Volume 7, Issue 5, May 2000
 LETTERS


Effect of field ionization on interaction of an intense subpicosecond laser pulse with foils
View Description Hide DescriptionThe effect of transient ionization in an intense, short laser pulse irradiated foil is investigated via a simulation based on the collisional particleincell method coupled with the atomic kinetics. A strong electric field induced by superthermal electrons causes significant plasma ionization at the rear surface of the target. The plasma induced field ionization is found to dominate the production of the multiplecharged ions at the rear surface of the foil, which are accelerated forward up to multi mega electron volts by irradiation of a subpicosecond laser pulse with intensities from to

Observation of shock wave elimination by a plasma in a Mach2.5 flow
View Description Hide DescriptionAn experimental study on the influence of a plasma on the structure of an attached conical shock front appearing at the front end of a missileshaped model has been carried out in a Mach2.5 flow. The tip and the body of the model are designed as the cathode and anode for gaseous discharge, which produces a spraylike plasma moving around the tip. It is observed that the plasma has caused the shock front to separate from the model. The shock wave moves upstream in the form of a detached bow shock a sensible distance away from the model tip. The detached shock front appears to be highly dispersed in its new location as seen in the shadow video graphs of the flow. As the discharge current increases, experimental evidence shown in the video further reveals a distinct state of the flow without the presence of any shock wave.

Zonal flow generation by parametric instability in magnetized plasmas and geostrophic fluids
View Description Hide DescriptionTwodimensional magnetized plasmas and geostrophic fluids exhibit a common nonlinearity due to the advection of vorticity. It is shown here that due to this nonlinearity, the propagation of small scale wave packets is accompanied by instability of a low frequency, long wavelength component. This instability is the coherent hydrodynamic generalization of the resonant type mean flow instability identified recently [P. H. Diamond, M. N. Rosenbluth, F. L. Hinton, M. Malkov, J. Fleischer, and A. Smolyakov, 17th IAEA Fusion Energy Conference, IAEACN69/TH3/1, Yokohama, 1998 (to be published, International Atomic Energy Agency, Vienna)]. The mechanism discussed here, along with the resonant type, constitutes the “hydrodynamic” and “kinetic” regimes of the same process, similar to the case of plasmabeam instabilities. It is suggested that this generic mechanism is responsible for the generation of mean flow in atmospheres of rotating planets and magnetized plasmas.

Offmidplane launch of electron Bernstein waves for current drive in overdense plasmas
View Description Hide DescriptionNumerical modeling shows that localized, efficient current drive is possible in overdense toroidal plasmas (such as reversed field pinches and spherical tokamaks) using perpendicular launch of electron Bernstein waves. The wave directionality required for driving current can be obtained by launching the waves above or below the midplane of the torus and is a geometric effect related to the poloidal magnetic field.Wave absorption is strong, a result of the electrostatic nature of the waves, giving efficient suprathermal tail formation and current drive.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

The compressible plane currentvortex sheet
View Description Hide DescriptionThe plane currentvortex sheet is a magnetohydrodynamic configuration in which a vortex sheet lies at the location of an electric current sheet. Most previous research on this structure has focused on the incompressible situation. In this paper some effects of compressibility on the linear stabilityproperties of the plane currentvortex sheet are examined. The relevant compressible equations are derived and then solved by a new magnetohydrodynamic extension of the SPEctral Compressible Linear Stability (SPECLS) algorithm, a Chebyshev collocation code. Of particular interest is an investigation of how the properties of the low sonic Mach number (M) analogs of previously investigated incompressible unstable modes vary as M is increased to supersonic values. It is found that, in general, the growth rates of these modes decrease as M increases. However, new unstable modes are found to appear at high M. These new modes, which have a finite phase velocity, are also found to be weakly evanescent and oscillatory in the crossstream direction. Further data is presented on the influence of the streamwise and spanwise wave numbers, and also the Alfvén number. The morphology of the perturbations is also discussed, with an emphasis on the temperature and mass density structure. A short discussion is also given of the effect of spatial variation of the zerothorder temperature and mass density fields, a situation that would arise when magnetofluids with different thermodynamic properties are brought into contact with each other.

Hydrodynamic regime of twodimensional electron magnetohydrodynamics
View Description Hide DescriptionA detailed numerical simulation studying certain aspects of turbulence in the electron magnetohydrodynamic (EMHD) model is presented. In particular, new studies have been carried out in a parameter regime where the typical turbulent length scales are comparable to or smaller than the electron skin depth. This is the regime where the EMHD turbulence has similar traits to the hydrodynamicturbulence and the wave propagationeffects due to whistlers are believed to be inconsequential.

Mechanism of dustacoustic instability in a direct current glow discharge plasma
View Description Hide DescriptionAn observation of low frequency waves spontaneously excited in a dc glow dischargedusty plasma is reported. To analyze possible reasons for the instability observed, a linear dispersion relation which takes into account collisions with neutrals, dust grain charge variations, ion drift, and forces acting on dust particles is derived. Numerical analysis of the dispersion relation shows that the observed instability is the result of dust charge variations in the presence of external chargedependent forces together with the ion drift effect.
 Nonlinear Phenomena, Turbulence, Transport

The splitweight particle simulation scheme for plasmas
View Description Hide DescriptionAn efficient numerical method for treating electrons in magnetized plasmas has been developed. The scheme, which is based on the perturbative gyrokinetic particle simulation, splits the particle electron responses into adiabatic and nonadiabatic parts. The former is incorporated into the gyrokinetic Poisson’s equation, while the latter is calculated dynamically with the aid of the charge conservation equation. The new scheme affords us the possibility of suppressing unwanted highfrequency oscillations and, in the meantime, relaxing the Courant condition for the thermal particles moving in the parallel direction. It is most useful for studying lowfrequency phenomena in plasmas. As an example, onedimensional drift wave simulation has been carried out using the scheme and the results are presented in this paper. This methodology can easily be generalized to problems in threedimensional toroidal geometry, as well as those in unmagnetized plasmas.

Nonlocal radiative forces in fullyionized plasmas
View Description Hide DescriptionThe solution of the Fokker–Planck equation which contains the contribution of highfrequency electric fields is presented. For this, the projection operators of the Krook collision operator and the semicollisional propagator computed with the continued fractions were used. Both elastic and inelastic momentum transfer from the highfrequency electric field to electrons have been considered. The contributions of such highfrequency electric fields to the stress tensor and to the mean change of momentum due to electron–ion collisions are derived in the whole collisionality range. As a result, in the collisionless range, these contributions led to a new force which is 1.6 (instead of 4/3) more important than the Miller force. In the collisional range, the radiative force due to the inverse bremsstrahlung absorption is recovered and a new contribution which arises from the mean change of momentum due to collisions is found.

Experimental study of fluctuations excited by a narrow temperature filament in a magnetized plasma
View Description Hide DescriptionA systematic study is made of the spontaneous growth of fluctuations in temperature, density, and magnetic field in a narrow (on the order of the electron skin depth) fieldaligned temperature filament embedded in a large magnetized plasma. Two broad classes of fluctuation (“low” and “high” frequency modes) have been identified and studied in detail. A highfrequency driftAlfvén mode grows at frequencies about one tenth the ion gyrofrequency in the region of the filament where the temperature gradient is large. The measured radial profiles of the density and magnetic fieldfluctuations associated with this mode agree well with theoretical predictions. The highfrequency mode has been observed to exhibit several interesting nonlinear features, including steepening wave form, progression in azimuthal mode number, coupling to the low frequency mode with subsequent sideband generation, and eventually a transition to broad band turbulence. The nature of the lowfrequency mode which has frequencies about one fiftieth of the ion gyrofrequency is less certain, but it has been identified as a spatially localized, azimuthally symmetric mode consisting primarily of temperaturefluctuations. Both the high and lowfrequency modes give rise to electron heat transport at rates in excess of the classical values.
 Magnetically Confined Plasmas, Heating, Confinement

Statistical characterization of fluctuation wave forms in the boundary region of fusion and nonfusion plasmas
View Description Hide DescriptionThe statistical properties of plasma fluctuations have been investigated in the plasma boundary region of fusion(tokamaks and stellarators) and nonfusion plasmas. Fluctuations in ion saturation current and floating potential have a nearGaussian character in the proximity of the velocity shear layer However, fluctuations deviate from a Gaussian distribution when moving inside of the plasma edge or into the scrapeoff layer region Furthermore, fluctuations show sporadic pulses that are asymmetric in time. The present analysis shows a coupling of those pulses and the averaged flow in the shear layer region.

Paths to ignition by radio frequency heating during the Bfield ramp
View Description Hide DescriptionTo conserve transformer voltseconds, power to toroidalmagnetic field coils, and to trigger an early transition into high confinement (H) mode, where the requirements on auxiliary power are lower, rf heating during the Bfield ramp phase of ignitionclass tokamaks is considered. The scheme is analyzed by modifying the usual plasma operating condition diagrams to apply to the ramp phase where the magnetic field,plasma current, and density are changing. It is shown that ion cyclotron rangeoffrequencies direct electron heating during the ramp phase of IGNITOR [B. Coppi, M. Nassi, and L. E. Sugiyama, Phys. Scr. 45, 112 (1992)], as proposed by Majeski [R. Majeski, in AIP Conference Proceedings 485—Radio Frequency Power in Plasmas, Annapolis, MD (AIP, New York, 1999), p. 353], may be useful in optimizing the operating condition path to ignition.

A hydrocarbon reaction model for low temperature hydrogen plasmas and an application to the Joint European Torus
View Description Hide DescriptionA model of collisional processes of hydrocarbons in hydrogen plasmas has been developed to aid in computer modeling efforts relevant to plasma–surface interactions. It includes 16 molecules (CH up to to and to and four reaction types (electron impact ionization/dissociative ionization, electron impact dissociation,proton impact charge exchange, and dissociative recombination). Experimental reaction rates or cross sections have been compiled, and estimates have been made for cases where these are not available. The proton impact charge exchangereaction rates are calculated from a theoretical model using molecular polarizabilities. Dissociative recombination rates are described by the equation where parameter A is fit using polarizabilities and B is estimated from known reaction rates. The electron impact ionization and dissociation cross sections are fit to known graphs using four parameters: threshold energy, maximum value of the cross section, energy at the maximum, and a constant for the exponential decay as energy increases. The model has recently been used in an analysis of the Joint European Torus [P. H. Rebut, R. J. Bickerton, and B. E. Keen, Nucl. Fusion 25, 1011 (1985)] MARK II carbon inner divertor using the WBC Monte Carlo impurity transport code. The updated version of WBC, which includes the full set of hydrocarbon reactions, helps to explain an observed asymmetry in carbon deposition near the divertor.

Fast particle destabilization of toroidicityinduced Alfvén eigenmodes in the National Spherical Torus Experiment
View Description Hide DescriptionToroidicityinduced Alfvén eigenmode (TAE) stability in the National Spherical Torus Experiment (NSTX) [S. M. Kaye, M. Ono, Y.K. M. Peng et al., Fusion Technol. 36, 16 (1999)] is analyzed using the improved NOVAK code [N. N. Gorelenkov, C. Z. Cheng, and G. Y. Fu, Phys. Plasmas 6, 2802 (1999)], which includes finite orbit width and Larmor radius effects and is able to predict the saturation amplitude for the mode using the quasilinear theory. A broad spectrum of unstable global TAEs with different toroidal mode numbers is predicted. Due to the strong poloidal field and the presence of the magnetic well in NSTX, better particle confinement in the presence of TAEs in comparison with tokamaks is illustrated making use of the ORBIT code [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)].

Poloidal flows and enhanced reverse shear bifurcation in tokamaks
View Description Hide DescriptionThe effect of poloidal flow shear on transition to enhanced reverse shear (ERS) mode in tokamaks is studied. A model is examined where the flowfluctuation equilibrium evolves in time as a function of pressure gradients. Under some conditions the transition to the ERS phase occurs via as fold catastrophe and is accompanied by a spike in poloidal flow generation where poloidal flow shear is reversed. The nature of the transition is found to depend critically on the ratio of factors controlling the damping and generation of poloidal flow shear. The turbulence level shows a periodic bursting behavior which is suppressed in the ERS phase. The relevance of these results to recent observations from the TokamakFusion Test Reactor [R. E. Bell et al., Phys. Rev. Lett. 81, 1429 (1998)] is briefly discussed.

Kinetic simulations of the formation and stability of the fieldreversed configuration
View Description Hide DescriptionThe FieldReversed Configuration (FRC) is a highbeta compact toroidalplasma confined primarily by poloidal fields. In the FRC the external field is reversed on axis by the diamagnetic current carried by thermal plasma particles. A threedimensional, hybrid,particleincell (zeroinertia fluid electrons, and kinetic ions), code FLAME, previously used to study ion rings [Yu. A. Omelchenko and R. N. Sudan, J. Comp. Phys. 133, 146 (1997)], is applied to investigate FRC formation and tilt instability. Axisymmetric FRC equilibria are obtained by simulating the standard experimental reversed thetapinch technique. These are used to study the nonlinear tilt mode in the “kinetic” and “fluidlike” cases characterized by “small” and “large” ratios of the characteristic radial plasma size to the mean ion gyroradius, respectively. The formation simulations have revealed the presence of a substantial toroidal (azimuthal) magnetic field inside the separatrix, generated due to the stretching of the poloidal field by a sheared toroidal electron flow. This is shown to be an important tiltstabilizing effect in both cases. On the other hand, the tilt mode stabilization by finite Larmor radius effects has been found relatively insignificant for the chosen equilibria.

Toroidal field effects on dipole equilibrium and stability at finite plasma pressure
View Description Hide DescriptionThe influence of a toroidalmagnetic field on the equilibrium and stability of a plasma confined by the magnetic field of a point dipole is considered, since it may be necessary to introduce a toroidalmagnetic field to smooth electron heating or other asymmetries in the toroidal direction that can lead to toroidalelectric fields and a loss of confinement due to radial drift. To begin, a toriodal magnetic field is shown to restore confinement by reducing electron heating departures from axisymmetry. Then the Grad–Shafranov equation for a point dipole in the presence of a toroidal field is solved to explicitly demonstrate that an equilibrium exists. Finally, it is shown that a toroidalmagnetic field destabilizes the otherwise stable point dipole ballooning modes at high plasma pressure. Consequently, if a toroidalmagnetic field is necessary to remove toroidal heating asymmetries in a dipole equilibrium, the heating may have to be performed transiently or at lower plasma pressures.

Effect of edge biasing on electrostatic fluctuations and particle transport in a nonfusion magnetoplasma
View Description Hide DescriptionThis paper concerns experiments about the role of a radial electric field on the plasma turbulence in a toroidal magnetoplasma, and the possibility of anomalous transport reduction through the application of external biasing potentials to the plasma edge. The experiments show, for positive applied bias potential, a progressive reduction of radial anomalous particle flux, as well as electrostaticfluctuations. Some results on statistical and spectral properties of plasma and density fluctuations are reported.

Poloidal field effects on fundamental minority ion cyclotron resonance heating in a tokamak plasma
View Description Hide DescriptionMinority ion fundamental cyclotron resonance is studied in a large tokamak in which the geometrical optics approximation applies off resonance and the minority average speed is less than the wave phase speeds. Poloidal equilibrium magnetic fieldeffects are included, which lead to nontrivially nonlocal integrodifferential equations for the wave fields. Exact reciprocity relation is given as well as explicit analytic solutions for the transmission coefficients for both the high and low field side incidences. Numerical solutions are needed only for the high field side incident reflection coefficient. Numerical schemes are described and numerical results are presented together with a reliable error bound. Typically, energy absorption increases with poloidal field. The energy absorption increases with minority density at low values of minority density. However, it decreases at high minority density. Poloidal field effects weaken the dependence of energy absorption on the toroidal wave number.

The radial and poloidal localization of fast magnetoacoustic eigenmodes in tokamaks
View Description Hide DescriptionEdgelocalized fast magnetoacoustic eigenmodes (FME) may be responsible for the observed superthermal ion cyclotron emission in recent deuterium/deuterium (DD) and deuterium/tritium (DT) experiments. These modes can be driven unstable by resonant interaction with a small population of energetic ions (e.g., fusion alphas), having an anisotropic distribution in velocity space, provided that the mode frequency is close to the ion cyclotron frequency. In the present paper, the radial and poloidal structure of these eigenmodes is analyzed, by solving the twodimensional (2D) eigenmode equation both numerically and analytically using a variational approach. In particular, the conditions for the mode localization and the dependence of the mode structure on aspect ratio and ellipticity are investigated. The eigenmode is found to be radially localized near the plasma edge. In a tokamak with finite aspect ratio, the mode is localized also poloidally near the outer midplane edge. The existence of localized solutions for both sign of the poloidal mode number is found to be sensitive to the ellipticity, the magnitude of parallel wave number and the plasma density profile.