摘要:

The nonlinear behavior of the two‐dimensional Be´nard problem with periodic boundary conditions in the horizontal direction is studied with particular emphasis on the role of self‐consistent chaotic advection. The results show a complex interplay between vortices driven by the Be´nard (Rayleigh–Taylor) instability and shear flow, which is driven by the vortices [J. Drakeetal., Phys. Fluids B4, 4881 (1992)] and which causes their decay. Chaotic advection occurs in the transition from the low Rayleigh number (Ra) regime to the high Ra regime [J. Finn, Phys. Fluids B5, 415 (1993)]. For the former, vortex flow and shear flow coexist, possibly with slow relaxation oscillations. In the high Ra regime there are vortices localized near the upper and lower boundaries with a shear flow in between. As Ra is decreased from the high Ra regime, these vortices broaden, eventually overlapping, causing self‐consistent Lagrangian chaos. This onset of chaos is responsible for several properties of the transition state between the low Ra and the high Ra regimes, most notably the damping of the relaxation oscillations involving vortex and shear flow. It is also observed that the Nusselt number Nu has a peak with respect to Ra in this transition regime characterized by Lagrangian chaos. In the low Ra regime, on the other hand, the relaxation oscillations are on a much slower time scale than the eddy turnover time and the Lagrangian behavior is described by separatrix crossing.

ISSN:0899-8221    

DOI:10.1063/1.860613

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The theory of Onsager symmetry is reconsidered from the point of view of its application to nonequilibrium, possibly turbulent steady states. A dynamical formalism based on correlation and response functions is used; understanding of its relationship to more conventional approaches based on entropy production enables one to resolve various confusions about the proper use of the theory, even near thermal equilibrium. Previous claims that ‘‘kinematic’’ flows must be excluded from considerations of Onsager symmetry are refuted by showing that suitably defined reversible and irreversible parts of the Onsager matrix separately obey the appropriate symmetry; fluctuating hydrodynamics serves as an example. It is shown that Onsager symmetries are preserved under arbitrary covariant changes of variables; the Weinhold metric is used as a fundamental tensor. Covariance is used to render moot the controversy over the proper choice of fluxes and forces in neoclassical plasma transport theory. The fundamental distinction between the fully contravariant Onsager matrixLijand its mixed representationLijis emphasized and used to explain why some previous workers have failed to find Onsager symmetry around turbulent steady states. The generalized Onsager theorem of Dufty and Rubi´ [Phys. Rev. A36, 222 (1987)] is reviewed. An explicitly soluble Langevin problem is shown to violate Onsager’s original symmetry but to obey the generalized theorem. The physical content of the generalized Onsager symmetry is discussed from the point of view of Nose´–Hoover dynamics. A set of extended Graham–Haken potential conditions are derived for Fokker–Planck models and shown to be consistent with the generalized Onsager relations. Finally, for quite general, possibly turbulent steady states it is argued that realizable Markovian statistical closures with underlying Langevin representations must also obey the generalized theorem. In the special case in which all state variables have even parity and there are no external parameters that change sign under time reversal, the steady‐state energy balance fully determines the Onsager matrix, which is guaranteed to be symmetric.

ISSN:0899-8221    

DOI:10.1063/1.860614

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The purpose of this paper is to formulate the transport problem for a multispecies rotating toroidal magnetoplasma in the so‐called runaway regime, which is defined by an appropriate ordering of relevant characteristic frequencies, in particular, the Larmor frequency, the characteristic acceleration frequency due to the applied electric field and the effective collision frequency, all evaluated at some characteristic speedv0. A suitable form of the gyrokinetic equation is obtained to describe the time‐dependent, multispecies plasma response to an applied electric field, in toroidal geometry and for a strongly rotating, quiescent, and collisional plasma. Its moment equations are proven to imply the reduction of the energy equation to Joule’s law, as well as consequences on the form of Ohm’s law and of the Grad–Shafranov equation. To construct an approximate solution of the gyrokinetic equation and to evaluate all relevant fluxes, appearing in the moment equations, a general variational solution method is developed.

ISSN:0899-8221    

DOI:10.1063/1.860615

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Application of a short current pulse on a nominal Ohmic discharge in the STOR‐M tokamak (Saskatchewan Torus‐Modified) [Phys. Fluids B4, 3277 (1992)] triggers the Ohmic H‐mode characterized by reduced H&agr;radiation, increased electron density, and reduced edge density/magnetic fluctuations. Measurements of plasma floating potential at the plasma edge and in the scrape‐off layer indicate that the Ohmic H‐mode is accompanied by negative plasma autobiasing, which leads to a steeper radial electric field profile at the edge. Since the duration of the current pulse (≤20 kA, 100 &mgr;sec) is shorter than the resistive skin time (&bartil;1 msec), preferential edge heating is expected, which is believed to be responsible for changes in the edge discharge condition favorable for inducing the Ohmic H‐mode. The electron density profile becomes steeper at the edge during the H‐mode, and clear formation of a density pedestal has been seen. The evolution of the density profile suggests the presence of particle pinch. An improved confinement phase (ICP) is induced by external negative electrode biasing. The ICP reveals some similarities as compared to the current pulse induced H‐mode. It is found that the electrostatic modes are dominant in the scrape‐off layer while electromagnetic modes dominate in the plasma edge during the normal Ohmic discharges.

ISSN:0899-8221    

DOI:10.1063/1.860616

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The theory of toroidicity‐induced Alfve´n eigenmodes (TAE) and kinetic TAE (KTAE) is generalized to arbitrary mode numbers for a large aspect ratio low‐beta circular tokamak. The interaction between nearest neighbors is described by a three‐term recursion relation that combines elements from an outer region, described by the ideal magnetohydrodynamic equations of a cylinder, and an inner region, which includes the toroidicity and the nonideal effects of finite ion Larmor radius, electron inertia, and collisions. By the use of quadratic forms, it is proven that the roots of the recursion relation are stable and it is shown how perturbation theory can be applied to include frequency shifts due to other kinetic effects. Analytic forms are derived which display the competition between the resistive and radiative damping, where the radiation is carried by kinetic Alfve´n waves. When the nonideal parameter is small, the KTAE modes appear in pairs. When this parameter is large, previously found scaling for the single gap case is reproduced analytically.

ISSN:0899-8221    

DOI:10.1063/1.860617

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new scheme of feedback suppression of the radiative condensation instability in the tokamak edge plasma is presented. The physics basis of this scheme is the modulation of the parallel electron thermal flux at the instability frequency via a feedback circuit. The latter consists of a temperature sensor, amplifiers, phase shifters, and a suitable form of insulated segmented poloidal limiter sections used as suppressors. The necessary feedback power is the order of a kilowatt for small tokamaks and a few megawatts for large reactor‐type machines.

ISSN:0899-8221    

DOI:10.1063/1.860618

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The purpose of this work is to determine ion temperature profiles in spheromaks [Nucl. Fusion19, 489 (1979)] for the first time. Knowledge of the ion temperature profile is necessary in the correct calculation of plasma confinement parameters. The work herein details the calculation of ion temperature profiles for the Compact Torus Experiment (CTX) [Nucl. Fusion28, 1555 (1988)] and S‐1 [Phys. Rev. Lett.46, 188 (1981)] spheromaks. Data from single chord Doppler ion temperature measurements in these devices have been analyzed with the aid of a one‐dimensional equilibrium charge state transport code. Using electron temperature and density profiles from Thomson scattering, and estimates for transport rates, a most probable position for the emission of line radiation can be determined and correlated with the measured Doppler ion temperature, thus generating an ion temperature profile. From this ion temperature profile determination, plasma confinement parameters for the small solid flux conserver CTX [Phys. Fluids B2, 1342 (1990)] spheromak can be determined, and confinement parameters for S‐1 can be reevaluated, eliminating the previous confinement calculations assumptionTi(r)=Te(r). The CTX device has a calculated volume averaged beta ⟨&bgr;⟩ ranging from 3% to 8% and a volume averaged energy confinement time ⟨&tgr;E⟩ between 14 and 35 &mgr;sec; while the S‐1 spheromak has ⟨&bgr;⟩ between 15% and 40% and ⟨&tgr;E⟩ ranging from 30 to 70 &mgr;sec.

ISSN:0899-8221    

DOI:10.1063/1.860619

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new model for the structure of the radial electric‐field profile in the edge during the high confinement (H mode) is proposed. Charge separation caused by the difference between electron and ion gyromotion, or more importantly in the tokamak, the banana motion (halo effect) can self‐consistently produce an electric dipole moment that causes the sheared radial electric field. The calculated results based on the model are consistent with DIII‐D [Fusion Technol.8, 441 (1985)] and Tokamak Experiment for Technology Oriented Research (TEXTOR) [J. Nucl. Mater.122,123, 1124 (1984)] experimental results.

ISSN:0899-8221    

DOI:10.1063/1.860620

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Toroidal drift mode features and stability are studied, using a fluid description, for a plasma with two ion species: impurity and main ions. Impurity and main ion temperature gradient (ITG) modes dominate for larger temperature gradients, &eegr;i≳1, while dissipative trapped electron (DTE) and impurity‐induced modes are present also for &eegr;i<1. Simple analytical expressions for the stability thresholds are derived from conditions given by the impurity and main ion fluids.

ISSN:0899-8221    

DOI:10.1063/1.860621

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theoretical description of the spontaneous (Stringer) poloidal spin‐up of tokamak plasmas is given. A set of reduced equations is derived to investigate various aspects of the phenomenon. A simple physical description of the instability mechanism is given. The theory of the instability is extended to show that the spin‐up persists even under conditions where the growth rate is sonic. A nonlinear evolution equation for unstable poloidal rotation is obtained which shows that the instability mechanism loses its efficacy when the poloidal speed exceeds the poloidal sound speed, thus resulting in nonlinear saturation of the rotation at this level.

ISSN:0899-8221    

DOI:10.1063/1.860622

出版商:AIP    

年代:1993

数据来源: AIP