E. J. T. Burns

Applied‐B ion diode experiments on the Particle Beam Fusion Accelerator‐I

A series of experiments was performed with an Applied‐B ion diode on the Particle Beam Fusion Accelerator‐I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied‐B diode operation from the sub‐TW level on single module accelerators to the multi‐TW level on a low impedance, self‐magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100‐TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of...

Applied‐B field ion diode studies at 3.5 TW

The performance of 7 and 14 cm radius, applied B‐field, focusing ion diodes is investigated on the Proto II accelerator at a diode power of 3.5 TW. The ion production efficiency of the 14‐cm diode is determined to be approximately 80% for the 30‐ns power pulse used. When a 2 Torr air background gas was used in the beam propagation region between the diode and the target, the ion beam was observed to be approximately 70% magnetically neutralized in regions of approximately 20 kG transverse B‐field and ≥95% in regions of no B‐field. The local and global divergence of the 1.4‐MV beam were each determined to be approximately 1°. Local source power brightness of >1.5×1013 W/cm2/rad2 is inferred.

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 lithium‐fluoride flashover ion source cleaned with a glow discharge and irradiated with vacuum‐ultraviolet radiation

We have studied methods of varying the ion species generated by a lithium‐fluoride overcoated anode in a 0.5‐MV magnetically insulated ion diode. We found that cleaning the anode surface with a 13.6‐MHz rf glow discharge or illuminating the anode with a pulsed soft x‐ray, vacuum‐ultraviolet (XUV) radiation source just before the accelerator pulse significantly altered the ion species of the ion beam produced by the diode. The glow‐discharge plasma removed adsorbates (carbon, hydrogen, and oxygen) from the surface of the LiF flashover source. The ions seen were lithium and hydrogen. Unfortunately, the diode impedance with a lithium‐fluoride anode was high and the ion efficiency was low; however, XUV irradiation of the surface dramatically lowered the impedance by desorbing neutrals from the ion source via photon‐stimulated desorption. Current densities of ten times the Child–Langmuir space‐charge limit were achieved under XUV irradiation. In particular, ion currents increased by over a factor of 3 when 12 ...

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.

Low energy x‐ray emission from light ion targets

The light ion fusion program is expected to achieve breakeven conditions in an inertial fusion target in experiments on the PBFA II accelerator. This goal is expected to require ion power densities of ≲1014 W/m2. The diagnostics which have been employed to diagnose this deposition are described and some soft x‐ray plasma measurements which have been made on targets on the 200 times smaller Proto I accelerator are presented. Plasma brightness temperatures of 20 eV have been observed in the deposition region and 35 eV (100 eV electron temperature) in the stagnation region of imploding conical Al targets on Proto I.The light ion fusion program is expected to achieve breakeven conditions in an inertial fusion target in experiments on the PBFA II accelerator. This goal is expected to require ion power densities of ≲1014 W/m2. The diagnostics which have been employed to diagnose this deposition are described and some soft x‐ray plasma measurements which have been made on targets on the 200 times smaller Proto I accelerator are presented. Plasma brightness temperatures of 20 eV have been observed in the deposition region and 35 eV (100 eV electron temperature) in the stagnation region of imploding conical Al targets on Proto I.