摘要:

A theory of fast‐ion generation in the 2XIIB mirror experiment due to a single resonant electrostatic ion‐cyclotron mode is presented. Assuming weak radial gradients of the magnetic induction, which appear to be consistent with experimental data, it is shown that ions with small parallel excursions can be accelerated into the 40 keV region. Allowance for a periodic temporal variation of the amplitude of the mode results in stochastic motion of those ions whose range of parallel excursions form a limited but significant fraction of the plasma length. The diffusion coefficients of this class of ions are computed in the quasilinear approximation. The corresponding diffusion times are in reasonable accord with those from the experiment.

ISSN:0031-9171    

DOI:10.1063/1.864361

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Particle–fluid hybrid simulations of electrostatic ion‐cyclotron instabilities in a mirror plasma have been extended to include axial ion bouncing in a nonuniform magnetic field. This model admits unstable ion‐bounce modes as well as drift‐cyclotron and drift‐cyclotron‐loss‐cone modes. Linear theory showed that ion‐bounce modes were as difficult to stabilize as drift‐cyclotron‐loss‐cone modes, but the ion‐bounce modes saturated at very low levels in simulations. Simulations have also followed the self‐consistent interaction of a mirror plasma with turbulent electric fields in the presence of warm plasma stream, ion drag on electrons, injection of energetic neutral beams, and transit losses. Driven by velocity diffusion due to the drift‐cyclotron‐loss‐cone instability, the plasma evolved to a stable configuration. The wave turbulence was moderated by streaming plasma and axial electron dissipation, was insensitive to the energy width of the neutral beam, and decayed when the neutral beam was removed. The simulation results confirmed many of the predictions of quasilinear theory and theories of orbital stochasticity, and were in qualitative agreement with many (but not all) of the observations in the Lawrence Livermore National Laboratory 2XIIB experiment.

ISSN:0031-9171    

DOI:10.1063/1.864362

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Toroidal corrections to the fields in a bumpy torus cause the particle drift surfaces to depart from the magnetic flux surfaces, thereby resulting in transport and direct radial losses. Such losses can be reduced by choosing field structures such that the drift and flux surfaces remain close everywhere. A technique is presented for finding, analytically, such optimized fields in the near‐axis, vacuum approximation.

ISSN:0031-9171    

DOI:10.1063/1.864363

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

An initial value numerical solution of the time‐dependent nonlinear ideal magnetohydrodynamic equations demonstrates that spheromak equilibria which are linearly unstable to nonresonant helical internal perturbations saturate at low amplitude without developing singularities. These instabilities thus represent the transition from an axisymmetric to a nonaxisymmetric equilibrium state, caused by a peaking of the current density.

ISSN:0031-9171    

DOI:10.1063/1.864364

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The tilting‐mode stability of field‐reversed configurations is analyzed taking into account plasma rotational effects that had not been included in previous theoretical treatments. It is shown that for a rigidly rotating plasma in stationary equilibrium, stability can be attained if the plasma rotational energy is of the same order as the thermal energy. Since presently available values of the rotational velocities are quite lower than required by the stabilization mechanism considered here, the contribution of this effect to the overall stability of the mode does not appear to be significant.

ISSN:0031-9171    

DOI:10.1063/1.864365

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The equilibrium of a class of field‐reversed configurations in an external magnetic field is studied analytically. Bifurcation points in parameter space appear when the elongation of the separatrix is equal to 1.61 or to 0.23. In general, for fixed parameters, two equilibria are possible, one of which is almost spherical while the other is highly prolate or, alternatively, highly oblate. The nearly spherical equilibrium corresponds to a lower energy condition.

ISSN:0031-9171    

DOI:10.1063/1.864366

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Optical excitation of electrostatic modes in front of an infinite step in the plasma density can lead to total absorption of the incident light. The conditions for complete mode conversion are analyzed for inhomogeneous and thermal surface layers and possible applications for laser plasma experiments are discussed.

ISSN:0031-9171    

DOI:10.1063/1.864367

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Computer simulations of laser light absorption and hot‐electron generation by the ion‐acoustic decay and oscillating two‐stream instabilities in nonlinearly steepened density profiles are presented. The steepened density scale length is estimated and used to estimate a condition for instability generation.

ISSN:0031-9171    

DOI:10.1063/1.864349

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Computer simulations and theory are used to illustrate the linear and nonlinear behavior or Raman backward and forward scattering in laser‐irradiated plasmas. The heated‐electron distributions, nonlinear Landau damping, bandwidth of the scattered light, and other results of nonlinear simulations in homogeneous and nonhomogeneous plasmas are described. The characteristics of Raman scattering in a plasma with a quite high background temperature (64 keV) are examined, which are relevant to CO2‐heated plasmas. Finally the effects of self‐generated magnetic fields are pointed out, which can reduce Raman scattering by inhibiting the transport of hot electrons, and the reduction of Raman scattering by use of laser light with a large bandwidth is described.

ISSN:0031-9171    

DOI:10.1063/1.864336

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

It has been observed experimentally that the CO2‐laser Gemini and Helios systems generate very high harmonics with nearly constant efficiency over several harmonics with a cutoff near the local plasma frequency at the upper density shelf. To understand the experimental results, the strongly nonlinear electron response to a time‐dependent inhomogeneous force is studied. The equation of the electron motion in Lagrangian variables is derived and the conditions on the density scale length and power intensities for wavebreaking are found. By transforming the solution back to Eulerian variables, it is shown that the source of a given high harmonic frequency is localized around the density region where the local plasma wave is at that frequency. Using the numerically obtained nonlinear current density we evaluate the intensity of the radiation coming out of the plasma and verify the nearly constant harmonic production efficiency with a sharp decrease at the local plasma wave frequency at the upper shelf density.

ISSN:0031-9171    

DOI:10.1063/1.864337

出版商:AIP    

年代:1983

数据来源: AIP