摘要:

For an electron plasma that is magnetically confined in a cylindrical field geometry, radial expansion occurs only if the angular momentum of the plasma changes. In the absence of external torques, the plasma can be confined forever. Confinement experiments have suggested that the small asymmetries in the confining fields are an important external source of angular momentum. Recent transport experiments show that such field errors may excite waves and thereby enhance radial transport. A detailed theory of the nonlinear couplings between the electrostatic collective modes and perturbing asymmetric fields is presented, with emphasis on three‐wave and induced scattering‐type interactions. Via these interactions, field errors may trigger instabilities. Thresholds for such instabilities are derived and the resulting transfer of angular momentum is analyzed. The theory is applied to a simple model of a long column in order to estimate the field error levels required by the thresholds. The theoretical formalism is based on a variational formulation of the Vlasov–Poisson equations that is well suited to discussing the nonlinear interactions of global modes in bounded equilibria.

ISSN:0031-9171    

DOI:10.1063/1.866143

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A set of nonlinear fluid equations that include the effect of finite ion Larmor radius is derived to describe the dynamics of flute modes in a rotating inhomogeneous plasma with a strong axial magnetic field. It is shown that these equations possess a solution in the form of a global pattern of two‐dimensional vortices, different from the solitary vortex structure found earlier for the nonlinear Rossby waves.

ISSN:0031-9171    

DOI:10.1063/1.866144

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

It is shown that high‐phase‐velocity compressional Alfve´n waves have the desirable features needed for efficient current drive in fusion‐reactor‐like conditions; the energy deposition is low on the &agr; particles, and high on the hot electrons in the plasma interior.

ISSN:0031-9171    

DOI:10.1063/1.866145

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The anomalous Doppler instability is investigated for two‐dimensional velocity distributionsf(v∥,v⊥) appropriate for lower‐hybrid current drive. In this case, in contrast to the runaway electron problem, this mode becomes unstable athighdensitywhen the ratio &ohgr;2pe/ &ohgr;2ceis sufficiently large. Increasing the maximum velocityv2of the rf (radio‐frequency) plateau,v1≤v∥≤v2, is also a destabilizing influence. In this paper expressions are given for the growth rate of the electrostatic waves obeying the dispersion relation &ohgr;=&ohgr;pe(1+&ohgr;2pe/ &ohgr;2ce)−1/2k∥/k, employing both analytic expressions forf(v∥,v⊥) and numerical solutions forffrom the two‐dimensional Fokker–Planck equation. The analytical and numerical results are in close agreement.

ISSN:0031-9171    

DOI:10.1063/1.866146

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A simple model of the crash phase of sawteeth in tokamaks is presented, which explains many of the significant experimental observations: small oscillations in the electron temperature prior to the crash (precursor), a short crash time, and large long‐lived oscillations in the electron temperature following the crash (successor). In tokamak discharges with good central confinement the temperature profile across the center of the discharge remains rather flat. In such profiles skin currents form that cause the safety factorqto fall below one at a finite radiusrswhileq(r=0)&bartil;1. The sawtooth crash is caused by them=n=1 tearing mode growing on such a profile. For a profile in whichqfalls below one by an amount &Dgr;q, the crash time is given by the helical Alfve´n time &tgr;A/&Dgr;q, independent of the resisitivity. The size of the magnetic island of the precursor can be a small fraction ofrs. In contrast, a remnant magnetic island of the order ofrssurvives the crash and causes the successor oscillation. Estimates of the decay time of the successor are given.

ISSN:0031-9171    

DOI:10.1063/1.866506

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A radial time‐dependent plasma transport code to be used for numerically modeling field‐reversed configurations is described. The plasma model assumes two‐fluid magnetohydrodynamics and includes a description of plasma on both the closed and open magnetic field lines, as well as such physics effects as axial contraction of the separatrix, flux surface averaging, and particle and energy losses along the open magnetic field lines. This code was applied to two typical discharges (5 and 20 mTorr initial fill pressures) of the FRX‐C field‐reversed configuration[Fusion Tech.9, 13 (1986); Phys. Fluids27, 1514 (1984)] experiment at Los Alamos. Simulation results and values of the transport coefficients are reported.

ISSN:0031-9171    

DOI:10.1063/1.866147

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Gyromotion is studied by applying averaging techniques to exact equations for the guiding‐center position, magnetic moment, and gyrophase. A static nonuniform forceFis shown to shift the gyrofrequency from &OHgr; to &ohgr;=&OHgr;−∇⊥⋅(F/2m&OHgr;). The effects of this gyrofrequency shift on heating are discussed, as is the prospect of experimentally observing the phenomenon. It is also shown that an oscillatory force of sufficient amplitude, whose frequency &ohgr;* is close to &OHgr;, can cause phase locking, i.e., can force the particle to gyrate at the driving frequency &ohgr;*. A rough criterion for an electrostatic wave with electric fieldEto cause phase locking is thatvE/&rgr;&dgr;&ohgr;≥1, wherevE=cE/B, &rgr; is the gyroradius, and &dgr;&ohgr;=&ohgr;*−&OHgr;. Thus a weak wave can cause phase locking if the frequency mismatch is small. Phase locking is enhanced if the magnetic field has a well, which provides a large volume over which ‖B‖ is nearly uniform. Comparison with experimental measurements on the 2XIIB magnetic mirror experiment [J. Phys., Coll. 6, Suppl. No. 12, 38, C6 (1977)] shows that phase locking may account for the puzzling observation of a highly monochromatic wave at the central cyclotron frequency.

ISSN:0031-9171    

DOI:10.1063/1.866148

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The influence of energetic electron rings, or disks, on the magnetohydrodynamic stability of an axisymmetric tandem mirror configuration is studied. Linear stability equations describing interchange and ballooning modes are solved both analytically and numerically using a two‐dimensional stability code for axisymmetric mirrors. Finite‐ion‐Larmor‐radius effects and the presence of a lateral wall are included in the calculations. Stable configurations can be obtained with either sufficiently high hot‐electron pressure or with sufficiently high center‐cell pressure if a lateral conducting wall is close enough to the plasma. In particular, hot electrons might be used to stabilize transiently the plasma during buildup to a high‐beta, wall‐stabilized configuration.

ISSN:0031-9171    

DOI:10.1063/1.866149

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An analytic solution for the force‐free magnetic field, ∇×B=&lgr;B, in the poloidal divertor configuration of the Tokapole II device [Nucl. Fusion19, 1509 (1979)] is presented. Experiments conducted on Tokapole II which provide equilibrium magnetic field profile measurements using a magnetic probe are then described. Observations include a non‐constant‐&lgr; profile and a large diamagnetic current near the magnetic separatrix.

ISSN:0031-9171    

DOI:10.1063/1.866150

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Spontaneous growth of magnetic reconnection is numerically studied in the tearing‐type field geometry. It is found that only in a sufficiently long system does magnetic reconnection fully couple to an anomalous resistivity and develop into the strong fast reconnection mechanism on nonlinear saturation. The supersonic plasma jet resulting from slow shocks causes a remarkable expansion of a magnetic island, and a fast shock stands at the interface between the plasma jet and the island.

ISSN:0031-9171    

DOI:10.1063/1.866514

出版商:AIP    

年代:1987

数据来源: AIP