摘要:

Solutions to the Woltjer–Taylor relaxed plasma equation ∇×B=&mgr;B, with &mgr; a constant, inside an infinitely long, circular cylinder with magnetically impermeable walls and no net axial magnetic flux, are obtained for complexk, where modal spatial dependence is exp i(m&thgr;−kz). Complexkyields the continuum of externally driven states between &mgr;=0 (currentless) and the finite ‖&mgr;‖ free‐states. The continuum states are related to formation of plasma from magnetized electrode sources.

ISSN:0031-9171    

DOI:10.1063/1.866515

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

F‐&thgr; pumping is studied in a 1‐D model including the hyperresistivity caused by tearing mode turbulence. When the hyperresistivity is sufficiently large compared to the resistivity, a positive, although time‐varying, toroidal current can be maintained without a net toroidal voltage. For less hyperresistivity, the toroidal current periodically changes sign.

ISSN:0031-9171    

DOI:10.1063/1.866164

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The ion rotational angular velocity &OHgr; and the ion temperatureTiof a translated field‐reversed configuration (FRC) plasma are measured using neutral beam probe spectroscopy. The value of &OHgr; is ∼(1.0∼1.2)×&OHgr;* at the onset time of then=2 rotational instability, where &OHgr;* is the ion diamagnetic frequency for a rigid‐rotor equilibrium. The ion rotational direction is the same as the ion diamagnetic direction. The value of &OHgr; is smaller than the angular frequency &ohgr;reof then=2 instability, which can yield experimental evidence of the ion kinetic effects on then=2 instability in the FRC plasma. When the octupole field is applied to the plasma in order to suppress then=2 deformation, &OHgr; is slightly reduced. The ion temperatureTiis ∼70 eV at the onset time of then=2 instability.

ISSN:0031-9171    

DOI:10.1063/1.866165

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The propagation of laser light in a thermally unstable plasma attributable to potential imbalances in the local heating and cooling rates caused by density or temperature perturbations is considered. The linear growth rates of laser filamentation and thermal instability are governed by a single fifth‐order dispersion relation. The behavior of different modes in various limits is discussed in detail and examples of potential applications to laser–target interactions are given.

ISSN:0031-9171    

DOI:10.1063/1.866166

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Experimental measurements of ablation parameters in a KrF laser produced aluminum plasma are presented for the laser intensity range of 1011–1013W/cm2. Plasma created from large focal spots shows single broad ion current pulses as compared to two groups of ions observed from small focal spots. Detailed analysis of the velocity distributions of emitted ions is carried out to understand this behavior and its effect on measurements of the ablation parameters. Scalings with laser intensity of the mass ablation rate (m˙∼I0.42L) and of the ablation pressure (Pa∼I0.81L) givingm˙=1.5×105g cm−2 sec−1andPa=5.5 Mbar at a laser intensity of 1013W/cm2are obtained. The results are found to be in good agreement with the predictions of a plasma ablation model based on inverse bremsstrahlung dominated absorption.

ISSN:0031-9171    

DOI:10.1063/1.866167

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Simultaneous measurements of x‐ray intensity and shock breakout at the target rear side showed a significant increase in the postshock x‐ray transmission through the target. This was attributed to target decompression by the unloading wave. The results corroborated previously observed delayed radiation heating of the target rear surface. They also indicated that time‐integrated x‐ray measurements on the target rear side could be affected by target decompression.

ISSN:0031-9171    

DOI:10.1063/1.866168

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A formulation is given for the parametric coupling of nonlinear space‐charge waves with transverse magnetic (TM) waveguide modes through a longitudinal wiggler field. The model is based upon the simplifying assumption of thin electron beam geometry, which requires only one‐dimensional dynamics of the space‐charge waves, and is an exact formulation in the limit of infinitesimally thin beams. A reduced model, obtained through the application of standard time‐averaging techniques, leads to an analytic expression for the parametric growth rate as a function of beam parameters, pump strength, and wave mismatch. A numerical code is developed and used to solve the unapproximated model equations, and comparisons are made with the analytic theory. Agreement is good in the domain of applicability of the theory. It is also found numerically that nonlinear steepening and associated slowing of the space‐charge waves—aspects usually neglected in ponderomotive theories—have important consequences on the evolution of the instability in the strongly nonlinear regime.

ISSN:0031-9171    

DOI:10.1063/1.866169

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Propagation of a nonrelativistic electron beam in a plasma in a strong magnetic field has been studied using electrostatic one‐dimensional particle simulation models. Electron beams of finite pulse length and of continuous injection are followed in time to study the effects of beam–plasma interaction on the beam propagation. For the case of pulsed beam propagation, it is found that the beam distribution rapidly spreads in velocity space generating a plateaulike distribution with a high energy tail extending beyond the initial beam velocity. This rapid diffusion takes place within a several amplification length of the beam–plasma instability given by (&ohgr;p&ohgr;2b)−1/3V0, where &ohgr;p, &ohgr;b, andV0are the target plasma, beam–plasma frequencies, and the beam drift speed. This plateaulike distribution, however, becomes unstable as the high energy tail electrons free‐stream, generating a secondary beam. A similar process is observed to take place for the case of continuous beam injection when the beam density is small compared with the total densitynb/nt<1. In particular, the electron velocity distribution is found monotonically decreasing in energy, having a high energy tail whose energy reaches twice the initial beam energy. Such an electron distribution is also seen in laboratory experiments and in computer simulations performed for a uniform, periodic system. When the beam density is increased so that the beam current exceeds the thermal return current,enbV0≳enevt, whereneandvtare the density and thermal speed of the ambient electrons, beam propagation becomes much slower due to the electric field generated by the excess charges associated with the beam electrons.Beam electrons are reflected from the ambient plasma as if they are bouncing off a rigid wall. When the beam velocity is increased while holding the beam density constant, simulations show that the beam current can exceed significantly the return current generated by the thermal electronsenevt. It is shown that the electric field generated by the beam–plasma instability accelerates the ambient electrons opposite to the beam propagation, thereby enhancing the return current.

ISSN:0031-9171    

DOI:10.1063/1.866171

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Injection of a nonrelativistic electron beam into a fully ionized plasma from a spacecraft including the effect of charging has been studied using a one‐dimensional particle simulation model. It is found that the spacecraft charging remains negligible and the beam can propagate into a plasma, if the beam density is much smaller than the ambient density. When the injection current is increased by increasing the beam density, significant spacecraft charging takes place and the reflection of beam electrons back to the spacecraft reduces the beam current significantly. On the other hand, if the injection current is increased by increasing the beam energy, spacecraft charging remains negligible and a beam current much larger than the thermal return current can be injected. It is shown that the electric field caused by the beam–plasma instability accelerates the ambient electrons toward the spacecraft thereby enhancing the return current.

ISSN:0031-9171    

DOI:10.1063/1.866172

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

In this paper the features of propagation of intense relativistic beams are discussed in the environment of low‐density (≲1 Torr), partially ionized gas. Beam phenomena in this ion‐focused regime (IFR) are found to be analogous to those at much higher density. Several instabilities and dissipative mechanisms have been identified and are compared to their high‐density counterparts. Scaling laws are derived for quantitative estimation of these effects. In general, it is found that long propagation distances are possible in the laboratory suggesting that this regime may be useful for controlled beam transport and conditioning.

ISSN:0031-9171    

DOI:10.1063/1.866173

出版商:AIP    

年代:1987

数据来源: AIP