摘要:

The existence of small‐scale magnetic islands having a characteristic width of the order of the skin depth is discussed in terms of nonlinear hydrodynamic equations for a nonuniform plasma in a sheared magnetic field. A resultant electron thermal diffusivity has sensitive dependence on the electron temperature gradient, &eegr;e.

ISSN:0899-8221    

DOI:10.1063/1.860510

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

When the previous H‐mode theory [K. C. Shaing and E. C. Crume, Jr., Phys. Rev. Lett.63, 2369 (1989)] is extended to include a bi‐Maxwellian ion distribution with both cold and hot ion components to model the effects of the non‐Maxwellian distribution, the collisionality of the cold ions &ngr;*icbecomes an irrelevant bifurcation parameter, as it is for the electrode‐induced H mode. Thus, the observation that the L–H transition can occur even when &ngr;*ic≫1 is consistent with the fundamental physics of the ion orbit loss model, including a hot ion component.

ISSN:0899-8221    

DOI:10.1063/1.860511

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Numerical investigations of a warm‐fluid model with an isothermal equation of state for the perpendicular dynamics of an axisymmetric, magnetically confined pure electron plasma predict an exponentially unstable,l=1, diocotron mode for hollow density profiles. The unstable mode can be identified with a stable, nonsmooth mode that exists in cold drift models but which is destabilized by finite temperature effects. The unstable mode has many properties similar to the experimental results reported by Driscoll [Phys. Rev. Lett.64, 645 (1990)].

ISSN:0899-8221    

DOI:10.1063/1.860512

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Results are presented of particle simulations of the time‐dependent behavior of the plasma sheath surrounding an electrode whose potential is abruptly changed from 0 to some negative value. The surrounding plasma is assumed isotropic and collisionless, and the electrode is a sphere or infinite cylinder. Potential steps up to −1000kTe/eare considered. The time histories and frequency spectra of sheath and plasma oscillations initiated by the potential step are studied. The energy in sheath oscillations (frequency <&ohgr;p) escapes by nonlinear coupling to plasma oscillations (frequency &ohgr;p). The time scale for sheath–plasma coupling depends strongly on the ion‐to‐electron mass ratio. Both linear and nonlinear ion acoustic waves are excited by the response to the potential step. Near a small (radius 1&lgr;D) cylindrical electrode, trapped ions are shown to make an important contribution to the sheath ion density. The height of the transient peak in collected current for a cylindrical electrode displays strong dependence on the ion‐to‐electron temperature ratio, for sufficiently large potential steps.

ISSN:0899-8221    

DOI:10.1063/1.860513

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The equilibration of spin temperatureTspinwith kinetic temperatureTis examined in a weakly correlated pure electron plasma in the strongly magnetized limit, where the distance of closest approach is large compared to the Larmor radius. In this limit, the spin precession frequency &OHgr;p=g&OHgr;c/2 is large so the component of spin along the magnetic field is an adiabatic invariant that is broken only by resonant magnetic fluctuations of frequency &OHgr;p. (Here &OHgr;cis the electron cyclotron frequency andg&bartil;2.002.) In this case, the most important spin flip mechanism stems from electron–electron collisions in a spatially inhomogeneous magnetic field. Such collisions cause an exchange of spin and cyclotron quanta, and consequently the conventional many‐electron adiabatic invariant (i.e., the total number of cyclotron quanta) is broken and is replaced by a new adiabatic invariant, equal to the sum of the spin and cyclotron actions. A quantum Boltzmann equation is derived to describe the equilibration ofTspintowardT.

ISSN:0899-8221    

DOI:10.1063/1.860514

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Particle transport across braided magnetic surfaces is studied by using simple field and collision models. Diffusion along and across field lines is taken into account. A general variational principle is established. It provides a useful guide for numerical computations. In a special case an explicit formula for the diffusion coefficient is obtained. Several diffusion regimes are clearly identified, and their domain of validity is determined. A new form of the diffusion coefficient has to be introduced in the long‐mean‐free‐path regime.

ISSN:0899-8221    

DOI:10.1063/1.860961

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

It has been found that ion acoustic solitons tunnel through the space charge sheath in front of the reflector, without time delay, but they are absorbed resonantly when the spatial width of the wave is close to the characteristic gradient scale length of the sheath. The empirical scaling laws for the reflection and transmission rates have been obtained as functions of (i) the bias voltages on the reflector, (ii) the wave width, and (iii) the density gradient scale length. The theoretical model for interpreting the observed phenomena is discussed to show the existence of the electrostatic tunnel effect in the space charge sheath area. In this model, the absorption of wave energy by the sheath, which is strong in the present experiments, was not taken into account.

ISSN:0899-8221    

DOI:10.1063/1.860515

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This paper discusses the interaction between perpendicular magnetohydrodynamic shocks (S) as well as tangential discontinuities (T). The main conclusions are as follows: (1) The collision between a forward shock (S&drarr;) and a reverse shock (S&dlarr;) produces a shock system consisting of a weaker reverse shock, a tangential discontinuity, and a weaker forward shock, namely,S&drarr;S&dlarr;→S&dlarr;TS&drarr;. (2) The merging of two forward shocks leads to a stronger forward shock, followed by a tangential discontinuity and a reverse wave. For a given &bgr;r(the ratio of gas pressure to magnetic pressure) of the background plasma, there exists a critical parameter &ggr;c. When the adiabatic index &ggr;<&ggr;c, the reverse wave is a rarefaction wave (R&dlarr;). When &ggr;≳&ggr;c, the reverse wave produced by merging of strong shocks is still a rarefaction wave but the reverse wave becomes a shock when weak shocks merge. (3) The interaction between a forward shock and a tangential discontinuity may formS&dlarr;TS&drarr;,R&dlarr;TS&drarr;, orTS&drarr;, depending on the strength of the initial forward shock and the property of the initial tangential discontinuity.

ISSN:0899-8221    

DOI:10.1063/1.860516

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The total momentum and energy conservation relations between particle kinetic energy and wave energy is satisfied for the plasma–maser instability. The Manley–Rowe relation for plasma waves is violated and as a result an efficient energy up‐conversion from the low‐frequency mode to the high‐frequency mode is possible even for a normal unreversed electron population in plasma turbulence. The entropy production rate for plasma turbulence with plasma–maser instability is estimated. The plasma–maser instability always coexists with the quasilinear interaction, thus it has a potential importance to interpret numerous experiments in fusion and astrophysical plasmas.

ISSN:0899-8221    

DOI:10.1063/1.860929

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The universal drift instability and other drift instabilities driven by density and temperature gradients in a toroidal system are investigated in both linear and nonlinear regimes via particle simulation. Runs in toroidal and cylindrical geometry show dramatic differences in plasma behavior, primarily due to the toroidicity‐induced coupling of rational surfaces through the poloidal mode numberm. In the toroidal system studied, the eigenmodes are seen to possess (i) an elongated, nearly global radial extent, (ii) a higher growth rate than in the corresponding cylindrical system, (iii) an eigenfrequency nearly constant with radius, and (iv) a global temperature relaxation and enhancement of thermal heat conduction. Most importantly, the measured &khgr;ishows an increase with radius and an absolute value on the order of that observed in experiment. On the basis of the present observations, it is argued that the increase in &khgr;iwith radius observed in experiment is caused by the global nature of heat convection in the presence of toroidicity‐induced mode coupling.

ISSN:0899-8221    

DOI:10.1063/1.860930

出版商:AIP    

年代:1993

数据来源: AIP