摘要:

The problem of a two‐component, collisionless plasma expansion into vacuum is investigated from the viewpoint of the Vlasov–Poisson model. The set of equations is treated both analytically (through the rescaling transformations) and numerically, using a one‐dimensional Eulerian code. In planar geometry, the rescaling allows to conjecture the existence of a self‐similar expansion over long times. Numerical results subsequently confirm the conjecture and show that the plasma becomes neutral over a smaller and smaller scale. A few thermodynamical properties are studied: the temperature is shown to decrease ast−2; the polytropic relation (d/dt)(pn−&ggr;)=0 (with &ggr;=3) is verified asymptotically via a semianalytical argument. Finally, the same problem is studied in a spherical one‐dimensional geometry. The time‐asymptotic solution is again self‐similar. Numerical simulations show that a non‐neutral, multiple‐layer structure appears, which is proved to be stable over long times.

ISSN:0899-8221    

DOI:10.1063/1.860524

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The existence of self‐sustained toroidal &eegr;i‐mode turbulence below the instability threshold has been shown by mode coupling simulations. Significant transport and a tendency for long‐wavelength condensation are observed. The subcritical behavior is explained as a result of nonlinear interaction between modes of positive and negative energy by analytical calculation of the coupling factors.

ISSN:0899-8221    

DOI:10.1063/1.860525

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A simple method for obtaining higher‐order solutions of a solitary wave is presented. The method is based on the higher‐order approximation of the reductive perturbation method. As a model equation, the Korteweg–de Vries (KdV) equation with fifth‐order dispersion term is employed and higher‐order solutions are obtained. In the case of ion‐acoustic solitary wave, the fourth‐order solution is given explicitly and is compared with numerical solutions. The fourth‐order solution is shown to be valid if a normalized wave amplitude is less than 0.5.

ISSN:0899-8221    

DOI:10.1063/1.860526

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Results on the nonlinear behavior of the Rayleigh–Taylor instability and consequent development of shear flow by the shear instability [Phys. Fluids B4, 488 (1992)] are presented. It is found that the shear flow is generated at sufficient amplitude to reduce greatly the convective transport. For high viscosity, the time‐asymptotic state consists of an equilibrium with shear flow and vortex flow (with islands, or ‘‘cat’s eyes’’), or a relaxation oscillation involving an interplay between the shear instability and the Rayleigh–Taylor instability in the presence of shear. For low viscosity, the dominant feature is a high‐frequency nonlinear standing wave consisting of convective vortices localized near the top and bottom boundaries. The localization of these vortices is due to the smaller shear near the boundary regions. The convective transport is largest around these convective vortices near the boundary and there is a region of good confinement near the center. The possible relevance of this behavior to the H mode and edge‐localized modes (ELM’s) in the tokamak edge region is discussed.

ISSN:0899-8221    

DOI:10.1063/1.860527

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Using the electron drift‐kinetic equation and a hydrodynamic description for the ions new nonlinear vortex equations are derived taking into account the parallel trapping of the electrons in the positive potential regions. It is shown that the usual integration procedure for finding the coherent vortex structures for theE×Bflows in the fluid description can be generalized to include the parallel accelerationeE∥∂f/∂v∥producing the electron holes in the phase space. An example is considered in some detail.  

ISSN:0899-8221    

DOI:10.1063/1.860528

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Propagation of ion‐acoustic solitary waves in a warm plasma with negative ions under the drifting effect of electrons is considered. Compressive and rarefactive solitons are shown to exist depending on the drift velocityv’eof electrons for different values ofQ’(mj/mi—negative to positive ion mass ratio) ≥1 or ≤1. The amplitude of the compressive soliton forQ’≥1 and rarefactive soliton forQ’≤1 but small, is found to be greatest nearv’e= 0 whenr(=nj0/ni0, negative to positive ion number density ratio) ≤0.5, showing thereby the characteristic change in solitons for the inclusion ofv’e.

ISSN:0899-8221    

DOI:10.1063/1.860529

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electron cyclotron emission can be a sensitive indicator of nonthermal electron distributions. A new, comprehensive ray‐tracing and cyclotron emission code that is aimed at predicting and interpreting the cyclotron emission from tokamak plasmas is described. The radiation transfer equation is solved along Wentzel–Kramers–Brillouin (WKB) rays using a fully relativistic calculation of the emission and absorption from electron distributions that are gyrotropic and toroidally symmetric, but may be otherwise arbitrary functions of the constants of motion. Using a radial array of electron distributions obtained from a bounce‐averaged Fokker–Planck code modeling dc electron field and electron cyclotron heating effects, the cyclotron emission spectra are obtained. A pronounced strong nonthermal cyclotron emission feature that occurs at frequencies relativistically downshifted to second harmonic cyclotron frequencies outside the tokamak is calculated, in agreement with experimental results from the DIII‐D [J. L. Luxon and L. G. Davies, Fusion Technol.8, 441 (1985)] and FT‐1 [D. G. Bulyginskyetal., inProceedingsofthe15thEuropeanConferenceonControlledFusionandPlasmaHeating, Dubrovnik, 1988 (European Physical Society, Petit‐Lancy, 1988), Vol. 12B, Part II, p. 823] tokamaks. The calculations indicate the presence of a strong loss mechanism that operates on electrons in the 100–150 keV energy range.

ISSN:0899-8221    

DOI:10.1063/1.860530

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Self‐similar solutions are given that represent an analytical theory of implosion and extension stages of a &THgr; pinch before the magnetic field lines reconnection in course of formation of field‐reversed configuration. Effects of Ohmic dissipation, thermal conductivity, and plasma turbulence are included. The self‐similar solutions, obtained in an explicit analytical form, demonstrate that magnetic flux is trapped during the implosion and expansion stages for a classical plasma, and that losses of magnetic flux are possible for a turbulent plasma during the expansion stage. The rate of flux trapping and diffusion is expressed in terms of experimental parameters.  

ISSN:0899-8221    

DOI:10.1063/1.860531

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Various kinds of movable limiters are inserted into the plasma and effects on plasma performance are studied in the REPUTE‐1 reversed‐field pinch (RFP) device [Plasma Phys. Controlled Fusion28, 805 (1986)]. The increases in one‐turn loop voltageVland magnetic fluctuations with nearly constant value of ion temperature are found with an advance of movable limiters, and drastic increases inVlare observed when the limiter is beyond a certain position. ThisVlincrease is discussed comparing with theories. The first pump limiter experiments in RFP are tried. Favorable effects such as reductions ofVland impurity intensities are observed when the limiter head is inserted by ∼2 cm from the plasma edge.

ISSN:0899-8221    

DOI:10.1063/1.860532

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Radial particle transport is considered in a plasma with ergodic and stochastic flux lines. The self‐consistent electric field is neglected and the particle flux is due to a succession of mass convections along flux lines, in series with very small‐scale‐length transverse diffusion between neighboring flux lines. The dependence of the average radial flux on the thermodynamic forces, namely the pressure gradient that controls the parallel convection, and the density gradient that controls the small‐scale transverse diffusion is investigated. The conditions for which the average radial flux is driven by the average pressure or density gradient are derived.

ISSN:0899-8221    

DOI:10.1063/1.860533

出版商:AIP    

年代:1993

数据来源: AIP