Volume 19, Issue 3, March 2012

An analysis of the dependence of transport on the safety factor profile in highperformance, steadystate scenario discharges is presented. This is based on experimental scans of and taken with fixed , toroidal field, doublenull plasma shape, divertor pumping, and electron cyclotron current drive input. The temperature and thermal diffusivity profiles were found to vary considerably with the qprofile, and these variations were significantly different for electrons and ions. With fixed , both temperature profiles increase and broaden as is increased and the magnetic shear becomes low or negative in the inner half radius, but these temperature profile changes are stronger for the electrons. Power balance calculations show the peak in the ion thermal diffusivity at increases with or . In contrast, the peak in the electron diffusivity decreases as is raised from to 1.5, and it is insensitive to . This is important for fully noninductive scenario development because it demonstrates that elevated and weak or reversed shear allow larger electron temperature gradients and, therefore, increased bootstrap current density to exist at . Chordaveraged measurements of long wavelength density fluctuation amplitudes are shown, and these have roughly the same dependence on qprofile as . This data set provides an opportunity for testing whether theory based transport models can provide insight into the underlying transport physics of high performance scenarios and if they can reproduce observed experimental trends. To this end, we applied the trapped gyroLandau fluid (TGLF) code to calculate the linear stability of drift waves and found that the resulting variation of growth rates with qprofile are mostly inconsistent with the observed trends of , , and with qprofile. TGLF simulations of the temperature profiles consistent with heating sources also have mixed agreement with the measured profiles, such that the simulated electron and ion heat flux in low discharges are too low and heat fluxes in high discharges are too high.
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Transport coefficients in strongly coupled plasmas
View Description Hide DescriptionTransport coefficients for Coulomb collision processes in correlated plasmas are calculated within the binary collision approximation. Considering plasmas with flowing Maxwellian distributions and Debye screened particle potentials as an example, it is shown that the friction and energy exchange densities are similar to those in weakly coupled plasmas, but a generalized Coulomb logarithm, , is required. This asymptotes to in the weakly coupled limit, but is significantly modified when due to large angle collisions. The results show excellent agreement with molecular dynamics simulations of temperature relaxation.

Relativistic acceleration of Landau resonant particles as a consequence of Hopf bifurcations
View Description Hide DescriptionUsing bifurcation theory on a dynamical system simulating the interaction of a particle with an obliquely propagating wave in relativistic regimes, we demonstrate that uniform acceleration arises as a consequence of Hopf bifurcations of Landau resonant particles. The acceleration process arises as a form of surfatron established through the locking in pitch angle, gyrophase, and physical trapping along the wavevector direction. Integrating the dynamical system for large amplitudes () obliquely propagating waves, we find that electrons with initial energies in the keV range can be accelerated to MeV energies on timescales of the order of milliseconds. The Hopf condition of Landau resonant particles could underlie some of the most efficient energization of particles in space and astrophysical plasmas.

Surfaceplasmonenhanced MeV ions from femtosecond laser irradiated, periodically modulated surfaces
View Description Hide DescriptionEnhanced emission of high energy ions is measured from subwavelength metallic grating targets under irradiation by intense (10^{15}−10^{16} W cm^{−2}), ppolarized, 50 fs, 800 nm laser pulses. The maximum ion energy is 55% higher and the ion flux is 60% higher for the modulated surface in comparison with polished surfaces of the same atomic composition. The ion emission, a result of enhanced light coupling and hot electron production in the grating targets, is correlated with enhancement in hard xray bremsstrahlung emission. The results are well reproduced by particleincell simulations. The study reveals that the enhanced laser coupling by surface plasmon excitation on metallic gratings is directly responsible for the enhancement of ion energies.

Measurement of virtual cathode structures in a plasma sheath caused by secondary electrons
View Description Hide DescriptionThe measured potential profiles of unmagnetized plasma sheath near a stainless steel plate exhibit deep virtual cathode structures caused by secondary electrons produced by highspeed ions hitting the surface of the plate. The depth and thickness of the virtual cathode depend on the ion streaming energy and gas pressure. The experimental results are in agreement with numerical calculations.

Smallscale dynamo action in multiscale magnetohydrodynamic and microturbulence
View Description Hide DescriptionNonlinear interplay between resistivemagnetohydrodynamic(MHD)magnetic island and drift wave microturbulence is investigated using direct Landaufluid simulations. A twisting oscillation of magnetic island associated with the driving force of microturbulence is observed, which is referred to as magnetic island seesaw. In the initiating phase of the seesaw oscillation, smallscale current and magnetic fieldfluctuations increase dramatically while the magnetic induction grows exponentially at smallscales corresponding to the spatial scale of microturbulence, showing a smallscale dynamo action. A minimal model consisting of reduced MHDturbulence and a microinstability is proposed to elucidate the underlying mechanism. It is identified that the island seesaw is driven by a net oscillatory electromagnetic torque, which results from smallscale dynamogenerated current and magnetic field. The dynamo mechanism may offer an important energy exchange channel between MHD and microturbulence in magnetic fusion plasmas.

Quasimonochromatic pencil beam of laserdriven protons generated using a conical cavity target holder
View Description Hide DescriptionA 7 MeV protonbeam collimated to 16 mrad containing more than particles is experimentally demonstrated by focusing a 2 J, 60 fs pulse of a Ti:sapphire laser onto targets of different materials and thicknesses placed in a millimeter scale conical holder. The electric potential induced on the target holder by laserdriven electrons accelerates and dynamically controls a portion of a divergent quasithermal protonbeam originated from the target, producing a quasimonoenergetic “pencil” beam.

Selective deuteron production using target normal sheath acceleration
View Description Hide DescriptionWe report on the first successful demonstration of selective deuteron acceleration by the target normal sheath acceleration mechanism in which the normally overwhelming proton and carbon ion contaminant signals are suppressed by orders of magnitude relative to the deuteron signal. The deuterium ions originated from a layer of heavy ice that was deposited on to the rear surface of a 500 nm thick membrane of Si_{3}N_{4} and Al. Our data show that the measured spectrum of ions produced by heavy ice targets is comprised of deuterium ions. With a laser pulse of approximately 0.5 J, 120 fs duration, and mean intensity, the maximum recorded deuterium ion energy and yield normal to the target rear surface were 3.5 MeV and , respectively.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Slow packets of electromagnetic waves in magnetized plasma with ferrite grains
View Description Hide DescriptionThe propagation of narrow Gaussian electromagnetic wave packets in the collisionless magnetized plasma with ferrite grains (MPFG) is studied. In this system, in the frequency range close to the frequency of the electron cyclotron resonance, which coincides with the frequency of the ferromagnetic resonance, the dispersion of the electric permittivity (electron subsystem) and magnetic permeability (ferrite grain subsystem) must be taken into account simultaneously. The refractive index of MPFG can be negative and with increase in frequency becomes positive passing through zero. The strong dispersion of and the group velocity results in the spreading of the wave packets, which propagate with velocity much smaller than the speed of light. Their phase velocity is negative if and positive if . For , the group velocity can have a maximum at some frequency. The wave packets centered at this frequency are weakly spreading. The existence of slow waves in the frequency range, where , may be a general property of left handed media.

Dispersion and damping of potential surface waves in a degenerate plasma
View Description Hide DescriptionPotential (electrostatic)surface waves (SWs) in a semibounded plasma with degenerate electrons are studied using the quasiclassical meanfield kinetic model. The SW spectrum and the collisionless damping rate are obtained numerically for a wide range of wavelengths. In the limit of long wavelengths, the SW frequency approaches the coldplasma limit with being the plasma frequency, while at short wavelengths, the SW spectrum asymptotically approaches the spectrum of zerosound mode propagating along the boundary. It is shown that the surface waves in this system remain weakly damped at all wavelengths (in contrast to strongly damped surface waves in Maxwellianelectronplasmas), and the damping rate nonmonotonically depends on the wavelength, with the maximum (yet small) damping occurring for surface waves with wavelength of , where is the ThomasFermi length. The applicability of the used approximations and of the obtained results is discussed in detail.

Study of high frequency parallel propagating modes in a weakly magnetized relativistic degenerate electron plasma
View Description Hide DescriptionUsing the VlasovMaxwell’s model, the generalized dispersion relations for parallel propagating electromagnetic modes (R and Lwaves) and electrostatic mode (Langmuir wave) for weakly magnetized relativistic degenerate plasma are derived. The propagation characteristics of these modes are analyzed both analytically and graphically in order to highlight the relativistic and the ambient magnetic fieldeffects in different degenerate plasma environments. Furthermore, the analytical expressions of these modes are extracted for the limiting cases, i.e., nonrelativistic ( and ultrarelativistic (. It is noted that the relativistic effects shift the cutoff point towards the lower value of frequency and resultantly broaden the propagation domain. Moreover, the cutoff point of Rwave/Lwave move towards the higher/lower frequency as the strength of ambient magnetic field is increased. The shifting of frequency due to the ambient magnetic field becomes more prominent in the nonrelativistic regime than in the highly relativistic one because the relativistic effect suppresses the contribution of the ambient magnetic field.

Nonlinear structures in a nonextensive electronpositronion magnetoplasma
View Description Hide DescriptionIn outer space physics and astrophysics, there is a considerable amount of anomalous phenomena that support nonextensive particle distribution. This is because of the relevance of gravitational forces (which are long ranged) as well as to a variety of dynamical nonlinear aspects. Here, we investigate the nonlinear properties and the existence conditions of a plasma system consisting of ion fluid as well as electrons and positrons modelled by nonextensive distributions. The numerical analysis of the evolution equation shows that both subsonic and supersonic electrostatic waves may exist. These nonlinear waves admit positive and negative localized structures. The dependence of the latter on the plasma parameters is investigated numerically.

Ionacoustic solitary waves in ionbeam plasma with Boltzmann electrons
View Description Hide DescriptionParticle simulations and a pseudopotential method were used to study ionacoustic solitary waves in a plasma composed of Boltzmannelectrons and kinetic beam ions. Pseudopotential theory was first applied to determine how ionacoustic solitary waves can exist in a twocomponent plasma. Then, particle simulations were carried out, wherein ionacoustic solitary waves were excited by modulating the bias grid voltage in a double plasma model. For the modulation, the potential of the grid bias was rapidly decreased, such that humptype ionacoustic solitary waves with good Gaussian shape were excited one after another, forming a train of waves. The simulation also showed that the phase velocities of ions decrease sharply when the solitary wave occurs, which indicates that the solitary ionacoustic wave is excited via the inverse Landau damping process.

Phase space analysis of multipactor saturation in rectangular waveguide
View Description Hide DescriptionIn certain high power RF systems multipactor cannot be avoided for all operating points, but its existence places limits on performance, efficiency, lifetime, and reliability. As an example multipactor in the input couplers of superconducting RF cavities can be a major limitation to the maximum RF power. Several studies have concentrated on rectangular waveguide input couplers which are used in many light sources. Most of these studies neglect space charge assuming that the effect of space charge is simply to defocus the electron bunches. Modelling multipactor to saturation is of interest in determining the performance of waveguide under a range of conditions. Particleincell modelling including space charge has been performed for 500 MHz halfheight rectangular waveguide. Phase plots of electron trajectories can aid understanding the processes taking place in the multipactor. Results strongly suggest that the multipacting trajectories are strongly perturbed by space charge causing the electrons to transition from twosurface to singlesurface trajectories as the multipactor approaches saturation.

Soliton propagation in a moving electronpositron pair plasma having negatively charged dust grains
View Description Hide DescriptionWe consider an electronpositron pair plasma that has negatively charged dust grains. Being the same mass of the electrons and positrons, both of these species contribute to the wave excitation in the plasma. Since the frequency of oscillations is high, the dust grains are considered to be fixed and they only provide the restoring force for the wave excitation. For the sake of completeness, we include the finite temperatures of the electrons and positrons species and derive the expressions for the phase velocities. Two types of modes propagating in opposite directions occur in this pair plasma model. Based on Kortewegde Vries (KdV) equation, it is realized that these modes evolve only into compressive solitons. However, usual KdV approach fails when both the species carry the same temperatures and also drift with the same velocity. Under this situation, modified KdV (mKdV) equation is derived that shows the occurrence of rarefactive solitons also, which carry the same amplitudes as the compressive solitons attain corresponding to mKdV equation. In addition to the expressions of peak amplitude and width of the solitons, we calculate soliton energy for examining their propagation characteristics in detail under the effect of positron to electrontemperature ratio σ, their drift velocities ( and ), dust concentration , and dust charge valency . The issue of nonvanishing coefficient of nonlinearity is also discussed in greater detail.

Examination of argon metastable atom velocity distribution function close to a conducting wall
View Description Hide DescriptionThe spatial evolution of the metastable argon atom velocity distribution function is recorded in the sheath and presheath regions of a metallic wall using laser induced fluorescence(LIF) spectroscopy. Metastable argon atom temperature and fluid velocity are computed from measured data. Owing to the loss of metastable argon atom after a collision with the surface, the atom temperature seemingly decreases and the velocity increases when approaching the wall. These artifacts are carefully examined and explained in terms of changes in the metastable argon atom distribution function. In addition, the atom nonelastic reflection coefficient is computed from the ratio of outward to inward atom flux to the surface. This study indicates less than 1% of metastable atoms survive a collision with the metallic wall.

Theoretical study of the surface waves in semibounded quantum collisional plasmas
View Description Hide DescriptionThe propagation of surface waves on a semibounded quantum plasma is investigated taking into account the collisional effects. The quantum hydrodynamic model includes Bohm’s quantum force, FermiDirac statistical, and collisional corrections are used to derive the dispersion relation of these waves. It is shown that the collisions play a significant role on the decay of surface wave amplitude. Furthermore, the surface waves can be unstable in the presence of collisional effects. It is also indicated that the growth rate of the surface waveinstability increases with the increase of collisional and quantum effects, especially in the high wavenumber region.

BernsteinGreeneKruskal waves in relativistic cold plasma
View Description Hide DescriptionWe construct the longitudinal traveling wavesolution [Akhiezer and Polovin, Sov. Phys. JETP 3, 696 (1956)] from the exact space and time dependent solution of relativistic cold electron fluid equations [Infeld and Rowlands, Phys. Rev. Lett. 62, 1122 (1989)]. Ions are assumed to be static. We also suggest an alternative derivation of the Akhiezer Polovin solution after making the standard traveling waveAnsatz.

Momentum transfer of solar wind plasma in a kinetic scale magnetosphere
View Description Hide DescriptionSolar windinteraction with a kinetic scale magnetosphere and the resulting momentum transfer process are investigated by 2.5dimensional full kinetic particleincell simulations. The spatial scale of the considered magnetosphere is less than or comparable to the ion inertial length and is relevant for magnetized asteroids or spacecraft with minimagnetosphere plasma propulsion. Momentum transfer is evaluated by studying the Lorentz force between solar windplasma and a hypothetical coil current density that creates the magnetosphere. In the zero interplanetary magnetic field(IMF) limit, solar windinteraction goes into a steady state with constant Lorentz force. The dominant Lorentz force acting on the coil current density is applied by the thin electron current layer at the windfilled front of the magnetosphere. Dynamic pressure of the solar wind balances the magnetic pressure in this region via electrostatic deceleration of ions. The resulting Lorentz force is characterized as a function of the scale of magnetosphere normalized by the electron gyration radius, which determines the local structure of the current layer. For the finite northward IMF case, solar wind electrons flow into the magnetosphere through the reconnecting region. The inner electrons enhance the ion deceleration, and this results in temporal increment of the Lorentz force. It is concluded that the momentum transfer of solar windplasma could take place actively with variety of kinetic plasma phenomena, even in a magnetosphere with a small scale of less than the ion inertial length.

Rotation induced nonlinear dispersive dust drift waves can be the progenitors of spokes
View Description Hide DescriptionRotation induced dispersive dust drift waves are suggested as the possible cause of the formation of spokes in the Saturn’s B ring. Using the plasma parameters found in the Saturn’s B ring, it has been shown that the theoretically predicted spatiotemporal scalelengths agree well with the satellites and Hubble Space telescope observations of the spokes.

VlasovMaxwell equilibria: Examples from highercurl Beltrami magnetic fields
View Description Hide DescriptionStationary solutions of VlasovMaxwell equations are obtained by exploiting the invariants of single particle motion and lead to linear or nonlinear functional relations between current and vector potential. The nonlinear relations support various special types of magnetic configurations including multiple current sheets and magnetic field discontinuities leading to singular current layers. It is demonstrated through the examples that in one dimension, the description of the equilibrium magnetic fields obeys double or highercurl Beltrami equation. For the linear case, such representation gives the advantage of obtaining exact analytic solutions that are expressed as a superposition of the singlecurl Beltrami fields.