摘要:

The distribution function of ions during ion cyclotron resonance heating (ICRH) is studied analytically using a simplified one‐dimensional Fokker–Planck equation incorporating ion–ion and ion–electron collisions and rf quasilinear diffusion. By including source and loss terms in the equation, steady‐state and time‐dependent solutions, which are regular near the origin and vanish at high energies, are found. It is shown that an initially Maxwellian distribution function is transformed by the ICRH into a non‐Maxwellian distribution with a tail of energetic ions. The tail is more pronounced when quasilinear diffusion dominates over collisions and losses.

ISSN:0031-9171    

DOI:10.1063/1.867020

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The two coupled diffusive equations for the perturbed electron temperature and poloidal magnetic field have been solved in the framework of the Kadomtsev model for the sawtooth activity. It is found that the time averaged toroidal electric field profile is radially nonuniform. A simple formula that describes the radial behavior of the toroidal electric field is proposed, and the effect on the power balance is presented.

ISSN:0031-9171    

DOI:10.1063/1.866842

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A one‐dimensional theoretical model for predicting the heating of a hydrogen plasma in open confinement systems by a relativistic electron beam is presented. Direct energy transfer of beam electrons via interaction with large amplitude waves of the two‐stream instability and Ohmic dissipation of plasma return current caused by classical and anomalous resistivities are considered as power input terms. For loss terms, various atomic processes and heat conduction mechanisms are considered. In the light of observed changes in the average scattering angle of the beam inside the plasma, criteria deciding the character of beam–plasma interaction and the estimation of direct power transfer are discussed. The numerical results are presented with a reference to the results of the beam–plasma heating experiments reported in the literature. Better agreement is observed between the experiment and the present analysis.

ISSN:0031-9171    

DOI:10.1063/1.866843

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Ion temperature gradient drift instabilities have been investigated using gyrokinetic particle simulation techniques for the purpose of identifying the mechanisms responsible for their nonlinear saturation as well as the associated anomalous transport. For simplicity, the simulation has been carried out in a shear‐free slab geometry, where the background pressure gradient is held fixed in time to represent quasistatic profiles typical of tokamak discharges. It is found that the nonlinearly generated zero‐frequency responses for the ion parallel momentum and pressure are the dominant mechanisms giving rise to saturation. This is supported by the excellent agreement between the simulation results and those obtained from mode‐coupling calculations, which give the saturation amplitude as ‖e&Fgr;/Te‖ &bartil;(‖&ohgr;l+i&ggr;l‖/&OHgr;i)/(k⊥&rgr;s)2, and the quasilinear thermal diffusivity as &khgr;i&bartil;&ggr;l/k2⊥, where &ohgr;land &ggr;lare the linear frequency and growth rate, respectively, for the most unstable mode of the system. In the simulation, the time evolution of &khgr;iafter saturation is characterized by its slow relaxation to a much lower level of thermal conduction. On the other hand, a small amount of electron–ion collisions, which has a negligible effect on the linear stability, can cause significant enhancement of &khgr;iin the steady state.

ISSN:0031-9171    

DOI:10.1063/1.866844

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In this paper a series of numerical simulations of field reversal in the reversed field pinch is in the simulations described using an incompressible magnetohydrodynamic (MHD) model and a reference set of plasma conditions. Field reversal and maintenance are observed, but require values of the pinch parameter &thgr; larger than in experiment. This discrepancy is shown to arise largely from the unrealistic resistivity profile in the reference conditions and may not be fundamental. Qualitative agreement with experiment is demonstrated in several areas. The view that field reversal occurs because of a simple MHD dynamo is therefore given support.

ISSN:0031-9171    

DOI:10.1063/1.866791

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The splitting of a tokamak with elliptical and bean‐shaped cross sections is studied for finite beta plasmas. When the plasma beta exceeds a critical value, an elliptical tokamak is subject to an ideal pressure‐driven instability, which deforms the ellipse in such a way that a thinned plasma current sheet is formed around the magnetic axis. As a result, magnetic reconnection is nonlinearly driven and the ellipse is split. The bean‐shaped tokamak, however, is stable against a splitting perturbation for limited equilibria that could be numerically constructed. An interesting similarity to the energy relaxation process in a force‐free plasma, namely, a two‐step evolution (initial occurrence of an ideal magnetohydrodynamic instability and subsequent occurrence of driven reconnection), is discussed.

ISSN:0031-9171    

DOI:10.1063/1.866792

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Recently, the propagation of an electron beam through an ion channel with periodically varying ion density has been proposed as a method of transporting or modulating an electron beam. A theoretical treatment indicates that, in the absence of an external magnetic field, cylindrically symmetric radial oscillations of the beam electrons are excited by the channel, with resonant excitation occurring when the wavelength of the ion density variation equals the betatron wavelength. An analysis of the ion focusing regime (IFR) ion hose instability indicates that thee‐folding length of this instability increases when the variations of the electron density are increased. In the absence of an external magnetic field and damping, moderate electron density variations (caused, for example, by a periodic ion channel or the nonrigidity of the beam and channel) result in a wavelength ande‐folding length on the order of the beam‐averaged betatron wavelength.

ISSN:0031-9171    

DOI:10.1063/1.866793

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The effect of a plasma filament on the characteristics of stimulated Raman scattering is examined. The model representation of the filament allows an assessment of the way in which Raman growth rates are influenced not only by the density inhomogeneity transverse to the beam direction, but by the nonuniform intensities of the incident laser and scattered light, as well as by Landau damping in the plasma. The model determinesglobaleigenvalues without recourse tolocal(WKB) approximations, or to phenomenological damping factors. In shallow filaments the concentration of the light gives rise to modest gains in growth, while for deeper filaments the effects of the inhomogeneity and of Landau damping are dominant and suppress the instability. In both cases the strongest growth occurs at the bottom of the filament. Waveguide effects in deep filaments can result in emission being constrained to wavelengths significantly shorter than twice that of the incident laser light.

ISSN:0031-9171    

DOI:10.1063/1.866794

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Magnetic fields in the megagauss range have been measured in experiments on plasmas generated by irradiating targets with high power lasers. A study of the morphology of these self‐generated fields is important not only for its intrinsic interest but for possible implications in laser–target physics. In this paper work on the numerical modeling of large magnetic fields generated in target experiments is reported. The results show generally satisfactory agreement with the fields measured experimentally both in terms of the magnitude of the peak fields and their morphology. In the numerical model the contribution from the Hall term in describing the evolution of the magnetic field is shown to be important especially in short pulse (&bartil;100 psec) experiments.

ISSN:0031-9171    

DOI:10.1063/1.866795

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A simple model for the heavy ion beam driven implosion of uniform spherical shell targets is constructed. The model equation is obtained by averaging the fluid equation of conservation of energy over the thickness of the shell. The energy balance contains the thermal and kinetic energies of the region absorbing the beam energy. The driving power is considered as a constant. The model gives the dependence of the implosion properties on the ion energy and the results are in agreement with published simulation data. The dynamics of a plane slab heated by an intense ion beam with range smaller than the slab thickness is described by a modified version of the model. It yields the temporal behavior of the hydrodynamics efficiency and its dependence on the ratio between the payload and the absorber masses such as is observed in simulation results.

ISSN:0031-9171    

DOI:10.1063/1.866796

出版商:AIP    

年代:1988

数据来源: AIP