A. J. Toepfer

Electron beam focusing and application to pulsed fusion

This paper reviews recent work on the focusing of high-power relativistic electron beams in diodes and discusses concepts for pulsed fusion based on this technology. The physics of high-current relativistic electron beam focusing using plasmas in high-current diodes is studied experimentally and with computer simulation. The physics of the beam interaction with dense targets and the requirements for break-even are briefly discussed.

Self-magnetic insulation in vacuum for coaxial geometry

Magnetic insulation obtained by employing the magnetic field of the line current in coaxial vacuum‐transmission lines is studied in experiments on two different relativistic electron‐beam accelerators, spanning the voltage range 0.4–10 MV. Effective magnetic insulation at fields up to 1.3 MV/cm is demonstrated. The self‐limiting impedance is measured and compared to a number of theories for magnetic insulation and it is found that none of the ’’standard’’ theories successfully describes the data. However, computer simulations using a self‐consistent two‐dimensional particle code give good agreement with the experimental data, as does a proposed modification of the parapotential flow model.

Plasma Instabilities in High‐Current Field‐Emission Diodes

Striations in electron‐energy‐deposition profiles from high‐current field‐emission diodes utilizing razor‐blade cathodes have been observed. The variation of wavelength in the striations with cathode geometry indicates that the current‐carrying plasma at the surface of the cathode undergoes a tearing instability during the initial stages of breakdown.

Pulsed power applications to intense neutron source development

Abstract The use of conventional and near term pulsed power technology to generate intense fluxes of neutrons for use in fusion reactor materials studies is discussed. Two types of neutron production mechanism are considered. For the immediate future, the use of single pulse or high rep rate intense ion beam sources is proposed to provide high fluxes of neutrons from beam-target interactions. Farther along in time, the use of intense ion or electron beams to initiate inertially confined fusion reactions will lead to intense sources of thermonuclear neutrons.

Shock focusing in implosions of inertial fusion model targets

Implosions of high‐gain targets for inertial confinement fusion must be highly spherically symmetric for efficient ignition. Using a single relativistic electron beam at low power (∼0.3 TW) and cylindrical targets, we have experimentally shown that loading asymmetries generate strong implosion asymmetries, resulting in poor convergence ratios. By introducing a nonspherical variation in the shell radius and thickness (shimming), the implosion symmetry is radically improved at one time during the implosion. The effect is explained by a shock focusing mechanism whereby ultrahigh pressure—0.9 Tpa (9 Mbar) —is achieved on one side of the target.

Use of image restoration and enhancement to improve x-ray pinhole camera resolution

Numerical image processing techniques are applied to the restoration of x‐ray pinhole photographs of solid targets irradiated by intense relativistic electron beams. The numerical and analytic bases for image restoration are briefly described. An experimentally determined point‐spread function for the pinhole camera is utilized to eliminate the effects of high‐energy x‐ray degradation of pinhole resolution. It is shown that penetration by hard x rays from filtered spectra through the pinhole edges can lead to apparent broadening of the electron‐beam pinch on high‐Z targets. Elimination of x‐ray shine‐through also permits determination of photon intensity from high‐Z target ablation products, and improves spatial resolution of the x‐ray source.

Implosion dynamics of a hemispherical target irradiated by an intense relativistic electron beam

An intense tightly pinched relativistic electron beam from the Hydra accelerator (250 kA, 800 kV, 80 nsec) was used to produce an ablation driven implosion of a dense hemispherical shell. A laser reflection measurement was used to measure the implosion time and the velocity of the jet of material produced following the implosion. X‐ray pinhole photography and optical holography were used to study the uniformity of target loading and ablator motion. Target dynamics was modeled by a two‐dimensional Eulerian hydrodynamic code. The results indicated 17–50% of the total beam energy was delivered to the target, where the total beam energies varied between 17 and 20 kj. The uniformity in target loading varied between 20 and 80% from pole to equator. Suggestions are given for improving beam accuracy and target loading uniformity.