摘要:

Dynamo action is demonstrated numerically in the forced Taylor–Green (TG) vortex made up of a confined swirling flow composed of a shear layer between two counter-rotating eddies, corresponding to a standard experimental setup in the study of turbulence. The critical magnetic Reynolds number above which the dynamo sets in depends crucially on the fundamental symmetries of the TG vortex. These symmetries can be broken by introducing a scale separation in the flow, or by letting develop a small non-symmetric perturbation which can be either kinetic and magnetic, or only magnetic. The nature of the boundary conditions for the magnetic field (either conducting or insulating) is essential in selecting the fastest growing mode; implications of these results to a planned laboratory experiment are briefly discussed. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872578

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Unique measurements of the basic plasma-flow characteristics in a low pressure (⩽53 mPa H2) spherically convergent ion focus are obtained using high-voltage (⩽5 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4–5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic∼1015m−3) are consistent with simple flow convergence models. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872110

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electron capture processes by positrons from one- and two-electron atoms in dense plasmas are investigated in accordance with the classical Bohr–Lindhard model using the modified hyperbolic-orbit trajectory method. The interaction potential in dense plasmas has been approximated by the Debye–Hu¨ckel model. The modified hyperbolic-orbit trajectory for the positron path is obtained by the combination of the hyperbolic-orbit and straight-line trajectories with the modified impact parameter. The maximum value of the electron capture probability per electron for two-electron (He-like) atoms is smaller than that for one-electron (H-like) atoms. The maximum position of the electron capture probability is slightly shifted to the target nucleus with an increase of the target charge. The plasma-screening effect on the electron capture probability for He-like atoms is more effective than that for H-like atoms. However, the capture probabilities obtained by the straight-line trajectory for H-like and He-like atoms are identical. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872129

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In dense plasmas, dynamic plasma screening effects are investigated on1s→2pdipole transition probabilities for electron-impact excitation of hydrogenic ions. The electron–ion interaction potential is considered by introduction of the plasma dielectric function. A semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to visualize the semiclassical transition probability as a function of the impact parameter, projectile energy, and plasma parameters. The transition probability including the dynamic plasma screening effect is always greater than that including the static plasma screening effect. When the projectile velocity is smaller than the electron thermal velocity, the dynamic polarization screening effect becomes the static plasma screening effect. When the projectile velocity is greater than the electron thermal velocity, then the interaction potential is almost unshielded. The difference between the dynamic and static plasma screening effects is more significant for low-energy projectiles. It is also found that the static plasma screening formula obtained by the Debye–Hu¨ckel model overestimates the plasma screening effects on the atomic excitation processes in dense plasmas. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872135

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In a large magnetized laboratory plasma(n≃1012cm−3,kTe⩾1eV,B0⩾10G, 1 m×2.5 m), the transient processes of switch-on currents to electrodes are investigated experimentally. The current rise time lies between the ion and electron cyclotron periods (electron magnetohydrodynamics). The initial current scales linearly with applied voltage and isnotlimited by the electron saturation current of the positive electrode, but by the ion saturation current of the return electrode. The collection of electrons in the flux tube of the positive electrode gives rise to a space charge electric field, which expels the unmagnetized ions, erodes the density, and disrupts the current. Repeated current oscillations arise from a feedback between current, density, and potential oscillations. The dependence of the transient and unstable electrode currents on externally variable parameters is investigated in the present paper. A companion paper [Urrutia and Stenzel, Phys. Plasmas4, 36 (1997)] presentsin situmeasurements of plasma currents, plasma parameters, and microinstabilities. These results are relevant to the physics of pulsed Langmuir probes, current collection from tethered electrodes in space, and plasma erosion switches. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872109

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In a magnetized laboratory plasma described in the companion paper [Stenzel and Urrutia, Phys. Plasmas4, 26 (1997)], a large positive voltage step(V≫kTe/e)is applied to electrodes. The current front propagates in the whistler mode in the parameter regime of electron magnetohydrodynamics. The topology of the current density is that of nested helices. Large transient currents in excess of the electron saturation current can be drawn. A transient radial electric field associated with the current rise, excites a compressional, large amplitude, radially outgoing sound wave, which leaves the current channel depleted of plasma. The current collapses due to the density erosion. Electric field reversal excites a rarefaction wave which leads to a partial density and current recovery. Periodic plasma inflow and outflow cause the current to undergo strong relaxation oscillations at a frequency determined by the electrode diameter and the sound speed. In addition, a broad spectrum of microinstabilities is observed in regions of high current density. For drift velocities approaching the thermal speed, the spectrum extends beyond the ion plasma frequency(&ohgr;pi)up to the electron plasma frequency(&ohgr;pe).Correlation measurements above&ohgr;pireveal modes propagating along the electron drift at speeds above the sound speed but well below the electron drift speed. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872492

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Computations of damped diocotron oscillations (quasi-modes) are described for non-neutral plasmas and inviscid fluids. The numerical method implements a suggestion made by Briggs, Daugherty, and Levy some 25 years ago [Phys. Fluids13, 421 (1970)] to push the branch line that forms the continuum into the complex&ohgr;-plane by solving the mode equation in the complexr-plane. For the special case of power-law density profiles the calculation finds the same quasi-mode frequencies found recently by Corngold [Phys. Plasmas2, 620 (1995)]. It is found that the feature of the continuum eigenfunctions which indicates the presence of a nearby quasi-mode is continuity of the derivative of the regular part of the eigenfunctions near the singularity. The evolution of Rayleigh modes, found in density profiles with steps, is also studied as the density steps are smoothed. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872497

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Observations are carried out in a double-plasma device where the instabilities are excited around the negatively biased grid when there is sufficient plasma density difference between the two chambers and an ion beam is injected from the source to the target section. If the density difference between the two chambers is slowly decreased, the instabilities exhibit oppositely different characteristics after a critical value. Below the critical value of the density ratio, the energy resonance of the background ions and the beam ions is a typical condition for the excitation of such instabilities. Chaotic phenomena occur when there is a hump in the plasma potential profile near the sheath edge. Above the critical value of the density ratio, the instability occurs due to the interaction of the three beams that arise due to asymmetry of the sheath potential. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872111

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Debye screening potential and wake potential for a moving dust grain in a collisionless plasma with ion flow is studied. When a relative velocity of the dust grain exceeds the ion acoustic velocity, the oscillatory wake potential is formed in a circular cone behind the particle and produces potential minima in a periodic manner. The ion acoustic collective effects on dust particles contribute to the formation of the periodic structure. The characteristic spacing between the potential minima are several times of Debye wavelength in height and in radius. Such a periodic structure may be relevant to the formation of Coulomb quasilattices (plasma crystals) observed in the dusty plasma laboratory experiments. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872112

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Linear dissipative magnetohydrodynamics (MHD) shows that driven MHD waves in magnetic plasmas with high Reynolds number exhibit a near resonant behaviour if the frequency of the wave becomes equal to the local Alfve´n or slow frequency of a magnetic surface. This near resonant behaviour is confined to a thin dissipative layer which embraces the resonant magnetic surface. Although the driven MHD waves have small amplitudes far away from the resonant magnetic surface, this near-resonant behaviour in the dissipative layer may cause a breakdown of linear theory. In the present paper we deal with the nonlinear behaviour of driven MHD waves in the slow wave dissipative layer. The method of matched asymptotic expansions is used to obtain the nonlinear equation for wave variables inside the dissipative layer. The concept of connection formulae introduced into the theory of linear resonant MHD waves by Sakurai, Goossens, and Hollweg [Sol. Phys.133, 227 (1991)] is extended to include nonlinear effects in the dissipative layer for slow resonant waves. The absorption of the slow resonant wave in the dissipative layer generates a shear flow parallel to the magnetic surfaces with a characteristic velocity of the order of&egr;1/2,where &egr; is the dimensionless amplitude of perturbations far away from the dissipative layer. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872113

出版商:AIP    

年代:1997

数据来源: AIP