摘要:

The collision operator for a relativistic plasma is reformulated in terms of an expansion in spherical harmonics. In this formulation the collision operator is expressed in terms of five scalar potentials that are given by one‐dimensional integrals over the distribution function. This formulation is used to calculate the electrical conductivity of a uniform electron–ion plasma with infinitely massive ions.

ISSN:0899-8221    

DOI:10.1063/1.858966

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Fluctuations in plasmas at frequencies near the electron plasma frequency (&ohgr;p) have been measured withinsituwire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne<1012cm−3,kTe<5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (lengthL≫radiusa≳Debye length &lgr;D) with a low‐noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (&ohgr;&ohgr;p) on both electric and magnetic antennas. In the evanescent regime (&ohgr;<&ohgr;p) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/&ohgr;p) around the antenna. Recalling that electron transit‐time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (&ohgr;<&ohgr;p) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric &egr;(&ohgr;,k)→0) produces a broad noise enhancement at &ohgr;≳&ohgr;pon electric antennas.It can only be observed in the open‐loop voltage of dipoles measured with a high‐impedance transformer between antenna and 50&OHgr; transmission line. The enhanced noise is electrostatic, randomly polarized, butnotsignificantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.

ISSN:0899-8221    

DOI:10.1063/1.858967

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

An integral expression for the gyrokinetic total energy of a magnetized plasma, with general magnetic field configuration perturbed by fully electromagnetic fields, was recently derived through the use of a gyrocenter Lie transformation. It is shown that the gyrokinetic energy is conserved by the gyrokinetic Hamiltonian flow to all orders in perturbed fields. An explicit demonstration that a gyrokinetic Hamiltonian containing quadratic nonlinearities preserves the gyrokinetic energy up to third order is given. The Poisson‐bracket formulation greatly facilitates this demonstration with the help of the Jacobi identity and other properties of the Poisson brackets.

ISSN:0899-8221    

DOI:10.1063/1.858968

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In the present paper the propagation of (internal) magneto‐acoustic‐gravity waves is analyzed in a compressible, isothermal, stratified, electrically conducting atmosphere that is permeated by a uniform, nearly horizontal magnetic field. The conversion, near the so‐called cusp resonance of a long acoustic‐gravity wave into a short slow magneto‐acoustic wave, is demonstrated by means of boundary layer theory based on the smallness of the vertical component of the magnetic field. The magneto‐acoustic wave subsequently carries the energy off upward, which in the limit of a horizontal magnetic field would be fed into the cusp resonanceadinfinitum. This gives rise to singular fields. The scaling of the tendency toward singular behavior of a field quantity with the obliqueness of the magnetic field is discussed, as well as the partial reflection of the long acoustic‐gravity wave.

ISSN:0899-8221    

DOI:10.1063/1.858969

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The propagation and absorption of oblique second‐harmonic electron‐cyclotron waves in a plasma slab are studied by means of propagation equations directly deduced from the Vlasov–Maxwell system. An electromagnetic wave injected from the vacuum into the plasma can be transformed into a quasielectrostatic forward mode, which, in turn, is converted into a Bernstein backward wave. The occurrence of this double conversion process and the global absorption and reflection are analyzed, and the dependence of these phenomena on the plasma parameters and on the wave characteristics is investigated.

ISSN:0899-8221    

DOI:10.1063/1.858970

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The nonlinear evolution of the magnetohydrodynamic (MHD) parametric instability of wave fluctuations propagating along an unperturbed magnetic field is investigated. Both a magnetohydrodynamic perturbation‐theoretical approach and a nonlinear MHD simulation are used. It is shown that high harmonic waves are rapidly excited by wave–wave coupling, and that the wave spectrum evolves from a state containing a small number of degrees of freedom inkspace to one which contains a large number of degrees of freedom. It is found that the spectral evolution prior to nonlinear saturation is well described by the perturbation theory. During this stage, the ratio of the growth rate of thenth harmonic wave to the linear growth rate of the fundamental wave isn. The nonlinear saturation stage is characterized by a frequency shift of the fundamental wave that destroys the wave–wave resonance condition which, in turn, causes the wave amplitude to cease its growth.

ISSN:0899-8221    

DOI:10.1063/1.858971

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Anomalous transport caused by fluctuations of resistive pressure‐driven modes is discussed within the framework of magnetohydrodynamics (MHD). The nonlinear‐reduced equations describing fluctuations localized near a particular magnetic field line are derived for tokamak and reversed‐field‐pinch (RFP) plasmas, taking into account nonzero viscosity and heat conductivity. For an ideally stable but resistively slightly unstable plasma, the anomalous transport is caused particularly by convective motions. The convection is studied as bifurcation from the linearly unstable equilibrium and the expression of the anomalous transport in a tokamak plasma is obtained as a function of the mean pressure gradient near the critical point. In order to evaluate the effects of the convection on the anomalous transport under various conditions, the reduced equations are also solved numerically. It is found that Nusselt number, that is, the ratio of the total heat conductivity including the anomalous heat transport to the classical collisional heat conductivity, is significantly large under some conditions. This partially accounts for the large heat losses in controlled thermonuclear fusion devices.

ISSN:0899-8221    

DOI:10.1063/1.858972

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The ideal stability of cylindrical plasma with mass flows is investigated using the guiding center plasma (GCP) model of Grad [ProceedingsoftheSymposiumonElectromagneticandFluidDynamicsofGaseousPlasmas(Polytechnic Inst. of Brooklyn, New York, 1961), p. 37]. For rotating plasmas, the kinetic treatment of the parallel motion in GCP gives significantly different results from the fluid models, where the pressures are obtained from equations of state. In particular, GCP removes the resonance with slow magnetoacoustic waves and the loss of stability that occurs in magnetohydrodynamics (MHD) for near‐sonic flows. Because of the strong kinetic damping of the sound waves in an isothermal plasma, the slow waves have little influence on plasma stability in GCP at low beta. In the large aspect ratio, low‐beta tokamak ordering, Alfve´nic flows are needed to change the ideal GCP stability significantly. At lowest order in the inverse aspect ratio, flow can be favorable or unfavorable for stability of local modes depending on the profiles, but external kinks are always destabilized by flow if the velocity vanishes at the edge. For high‐beta, reversed field pinch equilibria, numerical computations show that flow can be stabilizing for local modes, but external modes are destabilized by flow. In three dimensions, the MHD equilibrium problem becomes hyperbolic for arbitrarily small flows across the magnetic field, whereas the GCP equilibrium equation remains elliptic for sub‐Alfve´nic flows.

ISSN:0899-8221    

DOI:10.1063/1.858973

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A recent study suggested that the resistive ballooning modes are largely stable in the second region of the ideal ballooning stability, unlike in the first region. This problem is reexamined here by numerically solving the full resistive mode equation. It is shown that in the second region there are unstable resistive ballooning modes with larger growth rates compared to those in the first region. This behavior is shown to be caused by the fact that the curvature force significantly influences the dynamics in the resistive region, and the mode is mainly driven unstable from the resistive region and not from the ideal region through &Dgr;’.

ISSN:0899-8221    

DOI:10.1063/1.858974

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The stability of toroidicity‐induced drift waves in a tokamak equilibrium with magnetic separatrix is studied both analytically and numerically. In particular, the task of a proper determination of the complex ballooning parameter &thgr;0is performed by solving the stationarity condition for the eigenvalue. Results show qualitative dependence on the location of the x point in the meridian plane. Specifically, locating the x point in the equatorial plane, both on the outside and on the inside of the plasma, causes a deepening of the well structure in the potential for the eigenmode, thereby enforcing the inhibition of the shear damping and the marginal stability result obtained in the circular magnetic surfaces case. On the other hand, the location of the x point at the top of the plasma produces a flattening of the well and restores the shear damping, yielding stabilization of the mode. A new quasimarginally stable branch, corresponding to modes localized around the x point, is shown to exist at high values of the separatrix parameterkand x‐point location close to the equatorial plane.

ISSN:0899-8221    

DOI:10.1063/1.859201

出版商:AIP    

年代:1989

数据来源: AIP