摘要:

The development of the fast reconnection mechanism is systematically studied in a sheared (skewed) field geometry with no magnetic neutral (null) point. Initially, there are antiparallel fields (of componentBx0) with a current sheet in the middle, as well as a uniform sheared field component (Bz0). On the basis of the spontaneous fast reconnection model, a number of computations demonstrate that, initiated by a small disturbance, the fast reconnection mechanism can be fully established by releasing the energy associated with the antiparallel fields. Once the quasisteady fast reconnection mechanism is set up, the magnetic flux is drastically transferred, leading to an effective topological change of the skewed field configuration. The thin (shock) transition layer standing in the quasisteady fast reconnection region is divided into the intermediate wave region and the slow shock region. In the intermediate wave region, the magnetic field simply rotates without changing plasma quantities, whereas in the slow shock region a slow shock is combined with an intermediate wave. For the largerBz0, the finite‐amplitude intermediate wave becomes dominant so that the slow shock characteristics become obscure in the transition layer. The reconnection (flux transfer) rate is estimated to scale asE0/[1+tan2(&OHgr;/2)], where &OHgr;=2 tan−1(Bz0/Bx0), andE0is the reconnection rate for the coplanar case (Bz0=0) and tends to decrease as the plasma beta becomes larger.

ISSN:0899-8221    

DOI:10.1063/1.860688

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In the high gain strong pump regime, the radiation field amplitude and phase can vary substantially over a wiggler period. Also, due to the periodic transverse motion, the longitudinal electron energy suffers large variations over the same distance. Because of these considerations, the spatial average that leads to the classical one‐dimensional free‐electron laser equations is not applicable here. Thus the rederived equations suitable for the high gain strong pump regime contain 2&pgr;/kwperiodic terms. Comparisons with the Electron Laser Facility (ELF) experiments of the Lawrence Livermore National Laboratory [Phys. Rev. Lett.54, 889 (1985);57, 2172 (1986); Phys. Rev. A35, 2184 (1987)] on the operational characteristics including the relative phase of the radiation field are made.

ISSN:0899-8221    

DOI:10.1063/1.860689

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A relativistic collisionless electron beam is described through moments on the Vlasov equation. The closure is obtained both in the case of local thermodynamical equilibrium and for awarmplasma. Radial steady solutions are studied in the two closure approaches and compared: In particular, the generalization of the Bennett pinch solution is found in the warm closure.

ISSN:0899-8221    

DOI:10.1063/1.860690

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A nonlinear analysis of the wave–particle interaction in the output cavity of a frequency‐doubling magnicon amplifier is presented. The cavity is immersed in an axial magnetic field, wherein electrons interact with a rotating TM210mode via a gyroresonant mechanism. The electron equations of motion and the wave equation for the radio‐frequency (rf) field are derived and expressed in terms of scaled variables in order to study the general scaling of the output cavity. Single‐electron, time‐dependent simulation of the interaction in the cavity is performed to assess the accessibility and stability of high‐efficiency states for an X‐band magnicon. Results from multielectron, steady‐state simulation are presented to show the effect of spreads in electron‐beam radius, energy, and transverse momentum on the efficiency.

ISSN:0899-8221    

DOI:10.1063/1.860691

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theoretical model is developed for the two‐beam klystron, where two concentric annular electron beams propagate through a grounded conducting tube. The beams premodulated at the first cavity enter the second cavity, exciting it. The induced voltage at the second cavity returns a part of the inner beam toward the first cavity, establishing a mutual communication between two cavities. The in‐phase condition of the return current is obtained in terms of the intercavity distance. The maximum current‐modulation condition at the second cavity is also obtained. These two conditions determine an optimum intercavity distance. The oscillation behavior of the first cavity is investigated, including influence of the intercavity distance on the induced gap voltage appearing on the cavity opening. The relationship between the induced voltage and the modulated return current in the opening is found from an equivalent circuit representation of the cavity impedance. The boundary between the oscillator and amplifier regions is described in the parameter space defined by the normalized return‐current strengthhand intercavity distance represented by the phase angle &agr;. In the amplifier region (h sin &agr;<1), the amplification occurs only for a narrow range of the phase angle &agr;. The induced voltage grows exponentially in the oscillator region satisfyingh sin &agr;≳1. Introducing a nonlinear saturation term into the return current, the induced voltage is bounded in the oscillator operation.

ISSN:0899-8221    

DOI:10.1063/1.860638

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The propagation of an ion ring in a 2.5 m long, 0.30 m diam magnetized plasma column was studied using an axial array of magnetic probes and fast proton detectors, a microwave interferometer, and a grating spectrometer. The ring propagated with an initial axial velocity 3×106m/sec, about four times the Alfve´n speed in the plasma, and excited damped magnetosonic waves whose peak amplitude depended on the plasma return current. The plasma electrons were heated by about 1 eV in response to the ring, as measured by the H&ggr;line‐to‐continuum intensity ratio. This heating is primarily attributed to classical Coulomb collisions, although about 1%–10% can be attributed to the collective ring–plasma interaction. The collective interaction had no measurable effect on the dynamics of the ion ring, consistent with theoretical models, given the beam, plasma, and magnetic‐field parameters in the experiment.

ISSN:0899-8221    

DOI:10.1063/1.860639

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Three‐dimensional equations for the plasma wake fields in the presence of a single frequency, intense circularly polarized electromagnetic (CPEM) wave in a two‐component magnetoplasma are derived. For a given CPEM envelope, the axial electric fields of plasma waves are obtained for a one‐dimensional model problem in which all the waves are aligned along the magnetic‐field lines. It is found that the presence of an external magnetic field can significantly enhance the strength of the plasma wake fields when the right‐hand CPEM wave is used for the excitation purposes.

ISSN:0899-8221    

DOI:10.1063/1.860640

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Ion extraction from a plasma plays important roles in various technology fields, such as ion plantation, ion beam generation, and laser isotope separation. In this paper, the behavior of ions during the extraction process is investigated in detail using two types of particle simulation codes, and the ion‐extraction time is compared with the results of an analytical model. A two‐dimensional code, in which the electrons are assumed to be distributed, in accordance with the Boltzmann relation, while the ion behavior is calculated by the particle model, results in essentially the same values for ion‐extraction time, as with the analytical model. On the other hand, the results of the one‐dimensional full particle code partly differ from that of the two‐dimensional code and the analytical model. The use of the Boltzmann distribution of the electrons tends to overestimate the ion‐extraction time up to a factor of approximately 2 at its worst point.

ISSN:0899-8221    

DOI:10.1063/1.860641

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Observations carried out with a laser‐induced fluorescence diagnostic on several configurations of a multipolar discharge are presented. This diagnostic allows very accurate measurements of the ion velocity distribution function in the argon plasma and reveals the presence of double‐hump velocity distribution functions due to ion acceleration in the static plasma potential and ion reflection on a multipolar magnetic field. These observations give a new insight to ion temperature and ion drifts inside the device.

ISSN:0899-8221    

DOI:10.1063/1.860642

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The modulated vortex model introduced by Zabusky and McWilliams [Phys. Fluids25, 2175 (1982)] is applied to a cylindrical plasma to study dynamical behavior of a many‐vortex system. Anticyclones are shown to be long lived while cyclones are dissolved into the core and outer dresses which are left behind the core. This observation is in good agreement with the experimental results by Antipovetal. [Sov. J. Plasma Phys.14, 648 (1988)].

ISSN:0899-8221    

DOI:10.1063/1.860643

出版商:AIP    

年代:1993

数据来源: AIP