler (W) and electron temperature gradient (ETG) mode using twofluid model in high beta plasma. Nonadiabatic ion response, parallel magnetic field perturbation (), perpendicular magnetic flutter (), and electron collisions are included in the treatment of theory. A linear dispersion relation for whistlerelectron temperature gradient (WETG) mode is derived. The numerical results obtained from this relation are compared with the experimental results observed in large volume plasma device (LVPD) [Awasthi et al., Phys. Plasma17, 42109 (2010)]. The theory predicts that the instability grows only where the temperature gradient is finite and the density gradient flat. For the parameters of the experiment, theoretically estimated frequency and wave number of WETG mode match with the values corresponding to the peak in the power spectrum observed in LVPD. By using simple mixing length argument, estimated level of fluctuations of WETG mode is in the range of fluctuation level observed in LVPD.

Fluctuations in collisional plasma in the presence of an external electric field
View Description Hide DescriptionThe theory of largescale fluctuations in a plasma is used to calculate the correlations functions of electron and ion density with regard to particle collisions described within the BhatnagarGrossKrook (BGK) model and the presence of a constant external electric field. The changes of plasmaparticle distribution functions due to an external electric field and their influence on the plasma dielectric response are taken into account. The dispersion relations for longitudinal waves in such a plasma are studied in details. It is shown that external electric field can lead to the ionacoustic waveinstability and anomalous growth of the fluctuation level. Detailed numerical studies of the general relations for electron number density fluctuations are performed and the effect of external electric field on the fluctuation spectra is studied.

Numerical studies of the high power microwave breakdown in gas using the fluid model with a modified electron energy distribution function
View Description Hide DescriptionA modified electron energy distribution function(EEDF) is introduced into the fluid model and its effects on the high power microwave (HPM) breakdown in air and argon are investigated. A proper numerical scheme for the finitedifference timedomain method is employed to solve the fluid model. Numerical simulations show that the HPM breakdown time in argon predicted by the fluid model with the modified EEDF agrees well with the results of ParticleincellMonte Carlo collision simulations, while the MaxwellianEEDF results in faster HPM breakdown when the mean electron energy is less than 20 eV. We also confirm that the MaxwellianEEDF can be used in the fluid model for simulating the air breakdown at the low frequencies based on the reported experiments.

Role of the pressure force in the explosive dynamics of magnetic islands in double tearing modes
View Description Hide DescriptionThe evolution of magnetic islands at two resonant surfaces during the development of a global tearing mode is investigated via numerical simulations of a reduced set of magnetohydrodynamic equations in slab plasmas. The explosive dynamics of the islands resulting in an interchange follows a Rutherfordlike regime, also referred to as a weakly coupled double tearing mode. It is found that the latent mechanism of this dynamics is the reduction of the total pressure around the opposite Xpoint and the abrupt growth is associated with an imbalance of the pressure around the islands. Once this imbalance is well established, each plasmoid is pushed to the opposite Xpoint, increasing the drive for the field merging. A feedback loop then takes place as the reconnection itself decreases the strength of the magnetic field between the tearing layers, i.e., reinforces the pressure imbalance. This loop acceleratesreconnection, thus leading to the observed explosive growth.

Degenerate mixing of plasma waves on cold, magnetized singlespecies plasmas
View Description Hide DescriptionIn the coldfluid dispersion relation for TrivelpieceGould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination . As a result, for any frequency , there are infinitely many degenerate waves, all having the same value of . On a cold finitelength plasma column, these degenerate waves reflect into one another at the ends; thus, each standingwave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the manywave modes is that the shortwavelength waves often add constructively along resonance cones given by . Also, the presence of short wavelengths in the admixture for a predominantly longwavelength mode enhances the viscous damping beyond what the singlewave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of loworder, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid.

On the dispersion features of whistler waves in almost pure ion plasmas
View Description Hide DescriptionIt is shown that in a multiion plasma of moderate density enriched by a large amount of negatively charged ions and/or heavy particulates, the lower cutoff frequencies of the electron whistler and the Zmode (extraordinary) waves tend to each other, approaching the ion plasma frequency ω_{ pi }. The evolution of the dispersion curves ω(k, θ) of both wave modes is studied as a function of the relative electron density, beginning from the case of a moderate density plasma with comparable values of electron gyro and plasma frequencies and ω_{ pi } essentially exceeding the ions’ gyrofrequencies. When the fraction of free electrons is very small the transparency frequency domain of the electron whistler mode becomes very narrow, being located in the vicinity of ω_{ pi }. If the negatively charged ions have the smallest specific charges among other ion species then, under similar conditions, the socalled crossover effect and the accompanying polarization sense reverse can occur at frequencies essentially greater than the ions’ gyrofrequencies. The revealed effects are characteristic of plasmas with excess of electronegative gas molecules or dust particulates highly adhesive to electrons, i.e., almost pure ion plasmas and/or pairion plasmas with extra small fractions of free electrons. Moreover, it is found that the vanishingly small electron fraction providing the same value of the electron whistler ion cutoff frequency normalized to the ion plasma frequency is essentially less for pair fullerene than for pair hydrogen H ^{±}plasmas. The technique to determine ω_{ pi } using the evolution of the whistler wavemagnetic fieldpolarization ellipse is also described.

Anomalous skin effects in relativistic parallel propagating weakly magnetized electron plasma waves
View Description Hide DescriptionFully relativistic analysis of anomalous skineffects for parallel propagating waves in a weakly magnetized electron plasma is presented and general expressions for longitudinal and transverse permittivites are derived. It is found that the penetration depth for R and Lwaves increases as we move from nonrelativistic to highly relativistic regime. The ambient magnetic field reduces/enhances the skineffects for Rwave/Lwave as the strength of the field is increased. In general, the weak magnetic fieldeffects are pronounced for the weakly relativistic regime as compared with other relativistic cases. The results are also graphically illustrated. On switching off the magnetic field, previous results for field free case are retrieved [A. F. Alexandrov, A. S. Bogdankevich, and A. A. Rukhadze, Priniples of Plasma Electrodynamics (SpringerVerlag, Berlin, Heidelberg, 1984), Vol. 9, p. 106].

Solitary wave evolution in a magnetized inhomogeneous plasma under the effect of ionization
View Description Hide DescriptionA modified form of KortewegdeVries (KdV) equation appropriate to nonlinear ion acoustic solitary waves in an inhomogeneous plasma is derived in the presence of an external magnetic field and constant ionization in the plasma. This equation differs from usual version of the KdV equation because of the inclusion of two terms arising due to ionization and density gradient present in the plasma. In this plasma, only the compressive solitary waves are found to propagate corresponding to the fast and slow modes. The amplitude of the solitary wave increases with an enhancement in the ionization for the fast mode as well as for the slow mode. The effect of magnetic field is to enhance the width of the solitary structure. The amplitude is found to increase (decrease) with an enhancement in charge number of the ions for the fast (slow) mode. The tailing structure becomes more (less) prominent with the rise in ion drift velocity for the case of fast (slow) mode.