摘要:

An analytical explanation is provided for phenomena observed in gyrokinetic particle simulations used to study the nonlinear evolution of the universal instability. The nonlinearE×Badvection of nearly resonant electrons is related to the deactivation of the instability mechanism, leading to oscillations of the perturbation amplitude around a nonzero saturated level. An equivalent description is given in terms of nonresonant mode coupling. A self‐consistent evolution equation for the amplitude is obtained for the case where this mechanism predominates, and is successfully compared to the gyrokinetic simulations.

ISSN:0031-9171    

DOI:10.1063/1.865030

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The trapped ion instability appears as a single mode in the Columbia Linear Machine and is expected to appear as such in tandem mirrors. Using three‐wave, nonresonant mode coupling of radially nonlocal eigenfunctions, two mode‐coupling equations for the unstable fundamental and stable first‐order radial harmonics are derived and solved. Mode coupling to the damped higher‐order radial harmonics is the principal physical saturation mechanism of the unstable fundamental mode.

ISSN:0031-9171    

DOI:10.1063/1.865400

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Beginning with a linearized Coulomb collision operator, the derivations of bounce‐averaged energy and pitch angle scattering terms that have been suitably discretized for Monte Carlo use are described. The derivation assumes that the electrostatic potential is constant along a magnetic field line. The collision operators are applied to electron transport in ELMO Bumpy Torus.

ISSN:0031-9171    

DOI:10.1063/1.865031

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A linear, electromagnetic analysis is presented of the cavity mode structure and dispersion relations for waves with frequency near the electron‐plasma frequency in cylindrical plasmas with monotonically decreasing electron density and finite magnetic field. Analytic solutions are obtained which are exact in the limits when either the ratio of electron cyclotron to plasma frequency or the ratio of plasma size to parallel wavelength tends to infinity. Comparison with numerical solutions confirms that the analytic results are highly accurate even for fairly modest values of these ratios. Thermal effects are incorporated, including Landau (damping or) gain, which show the lowest‐order transverse modes to have greatest gain. These modes are highly localized near the cylinder axis so that the plasma itself acts as a cavity, regardless of edge boundary conditions. The theory thus enables an interpretation to be made of maser action in quasicylindrical plasmas such as tokamaks.

ISSN:0031-9171    

DOI:10.1063/1.865032

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The effect of frequency modulation during stochastic ion heating induced by lower‐hybrid waves is examined. The modulation occurs either in the ion‐cyclotron frequency because of the variation of the magnetic field in toroidal devices, or it can be externally imposed on the frequency of the lower‐hybrid waves. It has already been observed numerically [Phys. Fluids27, 184 (1984)] that a small variation in the ion‐cyclotron frequency can induce velocity diffusion for wave amplitudes well below the stochasticity threshold in a uniform magnetic field. Here a detailed study reveals that to the lowest order in the small parameters, the modulational effects can be incorporated in a two‐dimensional Hamiltonian. This allows the derivation of the new stochasticity thresholds. It is found that a small amount of modulation, &Dgr;&ohgr;/&ohgr;≲1%, produces an order of magnitude reduction in the stochasticity threshold relative to the constant frequency case. The stochastic regime in velocity space also grows in size, resulting in a considerable increase of number of heated particles in the case of devices with modest aspect ratio. Both ion‐cyclotron and wave‐frequency modulation lead to similar results. The modulation of the wave frequency offers the ability to control and optimize the modulation parameters and is proposed as a method to enhance radio frequency (rf) heating.

ISSN:0031-9171    

DOI:10.1063/1.865033

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

This paper considers the nonlinear interaction of a background warm plasma with intense, circularly polarized radiation along an external magnetic field. It is shown that the couplings of the adiabatic particle as well as the hydrodynamic plasma motions with the radiation field can lead to localized wave‐envelope packets. The resulting self‐consistent equilibria consist of finite‐amplitude electron density perturbations which are nonlinearly driven by electromagnetic fields.

ISSN:0031-9171    

DOI:10.1063/1.865034

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A linear and nonlinear kinetic theory of resistive ballooning modes that includes diamagnetic drifts and finite Larmor radius effects is presented. The linear stability of resistive ballooning modes is examined analytically and numerically. A renormalized resistive ballooning equation is derived, and the saturation level of the instabilities is analytically calculated. Finally, a calculation of the electron thermal conductivity for the large &ohgr;@B|regime is presented.

ISSN:0031-9171    

DOI:10.1063/1.865406

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The confinement properties of particles and energy in the ELMO Bumpy Torus in the collisionless‐ion regime, characterized by positive ambipolar potential in the core region, have been investigated using a one‐dimensional transport code. It is found that both particle and energy confinement can be significantly improved over the conventional negative‐potential‐regime values. A centrally peaked radial profile of microwave heat deposition is found to be an essential condition for positive potential equilibria.

ISSN:0031-9171    

DOI:10.1063/1.865035

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The macroscopic stability of an ignited ELMO Bumpy Torus (EBT) reactor is investigated by studying the effects of the alpha particles generated by the deuterium–tritium (D–T) fusion reaction on the background interchange mode, the interacting interchange mode, and the high‐frequency compressional Alfve´n and coupled modes. A fluid description is used for the background plasma while a kinetic treatment is utilized for the hot‐electron species and the alpha particles. It is shown that the alphas tend to mildly destabilize the interacting interchange while stabilizing the background interchange because of their sizable Larmor radii. The destabilization is most pronounced when the beta of the alpha particles is highest, i.e., at birth, and recovery of stabilization takes place as these particles slow down toward thermalization. It is also shown that the alphas completely stabilize the high‐frequency modes, so that it can safely be concluded that fusion alphas present no detrimental effects on the stability of an EBT reactor that possesses an appropriate hot‐electron ring for macroscopic stability.

ISSN:0031-9171    

DOI:10.1063/1.865036

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A theory of the modification of stellarator transport caused by the presence of finite plasma pressure is developed and applied to a range of stellarator configurations. For typical planar‐axis stellarators, transport can change by a factor of about 4–5 in the progression from zero pressure to the equilibrium &bgr; limit of the device. For transport‐optimized configurations, the factor can be over an order of magnitude. Thus, a stellarator with transport‐optimized vacuum fields can have poor confinement at the desired operating &bgr;. Without an external compensating field, increasing &bgr; tends to degrade confinement, unless the initial field structure is very carefully chosen. The theory permits correct determination of this vacuum structure, in terms of the desired structure of the field at a prescribed operating &bgr;. With a compensating external field, the deleterious effect of finite &bgr; on transport can be partially eliminated.

ISSN:0031-9171    

DOI:10.1063/1.865037

出版商:AIP    

年代:1985

数据来源: AIP