摘要:

A Thomson scattering diagnostic capable of achieving one‐discharge, high‐resolution spectra through use of a microchannel plate image intensifier coupled to an optical multichannel analyzer has been implemented on a theta‐pinch device. The improved resolution permits measurements of local average electron velocities and current densities. First‐order relativistic corrections and anomalous velocity‐space fluctuations are directly observable. The enhanced fluctuations are attributed to electrons interacting with unstable waves.

ISSN:0031-9171    

DOI:10.1063/1.865142

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Experiments indicate that ionization‐diffusion instabilities (striations) of the convective type, present in low‐pressure mercury/noble‐gas lamps, are generated by current fluctuations in the inhomogeneous part at the cathode end of the discharge. Self‐excitation of striations is made possible by feedback via the electrical supply circuit of the lamp. An experimental boundary condition is found at the point of excitation. A simple plasma model, with an extension that takes feedback into account, fully describes the laser‐like phenomenon of mode selection and the influence of the discharge length.

ISSN:0031-9171    

DOI:10.1063/1.865143

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Current and electric field measurements have been made near the magnetic neutral line in a double inverse pinch device. The current in the central region did not show any observable Petschek‐type shock structure, but instead had the appearance of a simple current sheet. A disruption of the induced current system, called the impulsive flux transfer event, was observed. Measurements of the electric field in the central region showed that the space‐charge field, −∇&fgr;, was significant, especially during the current buildup and disruption. When measurements of the total electric field,E=−A˙−∇&fgr;, were combined with current density measurements, the conductivity of the plasma in the central region remained sensibly constant throughout the current disruption.

ISSN:0031-9171    

DOI:10.1063/1.865144

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In this paper it is shown by an asymptotic method that the solitons which appear on a cylindrical magnetohydrodynamic (MHD) pinch with axial symmetry persist when the Hall current term is included in the governing equations. On the other hand, the results presented here indicate that, under the linear stability criterion for the cylindrical pinch, nonlinear waves that have an amplitude that does not decay with time can occur.

ISSN:0031-9171    

DOI:10.1063/1.865145

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Kinetic equations for the electromagnetic drift modes are derived and analyzed for the stability of tokamaks in the local approximation. In the dissipationless, hydrodynamic limit, the fifth‐order polynomial dispersion relation previously studied is recovered. The kinetic velocity space integrals in the ion dynamics are shown to modify the five principal modes of oscillation and their polarizations. It is shown that in kinetic stability theory the critical plasma pressure defined in magnetohydrodynamic theory determines a transition from microinstability to macroinstability.

ISSN:0031-9171    

DOI:10.1063/1.865146

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The spectra of the semicollisional kinetic Alfve´n modes in a sheared slab geometry are investigated, including the effects of finite ion Larmor radius and diamagnetic drift frequencies. The eigenfrequencies of the damped modes are derived analytically via asymptotic analyses. In particular, as one reduces the resistivity, it is found that, because of finite ion Larmor radius effects, the damped mode frequencies asymptotically approach finite real values corresponding to the end points of the kinetic Alfve´n continuum.

ISSN:0031-9171    

DOI:10.1063/1.865347

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The properties of electrostatic ion‐cyclotron waves excited in a single‐ended cesium Q machine with a nonuniform magnetic field are described. The electrostatic ion‐cyclotron waves are generated in the usual manner by drawing an electron current to a small exciter disk immersed in the plasma column. The parallel and perpendicular (toB) wavelengths and phase velocities are determined by mapping out two‐dimensional wave phase contours. The wave frequency  f  depends on the location of the exciter disk in the nonuniform magnetic field, and propagating waves are only observed in the region where  f≳fci, where  fciis the local ion‐cyclotron frequency. The parallel phase velocity is in the direction of the electron drift. From measurements of the plasma properties along the axis, it is inferred that the electron drift velocity is not uniform along the entire current channel. The evidence suggests that the waves begin being excited at that axial position where the critical drift velocity is first exceeded, consistent with a current‐driven excitation mechanism.

ISSN:0031-9171    

DOI:10.1063/1.865348

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Two‐dimensional dissipative magnetohydrodynamic turbulence is randomly driven at small spatial scales and is studied by numerical simulation in the presence of a strong uniform external magnetic field. A novel behavior is observed which is apparently distinct from the inverse cascade that prevails in the absence of an external magnetic field. The magnetic spectrum becomes dominated by the three longest‐wavelength Alfve´n waves in the system allowed by the boundary conditions: those which, in a box size of edge 2&pgr;, have wavenumbers (kx,ky) =(1,0),(1,1), and (1,−1), where the external magnetic field is in thexdirection. At any given instant, one of these three modes dominates the vector potential spectrum, but they do not constitute a resonantly coupled triad. Rather, they are apparently coupled by the smaller‐scale turbulence.

ISSN:0031-9171    

DOI:10.1063/1.865349

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The neoclassical theory of ion transport in rotating axisymmetric plasmas is formulated. The flow speed is allowed to be of the order of the ion thermal speed. It is shown that the ion distribution function becomes Maxwellian, with temperature uniform on a magnetic surface, and the poloidal flow decays, in a few transit or collision times, in general. A drift kinetic equation is derived which is a simple generalization of the drift kinetic equation for nonrotating plasmas. The radial gradient of the toroidal angular velocity appears as a driving term like the temperature gradient. Both gradients drive the transport of toroidal angular momentum and energy, in general; Onsager relations for the two‐by‐two transport matrix are derived. The off‐diagonal transport coefficients are shown to be zero if the magnetic field has up–down symmetry. A simple expression for the enhancement of the ion thermal conductivity in the banana regime, caused by rotation, is derived. The neoclassical viscosity is shown not to be enhanced by rotation in the banana regime, and to be small in the collisionality parameter in the collisional regime, assuming up–down symmetry. In the collisional regime, the thermal conductivity is shown to be suppressed by the effects of rotation.

ISSN:0031-9171    

DOI:10.1063/1.865350

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The lifetime of energetic electrons generated via second harmonic electron‐cyclotron‐resonance heating in a symmetric magnetic mirror configuration is investigated experimentally. The decay time is observed to be as much as a factor of 4 shorter than the value calculated for the most rapid classical loss mechanism, Coulomb drag. The nonclassical loss component is shown to be induced by the microwave power available for electron heating. The microwave power required to sustain the temperature and density of the energetic electrons is calculated from the observed lifetimes and stored energy. It is found to be only a few percent of the total injected power which, in this experiment, is equal to or less than 1 kW.

ISSN:0031-9171    

DOI:10.1063/1.865351

出版商:AIP    

年代:1985

数据来源: AIP