摘要:

The stability of a stratified shear layer is studied using the compressible magnetohydrodynamic (MHD) equations, including an effective gravity term to represent curvature effects of the flow and magnetic field line geometry. A general eigenvalue equation is derived for a two‐dimensional MHD fluid. For the case of a hyperbolic tangent shear flow and exponential density profile, it is found that in the Boussinesq approximation the compressibility raises the critical Richardson number from (1)/(4) to as much as (1)/(2) , with the exact value depending on the value of the magnetic field at infinity. Under the approximation of a strong asymptotic magnetic field, but without invoking the Boussinesq approximation, it is shown both analytically and numerically that the density gradient terms cause the shear instability to be dispersive. In particular, the long‐wavelength stability boundary for the case of Richardson numberJ=0 is characterized by a normalized phase velocityc=−1. This result implies that the solution for the stability boundary found by Satyanarayana, Lee, and Huba [Phys. Fluids30, 81 (1987)], which required thatc=−&bgr;/2 (where &bgr; is the ratio of the velocity and density gradient scale lengths), is not valid, in general.

ISSN:0899-8221    

DOI:10.1063/1.858906

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

If plasma leaks through a negatively biased grid from a positively biased source chamber into an outer region where its potential is free to oscillate, the potential can go into a large‐amplitude spontaneous oscillation at a frequency set by the ion transit time through the sheath. This is accompanied by density fluctuations that free stream away from the grid, spatially damped. The frequency and temporal growth rate are investigated under various plasma conditions.

ISSN:0899-8221    

DOI:10.1063/1.858907

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

At small amplitudes, resistive ballooning modes form a quasilinear sheet current, which opposes the magnetic flux reconnection. In the ensuing Rutherford regime [Phys. Fluids16, 1903 (1973)] the sheet current at each resonance surface decays. To remain in a quasistatic equilibrium, magnetic islands form and thereby flatten the pressure at each resonance surface. For local instabilities, a spectrum of modes are excited, each flattening the pressure gradient at its resonance surface. The resulting averaged quasilinear pressure profile has a Cantor setlike structure. Using the equivalence of harmonics, the calculation of the saturated amplitudes is reformulated as a nonlinear eigenvalue problem of an integral differential equation with the quasimode formalism.

ISSN:0899-8221    

DOI:10.1063/1.858908

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The effect of including the electron temperature gradient and higher‐order terms in the expansion of the density profile in the equations for nonlinear drift waves in plasmas is studied, as well as the effect of expanding the equation for nonlinear Rossby waves to higher orders in the amplitude. The two cases are very similar, and earlier studies have concluded that these effects would make a new kind of stationary monopole vortex solution possible. This solution would have a unique functional form in the whole plane, and for some parameter values no closed streamlines. It is shown that this conclusion is correct, and that such solutions do not exist. It is also pointed out that these additional effects become particularly important in the region of very long wavelengths, where they give rise to wave breaking and a transfer of energy toward shorter wavelengths. This can prevent the condensation to zonal flows that has been predicted earlier.

ISSN:0899-8221    

DOI:10.1063/1.858909

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The analytic theory of weak Langmuir turbulence is well known, but very little has previously been done to compare its predictions with numerical solutions of the basic dynamical evolution equations. In this paper, numerical solutions of the statistical weak turbulence theory are compared with numerical solutions of the Zakharov model of Langmuir turbulence, and good agreement in certain regimes of very weak field strength is found.

ISSN:0899-8221    

DOI:10.1063/1.858910

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

It is shown that second‐order (&rgr;/L) corrections in the expression for the equilibrium distribution function are required to obtain a self‐adjoint system of equations for ion cyclotron range of frequencies (ICRF) wave propagation in a nonuniform plasma (&rgr; denotes the ion‐Larmor radius andLthe gradient scale length). Self‐adjointness results in energy conservation in the absence of absorption mechanisms, as expected. A Lagrangian density is obtained, which yields both the wave operator and its adjoint, and directly results in the conservation law of the truncated system.

ISSN:0899-8221    

DOI:10.1063/1.858911

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A recent stability calculation solved the dispersion matrix to find three distinct unstable modes in three distinct regions of the field‐reversed configuration (FRC). In the present work the transport produced by these three low‐frequency modes is calculated, and both global transport scaling estimates and local transport coefficients are obtained. The result is that low‐frequency drift wave transport agrees with most present experimental measurements of particle, flux, and energy loss both in terms of magnitude and scaling, becoming less dominant as the collision frequency decreases.

ISSN:0899-8221    

DOI:10.1063/1.858912

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

It is pointed out that the turbulent tokamak edge can significantly affect the transmission of a wave in the ion cyclotron range of frequencies during auxiliary heating at the resonant layer. Because of the large edge density fluctuations, it is necessary to consider an order unity expansion parameter for the general propagator theory of wave propagation. Consequently, a renormalized theory for the scattering process is developed and utilized to determine the backscattering (one dimensional) and spectral wavenumber spread (two dimensional) of incident energy.

ISSN:0899-8221    

DOI:10.1063/1.858913

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Several classes of analytic equilibria of a toroidal axisymmetric plasma with toroidal mass flow are obtained for isothermal as well as isentropic magnetic surfaces under various pressure and toroidal current density profiles. The effects of flow on certain global equilibrium and stability characteristics are examined. It is shown that for a wide family of equilibria the average beta ⟨ &bgr;⟩ does not depend explicitly on flow and falls within the limits of static equilibria. In particular, for the case of field‐reversed configurations ⟨ &bgr;⟩ is totally independent of plasma flow. Calculations based on a specific equilibrium solution for a tokamak with fixed average toroidal magnetic energy show that plasma elongation is an increasing function of the Mach number whereas the safety factor is a decreasing function of it.

ISSN:0899-8221    

DOI:10.1063/1.858914

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The radiative collapse of a Z pinch in hydrogen or helium is examined from a practical viewpoint. The maximum density that may be achieved is calculated by modifying the Pease–Braginskii current to include the effect of opacity and by incorporating the pinch into a realistic circuit. It is found that the maximum density is increased by the addition of series inductance inside the vacuum chamber. In most cases the density limit is determined by the pinch becoming optically thick, although for low line density and large series inductance the limit may be set by the onset of electron degeneracy. Pinches in hydrogen lead to denser collapsed states than pinches in helium.

ISSN:0899-8221    

DOI:10.1063/1.858915

出版商:AIP    

年代:1989

数据来源: AIP