The impedance of a diode having an annular cathode and indented anode that terminates a coaxial MITL (magnetically insulated transmission line) is measured and compared with a semiempirical model developed from calculations made using the magic code. The measurements were made on the 16-Ω electron accelerator HELIA (high-energy linear induction accelerator) operating at 3 MV. The model agrees with the measurements within the 10% measuring error and shows that the diode operates in either a load- or line-dominated regime depending on AK (anode-cathode) gap spacing. In the load-dominated regime, which corresponds to small AK gaps, the diode impedance is controlled by an effective anode-cathode gap, and the flow is approximately axial. In the line-dominated regime, which corresponds to large AK gaps, the impedance is independent of the AK gap and corresponds to the impedance associated with the minimum current solution of the MITL, with the flow becoming more radial as the AK gap is increased.

HERMES III is an accelerator being designed as a uniform source of flash ..gamma..-rays. Diode designs are needed that efficiently convert electrical energy to ..gamma..-ray energy and that distribute that ..gamma..-ray energy uniformly over a large area. Code simulations of coaxial diode designs show that the electron beam in the anode-cathode gap forms a weak pinch, which results in excessive on-axis radiation. In this report, a diode concept is developed that can reduce the pinch at the anode converter and thus can improve the uniformity of the radiation. This concept employs an indentation in the anode to passively control the beam. For this diode, electron flow, impedance models, and scaling laws of the diode behavior as a function of the geometric parameters of the diode, voltage, and current are developed. We evaluate the subsequent radiation output and improvement in radiation uniformity relative to diodes with planar anodes. The analysis shows that not only is the indented-anode capable of significantly improving radiation uniformity, but that it is also capable of reducing the width of the radiation pulse.

Use of an indented anode in a cylindrical-cathode, planar anode diode placed at the exit of a coaxial magnetically insulated transmission line on the helia accelerator results in improved radiation uniformity. Measurements of the uniformity made on the accelerator at 3 MV, 150 kA are compared and shown to agree with theoretical calculations. The advantage of this scheme over that of the planar anode is that the diode impedance, pinch angle, and radial impact position at the anode can be partially decoupled.

: PROTO-II is a nominal 10 TW, 320 kJ accelerator which has been used to study imploding plasma physics for the last few years. The machine has been modified to make it useful as a bremsstrahlung radiation source and to lower the inductance for better energy coupling to gas puff loads. The triplate water transmission line has been converted to a 4-line horizontal 8-plate transformer section feeding a 4-layer insulator stack, using a multiple rod crossover network. Hinged plates allow a constant impedance transmission line for gas puff applications and make a 2:1 impedance transformer for bremsstrahlung applications. For Gas Puff operation, vertical MITLs connect the 4-layer stack to the load. For bremsstrahlung operation, conical MITL plates connect each of the four lines to feed one side of a 2-cathode ring electron beam diode. Circuit simulations of the power flow predict up to 270 kJ of energy at 1.0 MV into the Gas Puff diode and up to 230 kJ at 1 .5 MV into the electron beam diode. Accelerator performance under the new configuration is discussed.

Hermes III is a 20 MV 500 to 800 kA linear induction accelerator being designed for use as a large-area uniform source of flash ..gamma..-rays. Diode designs that can efficiently convert the electrical energy to ..gamma..-ray energy and distribute that ..gamma..-ray energy uniformly over a large area are needed. Code simulations of coaxial diode designs show that the electron beam at the anode/cathode gap forms a weak pinch resulting in excessive on-axis radiation. Modifications to the anode/cathode structure to reduce this pinch by controlling the radial electric field are discussed. Results of diode simulations using the PIC code MAGIC and the electron/photon transport code CYLTRAN are presented.

Collisionless single particle trajectories are modeled for a single plasma channel having one section curved in a circular arc. The magnetic field is developed by superposition of straight and curved channel segments. The plasma density gives charge and beam-current neutralization. High transport efficiencies are found for turning a relativistic electron beam 90° under reasonable conditions of plasma current, beam energy, arc radius, channel radius, and injection distributions in velocity and in position at the channel entrance. Channel exit distributions in velocity and position are found consistent with those for a straight plasma channel of equivalent length. Such transport problems are important in any charged particle-beam application constrained by large diode-to-target distance or by requirements of maximum power deposition in a confined area.

A model describing the transport of relativistic electron beams in plasma channels and their subsequent interaction with solid targets is developed and applied to single-beam and multiple-beam configurations. For single beams the targets consist of planar tantalum foils and, in some cases, cusp fields on the transmission side of the foils are employed to improve beam/target coupling efficiency. In the multi-beam configurations, several beams are arranged in wagon-wheel fashion so as to converge upon cylindrical targets, consisting of either hollow tantalum or solid graphite cylinders, located at the hub. For 0.3-cm beam radii that are less than or equal to the channel radii, mean specific power depositions up to about 17 TW/g per MA of injected beam current are obtained for single beams; 12-beam results are typically an order-of-magnitude less. The corresponding enhancements are up to five times the collisional stopping power for either single or multiple beams. Substantial improvement is predicted for the multi-beam interaction should future channel technology permit transport at higher current densities in smaller channels.

$\ensuremath{\nabla}B$ transport, bunching, and focusing of relativistic electron beams give power deposition levels which may provide the absorbed fluxes of 100 TW/${\mathrm{cm}}^{2}$ believed necessary to drive breakeven inertial-confinement-fusion targets. Predicted depositions in excess of 100 (TW/g)/MA are presented here. These levels are up to two orders of magnitude higher than those previously calculated and appear to meet the absorbed-flux requirement.

Expressions are derived for the distance over which an ion beam will pinch to its first node for four cases: A—axially injected solid cylindrical beam; B—solid beam with geometric focus; C—hollow cylindrical beam; D—converging disk beam.

Two aspects of double sheaths (nonneutral regions) in plasmas are studied via analytic models. First, the motion of the structure through the plasma is analyzed by considering the force balance among the various species. A general proof is presented which shows the net acceleration of the sheath is zero in the absence of dissipative processes. A specific example is given which shows that a pressure difference between the two sides of the sheath results in the enhancement of the beam current into the low pressure side rather than acceleration of the sheath. The effect of beam-plasma interactions is also discussed. The second part of the analysis is the derivation of separate (though nearly identical) density criteria for sheath nucleation and sheath maintenance in equilibrium. These are compared with experimental results of Lutsenko, Sereda, and Kontsevoi and reasonable agreement is obtained.

Single-particle trajectory calculations have been used to study the effectiveness of beam concentration near a target pellet in two relativistic electron-beam-driven inertial fusion concepts: magnetic cusps and multiple current-carrying channels in a wagon-wheel configuration. Angular momentum constraints favor the multichannel transport scheme. The role of angular momentum nonconserving reflections from plugged-cusp loss cones is discussed. Minimum radii are calculated for various injection conditions in a 40-channel configuration. The resulting superposition of beams may provide an order-of-magnitude improvement in beam concentration over single-beam results.

The PULSAR compressed magnetic flux, pulse power generator system has applications as a topping stage in commercial power generation. A plasma armature mode of operation is described which could be powered by an inertially confined fusion reactor. The paper summarizes the development of a plasma armature and experimental results from its application in a PULSAR generator which utilizes a 0.5 m diameter superconducting magnet. A larger generator under construction which utliizes a l m diameter magnet is briefly described and estimates of its performance are given.

Two sets of finite-temperature relativistic fluid equations are obtained by taking moments of the Vlasov equation, using equilibrium and monoenergetic distribution functions. The closed sets of fluid-Maxwell equations are reduced to a simple set of equations under steady-state conditions, using two fluid constants of the motion derived for each set. The set obtained using the monoenergetic distribution is parametrized in cylindrical coordinates for high-current diode studies. The radial scale length for a radial equilibrium superpinch is obtained in terms of macroscopic diode parameters, and radial profiles of the pinch are obtained by solution of the one-dimensional system. It is found that high-current superpinches are characterized by a hot uniform-density core surrounded by a hollow current sheet, and that there is a limit to the current which can be propagated for a given pinch radius, given the macroscopic diode parameters. The minimum pinch radius obtainable in a diode in the steady state is obtained from the hot pinch limit, and diode scaling laws are presented, assuming Child-Langmuir or parapotential flow.

The plasma response to a strong high−frequency electric field has been reformulated in a way that shows that the basic nonlinear parameter is α = Λη0 (ωp/ω0)2, where Λ is essentially the number of particles in a Debye sphere and η0 = E0 (4πn0kBT)−1/2 is the usual electric field parameter. Numerical integration has produced results which show such phenomena as locally increased absorption, lower dissipation rates for strong fields, and somewhat deeper penetration of the electromagnetic wave turning point (compared with weak field results).