摘要:

The stability of a stratified shear layer is investigated using an exponential density profile and a laminar shear flow with a continuous velocity distribution. It is shown that an exact stability boundary can be obtained for an inhomogeneous inviscid fluid under the action of gravity without the need to impose the Boussinesq approximation. The stability boundary is given byJ=kˆ2(1−&bgr;2/4−kˆ2), where &bgr; is the ratio of the velocity and density gradient scale sizes,Jis the Richardson number, andkˆ is the perpendicular wavenumber normalized to the velocity gradient scale size; this reduces to the stability boundary derived by Drazin [J. Fluid Mech.4, 214 (1958)] in the limit &bgr;=0. The solution allows forc=&bgr;/2, wherecis the normalized phase velocity.

ISSN:0031-9171    

DOI:10.1063/1.866063

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The linear magnetohydrodynamic stability of plasmas with sub‐Alfve´nic flows aligned parallel to the equilibrium magnetic field is treated. Subject to the constraints that the fluid is incompressible, inviscid, and perfectly conducting, it is demonstrated that stabilizing effects appear as the flow speed approaches the Alfve´n speed. The screw pinch plasma is treated explicitly, but it is conjectured that the conclusions are applicable to other equilibrium configurations.

ISSN:0031-9171    

DOI:10.1063/1.866064

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An analytical theory of the nonlinear stability of drift‐tearing modes in the semicollisional regime is presented. It is shown that the overall stability depends critically on the initial island width and the plasma beta. Linear and quasilinear treatments demonstrate that islands smaller than the linear tearing layer are stabilized completely by the influence of the electron temperature gradient. A nonlinear theory is developed for larger islands. Standard algebraic growth is obtained unless diamagnetic effects are strong, in which case the time‐asymptotic stability is dependent on the density and temperature profiles.

ISSN:0031-9171    

DOI:10.1063/1.866065

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The influence of a weak normal component of the magnetic fieldBnon the resistive tearing mode in a sheet pinch is examined. The dispersion relation in the high conductivity limit is derived in closed analytical form and it is shown that the normal magnetic field exerts a stabilizing influence. For values ofBnscaling like &eegr;2/5, where &eegr; is the resistivity, the results obtained relate to recent numerical calculations of Nishikawa and Sakai [Phys. Fluids25, 1384 (1982)] for finiteBn. In the very weak ordering withBnscaling like &eegr;4/5, the previous analytical result of Janicke [Phys. Fluids23, 1843 (1980)] is recovered.

ISSN:0031-9171    

DOI:10.1063/1.866175

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A one‐dimensional electromagnetic particle‐in‐cell (PIC) model is used to investigate the Alfve´n ion‐cyclotron (AIC) instability in high‐&bgr; plasmas with large ion temperature anisotropies. The simulation results are compared with linear and quasilinear theory. Nonlinear effects, such as the rate at which the anisotropy relaxes and the level of perturbed magnetic wave energy at saturation, are compared as a function of &bgr; and of the initial ion anisotropy. It is found that for large &bgr;⊥(&bgr;⊥>3) the anisotropy decays to a residual level in a time comparable to an ion‐cyclotron period. Simulations designed to drive the instability by replenishing particles from an anisotropic temperature reservoir are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.866176

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Propagation of extraordinary mode waves in nearly parallel stratified plasmas (magnetic beach geometry) is investigated. Since these waves are very heavily damped WKB theory is unreliable and kinetic effects make the problem inherently nonlocal. The resonance region is treated by a boundary layer expansion which reduces the problem to an integrodifferential system in one dimension. It is proved analytically that for moderate to high density, waves incident from the high field side are totally absorbed with no reflected wave. At very low density some transmission is possible, where the transmission coefficients are being given correctly by cold plasma theory. Numerical solution of the integrodifferential system shows that the power deposition profile can differ significantly from that predicted from a local WKB theory.

ISSN:0031-9171    

DOI:10.1063/1.866177

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The correlation integral has been studied in detail for broadband magnetic and density fluctuations observed in the TCA tokamak [Proceedingsofthe11thSymposiumonFusionTechnology(CEC, Luxembourg, 1981), Vol. I, p. 601]. These studies, which provide information on the correlation dimension &ngr;, have shown no evidence for a low‐dimensional attractor. It has therefore been concluded that the broadband fluctuations are associated either with a high‐dimensional system (&ngr;>10), or with a nonstationary state for which the concept of the correlation dimension has no meaning.

ISSN:0031-9171    

DOI:10.1063/1.866178

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The thermal and particle diffusivities driven by resistive fluid turbulence in diverted tokamak edge plasmas are calculated. Diverted tokamak geometry is characterized by increased global shear near the separatrix and the tendency of field lines to linger near the x point. For resistive fluid turbulence, the dominant effect is increased global shear, which causes a reduction in the effective step size of the turbulent diffusion process and corresponding improvements in heat and particle confinement close to the separatrix. Stability of resistive kink modes resonant near separatrix is also ensured by the increased global shear. The relevance of these considerations to the L→H transition and to the edge transport barrier in H‐mode plasmas is discussed.

ISSN:0031-9171    

DOI:10.1063/1.866179

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A strong modification of plasma potential was observed at the plasma surface with the application of a helical field produced by a set of local helical coils. The structure of magnetic field lines was computed by a field line tracing code for a tokamak plasma. Such a field structure was examined in the experiment using electron beam probing. The potential formation and the magnetic structure are discussed referring to the scheme of an ergodic magnetic limiter.

ISSN:0031-9171    

DOI:10.1063/1.866180

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A cylindrical plasma model is used to study the stabilizing effect of electromagnetic ion cyclotron frequency range (ICRF) waves on them=1 magnetohydrodynamic interchange mode. The fast wave eigenmodes of the column and the near‐field antenna pattern are calculated numerically for a diffuse plasma profile when &ohgr;>&OHgr;i. The resulting ponderomotive force and sideband contributions to global interchange stability are then determined using a rigid shift trial function. For far‐field stabilization it is verified that the direct ponderomotive and sideband contributions cancel exactly as the conducting wall supporting the fast wave eigenmode moves out to infinity. The near‐field stabilization effect is related numerically to the drivenk∥spectrum of the waves and their radial profiles. The numerical model is employed to calculate threshold ICRF wave amplitudes for the stabilization experiments in the Phaedrus tandem mirror [Phys. Rev. Lett.51, 1955 (1983)].

ISSN:0031-9171    

DOI:10.1063/1.866163

出版商:AIP    

年代:1987

数据来源: AIP