Electron Beam Focusing Using Current-Carrying Plasmas in High-νγDiodes

Abstract

A technique has been demonstrated for concentrating electron beams to 5xl0 6 A/cm 2 plasmas on the axis of diodes. A two-dimensional particle code has been used to illustrate the importance of both the ExB motion in vacuum and the self-pinch of the beam within the plasma.Interest has increased recently in applying intense electron beams to the problem of pulsed fusion. One of the fundamental requirements in achieving such a goal is the focusing of multimegampere currents onto submillimeter targets. For the last five years or more, fairly extensive beam research has been carried out in propagation and compression of 50~500-kA beams beyond the diode, with current densities of % 10 5 A/cm 2 achieved. Theoretical and experimental studies have indicated the futility of such an approach insofar as the necessary current densities in the 10 9 -10 10 A/cm 2 range are concerned. The goal of the work described here is to utilize the combined effects of azimuthal magnetic fields together with the applied electric field within the beam-generating diode to accomplish the desired results. We will discuss in this paper experimental and computational results giving the properties of self-pinched beams in diodes, and the focusing improvement obtained as a result of placing a current-carrying plasma along the diode axis.It was observed in early experiments with highv/y diodes that when a critical value of the current was reached, self-pinching of the beam occurred. Although the resultant current density at the anode (~ 10 5 A/cm 2 ) was in excess of the space-charge-limited value, the pinch was not reproducible and formed late in the pulse with a radial collapse velocity of -0.1 cm/nsec. Ecker 1 found that impedance values could be correlated at current maximum with a "parapotential" model originated by De Packh. 2 This model treated the electron trajectories as lying along conical equipotentials with an arbitrarily imposed current along the axis, and it did not treat motion across equipotentials at the cathode edge nor at the anode.Recent numerical studies of this problem 8 have shown that although the dominant mechanism in such a self-pinched diode is EXR motion toward the axis, the assumptions of the parapotential model do not apply. These calculations show that a substantial concentration of current will occur on axis of such a diode if a layer of plasma exists near the anode. They also show that the VB drift away from the anode, as well as space charge near the axis, will tend to inhibit the pinch formation. The primary effect of the spacecharge buildup is to distort the equipotentials sufficiently to force electrons into the anode before they reach the diode axis. Experiments have been conducted to minimize both of these detrimental effects in order to further enhance the diode self-pinch.Mosher et al.* showed that exploding wires could be used in low-impedance diodes to achieve output loads in the l-£2 range, and that under certain circumstances an energetic beam could be generated within such a wire. After we noted that in ...

Electron beam focusing and application to pulsed fusion

“…Experimental attempts to obtain both highly pinched beams and low-impedance diodes have been carried out employing the two types of phenomena discussed above: (a) self-pinch in large-aspect-ratio (low-impedance) vacuum diodes [25], and (b) further beam focusing within a space-charge-neutralizing plasma [35]. The concept, shown schematically in Fig.…”

Production and Focusing of High‐current Beams of Relativistic Electrons Up to High Densities

“…I ,2 In order to realize recent proposals, an electron beam with a current density of about 10 8_109 afcm 2 , a pulse duration of (5)(6)(7)(8)(9)(10) ns, and a beam energy of 3 MJ has to be generated. We have no such beam today.5-8 Therefore, the main experimental investigations of high-current beams being carried out at I. V. Kurchatov Institute are concerned with the problems of focusing and decreasing pulse duration.…”

“…During the last few years, ideas regarding heating solid D-T mixtures up to thermonuclear temperatures'v' by high-power electron beams have been further developed.v" It is important that the requirements for beam focusing and beam duration have essentially decreased, as compared with results of earlier works. I ,2 In order to realize recent proposals, an electron beam with a current density of about 10 8_109 afcm 2 , a pulse duration of (5)(6)(7)(8)(9)(10) ns, and a beam energy of 3 MJ has to be generated. We have no such beam today.5-8 Therefore, the main experimental investigations of high-current beams being carried out at I. V. Kurchatov Institute are concerned with the problems of focusing and decreasing pulse duration.…”

Two-Dimensional Calculation of Electron-Beam-Target Interaction

“…5 Such a contribution would have an intensity and spectral line shape that would be essentially temperature independent. Although this would cause the /yv" 1 data to bend away from the 1/VH" 1 line in Fig.…”

“…1 " 4 Considerable effort is devoted to obtaining highly pinched electron beams. 5 " 10 The process of energy deposition in the high-current, high-energy-density regime differs from the classical cold-target process in a number of ways. One of these is the effect of target expansion due to the high pressures which develop during the energy-deposition process.…”