bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on ‘‘Three-dimensional stability of drift vortices’’ [Phys. Plasmas 3, 160 (1996)]
Authors: Shoucri Magdi
Published: July 2002

Three-dimensional stability of drift vortices, in which the three-dimensionality is caused by a coupling of the vortex to ion-acoustic waves that propagate along the magnetic field, has recently been studied [Phys. Plasmas 3, 160 (1996)]. It is the purpose of the present Comment to draw attention to an important body of numerical results on this problem published in Plasma Physics 18, 187 (1976).
Effective charge in heavy ion stopping in classical collisionless plasmas
Authors: D’Avanzo J. , Hofmann I. , Lontano M.
Published: July 2002

The Brandt–Kitagawa (B–K) model for the effective charge is revisited in order to account for the effect of trapped electrons on the energy loss of a slow ion moving in a classical and collisionless plasma. The electrons trapped in the potential created in the plasma by the projectile are treated as classically bound, and it is shown that their main effect is to reduce the effective charge state of the projectile. This effect is expected to be important when the strength of the perturbation is non-negligible.
Direct determination of ion wave fields in a hot magnetized and weakly collisional plasma
Authors: Sarfaty M. , De Souza-Machado S. , Skiff F.
Published: July 2002

Electrostatic ion cyclotron wave fields are determined in a magnetized and weakly collisional plasma. A phased-locked Laser Induced Fluorescence (LIF) diagnostic is used to directly measure the wave perturbed ion velocity distribution. Comparing these local LIF measurements with a theoretical model uniquely determines the wave parameters, such as the wave potential, the three-dimensional wave vector, and the effective wave damping. The self-consistent wave–particle interaction is modeled by Boltzmann–Poisson equations in the limit of weak collisions. The wave parameters determined from local measurements agree with those determined from spatial scans.
ℓ=1 electrostatic instability induced by electron-neutral collisions in a nonneutral electron plasma interacting with background neutral gas
Authors: Davidson Ronald C. , Chao Edward H.
Published: July 2002

This paper investigates theoretically the electrostatic stability properties of a nonneutral electron plasma interacting with background neutral gas through elastic collisions with constant collision frequency νen. The model treats the electrons as a strongly magnetized fluid (ωpe2/ωce2≪1) immersed in a uniform magnetic field B0êz, and assumes small-amplitude perturbations with azimuthal mode number ℓ=1 and negligible axial variation (∂/∂z=0). The analysis also assumes weak electron collisions with νen/ωce=ε≪1, and that the process of heat conduction is sufficiently fast that the electrons have relaxed through electron-electron collisions to a quasiequilibrium state with scalar pressure P(r,θ,t)=n(r,θ,t)T, and isothermal temperature T. Assuming that perturbed quantities vary with time according to exp(−iωt), the detailed stability analysis carried out to first order in νen/ωce≪1 shows that the real oscillation frequency and growth rate for the ℓ=1 diocotron mode are given, respectively, by the simple expressions Re ω=ω0 and Im ω=(νen/ωce)ω0. Here, ω0=Nec/r2wB0, where rw is the perfectly conducting wall radius, and N=∫d2x n is the number of electrons per unit axial length. This analysis suggests that a measurement of the oscillation frequency and growth rate for the ℓ=1 diocotron mode can be used to infer νen, and thereby serve as a sensor for the background neutral pressure.
Orbit squeezing in a magnetic well
Authors: Shaing K. C. , Zarnstorff M. C. , Hsu C. T.
Published: July 2002

It is shown that the size of a charged particle’s drift orbit is squeezed, i.e., reduced, in a magnetic well in tokamak plasmas. The squeezing factor depends on the energy of the particle, being larger for higher-energy particles. Therefore, for high-energy particles, the size of the drift orbit depends only on the magnetic geometry. These energy-independent drift orbits are also observed [J. A. Rome and Y.-K. M. Ping, Nucl. Fusion 19, 1193 (1979)]. The implications on the core confinement improvement physics and the confinement of the advanced fuel are discussed.
Study of microinstabilities in toroidal plasmas with negative magnetic shear
Authors: Dong J. Q. , Zhang Y. Z. , Mahajan S. M. , Guzdar P. N.
Published: July 2002

The microinstabilities driven by a parallel velocity shear, and a temperature gradient of ions are studied in toroidal plasmas with negative magnetic shear. Both the fluid and the gyrokinetic formulations are investigated. It is found that for a broad range of parameters, the linear growth rates of the modes are lower and the threshold temperature gradient ηicr is higher for plasmas with negative magnetic shear compared to plasmas with positive magnetic shear of equal magnitude. The reduction in the growth rate (with negative shear), although not insignificant, does not seem to be enough to account for the dramatic improvement in the confinement observed experimentally. Other possible physical mechanisms for the improved confinement are discussed.
Uphill acceleration of electrons and secular fields in laser produced plasmas
Authors: Ruhl H.
Published: July 2002

Secular (dc) field generation by a single, obliquely incident p-polarized intense laser beam of ultrashort pulse duration is studied for steep density profiles. It is demonstrated that a secular uphill acceleration mechanism exists which could be observed experimentally as a negative secular electrostatic field in the skin layer.
Parallel transport in ideal magnetohydrodynamics and applications to resistive wall modes
Authors: Finn John M. , Gerwin Richard A.
Published: July 2002

It is shown that in magnetohydrodynamics (MHD) with an ideal Ohm’s law, in the presence of parallel heat flux, density gradient, temperature gradient, and parallel compression, but in the absence of perpendicular compressibility, there is an exact cancellation of the parallel transport terms. This cancellation is due to the fact that magnetic flux is advected in the presence of an ideal Ohm’s law, and therefore parallel transport of temperature and density gives the same result as perpendicular advection of the same quantities. Discussions are also presented regarding parallel viscosity and parallel velocity shear, and the generalization to toroidal geometry. These results suggest that a correct generalization of the Hammett–Perkins fluid operator [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)] to simulate Landau damping for electromagnetic modes must give an operator that acts on the dynamics parallel to the perturbed magnetic field lines.
Theoretical study on nonstationarity of electron energy distribution function for discharge-pumped XeCl laser
Authors: Zhu Qifeng , Imada Go , Masuda Wataru , Yatsui Kiyoshi
Published: July 2002

The nonstationarity of electron energy distribution function (EEDF) has been studied for a discharge-pumped XeCl laser, where a self-consistent model is considered. The energy relaxation time has been observed by solving a time-dependent Boltzmann equation. The importance of a nonstationary treatment in determining EEDF, electron average energy, electron-impact rate coefficients, and species densities has been discussed. It has been found that a pronounced nonstationarity is likely to occur at a relatively low electric field (< 2 Td). The quasi-stationary treatment tends to underestimate the electron average energy and some electron-impact rate coefficients around the inversion points of the discharge voltage. Electron–electron collisions induce a strong nonstationarity when the electron density exceeds a certain value. The electron-impact rate coefficients selected by neglecting the nonstationarity result in an incorrect determination of species densities.
Radial thermal stability of the radiative mantle
Authors: Stacey W. M.
Published: July 2002

Sufficient conditions for the thermal stability against radial excursions of a cylindrical equilibrium temperature distribution in a plasma with a strongly radiative edge, or mantle, are established in terms of the minimum allowable ratio of the core heating power density to the edge radiation power density. A positive dependence of the thermal conductivity upon temperature is found to be stabilizing, and a divertor separatrix boundary condition for the temperature is found to be destabilizing relative to a fixed separatrix temperature condition. The stability conditions are consistent with the conditions extant in a stable radiative mantle experiment and in a stable radiative mantle simulation. Expressions are developed for a radiation edge density limit and for the maximum fraction of the plasma heating power which can be radiated from the mantle.
Spontaneous emission effects in nonlinear interactions of nonresonant waves with resonant plasma fluctuations
Authors: Vladimirov Sergey V. , Ishihara Osamu
Published: July 2002

Spontaneous emission effects on propagation of nonresonant waves in plasmas in the presence of resonant fluctuations are studied. It is demonstrated that in closed plasma systems the number of nonresonant quanta is conserved as an adiabatic invariant. The conservation is due to the vanishing polarizational contribution that resulted from the symmetry of the system as well as to the balance of the direct nonlinear coupling and reverse absorption by particle collisions. Energy of the nonresonant waves as well as their amplitudes may vary with time even when the resonant field fluctuations are at the thermal level.
Hydromagnetic waves in a plasma of isotropic thermal and anisotropic suprathermal components
Authors: Kalra G. L. , Ghildyal V.
Published: July 2002

Low frequency plane waves supported by a medium containing a thermal plasma of isotropic pressure and a suprathermal collisionless plasma having anisotropic pressure are investigated. The usual Alfvén, slow and fast modes of isotropic pressure magnetohydrodynamics persist. In addition, a suprathermal mode appears which displays a rich variety of behavior due to an additional degree of freedom compared to the analogous mode when both the plasma components are described by collision-dominated magnetohydrodynamics. Since these modes are significant in a number of situations, they are extensively investigated by computing their phase speeds for wide-ranging numerical parameters.
Transport in a toroidally confined pure electron plasma
Authors: Crooks S. M. , O’Neil T. M.
Published: July 2002

O’Neil and Smith [T.M. O’Neil and R.A. Smith, Phys. Plasmas 1, 8 (1994)] have argued that a pure electron plasma can be confined stably in a toroidal magnetic field configuration. This paper shows that the toroidal curvature of the magnetic field of necessity causes slow cross-field transport. The transport mechanism is similar to magnetic pumping and may be understood by considering a single flux tube of plasma. As the flux tube of plasma undergoes poloidal E×B drift rotation about the center of the plasma, the length of the flux tube and the magnetic field strength within the flux tube oscillate, and this produces corresponding oscillations in T∥ and T⊥. The collisional relaxation of T∥ toward T⊥ produces a slow dissipation of electrostatic energy into heat and a consequent expansion (cross-field transport) of the plasma. In the limit where the cross section of the plasma is nearly circular the radial particle flux is given by Γr=1/2ν⊥,∥T(r/ρ0)2n/(−e∂Φ/∂r), where ν⊥,∥ is the collisional equipartition rate, ρ0 is the major radius at the center of the plasma, and r is the minor radius measured from the center of the plasma. The transport flux is first calculated using this simple physical picture and then is calculated by solving the drift-kinetic Boltzmann equation. This latter calculation is not limited to a plasma with a circular cross section.
Induced magnetic-field effects in inductively coupled plasmas
Authors: Cohen Ronald H. , Rognlien Thomas D.
Published: July 2002

In inductive plasma sources, the rapid spatial decay of the electric field arising from the skin effect produces a large radio frequency (RF) magnetic field via Faraday’s law. It was previously shown that this magnetic field leads to a reduction of the electron density in the skin region, as well as a reduction in the collisionless heating rate. The electron deficit leads to the formation of an electrostatic potential which pulls electrons in to restore quasineutrality. Here the electron density calculation is extended to include both the induced and electrostatic fields. If the wave frequency is not too low, the ions respond only to the averaged fields, and hence the electrostatic field is oscillatory, predominantly at the second harmonic of the applied field. The potential required to establish a constant electron density is calculated and compared with numerical orbit-code calculations. For times short compared to ion transit times, the quasineutral density is just the initial ion density. For timescales long enough that the ions can relax, the density profile can be found from the solution of fluid equations with an effective (ponderomotive-like) potential added. Although the time-varying electrostatic potential is an extra source of heating, the net effect of the induced magnetic and electrostatic fields through trapping, early turning, and direct heating is a significant reduction in collisionless heating for parameters of interest.
Flexible simple-pole expansion of distribution functions
Authors: Löfgren T. , Gunell H.
Published: July 2002

A method for parametrization and expansion of distribution functions is presented. The expansion has a finite number of simple poles, which gives efficient numerical calculations and control of all converging moments. The low velocity region is Maxwell-like and the high-velocity tail follows an inverse power law. The method is applied to Maxwell-like distributions with and without suppressed tails. Dispersion relations can be obtained for a wide class of distributions, using building blocks available in any numerical library. Dispersion relations, for ordinary Langmuir waves and for beam-plasma interactions with intermediate temperature and beam to plasma density ratio, are derived. The Landau damping, obtained in the long wavelength regime, is of the same order but smaller than for the generalized Lorentzian distributions, for a given degree in the power law.
Particle diffusion in the presence of trapping
Authors: Spineanu F. , Vlad M.
Published: July 2002

The diffusion of particles in the presence of randomly distributed trapping centres is examined. An analytical approach is developed for three simple models of the trap-release processes. It is shown that the particle motion remains diffusive on the average, but the diffusion coefficient can have large fluctuations. The results of the numerical simulations confirm the main qualitative trends found in the analytical study. Although they are very simple, the models can be useful for the examination of the diffusion in tokamak plasma in the presence of quasi-coherent structures which act as trapping centres.
Recovery of electron energy in cyclotron autoresonance masers
Authors: Bratman V. L. , Phelps A. D. R. , Savilov A. V.
Published: July 2002

Possibilities of efficiency enhancement in cyclotron autoresonance masers (CARMs) due to the use of recovery of the electron energy with a single-step depressed collector scheme are studied. The simple and very clear method of using the phase plane is developed for the analysis of the motion of electrons with different initial velocities in the CARM, operating in various regimes at both fundamental and higher cyclotron harmonics. The total efficiency can exceed 25% for a CARM with a low electron efficiency operating in the regime of electron bunching and 50% for a CARM in the regime of trapping and adiabatic deceleration of particles.
Large-amplitude solitary waves in finite temperature dusty plasma
Authors: Roychoudhury R. , Mukherjee Soma
Published: July 2002

Large-amplitude solitary waves in dusty plasma are investigated, taking into account the dusty particle temperature. It is shown that finite dusty temperature restricts the region for the existence of solitary waves.
Electric field spikes formed by electron beam–plasma interaction in plasma density gradients
Authors: Gunell H. , Löfgren T.
Published: July 2002

In the electron beam–plasma interaction at an electric double layer the beam density is much higher than in the classical beam–plasma experiments. The wave propagation takes place along the density gradient that is present at the high potential side of the double layer. Such a case is studied experimentally by injecting the electron beam from a plane cathode, without any grids suppressing the gradient, and by particle simulations. The high frequency field concentrates in a sharp “spike” with a half width of the order of one wavelength. The spike is found to be a standing wave surrounded by regions dominated by propagating waves. It forms at a position where its frequency is close to the local plasma frequency. The spike forms also when the electric field is well below the threshold for modulational instability, and long before a density cavity is formed in the simulations. Particle simulations reveal that, at the spike, there is a backward traveling wave that, when it is strongly damped, accelerates electrons back towards the cathode. In a simulation of a homogeneous plasma without the density gradient no spike is seen, and the wave is purely travelling instead of standing.
Production of sheared flow during ion cyclotron resonance heating in tokamak plasmas
Authors: Liu C. G. , Yamagiwa M. , Qian S. J.
Published: July 2002

An approach of producing the poloidal ion rotation by using ion cyclotron resonance heating (ICRH) is presented in core tokamak plasmas. The mechanism employed here is inducing a poloidal density inhomogeneity by rf cyclotron heating and then destabilizing the anomalous Stringer spin-up. A criterion for destabilization of the poloidal ion rotation in the presence of a rf wave is given, which depends on the ratio of the characteristic time of inhomogeneous density formation to the ion collision time. The numerical results have shown that the poloidal ion rotation can be destabilized in the present ICRH power level.
Nonlinear modulation of ion plasma waves via parametric processes
Authors: Honzawa Tadao , Hattori Minoru , Saitou Yoshifumi
Published: July 2002

Nonlinear modulation of the “slow” ion-beam modes in a beam-plasma system is experimentally shown to occur above a threshold of wave amplitude and to progress during propagation. The threshold is found to change depending on the wave frequency at a fixed plasma density and on the plasma density at a fixed frequency. The wave modulation is also studied by means of analysis of frequency spectra. The result reveals that the frequencies of the lower or upper sidebands, generated as a result of the wave–wave interaction, vary roughly in proportion to the ion plasma frequency fpi, when the plasma density is changed. Finally, on the basis of experimental data obtained here, the nonlinear modulation of the wave is tentatively interpreted to result from the parametric process, based on the three wave interaction satisfying relations ω0=±ω1+ω2 and k0=±k1+k2.
Charged particle confinement in torsatrons and heliotrons with toroidal field coils
Authors: Smirnova M. S.
Published: July 2002

The role of a strong and high-periodic toroidal magnetic-field ripple (the TF ripple) in charged particle confinement is investigated for the torsatron/heliotron magnetic devices [K. Uo, J. Phys. Soc. Jpn. 16, 1380 (1961)] with N discrete toroidal field coils in the helical field period. Caused by currents in these coils, such a ripple involves the appearance of additional toroidal ripple wells along the field line, in which charged particles may be trapped. To analyze the confinement character of the TF ripple-trapped particles, the method of a double averaging of their equations of motion is developed. It is shown that the suprathermal particles, which are trapped in the separately located TF ripple wells of the magnetic field, are the most dangerous for confinement because the radial deviations of these particles are N2 times higher than the corresponding values for the helically ripple-trapped particles. An additional type of superbanana orbit caused by the TF ripple perturbation of the magnetic field is also found.
Energy related conservation law for fluids and multi-fluid plasmas with equilibrium flow
Authors: Unverzagt M.
Published: July 2002

Within the framework of dissipationless multi-fluid theory for plasmas, all nonlinear perturbations of a stationary equilibrium that could be created from the equilibrium without breaking local entropy conservation are considered. Describing the fluid perturbations by Eulerian displacement vectors reveals a general symmetry and through Noether’s theorem leads to a conservation law. Its relation to the law of energy conservation is discussed since it will be important for the generalization of the energy principle known for static equilibria in magnetohyrodynamics to an energy principle of stationary equilibria in the framework of a multi-fluid theory.
Interacting eigenmodes of a plasma diode with a density gradient
Authors: Löfgren T. , Gunell H.
Published: July 2002

The formation of narrow high frequency electric field spikes in plasma density gradients is investigated using one-dimensional particle in cell simulations. It is found that the shape of the plasma density gradient is very important for the spike formation. A coupling to the ion motion, is not necessary for the formation of hf spikes. However, the hf spike influences the ion motion, and ion waves are seen in the simulations. Dispersion relations are calculated using realistic, non-Maxwellian, distribution functions. The spike can be seen as a coupled system of two eigenmodes of a plasma diode fed by a beam-plasma interaction. Based on a simplified fluid description of such eigenmodes, explanations for the localization of the spike, spatially and in frequency, are given. The amplitude of the oscillating density is comparable with the dc level close to the cathode. This sets a limit of a wave amplitude in the whole system.
Resonant excitation and control of high order dispersive nonlinear waves
Authors: Friedland L.
Published: July 2002

Autoresonant excitation of high order nonlinear waves with space–time varying parameters is investigated. A class of driven, two-component nonlinear waves described by the variational principle is studied in detail. The autoresonance in the system proceeds as an external eikonal pump wave excites a nonlinear daughter wave after crossing the linear resonance surface. Beyond the linear resonance, the pump and the daughter waves stay phase locked in an extended region of space–time despite the variation of the system’s parameters. The theory of the autoresonance is developed on the bases of the averaged variational principle and comprises a generalization of the formalism for scalar fields. The relation of the wave autoresonance problem to an associated two degrees of freedom problem in nonlinear dynamics is discussed. The conditions for the stable autoresonant solutions are (a) the adiabaticity of the driven system and (b) a sufficient nonlinearity. The theory is applied to the problem of resonant excitation and control of a Korteweg–de Vries (KdV) wave by means of launching an external pump wave with space–time varying frequency and wave vector. Numerical examples for temporal and spatial autoresonance in this system are presented.
Equations for the evolution of the radial electric field and poloidal rotation in toroidally symmetric geometry
Authors: Peeters A. G.
Published: July 2002

A set of three coupled ordinary differential equations that give the evolution of the density, the radial electric field, and poloidal plasma rotation are derived from the continuity equation and momentum balance. They include the effect of those processes considered to be of importance for the L- (low) to H- (high) mode transition: neutral friction, neoclassical viscosity, the radial pressure gradient, orbit losses, a radial current through a probe, anomalous stresses, and Stringer spin up. The equations are valid for arbitrary toroidally symmetric geometry and include effects of non-uniformity (of for instance the neutral friction) in the magnetic surface. As an example, non-uniform neutral friction in an elongated geometry is discussed.
Electron captures from hydrogenic ions by fully stripped ions in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

In strongly coupled plasmas, plasma-screening effects are investigated on electron capture from hydrogenic ions by fully striped ion projectiles using the classical Bohr–Lindhard model. The interaction potential in strongly coupled plasmas is described by the ion-sphere model potential. The electron capture radius is obtained by the ion-sphere interaction potential and the kinetic energy of the released electron in the frame of the projectile ion. The classical straight-line trajectory approximation is applied to the motion of the projectile ion in order to visualize the electron capture probability as a function of the impact parameter, projectile energy, and ion-sphere radius. It is found that the electron capture probability is quite sensitive to the projectile velocity and the ion-sphere radius. The maximum position of the electron capture probability is shifted to the target nucleus as the projectile energy increases and is receded from the target nucleus as the ion-sphere radius increases. It is also found that the maximum value of the capture probability is increased with an increase of the ion-sphere radius. The scaling relationship for the capture cross section in the screened case and compare with the existing one for the unscreened case is also obtained.
Propagation of a disturbance created by a probe in electron collection
Authors: Winslow D. L. , Bengtson Roger D. , Richards B. , Wootton A. J.
Published: July 2002

Experiments in the edge plasma of TEXT-U (the Texas Experimental Tokamak-Upgrade) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)] have used a positively biased Langmuir probe in conjunction with two triple probes to examine the “wake” or disturbance in the plasma caused by the electron current collected by the driven probe. Spatial measurements of the disturbed volume have been made in the radial and poloidal directions at two points along the disturbed flux tube, at 12 meters and 28 meters away from the driven probe. Both non-time dependent and time-dependent experiments are used to determine the dispersion of the driven wave. A classical collisional two-fluid model of electron collection by a biased surface, which accounts for electric potential perturbations and the associated currents only, successfully predicts many of the qualitative features of the disturbance. The magnitudes of the collision rates, however, have to be increased beyond the best estimates in order to achieve quantitative agreement, indicating the possible involvement of anomalous cross-field current transport processes.
Response to “Comment on ‘Angular dependence of stimulated Brillouin scattering in homogeneous plasma’ ” [Phys. Plasmas 5, 1215 (1998)]
Authors: Giacone R. E. , McKinstrie C. J. , Betti R.
Published: July 2002

In their comment ,Reference 1 Gupta and Sinha extend the analysis given by the authorsReference 2 of stimulated Brillouin scattering in a stationary plasma to a flowing plasma. The authors content that their formula including ion-temperature contribution is more accurate than that of Gupta and Sinha. (AIP)
Emission of magnetosonic waves by vortices in high shear flows
Authors: Tevzadze A. G.
Published: July 2002

Unbounded magnetohydrodynamic flows with constant shear of velocity and external uniform magnetic field directed perpendicular to the shear plane are considered. Vortex linear dynamics is studied by the tracing of spatial Fourier harmonics of perturbations in time. It is shown that vortices are able to emit magnetosonic waves even in the linear regime. Wave emission from vortices is efficient at high shear rates and at weak or moderate external magnetic fields.
Envelope equation for nonlinear transverse waves in a warm two-fluid plasma
Authors: Lee Nam C. , Parks George K.
Published: July 2002

An equation describing the nonlinear development of an envelope of weakly dispersive transverse waves propagating along the ambient magnetic field in a warm two-fluid plasma is derived by using the Fourier transformation method. This equation contains three nonlinear terms, a cubic, a derivative of cubic, and a quadratic multiplied by a derivative term. For the case of small wave amplitude, the equation is of non-Schrödinger type that includes a linear term involved with mixed second-order derivatives (space and time), in addition to the usual two terms of the Schrödinger-type equation. This mixed derivative term arises from the inclusion of the displacement current in the formulation. The nonlinear terms in the derived equation explicitly contain a smallness parameter which vanishes in the limit of zero dispersion. The low frequency limit of the equation is compared with the standard derivative nonlinear Schrödinger equation (DNLS), and differences are discussed. The new equation predicts that the condition for the modulational instability for the low frequency circularly polarized waves does not depend on the sense of polarization, a result that differs from the usual DNLS equation. The high and intermediate frequency limits of the equation are also considered.
Observation of collisionless thermalization of a plasmoid with a field-reversed configuration in a magnetic mirror
Authors: Himura H. , Ueoka S. , Hase M. , Yoshida R. , Okada S. , Goto S.
Published: July 2002

A systematic translation study of field-reversed configurations (FRCs) has been conducted on the FRC Injection Experiment (FIX) machine [Okada et al., in Fusion Energy 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 2, p. 229]. Plasma density and temperature of a translated FRC moving at supersonic speed are measured in the downstream magnetic mirror of FIX to verify a shock jump there when the FRC is reflected. A significant jump is observed. Moreover, the time evolution of the Carbon V Doppler profile is measured both quasi-parallel and perpendicular to the direction of FRC motion. Distinct transitions from Gaussian to non-Gaussian shapes are clearly seen in both profiles before and after the shock jump. Also, the ion mean-free path in the downstream magnetic mirror is calculated to be much longer than the characteristic width of the shock jump. These results indicate that the thermalization of flow energy in the translated FRC in the mirror is produced by a collisionless process, implying that this heating mechanism can be realized even in a reactor regime.
Whistler wave emission by a modulated electron beam through transition radiation
Authors: Starodubtsev M. , Krafft C. , Thévenet P. , Kostrov A.
Published: July 2002

Measurements have been performed in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving beam injection. Both whistler emission through Cherenkov resonance and a nonresonant mechanism of transition radiation from the point of beam injection into the plasma have been observed. Electrons injected from the gun into the plasma pass from one medium (gun chamber) into another (plasma volume) and electromagnetic fields change as charges cross the metallic interface between both media, giving rise to transition radiation. This type of beam radiation, observed separately from the resonant Cherenkov emission owing to adequate choices of the physical conditions, has been characterized as a function of the beam and plasma parameters. Moreover, in the case of beams injected from satellites in the ionospheric and magnetospheric plasmas, this nonresonant emission, mainly located in the near gun region, can be governed by an adequate control of the radiator parameters and separated from resonant emissions.
Propagation of fast surface waves in an ideal Hall-magnetohydrodynamic plasma slab
Authors: Zhelyazkov Ivan , Mann Gottfried
Published: July 2002

The oblique and parallel propagation of fast sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm’s law. It is found that, in the case of incompressible plasmas, the combining action of the Hall effect and the oblique wave propagation makes possible the existence of multivalued solutions to the dispersion relations of both MHD surface modes—some of them, corresponding to positive values of the transverse wave number (ky), undergo a “propagation stop” at specific (numerically found) full wave numbers. It is curious that such multivalued dispersion curves appear even at purely parallel propagation in low-β plasmas (β→0). Another peculiarity of these surface modes (in both cases) is that with the growing of the wave number they can change their nature—from bulk to pseudosurface (or pure surface) and leaky waves.
Resistive drift wave turbulence in a three-dimensional geometry
Authors: Korsholm So/ren B. , Michelsen Poul K. , Naulin Volker
Published: July 2002

The Hasegawa–Wakatani model describing resistive drift waves is investigated analytically and numerically in a three-dimensional periodic geometry. After an initial growth of the energy the drift waves couple nonlinearly to convective cells, which eventually dominate the system completely. An approach to include more physical boundary conditions to the system is presented. This changes the results of the simulations significantly.
Test particle transport in perturbed magnetic fields in tokamaks
Authors: de Rover M. , Schilham A. M. R. , Montvai A. , Cardozo N. J. Lopes
Published: July 2002

Numerical calculations of magnetic field line trajectories in a tokamak are used to investigate the common hypotheses that (i) field lines in a chaotic field make a Gaussian random walk and (ii) that the poloidal component of the magnetic field is uniform in regions with a chaotic magnetic field. Both hypotheses are found invalid in typical tokamak conditions. A test particle transport model in the so-called “collisionless diffusion” limit is presented, based on the field line excursions in numerical simulations. Decorrelation mechanisms that effectively enhance the transport in a stochastic field are investigated.
Avalanche runaway growth rate from a momentum-space orbit analysis
Authors: Parks P. B. , Rosenbluth M. N. , Putvinski S. V.
Published: July 2002

The growth of avalanche runaway electrons, a potentially serious issue for disruptions in high-current tokamaks, was recently studied by Monte Carlo and numerical solutions of the relativistic Fokker–Planck equation. Here a new analytical formulation based on the analysis of the particle trajectories of the knocked out secondary electrons offers insight and yields an analytical expression for the growth rate over the entire parameter range of interest. The growth rates are compared with the numerical simulations and analytical solutions of the Fokker–Planck equation in various limits [M. N. Rosenbluth and S. V. Putvinski, Nucl. Fusion 37, 1355 (1997)], and are found to be in good agreement.
Experimental study of laser penetration in overdense plasmas at relativistic intensities. I: Hole boring through preformed plasmas layers
Authors: Fuchs J. , Adam J. C. , Amiranoff F. , Baton S. D. , Blanchot N. , Gallant P. , Gremillet L. , Héron A. , Kieffer J. C. , Laval G. , Malka G. , Miquel J. L. , Mora P. , Pépin H. , Rousseaux C.
Published: July 2002

Propagation of a high-contrast subpicosecond (400–600 fs) relativistic (Iλ2 up to 2×1019 W⋅cm−2⋅μm2) laser pulse through long preformed overdense plasmas is experimentally studied. Transmission values up to 10% are measured in plasmas with initial peak densities above 10×nc. We compare these results with one-dimensional (1D) analytical calculations of the hole boring effect. It is shown that the ponderomotive force of the laser beam can push forward the overdense plasma, create a channel, and lead to transmission levels similar to the experimental ones.
Z-pinch discharges in aluminum and tungsten wires
Authors: Ruiz-Camacho J. , Beg F. N. , Dangor A. E. , Haines M. G. , Clark E. L. , Ross I.
Published: July 2002

A series of experiments on Z-pinch plasmas, driven by a pulsed power generator that delivers 160 kA with a rise time (10%–90%) of 65 ns are reported. Tungsten wires of various diameters were used and results are compared with 15 μm diameter aluminum wire. The expansion of the pinch is studied as a function of wire diameter and material. Schlieren observations show that the coronal plasma of various diameters of tungsten wires expands with the velocity of (9.4±1.0)×103 m/s. The aluminum pinch expands at least a factor of 2 faster. The m=0 perturbations appear at about 8 ns for the aluminum compared with 20 ns for the tungsten pinch. The wavelength and diameter of the perturbations increase with time for both types of wires, and relatively faster for the aluminum pinch. The short wavelength perturbations (∼200 μm) persist for a longer time for larger diameter tungsten wires. Bright spots are seen to appear after 60 ns from the current start for tungsten wires, whereas for aluminum wires, bright spots appear after 40 ns. The decay time of bright spots is 40 ns for the smallest diameter tungsten wire compared with only a few nanoseconds for larger diameter wires. Hard x-ray emission above 6 keV was observed from tungsten wire pinches, but it was not observed from either bright spots or the plasma column for the aluminum pinch. However, hard x-ray emission from the anode due to an electron beam was observed for wires of both materials.
Mach numbers for gases and plasmas in a convergent-divergent cascaded arc
Authors: Burm K. T. A. L. , Goedheer W. J. , Schram D. C.
Published: July 2002

For a plasma, flowing through a cascaded arc channel with a varying cross-section, and flowing from a subsonic to a supersonic state, the sonic condition moves downstream and the plasma Mach number at the smallest cross section is less than one, although in case of a transonic isentropic gas flow the sonic condition is found at the smallest cross section. This shift in sonic condition is due to the lack of isentropic behavior of the plasma flow. Sources causing the anisentropy are viscosity, heat and ionization, of which ionization is vital for a plasma. It is found that the plasma Mach number is always lower than the corresponding gas Mach number. A quasi one-dimensional analysis and simulations with a two-dimensional plasma model, which support the analysis, are presented.
Effective temperatures, sawtooth mixing, and stochastic diffusion ripple loss of fast H+ minority ions driven by ion cyclotron heating in the Tokamak Fusion Test Reactor
Authors: Petrov M. P. , Bell R. , Budny R. V. , Gorelenkov N. N. , Medley S. S. , White R. B. , Zweben S. J.
Published: July 2002

This paper presents studies of the H+ minority ions driven by Ion Cyclotron Radio Frequency (ICRF) heating in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Phys. Plasmas 5, 1577 (1998)] deuterium plasmas using primarily passive H° flux detection in the energy range of 0.2–1.0 MeV. The measured passive H+ energy spectra are compared with active (Li pellet charge exchange) results. It is shown that in the passive mode the main donors for the neutralization of H+ ions in this energy range are C5+ ions. The measured effective H+ tail temperatures range from 0.15 MeV at an ICRF power of 2 MW to 0.35 MeV at 6 MW. Radial redistribution of ICRF-driven H+ ions was detected when giant sawtooth crashes occurred during the ICRF heating. The redistribution affected ions with energy below 0.7–0.8 MeV. The sawtooth crashes displaces H+ ions outward along the plasma major radius into the stochastic ripple diffusion domain where those ions are lost in ∼10 msec. These observations are consistent with the model of the redistribution of energetic particles developed previously to explain the results of deuterium-tritium (DT) alpha-particle redistribution due to sawtooth oscillations observed in TFTR. The experimental data are also consistent with numerical simulations of H+ stochastic ripple diffusion losses.
Ion cyclotron resonance frequency heating of deuterium plasmas in the Joint European Torus: Interpretation of measurements of minority hydrogen isotope ion distribution functions
Authors: Testa D. , Core W. G. F. , Gondhalekar A.
Published: July 2002

An iterative method for solving the hot-plasma dispersion relation and computing power deposition during multispecies minority ion cyclotron resonance frequency (ICRF) heating experiments has been presented in a companion paper [D. Testa, W.G.F.Core, and A.Gondhalekar, Phys. Plasmas (this issue)]. Here this method is further developed to model and validate the measurements of minority hydrogen isotope ion distribution functions during multispecies second harmonic ICRF heating of deuterium plasmas in the Joint European Torus [J.Wesson, Tokamaks (Clarendon, Oxford, 1997), pp. 581–603].
On the stabilization of neoclassical tearing modes by electron cyclotron waves
Authors: Ramponi G. , Lazzaro E. , Nowak S.
Published: July 2002

The control of neoclassical tearing modes in tokamaks by means of electron cyclotron current drive and heating is investigated. The nonlinear evolution of the amplitude in absence and in presence of the stabilizing terms of an auxiliary current inside the island and of the associate heating is solved self-consistently with the evolution of the rotation frequency for International Thermonuclear Experimental Reactor (ITER) reference magnetic equilibrium [ITER-JCT and Home Teams, Plasma Phys. Controlled Fusion 37, A19 (1995)]. It is shown that, unless the wall braking torque is neutralized by external means, neoclassical tearing modes in ITER will be locked in a very short time. On the other hand, for rotating islands, the beneficial effect of modulating the current source in phase with the island rotation is pointed out, after an analysis of the time scales of the relevant phenomena (time response of the driven current, island rotation frequency, power pulse duration, and inductive response of the plasma). Consideration is given to different effects that may reduce the efficiency of the control of the flux reconnection rate and to the benefits of wall stabilization associated to the island rotation frequency. A quantitative assessment of the EC (electron cyclotron) power required to keep the island width at a reasonable level is given, both in absence and in presence of wall stabilization.
Surface wave propagation in a magnetized dusty plasma with charge fluctuation
Authors: Alam Md. Khurshed , Chowdhury A. Roy , Paul S. N.
Published: July 2002

Propagation of a surface wave in a semi-infinite magnetized dusty plasma is analyzed by taking into account the dust-charge fluctuation and assuming that the magnetic field is weak. The dispersion relation is deduced and analyzed to show the existence of a modified low frequency mode, which reduces to the result obtained earlier in the zero fluctuation limit. The two different cases of electrostatic and electromagnetic mode are discussed separately. Last, numerical estimates of the attenuation coefficient as a function of the frequency for its various range of values have been done and are displayed graphically.
Temporal evolution of linear drift waves in a collisional plasma with homogeneous shear flow
Authors: Mikhailenko V. S. , Mikhailenko V. V. , Stepanov K. N.
Published: July 2002

Temporal evolution of linear drift waves in a collisional plasma with a homogeneous shear flow is treated analytically. The explicit solutions for the linearized Hasegawa–Wakatani system of equations, as well as for linearized Hasegawa–Mima equation, are obtained for this case on the basis of the nonmodal approach. In the weak-collision regime, the homogeneous shear flow is found to be a factor impeding the development of the ordinary modal resistive drift instability. This instability is excited only in the case of a weak velocity shear. For a stronger shear, the nonmodal effects, such as the blocking of drift wave packets and the linear transformation of drift waves into convective cells, determine the temporal evolution of drift-like perturbations. A nonmodal solution is found in the limit of strong collisions or sufficiently strong flow shear. The solution at the asymptotically large time possesses a convective-cell pattern.
Formation of vortex streets due to nonlinearly interacting ion–temperature–gradient modes
Authors: Farid T. , Mirza Arshad M. , Shukla P. K.
Published: July 2002

Stationary solutions of the nonlinear equations, which govern the dynamics of ion–temperature–gradient (ITG) electrostatic modes, are re-examined without and with magnetic field inhomogeneities. It is shown that for both cases the nonlinear equations admit vortex street solutions in those parameter regimes where spatially bounded double vortex solutions are forbidden. The ITG-driven vortex streets can be associated with coherent nonlinear structures that are observed in laboratory and computer simulation experiments. Furthermore, vortex streets might play a detrimental role in the study of anomalous ion thermal transport in magnetically confined plasmas.
The spatial location of laser-driven, forward-propagating waves in a National-Ignition-Facility-relevant plasma
Authors: Cobble J. A. , Fernández J. C. , Kurnit N. A. , Montgomery D. S. , Johnson R. P. , Renard-Le Galloudec N. , Lopez M. R.
Published: July 2002

Ion acoustic and electron plasma waves, associated with backward-propagating stimulated Brillouin scattering and stimulated Raman scattering, have been diagnosed in a long-scale-length, nearly homogenous plasma with transverse flow. Thomson scattered light from a probe beam is employed to show that these waves are well localized in space and for a time much shorter than the laser pulse duration. These plasma conditions are relevant to hohlraum design for the National Ignition Facility inertial confinement fusion laser system. [R. Sawicki et al., Fusion Technol. 34, 1097 (1998)].
The role of the lower hybrid resonance in helicon plasmas
Authors: Cho Suwon
Published: July 2002

A numerical study is carried out to investigate the eigenmode characteristics of helicon plasmas near the lower hybrid resonance using the analytic solutions of the wave equation for uniform plasmas. It is shown that there are innumerable or a few isolated eigenmodes depending on the value of the wave frequency whether it is higher or not than that of the lower hybrid frequency. The plasma resistance is usually large with a sharp peak near the lower hybrid frequency, but it depends on the plasma density. Accordingly, the wave equation is solved self-consistently with the particle and power balance equations, and it is shown that there exists a threshold frequency for efficient ionization near the lower hybrid resonance. This may explain the existence of the optimum frequencies of helicon discharge which have been experimentally found to be near the lower hybrid frequencies.
Energy loss of a test charge in partially ionized dusty plasmas
Authors: Nasim M. H. , Qaisar M. S. , Mirza Arshad M. , Murtaza G. , Shukla P. K.
Published: July 2002

The energy loss of a test charge particle in an unmagnetized dusty plasma is estimated, by incorporating the dust–neutral collisions. A slowly damping large amplitude wake field is observed which moves ahead of the test charge position for large dust–neutral collision frequencies. A critical test charge velocity is determined for a particular dust–neutral collision frequency below which the test charge gains energy instead of losing. The collisions enhance the energy loss only for the test charge velocities greater than the dust acoustic speed.