摘要:

A systematic experimental study is conducted in order to measure the density and configuration of the plasma by holographic interferometry, and the ion temperature and level of turbulence by cooperative laser scattering. The two measurements are done in various phases of the plasma, before, during, and after the filament breakup. Suprathermal scattering during the filament phase and the successive development of an ionizing wave are evidenced for the first time. The anisotropy of the neutron emission is found to exist in all regimes by the simultaneous measurement of the energy spectra in two directions. A possible explanation of the neutron emission is given. It is concluded that nonthermal processes play an essential role in the plasma focus.

ISSN:0031-9171    

DOI:10.1063/1.861101

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The plasma response to a strong high−frequency electric field has been reformulated in a way that shows that the basic nonlinear parameter is &agr; = &Lgr;&eegr;0(&ohgr;p/&ohgr;0)2, where &Lgr; is essentially the number of particles in a Debye sphere and &eegr;0=E0(4&pgr;n0kBT)−1/2is the usual electric field parameter. Numerical integration has produced results which show such phenomena as locally increased absorption, lower dissipation rates for strong fields, and somewhat deeper penetration of the electromagnetic wave turning point (compared with weak field results).

ISSN:0031-9171    

DOI:10.1063/1.861125

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The so−called stimulated Compton effect is investigated on a one−and−two−halves−dimensional electromagnetic plasma model. This effect results in a parametric instability in which an electromagnetic wave is backscattered off bunches of particles rather than off coherent plasma waves. Numerical simulations were carried out with pump frequency ranging from 1.8 &ohgr;peto 5 &ohgr;pe. The momentum imparted by the pump wave to the plasma produces an averaged plasma drift motion in the direction of the pump wave and with velocity comparable to the electron thermal velocity (V0≃VT≃ 0.1c). In some cases a multiple backscatter process is believed to be the saturation mechanism of the instability. Stimulated Compton scattering has been observed even for frequencies below 2 &ohgr;pewhere the Raman instability cannot occur. However, in this region, a sufficiently strong pump can overcome the frequency mismatch and drive the system hydrodynamically unstable. In this situiation strong acceleration of some electrons occurs producing very energetic particles.

ISSN:0031-9171    

DOI:10.1063/1.861103

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A linear stability theory is presented for stimulated backscattering of zero−mode radiation from ’’diffusion modes.’’ The uncoupled diffusion mode is an electrostatic spatially periodic disturbance which relaxes at the parallel or cross−magnetic field diffusion rate. For ionospheric modification experiment parameters, the most unstable diffusion mode is within one degree of orthogonal to the geomagnetic field, and is characterized by the parallel diffusion rate. It is found to have a maximum temporal growth rate of 20 sec−1and a wavelength of 100 m or more at altitudes several tens of kilometers below the zero−wave reflection point. The total spatial amplification is calculated for special geometries.

ISSN:0031-9171    

DOI:10.1063/1.861104

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Experiments with large−amplitude electron−wave signals demonstrate the existence of nonlinear Floquet−Bloch harmonics and of a passive four−wave coupling interaction. Propagation of the first spatial−temporal harmonic is verified by direct dispersion measurements. The parametric process acts to couple test−wave excitations at nearby frequencies symmetric about a high−level pump. Several possible models for this effect are examined, and it is shown that a four−wave mode−coupling process provides the only consistent explanation. The observed passive interaction operates at spatial positions and at pump−wave levels preceding the turn−on of the trapped−electron sideband instability, and also in the presence of detrapping interference. These results indicate the indifference of the four−wave mechanism to particle trapping, and lead to the inference that the unstable electron sidebands must also be coupled by this competing nonlinear process. Implications of this conclusion for studies of the sideband instability are discussed.

ISSN:0031-9171    

DOI:10.1063/1.861105

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A general nonlinear theory of coherent instabilities is presented and applied to the generation of microwaves by a relativistic beam. The instability is driven both by ∂f/∂p⊥≳ 0 and the relativistic dependence of cyclotron frequency on particle energy. This dependence can cause a bunching in the azimuthal angle in such a way as to cause net particle energy to decrease. The nonlinear evolution of this system is studied by writing the orbit equations for a small number of particles averaged over the fast time scale. The orbit equations are then integrated numerically. The equation for the mode amplitude is found by conserving energy between waves and particles. A simple analytic theory is also presented which successfully predicts the maximum field amplitude and also the behavior of the system for times long after saturation.

ISSN:0031-9171    

DOI:10.1063/1.861106

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Stability boundaries for shear stabilization of the collisionless drift mode are calculated for experimental situations in which the unstable modes are broad and of the dimensions of the system. A universal diagram is obtained which enables one to easily calculate the required finite length, or magnetic field strength or shear parameter for stabilization of a given experiment. There is also an absolute value of the shear which stabilizes all these modes.

ISSN:0031-9171    

DOI:10.1063/1.861107

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Phase−space density granulations called clumps are simulated by computer experiments. The experimental results compare well with the theoretical predictions. In particular, we show that these clumps can be as large as the linear response. The criteria for steady−state clump turbulence are also examined numerically.

ISSN:0031-9171    

DOI:10.1063/1.861108

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A detailed numerical study has been made of the turbulent damping of the convective cells and of lower hybrid waves for a plasma near thermal equilibrium. It is found that some of the theories of strong turbulence, taking into account the diffusing orbits of particles, are inadequate for describing the observations; observations are, however, in good agreement with the linear dispersion relation with a turbulent diffusion coefficient.

ISSN:0031-9171    

DOI:10.1063/1.861109

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A calculation is made of the enhanced ion heating rate &ngr;+, electron heating rate &ngr;−, and dissipation, produced by ion acoustic turbulence. The effects of turbulence on ion motions are accounted for. It is found that &ngr;+≳ &ngr;−for some experiments, leading to a decreasing ratio of electron to ion temperatures and cessation of heating. It is also shown that &ngr;+∝m+−1/2(ion mass).

ISSN:0031-9171    

DOI:10.1063/1.861110

出版商:AIP    

年代:1975

数据来源: AIP