摘要:

The possibility of parametric excitation of high‐frequency electromagnetic waves by lower‐frequency dipole pumping is studied. It is shown that the obtained general dispersive equation may be reduced to the Mathieu equation, provided the case of the flux instability is neglected. In the framework of the developed approach, the excitation of magnetohydrodynamic waves and whistler oscillations is examined.

ISSN:0899-8221    

DOI:10.1063/1.860973

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The investigation of the energy diffusion process of the electrons in a magnetized plasma due to an electron cyclotron wave in perpendicular propagation with respect to the magnetic field is performed. Starting from the description of the relativistic electron motion by means of an action‐angle HamiltonianH(I,&thgr;,t), the Fokker–Planck–Kolmogorov (FPK) approach to the diffusion is considered for a globally stochastic regime of the system. In this regime, the phase correlation process is analyzed, and the characteristic decay time is estimated analytically. The action diffusion coefficientD(I) is derived and compared with a local quasilinear expression. With explicit reference to the low‐density regime, whereDincreases withI, and is very close to the quasilinear coefficient, the solution of the diffusion equation is compared with the results obtained by direct numerical integration of the motion equations for a statistically significant ensemble of particles. A good agreement with the local quasilinear FPK diffusion is observed. In addition, when the amplitude of the perturbation is varied, oscillations of the average action values around the quasilinear results are found on the long time scale.

ISSN:0899-8221    

DOI:10.1063/1.860857

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Nonlinear diffusive interpenetration of a finite‐&bgr; plasma and a magnetic field is considered in the framework of the one‐fluid magnetohydrodynamics (MHD) in the slab geometry. If the characteristic interpenetration time is much longer than the magnetoacoustic time, the process proceeds under the force equilibrium. Then the problem can be reduced to a single second‐order nonlinear diffusion equation in the Lagrangian coordinates. Two examples of the interpenetration are considered. The first presents the diffusion of a plasma into a plasma in a perpendicular magnetic field. The problem proves to be self‐similar, and its solution is found. The second one concerns the diffusive expansion of an initially confined plasma slab across a perpendicular magnetic field. The problem is solved numerically and the results are compared with an asymptotic self‐similar solution, found earlier in the low‐&bgr; limit.

ISSN:0899-8221    

DOI:10.1063/1.860858

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effects of self‐consistently interacting internal currents are modeled in a tokamak plasma with given external toroidal and poloidal magnetic fields. The unperturbed external magnetic surfaces are described through the well‐known nonlinear ‘‘standard map.’’ When the magnetic field is allowed to carry an internal current, the self‐interaction of these currents disturbs the integrity of the magnetic surfaces. A computational study of the effects of the interacting internal current filaments measures the diffusion of field lines from the unperturbed surfaces, and finds the self‐interaction to be a significant effect that always serves to increase diffusion. Perfect surfaces are not maintained even when magnetic islands would not otherwise overlap. Diffusion from the current interaction dominates when current fluctuations reach a fraction of the applied field.

ISSN:0899-8221    

DOI:10.1063/1.860859

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Convenient mathematical models appropriate for simultaneously measuring the thermal and energetic electron diffusivities by their responses to internal disruptions are presented and compared to simultaneous soft and hard x‐ray sawtooth data from the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion1, 479 (1981)]. The eigenfunction expansion technique employed is first illustrated on a constant diffusivity, single sawtooth crash model that is unable to give a satisfactory fit to the TEXT soft x‐ray data at disparate radii for an initial condition of a flattened temperature profile out to the mixing radius. A more sophisticated single crash model having a parabolic radial dependence for the thermal diffusivity and a flattened profile initial condition substantially improves the fit to the TEXT soft x‐ray data. Simultaneous measurements of the sawtooth oscillations on the hard x‐ray signal caused by runaway electrons hitting the limiter are interpreted to obtain a measure of the diffusivity of the energetic electrons in TEXT following the same crash used to measure the thermal diffusivity. A single sawtooth crash model is no longer adequate because the diffusivity of the energetic electrons may be substantially less than the thermal value. Therefore the eigenfunction expansion technique is extended to a periodic sawtooth crash model. Measurements of diffusivities far smaller than those that can be measured by a single crash model are possible and more than one value of diffusivity can fit the simultaneous TEXT hard x‐ray data. The lower values of diffusivity could not have been obtained by a single crash model. Additional hard x‐ray measurements having differing sawtooth periods are needed to remove the ambiguity. As more extensive hard x‐ray data become available on TEXT, it may become possible to simultaneously and unambiguously measure the thermal and energetic electron diffusivities during the same crash.

ISSN:0899-8221    

DOI:10.1063/1.860860

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Studies of stimulated Raman scattering (SRS) based on the one‐dimensional Zakharov–Maxwell equations for homogeneous, finite plasmas are presented for a wide range of parameters relevant to current laser–plasma interaction experiments. It was found that the primary mechanism responsible for saturation of SRS is the parametric decay instability (PDI) of the resonantly driven Langmuir wave. The different spatiotemporal evolution of SRS (convective) and PDI (absolute) leads to new physics in SRS nonlinear evolution, including disruption of PDI cascade, localization of Langmuir fields, and burstlike behavior of SRS reflectivity. The asymptotic saturation levels are well approximated by a simple scaling law, which is proportional to the PDI threshold and depends on the SRS convective gain factor.

ISSN:0899-8221    

DOI:10.1063/1.860861

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The problem of the existence of a minimum energy state is studied in the setHof all the magnetic fieldsB: (i) occupying the half‐cylinderD={r<R,z≳0}; (ii) having a normal component vanishing on the vertical part {r=R} of the boundary ofDand taking given valuesQ(r) on its horizontal part {z=0}; (iii) having a relative helicity equal to a prescribed valueH. It is first shown that the only field that may possibly be an energy minimizer inHis the unique (and therefore axisymmetric) constant‐&agr; force‐free fieldB&agr;contained in that set. Thus it is proved thatB&agr;minimizes the energy indeed if and only if 0≤‖H‖≤Hc≤∞, whereHcis an estimated critical value. ForHc≤‖H‖≤∞, on the contrary, it is possible to construct inHnonaxisymmetric fields with an energy smaller than that ofB&agr;and no minimum energy state does exist. However,B&agr;still minimizes the energy forHc≤‖H‖ ≤Haxc(with possiblyHaxc= ∞) if attention is restricted to the axisymmetric fields ofH. These results are used to put a limit on the validity of a popular model of the heating of the solar corona, in which the field of a coronal structure is supposed to release sporadically, by Taylor’s relaxation, a part of the energy it continuously extracts from the kinetic energy of the photospheric motions. It is argued that, as a consequence of the results above, one of the basic assumptions of the model breaks down when the field becomes highly sheared. It is speculated that, in such a situation, a completely new regime should set up, in which helicity and energy are continuously ejected at large distances by the system.

ISSN:0899-8221    

DOI:10.1063/1.860862

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The coherent corrections to the radiation spectrum of a relativistic electron beam, propagating along an external magnetic field are investigated. In the beam reference frame the cyclotron radiation from a hot magnetoplasma is calculated taking into account the electron correlations. The fluctuation‐dissipation theorem for the case of an equilibrium plasma is used to find the spectrum of current‐density fluctuations. At harmonics of the cyclotron frequency ≥2, the collective distortion of this spectrum decreases the radiation intensity compared to the spontaneous one. The coherent effects manifest themselves at wavelengths of the order of the Debye shielding length and become important when the electronic density in the laboratory frame is higher than 1013cm−3for typical experimental beam parameters.  

ISSN:0899-8221    

DOI:10.1063/1.860849

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A kinetic formalism of mode coupling in a whistler‐pumped free electron laser (FEL) is developed. The analysis includes the effect of finite space charge field of the electron beam. Kinetic effects appear through the beam plasma susceptibility in the coupling coefficient. For a Gaussian distribution in beam axial momentum, the growth rate of FEL instability falls off rapidly with increasing momentum spread. The growth rate is also a sensitive function of pump frequency and plasma density. Feasibility of this device in the low‐current Compton regime is also examined.

ISSN:0899-8221    

DOI:10.1063/1.860850

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Az‐discharge plasma channel is used to confine and transport an intense proton beam. A pinch‐reflex ion diode on the NRL Gamble II accelerator focuses a proton beam onto the entrance aperture of a 2.5 cm diam, 1.2 m longz‐discharge transport system. The beam ions are charge and current neutralized in the discharge plasma, and execute betatronlike orbits in the magnetic field of the discharge. Ion beam diagnostics include shadowbox imaging and prompt‐&ggr; radiation measurements from LiF targets. Under appropriate conditions, 95% particle transport and 90% energy transport are observed, with the only energy loss attributed to classical stopping in the channel gas. The transverse phase‐space distribution of the beam measured by the shadowbox is consistent with full charge and current neutralization of the transported beam.

ISSN:0899-8221    

DOI:10.1063/1.860851

出版商:AIP    

年代:1993

数据来源: AIP