摘要:

The magnetic trapping effects of weakly relativistic electron on electron cyclotron maser instability in the magnetic mirror fusion devices are studied. The integrodifferential eigenmode equation is solved perturbatively with a small part of weakly relativistic electron population. Compared with the previous local model, the results indicate that the trapped effects provide stabilization to the electron cyclotron maser propagating along the magnetic field.

ISSN:0899-8221    

DOI:10.1063/1.860544

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A quantitative theory of laser hot spots, which control plasma instabilities in real laser–plasma interactions, is presented in the case of random phase plate (RPP) optics. It is shown that the probability density of intense hot spots with intensityI,Phot(I), is given byPhot(I)∼(I/I02)exp(−I/I0) whereI0is the average intensity, and that the detailed amplitude and phase variation of the laser field in the vicinity of an intense hot spot is uniquely specified by the optics and isdeterministic. These hot spots may be the source of below threshold stimulated Raman scattering (SRS) and its variation withI0is shown to be super exponential. A brief preview of a quantitative nonlinear theory of hot‐spot‐induced laser filamentation is presented.

ISSN:0899-8221    

DOI:10.1063/1.860545

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The generation of long and homogeneous plasmas by multiphoton ionization of low‐pressure gases—H2, D2, and N2at pressures of a few Torr—with a high‐power laser at &lgr;=0.53 &mgr;m and intensities of the order of 1015W/cm2has been studied. The temperature and density of the plasma are measured by Thomson thermal scattering. After the initial full ionization the hydrodynamic density evolution with time is observed. The comparison with a simple model shows that this evolution is mainly due to the ponderomotive force of the laser beam itself. Typical density variations range from 1% to 5% per 100 psec.

ISSN:0899-8221    

DOI:10.1063/1.860491

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theory is presented describing the electron cyclotron interaction at frequencies near cutoff, followed by a Cˇerenkov interaction region. In such a case, the cyclotron interaction withdraws only the orbital component of electron momentum, while in the Cˇerenkov interaction the electrons lose their axial momentum. It is shown that the addition of the Cˇerenkov interaction significantly enhances the total electronic efficiency. Since both kinds of operation are relatively insensitive to electron velocity spread, the efficiency of the combined interaction is also rather tolerant to velocity spread. Thus, rather efficient sources of electromagnetic radiation based on poor quality electron beams may be developed.

ISSN:0899-8221    

DOI:10.1063/1.860492

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The recombination of the major charged species on a metallic wall has been analyzed in a low‐pressure and highly dissociated nitrogen plasma jet. The electron temperature and density in the boundary layer have been measured by electrostatic probes. Information about the temperature and density of some excited species has been deduced from emission spectroscopic measurements. It is shown that the metallic surface is not completely catalytic. The recombination rate has been determined and follows a 0.5 reaction order near the leading edge. The experimental results show that the Bohm criterion overestimates the value of the ion drift velocity at the sheath entrance and confirm the recent value proposed by Zawaideh [Phys. Fluids B2, 647 (1990)]. At last, a strong enhancement of the N2+(B) emission has been observed near the wall.

ISSN:0899-8221    

DOI:10.1063/1.860493

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The use of surfaces that emit cold electrons into a plasma has been shown to reduce the sheath potential drop by at least a factor of 2.6. This effect can be used to mitigate sputtering by causing a reduction of the mean energy of ions striking the emissive surface. The present study has been motivated by the severe sputtering problems that may occur at the divertor plate of a full power fusion reactor. In addition, secondary electron emission is a common occurrence at the bounding surface of a plasma, and so should be accounted for in the plasma analysis. Kinetic theory shows that, for a hydrogen plasma (singly charged, mean ion mass of 2.5 amu) with an electron temperature ofTeand ion temperature of 0.1Te, emission reduces the sheath potential drop from 3.9Teto 1.5Te. This reduction occurs when the surface emits a cold electron current that is increased to nine times the incident ion current. For comparison with theory, thermionic electron emission has been studied in a hydrogen plasma (composed mostly of H3+) powered by inductive coupling from an rf coil. The plasma was in contact with an electrically isolated surface of tungsten impregnated with barium scandate. Plasma potential was measured with a Langmuir probe and with a mass and energy analyzer that sampled the energy distribution of ions passing through an aperture in the emissive surface. As the surface temperature was increased, the surface potential floated up to the plasma space potential so that the ion energy at the surface dropped from 5Tefor temperatures below 570 °C to less than 1Tefor temperatures above 680 °C. The Richardson equation, which determined emission current as a function of surface temperature, and the above kinetic theory, was used to predict that the reduction in potential drop should occur from 420 °C to 510 °C. The abrupt reduction in ion energy over a narrow range of surface temperature was observed experimentally as predicted; however, the sudden reduction began at a surface temperature of 600 °C rather than 420 °C. The delayed onset of emission was probably caused by the sputtering removal rate of the impregnate at the surface exceeding the replenishment rate from the bulk for disk temperatures below the normal operating range.

ISSN:0899-8221    

DOI:10.1063/1.860494

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The region between a Maxwellian plasma source and an absorbing surface that emits cool electrons is modeled numerically with dynamic, electrostatic particle simulation and theoretically with a static, kinetic plasma‐sheath model. Steady‐state emission results are applied specifically to secondary electrons that are induced by either incident ions or electrons, but are also valid for thermionic and photoelectrons. The ratio of the emitted electron current to incident electron current is varied up to and beyond the critical emission coefficient (ratio) that causes electric field reversal at the collector. Results from these models agree very well over the range from zero to five times the critical emission coefficient. Increasing the secondary emission coefficient is found to reduce the collector potential and decrease the ion energy deposited, yet increase the total energy flux to the collector. In the simulation, some heating of the secondary electron stream is observed to gradually evolve over many Debye lengths, possibly because of a beam–plasma interaction. This heating increases potential fluctuations but causes only small deviations from the predictions with static theory.

ISSN:0899-8221    

DOI:10.1063/1.860495

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An accurate and comprehensive self‐consistent kinetic model is applied to a helium radio frequency (rf) discharge which in experiments exhibits an ‘‘&agr; to &ggr;’’ transition. The main conclusions are (i) there is good agreement (factor of 2) between the experiment and simulation in plasma density, power density, and current density across two orders of magnitude of the applied voltage. (ii) The bulk electron temperature exhibits a transition from high at low driving voltages to low at high voltages. The low‐voltage regime exhibits a larger bulk electric field in the simulation. (iii) Multistep ionization is important under all conditions studied.

ISSN:0899-8221    

DOI:10.1063/1.860496

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A pedagogical calculation based on the thermal‐equilibrium guiding‐center plasma model is used to illustrate deep consistency relations that the nonlinear dielectric response must obey. It is shown that these are satisfied by both the direct‐interaction approximation and a related Markovian statistical closure.

ISSN:0899-8221    

DOI:10.1063/1.860497

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

an algebraic error is found in Equation (1) of the paper by Mizunoet al.1Their measured threshold is analyzed following the corrected from of Equation (1). (AIP)

ISSN:0899-8221    

DOI:10.1063/1.860498

出版商:AIP    

年代:1993

数据来源: AIP