J.R. Boller

Theoretical modeling and experimental characterization of a rod-pinch diode

The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even f...

Triggered vacuum flashover switch for high‐power applications

A command triggered, high‐power, surface‐flashover closing switch that operates in vacuum has been developed for use on a prototype inductive‐storage pulsed power generator, Pawn. This vacuum flashover switch isolates the high‐pressure‐gas tamped wire fuse from a second opening switch. The switch consists of an insulating ring sandwiched between electrodes. Plasma and ultraviolet light from eight small spark discharges driven by a 5 keV pulse initiate a flashover across the switch insulator. The entire triggering unit resides inside one of Pawn’s metallic conductors. The switch can be triggered after holding off voltage for ≂15 μs. Normally, switch closure occurs at 22–45 kV. Time to closure at a voltage of ≂30 kV is ≂320 ns, with a typical jitter of ±50 ns. Peak current is typically ≂1 MA. Current density in the switch is approximately 25 kA/cm2. The average risetime of the fuse output current pulse can be varied by a factor of 2 by triggering the switch at different closing voltages.

Plasma Opening Switch Experiments On Hawk With An E-beam Diode Load

Successful application of inductive energy storage depends critically on the performance of the opening switch. The new Hawk generator at NRLI is used in plasma opening switch (POS) experiments in the 1-/spl mu/s conduction time regime to study long conduction time POS physics. In this experiment, different POS configurations were used, including various switch to load distances and different cathode center conductor radii. The load was an e-beam diode. Peak load powers of 0.5 TW, with load current risetimes of 20 ns and current transfer efficiencies of 80%, were achieved with a POS conduction time of 0.75 /spl mu/s using a 5 cm diam cathode. Typically, 40 kJ were coupled into the diode, which is 20% of the energy stored in the Hawk capacitance. The data indicate that above a critical load impedance the final switch gap, as determined from magnetic insulation arguments, is fixed to 2.5-3 mm, independent of conduction current and center conductor radius. Above this critical load impedance, current is shunted into the transition section between the switch and the load such that the voltage remains constant. At lower impedance values, the load voltage decreases in proportion to the load impedance. This critical load impedance is then the optimum impedance for maximum load power. Increasing the cathode magnetic field by conducting more current (up to a limit) or by decreasing the cathode center conductor radius at a given current level allows the switch to remain insulated at a higher voltage. Peak load voltages up to 1.7 MV were achieved using a 5 cm diam center conductor, a factor of 2 higher than that obtained with a 10 cm diam center conductor and 2.7 times higher than the erected Marx voltage (640 kV).

Rod-pinch diode development for short-pulse radiography using extended-length cathodes

Rod-pinch diodes utilize a small-diameter anode rod extending through and beyond the plane of a thin annular cathode. At low voltage (< 1 MV) and short pulse duration (< 20 ns), it is difficult for the space-charge-limited current (SCL) to exceed the critical current necessary for electron beam self-pinching using cathodes thinner than the anode-cathode gap spacing. Because the SCL is proportional to the cathode length and the critical current is independent of it, we have studied multiple-disk, extended-length cathodes for the purpose of achieving critical current early in the pulse. Experiments were conducted on the TriMeV facility at 0.8 to 1.2 MV and 20 to 40 kA with a 15-ns pulse duration. The cathode radius was typically 4 mm with cathode lengths varying from 3 mm to 3 cm. The anode radius was 0.25 mm. As expected, the longer cathodes achieved critical current earlier in the pulse and continued to operate at critical current for the pulse duration. Source diameters measured in the forward direction were about 0.5 mm with doses 1 m from the source in the 0.3- to 0.5-Roentgen range.

RADIACHROMIC FILM AS A DETECTOR FOR INTENSE MEV PROTON BEAMS

A simple technique has been developed to image intense pulsed proton beams with radiachromic film and to measure their spatial distribution. The optical density (OD) of 50 μm thick film, sufficient to stop 1.5 MeV protons, is measured to infer the incident beam fluence. The OD increases nearly linearly with fluence up to 0.25 cal/cm2. This film is used to measure the uniformity of large-area beams as well as the detector in a multiple-pinhole camera to determine the source uniformity and divergence of applied-magnetic field and pinched-beam ion diodes.

Radius scaling of X-radiation from gas-puff implosions on an inductive driver

The output of X radiation from gas-puff implosions is studied in reference to its theoretical prediction by simple model. Results show that the gas-puff radius, but not implosion time, controls radiation yield. Radius-scaling models being fairly reliable may lead to an overestimation sometimes. (AIP)

Absolute calibration of nuclear activation diagnostics for intense pulsed ion beams

Absolute calibration of an annihilation‐radiation NaI coincidence detection system for short‐lived ion‐induced radioactivity is described. Calibration with a 22Na radioactive source is compared with an independent calibration using the 12C(p,γ)13N(β+) reaction with 1 MeV protons from a Van de Graaff accelerator. These calibrations are consistent, provided correction for summing of the 0.51 and 1.27 MeV gamma rays from the 22Na source in the NaI detectors is included. This summing correction depends on the total efficiency for detecting 1.27 MeV gamma rays and can alter the calibration by as much as a factor of 2, depending on the size and separation of the detectors. A procedure to determine this correction using 60Co and 22Na radioactive sources is described. These two calibrations give a thick‐target yield for the 12C(p,γ)13N reaction at 1 MeV of (8.0±0.4)×10−10 reactions/proton. Intense pulsed proton fluences of 0.08 to 3.7 cal/cm2, determined by coincidence counting β+ annihilation radiation produced ...

Investigation of power flow to a plasma opening switch driven electron-beam diode

Summary form only given, as follows. Experiments are being conducted on the HAWK inductive storage generator at the Naval Research Laboratory using a plasma opening switch (POS) to drive an electron-beam (e-beam) diode load. Utilizing a suite of current, voltage, and radiation diagnostics to measure power flow out of the POS as it opens into an e-beam diode, these experiments aim to elucidate the switch-load coupling, a phenomenon which is not currently well understood. Localized, shielded B-dot pickup coils located in the cathode and anode are used as current monitors to measure generator and load currents, to determine magnitude and location of current losses, and to determine the partitioning between electron-flow and cathode currents. The radiation diagnostics include a time resolved bremsstrahlung spectrometer, a Compton-Hall voltmeter, a scintillator photodiode, two pinhole cameras, and thermoluminescent dosimeters. The spectrometer is used to measure the high-energy tail of the bremsstrahlung spectrum, from which the electron energy may be inferred. This is compared with a direct load-voltage measurement using a vacuum voltmeter. By comparing the measured currents, load voltage, and spectrometer data, the effect of electron-flow current on the bremsstrahlung spectrum may be investigated. The effects of varying the POS conduction time, the anode-cathode gap, and the POS-to-load distance on POS-diode coupling will be reported.

Development of an applied-magnetic-field diode for ion-beam-transport experiments

Summary form only given. An applied-magnetic-field ion diode (ABD) is being developed to study the transport of intense ion beams for light-ion inertial confinement fusion. Initially, the beam from this diode will be used to test the concept of self-pinched transport (SPT). The design goal is diode operation at 1.5 MV and 250-kA total current on the Gamble II generator at NRL. This goal is based on Gamble II experiments with a 50-cm/sup 2/ area ABD, and a grooved-anode flashover source which produced 55 kA at 1.25 MV and V/sub crit//V/spl ap/2.5. Also, a measured beam divergence of 25 mrad is used in this design. A diode with an anode area of 160 cm/sup 2/ is required. To match this diode to Gamble II, a parallel electron-beam load upstream of the ABD is used to clamp the voltage, producing a more monoenergetic beam with improved focusing. For SPT experiments, the beam is extracted from the diode and focused into a transport channel. The ATHETA code (Sandia National Laboratories) is used to calculate B-field configurations in the diode and ion-beam trajectories.

Simple cable delay multiplexer for displaying multiple nanosecond signals on a single oscilloscope channel

A simple multiplexer circuit is described which allows the displaying of two or more coincident wide band pulse−type signals on a single horizontal oscilloscope sweep by means of appropriately clamping, delaying, and mixing the input signals.