摘要:

The interaction of an intense relativistic electron beam (REB) with preformed channels in gaseous atmospheres has been analyzed in order to delineate the effects of reduced density, avalanche ionization, preexisting conductivity, and channel currents. The REB for these experiments was produced from a field emission diode driven by the ≈1.4 MV pulse from a pulse forming line. Relativistic electron beam currents up to ≈16 kA with current densities up to ≈2 kA/cm2were achieved and the REB current was approximately a half sine wave of width 27 nsec (FWHM). Preformed channels in the atmosphere were created using laser‐guided electric discharges. Current‐carrying reduced density channels were produced by applying a second discharge to the reduced density channel produced by the first discharge. Reduced density (≤&rgr;0/80), nonconducting channels were produced by the absorption of radiation from a pulsed CO2laser in ammonia gas at background pressures of ≈40 Torr (&rgr;0/20). The results show that reduced density had little effect on REB propagation except for a decrease in scattering until the density within the channel had been reduced to such a low level that the dominant mechanism by which conductivity is generated shifted from direct collisional ionization to avalanche ionization. Avalanche ionization in a uniform atmosphere increases the growth of REB instabilities but when it is limited to the reduced density channel region the REB was always repelled or expelled from the channel. Preexisting channel conductivity (&sgr;≥0.1 S/m) also caused the REB to be repelled or expelled from the channel. The presence of a parallel channel current permitted the REB to be readily injected into the channel and guided along it with minimal losses. All of these effects and the thresholds at which they occurred are consistent with the present understanding of the interaction of intense REB’s with gaseous atmospheres.

ISSN:0031-9171    

DOI:10.1063/1.866174

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A two‐dimensional model for the electron flow in the presence of a virtual cathode is presented. The model allows for electron reflexing and velocity distribution spread. Solutions with substantial radial flow agree closely with high‐power microwave emission observed in experiments. The radial motion is the result of the self‐magnetic field of the beam, which bends some of the electrons from propagating to side walls to reflex between real and virtual cathodes. The resulting plasma frequency solution agrees with the measured microwave frequency, which scales linearly with the diode current instead of the square‐root scaling for a one‐dimensional flow. Scaling laws are derived and agree with experimental measurements.

ISSN:0031-9171    

DOI:10.1063/1.866181

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Under the assumptions of quasineutrality and small perturbation to the applied solenoidal magnetic field, collective focusing of a neutralized, intense pulsed ion beam injected into a solenoidal magnetic field region is demonstrated. The focal point was observed to occur within the solenoidal magnetic field and was at a location in between the single‐particle electron and ion focal points. The focused beam current density was an order of magnitude above the 20 A/cm2of the incident beam. The experimental results were found to be in good agreement with a theoretical model developed under the above assumptions. The presence of background plasma or a conducting liner was observed to remove the collective focusing, confirming the physical picture of the theoretical model.

ISSN:0031-9171    

DOI:10.1063/1.866182

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A general analysis of the electromagnetic wakefields for an axisymmetric charge distribution moving through a cold uniform plasma is presented. Particular attention is given to the electromagnetic and relativistic effects influencing the waves. It is shown that the plasma provides shielding of the transverse wakefields on a scale length of a few electromagnetic skin depths (c/&ohgr;p). The implications for plasma wakefield accelerators are that driving beams with radii of a few skin depths will experience severe pinching, whereas larger beams will be subject to filamentation instabilities. These conclusions are supported by self‐consistent particle‐in‐cell simulations. The deleterious effects of transverse wakefields will severely limit the accelerating gradients that can be obtained even if a magnetic field is used to guide the driving beam.

ISSN:0031-9171    

DOI:10.1063/1.866183

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The stability of two‐dimensional infinitesimal disturbances of soliton solutions of the two‐dimensional Davey–Stewartson equations [Proc. R. Soc. London Ser. A338, 101 (1974)] is considered. It is shown that a previously developed generalized variational method, which allows the calculation of the maximum growth rates close to the cutoff, is applicable here. A numerical evaluation of the variational principle is presented.

ISSN:0031-9171    

DOI:10.1063/1.866184

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A particle‐in‐cell technique is used to model a plasma sheath in which the magnetic field intersects the surface at an oblique angle. The angle of impact of deuterium and impurity ions striking the surface is examined and found to depend on the relative sizes of the ion gyroradii and the Debye length.

ISSN:0031-9171    

DOI:10.1063/1.866185

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A hydrodynamic treatment of ponderomotive interactions in a collisionless plasma is presented and it is shown that consistent hydrodynamics leads to the correct expression for the solenoidal ponderomotive electron current density, a result previously thought to be derivable only in the framework of the warm‐plasma kinetic theory.

ISSN:0031-9171    

DOI:10.1063/1.866186

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The evolution of a plasma with a localized electric field perpendicular to an external magnetic field is shown to be dominated by the Kelvin–Helmholtz instability. For small ion gyroradius, the instability is similar to the fluid mode. When the ion gyroradius is an appreciable fraction of the spatial extent of the electric field, the plasma is not in equilibrium, and the initial drift profile relaxes. Subsequent evolution still leads to vortex flows.

ISSN:0031-9171    

DOI:10.1063/1.866187

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A general expression for the effective collision frequency associated with current‐driven ion sound turbulence in an inhomogeneous plasma is given based on generalized ponderomotive forces. The order of magnitude is estimated for each of the terms for the generalized ponderomotive forces.

ISSN:0031-9171    

DOI:10.1063/1.866188

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Oscillatory flow in a tube of slowly varying cross section is investigated in the presence of a uniform magnetic field in the axial direction. A perturbation solution including steady streaming is presented. The pressure and shear stress on the wall for various parameters governing the flow are discussed.

ISSN:0031-9171    

DOI:10.1063/1.866189

出版商:AIP    

年代:1987

数据来源: AIP