摘要:

Experimental measurements show that coherent, azimuthal density fluctuations (1) focus lower hybrid resonance cones azimuthally, and (2) modulate the radial location of the resonance cones. A simple theory based on wave refraction is presented; this theory is in good agreement with the experimental observations. The theory is extended to give a physical description of wave propagation through turbulent, isotropic (perpendicular to B) noise such as has been observed in tokamaks; it is found that the turbulence causes the lower hybrid wave vectors to have an angular spread in the plane perpendicular to B.

ISSN:0031-9171    

DOI:10.1063/1.862267

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Fine structures of the unstable waves and the electron density profiles resulting from nonlinear spatial transport are studied numerically for a collisional flute mode instability. It is confirmed that the nonlinear transport produces regular pulse trains in both the electron density and wave amplitude profiles in the direction perpendicular to the electron drift (in the direction of the density gradient). Fourier spectral analysis of the nonlinear structure in that direction indicates two major spectral peaks. The main peak is the fundamental mode which leads to overall flattening of the density distribution in the entire unstable region. The second peak corresponds to the generated small‐scale pulses resulting from a nonlinear instability associated with a strong wave‐induced transport. In the steady state the pulses tend to merge and be smeared out to form a plateau in the density profile. These indicate the presence of two different, steady and pulsative, transports. Examination of the spatial structure finds that the pulse width is roughly the same size as the dominant wavelength of the primary wave which propagates in the direction of the drift. It is further found that the observed nonlinear mode can be explained by a nonlinear transport equation of the formnt+(n2)z+ky−2(n2)zzz@qL=0, wherekyis the wavelength of the dominant primary wave propagating in the direction of the drift andzdenotes the coordinate perpendicular to the drift.

ISSN:0031-9171    

DOI:10.1063/1.862268

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The current driven ion acoustic instability has been studied in a quiescent magnetized argon plasma for electron densities in the range from 1010to 1011cm−3, an electron temperatureTeof 2.2 eV, an electron‐ion temperature ratioTe/Tiof 15, a homogeneous magnetic field of 400 G, and neutral gas pressures in the range from 1×10−3to 5×10−3mm Hg. Coulomb collisions are important for these experimental conditions. The instability occurs for dc currents in the range from 100 to 600 mA. The saturation appears to be explained by electron trapping. The nature of the instability is identified from measurements of the cross power density spectrum and the characteristic parameters like electron drift velocity, collisional damping, electron temperature, and electron‐ion temperature ratio are determined precisely from an observation of low frequency ion acoustic test waves using an unperturbative method of microwave interometry.

ISSN:0031-9171    

DOI:10.1063/1.862269

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Nonlinear behavior of the collisionless drift instabilities are studied for the universal and current driven modes by means of electrostatic particle simulations in two‐and‐a‐half dimensions. Realistic mass ratios of electrons to the ions are used in the simulations, where the guiding‐center approximation for the electrons and the exact dynamics for the ions are employed. Several nonlinear effects including the quasi‐linear diffusion of the particle density, the frequency shift due to the ambipolar field, the mode competition among the unstable waves, and the quasi‐linear diffusion in the velocity space are found to be dominant mechanisms for the saturation. The stabilization of the collisionless drift instabilities by the magnetic shear has also been studied.

ISSN:0031-9171    

DOI:10.1063/1.862270

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

An analytic technique for calculating magnetic‐moment jumps &Dgr;&mgr; of particles in magnetic traps, previously derived for particular two‐dimensional vacuum fields, is generalized to nonvacuum fields of arbitrary complexity and applied to high‐&bgr; mirror machines. The size of a jump depends on the behavior of the magnetic‐field strengthB(s) near the singularities ofBin the complexsplane, where realsmeasures position along a field line. It is demonstrated that an intrinsic complication of mirror‐machine magnetic configurations is the presence of multiple singularities ofB, which become closely spaced for field lines near the axis. An expansion inr2is used to determine &Dgr;&mgr; in the closely spaced regime. The analytic theory is compared with results from a particle‐orbit code for several axisymmetric nonvacuum fields, and is found to be in excellent agreement in both the well separated and closely spaced singularity regimes. Finite‐&bgr; effects are examined using axisymmetric model fields derived from the long, thin equilibrium approximation, with parameters representative of present (2XIIB) and future (MFTF) mirror experiments. It is shown that increasing &bgr; appreciably lowers the maximum energy at which particles are confined. The effect is particularly significant if the plasma length is small compared with the vacuum field axial scale length and if the gyroradius is small compared with the plasma radius. The degradation is substantially reduced when the gyroradius at the energy limit is comparable to the plasma radius.

ISSN:0031-9171    

DOI:10.1063/1.862271

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Wave absorption for frequencies near the electron cyclotron frequency for the ordinary and extraordinary modes is investigated. The role of finite electron Larmor radius effects is discussed. A numerical analysis of the relevant dispersion relation in the weakly relativistic and nonrelativistic limit is presented. The density dependence (1/ne) of the damping of the extraordinary wave for not too small densities and the strong absorption of the ordinary wave for quasi‐perpendicular propagation in a hot plasma (Te≳1 keV) are investigated numerically for several angles of incidence.

ISSN:0031-9171    

DOI:10.1063/1.862272

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A soluble theory of the post‐saturation portion of a beam‐plasma interaction is developed, concentrating on explaining the results of O’Neil, Winfrey, and Malmberg. Analytic progress is made possible by applying a certain constraint procedure, characterized by the ’’rotating‐bar’’ approximation, to a Hamiltonian formulation of the problem. The procedure yields, from the originalN‐particle HamiltonianH, a new, reduced Hamiltonian H˜, which has only two particle‐related degrees of freedom, and which maintains the conservation laws of energy and momentum possessed byH. The equations of motion coming from H˜ still describe the self‐consistent interaction of a mode of the plasma with the beam particles, as opposed to previous work, and, because of the great reduction in the number of degrees of freedom, explicit expressions for the nonlinear frequency shift, and growth rate, of the mode can be obtained, which are in very good agreement with the simulation results of O’Neil, Winfrey, and Malmberg.

ISSN:0031-9171    

DOI:10.1063/1.862273

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The dispersion relation for the trapped electron mode in noncircular cross‐section tokamaks is formulated and evaluated. It is shown that modest reductions in the growth rate result from cross‐sectional elongation.

ISSN:0031-9171    

DOI:10.1063/1.862274

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Ion energy distributions along the axis of the Maryland linear theta pinch were measured by a parallel‐plate electrostatic deflection type energy analyzer. Two possible sources of end loss are discussed. The first is acceleration in the axial potential which is a result of the cooling of electrons near the end of the tube. The second is the axial ion temperature that results from relaxation due to different microinstabilities among ion energy components. The particle measurement suggests that the second one is dominant. A two‐temperature Maxwellian distribution is observed along the axis. The absence of observable ions from farther than 10–20 cm into the plasma also implies a strong turbulent scattering process is operating.

ISSN:0031-9171    

DOI:10.1063/1.862275

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Under nonadiabatic conditions, rf plugging is accompanied by effective ion heating, which in turn causes increased particle losses from a cusp configuration. An empirical scaling relation for rf plugging of line cusp loss is obtained among the required rf electric field, the static magnetic field, and the plasma density, and is compared with the relation obtained by a linear theory.

ISSN:0031-9171    

DOI:10.1063/1.862276

出版商:AIP    

年代:1978

数据来源: AIP