摘要:

The Hall effect is studied as a correction to ideal magnetohydrodynamics (MHD) in the context of how it affects the linear stability of the cylindrical pinch in the simple case of a static, homogeneous equilibrium state. The effects of compressibility and electron pressure are included. The presence of the electron pressure gives rise to an electric field tangent to the boundary of the plasma. This introduces an additional boundary condition in the case of a perfectly conducting plasma boundary. Imposing this boundary condition eliminates the wave solutions presented in this paper. With respect to large radial wavenumber, the accumulation point of the slow magnetoacoustic wave frequency spectrum is changed from its finite value in ideal MHD to infinity by the Hall effect. The Hall effect gives rise to linear waves that do not exist in ideal MHD. Specifically, the Hall effect induces azimuthally symmetric, compressible, Alfve´n‐type wave propagation. The frequency spectrum of these waves is discrete and infinite and is a singular perturbation of the incompressible Alfve´n wave spectrum in ideal MHD. These Alfve´n‐type waves do not exist if the plasma is incompressible or ifH≡K⋅B=0, whereKis the wave vector of the perturbation andBis the equilibrium magnetic field.

ISSN:0899-8221    

DOI:10.1063/1.859047

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The relaxation time of potential formation across the magnetic field in a collisionless plasma is experimentally studied. The time variation of the local potential is directly measured with a high time resolution technique when a pulsed potential is applied between magnetized coaxial cylindrical double plasmas. It is found that the relaxation times of the potential and density decrease with the applied voltage and the density of the target plasma, and increase with the magnetic field strength and the density of the driver plasma. These results could be understood consistently by the diffusion of ions in the driver plasma to the target plasma.

ISSN:0899-8221    

DOI:10.1063/1.859048

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Electrostatic ion‐cyclotron (EIC) waves have been investigated in plasmas containing K+positive ions, electrons, and SF−6negative ions. Two EIC wave modes are generally present, the K+and SF−6modes. Their frequencies increase with increasing &egr;, the percentage of negative ions, while the critical electron drift velocities for excitation of either mode decrease with increasing &egr;. The observations are discussed on the basis of available theories.

ISSN:0899-8221    

DOI:10.1063/1.859049

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The evolution of unmagnetized beam‐driven strong Langmuir turbulence is studied in two dimensions by numerically integrating the Zakharov equations for systems pumped by monochromatic and broadband negative‐damping drivers with nonzero central wavenumber. Long‐time statistically steady states are reached for which the dependence of the evolution on the driver wavenumber, growth rate, and bandwidth is examined in detail. For monochromatic drivers, a quasiperiodic cycle is found to develop if the driver wavenumber is sufficiently large. In this cycle, energy from the driven mode undergoes a sequence of weak‐turbulence backscatter decays, which transfer energy to an approximately isotropic long‐wavelength condensate. During this phase, beam‐aligned chains of propagating beat waves develop and perpendicular density waves are also excited. Subsequently, nucleation of waves in density cavities causes a series of wave collapses (involving coherent wave–wave interactions) to occur, during which short‐wavelength damping reduces the system energy in discrete steps. Finally, the cycle restarts. The characteristic frequency of the quasiperiodic cycle and the average system energy are both approximately proportional to the growth rate. Broadening of the driver in wavenumber tends to degrade the system‐wide coherence of the cycle, but its main features appear to survive on the scale of the coherence length of the driver.

ISSN:0899-8221    

DOI:10.1063/1.859050

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Direct numerical simulations of decaying two‐dimensional magnetohydrodynamic flows at Reynolds numbers of several thousand are performed, using resolutions of 10242collocation points. An inertial range extending to about one decade is observed, with spectral properties depending on the velocity–magnetic field correlation. At very small scales, resistive tearing destabilizes current sheets generated by the inertial dynamics and leads to the formation of small‐scale magnetic islands, which may then grow and reach the size of inertial scales.

ISSN:0899-8221    

DOI:10.1063/1.859051

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A general form of the derivative nonlinear Schro¨dinger (DNLS) equation, describing the nonlinear evolution of Alfve´n waves propagating parallel to the magnetic field, is derived by using two‐fluid equations with electron and ion pressure tensors obtained from Braginskii [inReviewsofPlasmaPhysics(Consultants Bureau, New York, 1965), Vol. 1, p. 218]. This equation is a mixed version of the nonlinear Schro¨dinger (NLS) equation and the DNLS, as it contains an additional cubic nonlinear term that is of the same order as the derivative of the nonlinear terms, a term containing the product of a quadratic term, and a first‐order derivative. It incorporates the effects of finite beta, which is an important characteristic of space and laboratory plasmas.

ISSN:0899-8221    

DOI:10.1063/1.859052

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

It is shown that a recently constructed exact solution of the Vlasov equation describing a plasma with density and temperature gradients can be expressed in terms of the constants of motion. The distribution function is then used to illustrate the differences between a Vlasov and a one‐fluid description. In fluid theory, only the pressure profile is determined (unless one postulates an equation of state), while the Vlasov description leads to a separate determination of density (g) and temperature (&psgr;2) profiles; the equation of state,g=&psgr;3−2/&bgr;, comes out naturally in the latter case.

ISSN:0899-8221    

DOI:10.1063/1.859053

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In a recent paper, Nakayama, Sato, and Matsuoka [Phys. Fluids31, 630 (1988)] suggested that elliptical cross‐section tokamaks with aspect ratioR/a=3.2 and with elongation &kgr;=2.6 are unstable to a splitting (m=2,n=0) instability for plasma &bgr;>5%, and that &kgr;≥4.0 plasmas are unstable to a splitting for &bgr;≥1%. The magnetohydrodynamic evolution codetsc[J. Comput. Phys.66, 481 (1986)] indicates, however, that such plasmas are robustly stable with respect to this splitting. In fact, a &kgr;=3.7 plasma with &bgr;=23.0% shows no tendency to split. However, the addition of pinching coils at the waist will cause the plasma to split if the current in these coils exceeds a critical valueIc, which decreases with increasing beta.

ISSN:0899-8221    

DOI:10.1063/1.859054

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Kinetic effects of Alfve´n wave spatial resonances near the plasma edge are investigated numerically and analytically in a linearized cylindrical tokamak model. In Part I, cold plasma surface Alfve´n eigenmodes (SAE’s) in a pure plasma are examined. Numerical calculations of antenna‐driven waves exhibiting absorption resonances at certain discrete frequencies are reviewed first. From a simplified kinetic equation, an analytical dispersion relation is then obtained with the antenna current set equal to zero. The real and imaginary parts of its roots, which are the complex eigenfrequencies, agree with the central frequencies and widths, respectively, of the numerical antenna‐driven resonances. These results serve as an introduction to the companion paper, Part II [Phys. Fluids B1, 2364 (1989)], in which it is shown that, in the presence of a minority species, certain SAE’s, instead of heating the plasma exterior, can dissipate substantial energy in the two‐ion hybrid layer near the plasma center.

ISSN:0899-8221    

DOI:10.1063/1.859055

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The effects of a heavy minority on Alfve´n wave heating are examined in a linearized circular cylindrical tokamak model. It is shown that a suitable minority concentration and radially varying charge profile produces a two‐ion hybrid resonance in the plasma interior and an Alfve´n wave resonance near the edge. The latter situation, which gives rise to cold plasma surface Alfve´n eigenmodes (SAE’s), was described in Part I [Phys. Fluids B1, 2353 (1989)] for a pure plasma. Here, numerical results, exhibiting substantial absorption in the hybrid layer as a result of the dissipation of these modes, are presented. It is shown that the poloidal mode number strongly affects the amount of the energy absorbed in the hybrid layer. Generally, electron heating due to electron Landau and collisional damping of SAE’s and two‐ion hybrid fields [&ohgr;>(&ohgr;c2)max, with &ohgr;c2being the fundamental minority gyrofrequency] dominates minority heating excepting for cases whenk∥, the parallel wavenumber, is large enough that no SAE occurs near the plasma edge. For the largerk∥system, there occurs a stronger coupling between the two‐ion hybrid and fundamental minority gyroresonance layers, resulting in comparable electron and minority heating. The majority heating is zero everywhere because &ohgr;<(&ohgr;c1)min, where &ohgr;c1is the fundamental majority gyrofrequency.

ISSN:0899-8221    

DOI:10.1063/1.859171

出版商:AIP    

年代:1989

数据来源: AIP