A. V. Farnsworth

Anode plasma behavior in a magnetically insulated ion diode

The nature and time evolution of the ’’surface flashover’’ anode plasma in a magnetically insulated ion diode is studied. Holographic interferometric and spectrographic measurements indicate a plasma with an electron density of 5×1016 cm−3 and a temperature of approximately 5 eV which is created by electric breakdown along a surface parallel to the imposed pulsed electric field. The divergence of the ion beam accelerated from this plasma is governed by the spatial nonuniformities of the plasma. The beam is composed primarily of protons for the experiments studied. Contrary to previous expectations, a substantial C+4 beam component was not observed.

A radial ion diode for generating intense focused proton beams

A magnetically insulated light ion diode which has produced an ion beam with total current exceeding 400 kA for 25 ns and generated a proton current density approaching 500 kA/cm2 is described. This intense beam current is achieved in a noncurrent neutralized mode via geometric focusing and a balance between self‐magnetic field and space‐charge forces. A number of techniques are described which have been used to diagnose the beam production, transport, and focusing. These include observation of Kα emission due to beam‐induced atomic excitation, prompt‐γ radiation due to beam‐induced nuclear reactions, and thermal emission due to beam‐target heating.

A dual‐current‐feed magnetically insulated light‐ion diode

A 3×1011‐W modular magnetically insulated ion diode for inertial‐confinement fusion studies is described. Power is supplied to the diode, which is centered in an elongated solenoidal B‐field coil structure, by two self‐magnetically insulated transmission lines. A circular ion beam is emitted preferentially from one surface of the curved anode plate and extracted through a 13‐cm‐diam virtual cathode and exposed B‐field coils. The virtual cathode is formed by electrons which spiral along the B‐field lines that conform to the anode surface. The ion beam converges onto a planar target, located 13‐cm from the anode, through a 1‐Torr air plume which allows space‐charge and partial current neutralization of the beam. The beam is initially primarily composed of protons, but later carbon ions dominate. The proton beam has a divergence of 2.1°±0.3° and peak focused proton‐current density of 60 kA/cm2.

Soft x‐ray and vacuum‐ultraviolet spectroscopy of ion‐beam‐heated thin targets

XUV spectroscopy utilizing a 1‐m grazing incidence spectrograph and photoelectric diodes is used to determine the response of approximately one‐proton‐range‐thick planar targets to an intense beam of hydrogen and carbon ions. Electron temperature, brightness temperature, and total radiated power are then compared with radiation‐hydrodynamic calculations to determine the ion‐beam energy deposition and incident current density. Incident current densities of 25–35 kA/cm2 with 80% proton current and 20% singly ionized carbon ion current are consistent with the spectroscopic measurements.

Soft x-ray vacuum ultraviolet spectroscopy of thin targets heated by enhanced electron energy deposition

Soft x‐ray vacuum ultraviolet spectroscopy (10–600 eV) has been used to study relativistic electron beam energy deposition in 6‐μm Au foils mounted in two different anode geometries. Experimentally determined thermal electron temperatures of 173 eV and 306 eV are in agreement with calculations assuming electron energy deposition rates of 5×1013 W/g and 1014 W/g for the two geometries. These results are consistent with electrons multipassing through the thin foils several times.