摘要:

In a spherical tokamak, the 1/Rvariation of the toroidal field is extreme, and for a given value of the safety factor a relatively low average toroidal field can be used, together with large plasma current and large plasma minor radius and elongation. The poloidal and toroidal fields are then of similar size. In consequence, the orbits of fast ions depart considerably from the guiding center orbits because of gyromotion in the small magnetic fields on the low‐field side. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871741

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A kinetic theory for strongly coupled dusty plasmas is developed from the Bogolyubov–Born–Green–Kirkwood–Yvon (BBGKY) hierarchy. In these plasmas, interparticle distances are typically much smaller than the Debye length, and the coupling parameter &Ggr;d≡q2d/ddTdis much larger than unity.(Hereqdis the dust charge,Tdis the temperature, anddd≡[3/(4&pgr;nd)]1/3is the inter‐dust‐particle distance.)For such quasi‐steady‐state plasmas, a theoretical argument is given for the validity of the Vlasov equation in the regimes &Ggr;d≪1 and &Ggr;c≫&Ggr;d≫1, where &Ggr;cdesignates the coupling parameter when Coulomb crystallization occurs. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871742

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Measurements of basic plasma parameters in a low‐density single‐ended Q machine with a negatively biased cold plate are compared with pertinent predictions of time‐independent collisionless plane‐diode theory. As was also found in particle simulations [Phys. Fluids B3, 244 (1991)] and previous experiments, theory and experiment agree well if space‐charge conditions near the hot‐plate surface are electron rich, in which case the potential distributions are monotonically decreasing. For ion‐rich conditions, however, where the potential distributions exhibit a single maximum located in front of the hot plate, the plasma properties are found to depend very sensitively on the amount and distribution of the electrons trapped in the region of positive potential. In this regime, the experimental findings agree very well with a theoretical model in which a certain amount of trapped electrons is assumed. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871743

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of negative ions on the collisionless electrostatic ion cyclotron instability (EICI) is investigated analytically and numerically. The singly ionized negative ion species has mass greater than or equal to the mass of the singly ionized positive ion species. Standard linear Vlasov theory is used to estimate the critical electron drift velocity. It is shown that the critical drifts for the excitation of both the light and heavy ion EIC modes decrease as the relative density of negative ions increases. Our results are compared with available negative ion plasma experiments concerning the EICI, and possible applications to experiments in the ionosphere are briefly discussed. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871744

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Images of a cloud of grains in a dusty plasma reveal a pair of very low‐frequency modes, termed here the filamentary and great void modes. The plasma was a radio‐frequency discharge formed between parallel‐plate graphite electrodes. A cloud of 100 nm carbon particles was produced by accretion of carbon atoms produced by sputtering the graphite. The cloud was illuminated with a laser sheet and imaged with a video camera. The great void mode was a spoke‐shaped region of the cloud that was free of dust and rotated azimuthally in the discharge. The filamentary mode had the appearance of turbulent striations, with a smaller amplitude than the great void. The filamentary mode sometimes appeared as a distinctive vortex, curling in the poloidal direction. Both modes had a very low frequency, on the order of 10 Hz. Two possible causes of the modes are discussed. The low phase velocity of the modes may be consistent with a dust‐acoustic wave. Alternatively, the great void may be an ionization wave that moved the dust about, since a modulation in the glow was seen moving at the same speed as the void. It is argued that existing theories of waves in dusty plasmas assume weakly collisional plasmas, which may be unsuitable for explaining experimental results in laboratory dusty plasmas, since they are often strongly coupled. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871745

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this paper we present a theoretical framework for the Magnetic Reconnection Experiment (MRX) [M. Yamadaetal., Bull. Am. Phys. Soc.40, 1877 (1995)] in order to understand the basic physics of the experiment, including the effect of the external driving force, and the difference between co‐ and counterhelicity cases of the experiment. The problem is reduced to a one‐dimensional (1‐D) resistive magnetohydrodynamic (MHD) model. A special class ofholonomicboundary conditions is defined, under which a unique sequence of global equilibria can be obtained, independent of the rate of reconnection. This enables one to break the whole problem into two parts: a global problem for the ideal region, and a local problem for the resistive reconnection layer. The calculations are then carried out and the global solution for the ideal region is obtained in one particular case of holonomic constraints, the so called ‘‘constant force’’ regime, for both the co‐ and counterhelicity cases. After the sequence of equilibria in the ideal region is found, the problem of the rate of reconnection in the resistive reconnection region is considered. This rate tells how fast the plasma proceeds through the sequence of global equilibria but does not affect the sequence itself. Based on a modified Sweet–Parker model for the reconnection layer, the reconnection rate is calculated, and the difference between the co‐ and counterhelicity cases, as well as the role of the external forces is demonstrated. The results from the present analysis are qualitatively consistent with the experimental data, predicting faster reconnection rate for the counterhelicity merging and yielding a positive correlation with external forcing. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871746

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The quasi‐thermal electrostatic field fluctuations of a stable unmagnetized electron beam‐plasma system is considered. Both the beam and the background plasma are modeled as isotropic Maxwellians. Both the normal modes of the plasma and the contributions from other modes are considered. The dependence of these fluctuations on beam velocity, density, and temperature are examined. The implications of these waves for real beam‐plasma systems are also discussed. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871747

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Evolution equations for the coupled relaxation of densities and temperatures for the components in nonideal partially ionized plasmas are given. In these equations many‐body effects, such as screening, self‐energy, and lowering of the binding energy, are included. The coupled equations are solved numerically for a hydrogen plasma consisting of electrons, protons, and atoms. Impact ionization, three‐body recombination, and elastic processes are taken into account. Thermal relaxation times are determined and the results are compared with those from the literature. The influence of many‐body effects on the evolution process are discussed. In some cases, a significantly increased lifetime of the two‐temperature regime is found. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871748

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Plasma temperatures in the range 25 to 2×106K have been measured using a cryogenic, ultra‐high vacuum, pure‐electron plasma trap. The rate &ngr; at which the temperatures parallel and perpendicular to the applied magnetic field relax to a common value has been measured over the temperature range 28 to 3.8×105K and the magnetic field range 20 to 60 kG. This rate &ngr; is closely related to the plasma collision frequency. When the cyclotron radiusrcis large compared to the classical distance of closest approachb(rc/b≫1), the measured values of &ngr; are in agreement with conventional collision theory. When the cyclotron radius is small compared to the classical distance of closest approach (rc/b≪1), &ngr; drops precipitously asrc/bis decreased, in agreement with the many‐electron adiabatic invariant theory of O’Neil and Hjorth. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871749

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The nonlinear evolution of the Kelvin–Helmholtz instability in a narrow stratified velocity‐shear layer has been computed using numerical and analytical techniques. It is found that the Kelvin–Helmholtz instability develops, in the nonlinear regime, in a distinctly different manner in configuration andkspace when finite temperature effects are included in the fluid approximation. Vortex generation is inhibited with the vortices becoming striated and anisotropic. In addition, large density and electric field fluctuations are produced in the nonlinear regime. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871750

出版商:AIP    

年代:1996

数据来源: AIP