R.J. Allen

Rod-pinch diode operation at 2 to 4 MV for high resolution pulsed radiography

The rod-pinch diode is operated successfully at peak voltages of 2.4–4.4 MV for peak electrical currents of 55–135 kA delivered to the diode. At 4 MV, tungsten anode rods of 1 or 2 mm diam produce on-axis doses at 1 m of 16 rad(Si) or 20 rad(Si), respectively. The on-axis source diameter based on the full width at half-maximum (FWHM) of the line-spread function (LSF) is 0.9±0.1 mm for a 1 mm diam rod and 1.4±0.1 mm for a 2 mm diam rod, independent of voltage. The LANL source diameter, determined from the modulation transfer function of the LSF, is nearly twice the FWHM. The measured rod-pinch current is reproduced with a diode model that includes ions and accounts for anode and cathode plasma expansion.The rod-pinch diode is operated successfully at peak voltages of 2.4–4.4 MV for peak electrical currents of 55–135 kA delivered to the diode. At 4 MV, tungsten anode rods of 1 or 2 mm diam produce on-axis doses at 1 m of 16 rad(Si) or 20 rad(Si), respectively. The on-axis source diameter based on the full width at half-maximum (FWHM) of the line-spread function (LSF) is 0.9±0.1 mm for a 1 mm diam rod and 1.4±0.1 mm for a 2 mm diam rod, independent of voltage. The LANL source diameter, determined from the modulation transfer function of the LSF, is nearly twice the FWHM. The measured rod-pinch current is reproduced with a diode model that includes ions and accounts for anode and cathode plasma expansion.

High-Power Self-Pinch Diode Experiments for Radiographic Applications

We report here on self-magnetic-pinch diode experiments at voltages from 3.5 to 6 MV. In addition to electrical diagnostics, the diode is characterized as a radiation source by dose and spot-size measurement. As the operating voltage increases, we find that a given diode geometry tends to produce a smaller spot but suffers from the reduced impedance lifetime. Optimization involves increasing the cathode diameter and diode gap as the voltage increases. We find a good quantitative agreement with the Monte Carlo code integrated tiger series over the entire data set, assuming an effective electron incidence angle of 20deg. Over this range, we observe favorable dose and spot scaling of optimized diode performance with voltage. Our best results are roughly 200-rad at 1 m with an ~2-mm-diameter spot. These were obtained at diode parameters of roughly 6 MV, 150 kA, and 30-ns radiation full-width at half-maximum.

Initialization and Operation of Mercury, A 6-MV MIVA

Mercury became operational in a stepwise manner to test the machine components after modifications and reassembly at NRL. To avoid damaging the MIVA, extensive testing of the laser and PFL output switches was performed using dummy loads. Finally, the PFLs were connected to the MIVA and Mercury was fired into a simple cylindrical diode load with a Marx charge voltage up to 75 kV. Measured MIVA currents and voltages compare well with a circuit model of the MIVA fed by the measured PFL outputs and with PIC simulations of the MIVA and the diode load.

Time-resolved voltage measurements of Z-pinch radiation sources with a vacuum voltmeter

A vacuum-voltmeter (VVM) was fielded on the Saturn pulsed power generator during a series of argon gas-puff Z-pinch shots. Time-resolved voltage and separately measured load current are used to determine several dynamic properties as the load implodes, namely, the inductance, L(t), net energy coupled to the load, E(coupled)(t), and the load radius, r(t). The VVM is a two-stage voltage divider, designed to operate at voltages up to 2 MV. The VVM is presently being modified to operate at voltages up to 6 MV for eventual use on the Z generator.

Status of the Mercury pulsed-power generator, a 6-MV 360-kA, magnetically-insulated inductive voltage adder

Mercury is a nominal 6-MV, 360-kA, 2.2-TW magnetically-insulated inductive voltage adder that is being assembled at the Naval Research Laboratory. Mercury, originally known as KALIF-HELA, was located at the Forschungszentrum in Karlsruhe, Germany. Once assembled, Mercury will be used as a testbed for development of high-power electron- and ion-beam diodes. Applications include source development for high-resolution flash radiography, nuclear weapons effects simulation, and particle-beam transport research. This paper highlights the progress of the Mercury assembly and supporting activities, including modifications from the original design, circuit modeling to optimize the Mercury circuit, power-flow simulations to understand and optimize Mercury power flow and load coupling, and MITL theory and modeling to develop a transmission-line code capability for modeling transient effects in MITLs.

A Battery Powered, 200-KW Rapid Capacitor Charger for a Portable Railgun in Burst Mode Operation at 3 RPS

A portable power supply is being developed to rapidly charge the capacitor bank of a low velocity railgun system for countermeasure deployment from aircraft and watercraft. The goal is charge a 15-mF capacitor bank to 2.3 kV in 200 ms to allow countermeasure deployment in bursts of several rounds at a rate of 3 RPS (rounds per second). Due to possible use aboard aircraft, components where chosen to minimize weight and volume. For this reason, as well as for simplicity and to reduce cost, a series bank of special, high-current lead-acid batteries was chosen as the source of prime power. The 192-V voltage of the battery bank is boosted to 2 kV using a forward converter comprised of four IGBT switches in an H-bridge configuration, a ferrite-core step-up transformer, and a full wave rectifier.

Overview of the 6-MV, rod-pinch experiment on ASTERIX

This work presents the highlights of high-voltage, positive-polarity experiments on the ASTERIX generator characterizing the rod-pinch diode at up to 6 MV as a source for high-resolution, flash radiography. The paper reviews experimental results and analyses including conversion of ASTERIX to positive polarity, rod-pinch electrical and radiation characteristics, comparisons to numerical simulation, and composite rod-pinch performance. A minimum LANL source diameter of 1.32 mm with a dose of 23.7 rad(air) at 1 m was achieved at a peak voltage of 5.9 MV, leading to a radiographic figure-of-merit of 13.6 rad(Si)/mm/sup 2/. The results demonstrate the utility of the rod pinch for high-power pulsed radiography at up to 6 MV.

Characterization and optimization of a compact, 1-MV, 6-kA radiography source

The hybrid radiation source (HRS) is a compact pulsed power generator consisting of a commercial flash X-ray system that has been retrofit with a custom front-end assembly, replacing a sealed glass tube. The HRS diode hardware consists of a tapered tungsten anode extended through an annular stainless steel cathode. The HRS has been successfully fielded, however, some source parameters, such as voltage, current, and source size, were not known. These parameters as well as the dose have now been measured with an array of diagnostics. Also, a circuit model has been developed to enable optimization and analysis of the complete system. The source size has been dramatically reduced by using a 1-mm-OD tapered anode. Anode heating has been shown to mitigate anode disintegration, allowing for multiple shots on a 1-mm-OD anode.

Improved bremsstrahlung from diodes with pulse-heated tantalum anodes

Bremsstrahlung from high-power electron-beam diodes increases and becomes spatially more uniform when the tantalum anode is first pulse-heated to remove gas from the surface and interior, and then pulse-heated again to white hot just prior to and during the high-power pulse. Heating the tantalum eliminates protons, retards beam pinching, and increases the far-field X-ray dose relative to unheated tantalum. The radiation pattern becomes symmetric and hollow, producing a more uniform near-field dose distribution than for unheated tantalum. With a white-hot anode, the diode current is single-species Child-Langmuir until the voltage exceeds 1 MV, at which point it reaches critical current. These phenomena demonstrate reduced effects from ions in the diode. The increase in dose is a result of both reduced ion current and enhanced electron reflexing through the subrange tantalum foil. The heating technique is compatible with high-power generators such as Decade, whose x-radiation output would increase by as much as 30%.

Detectors for intense, pulsed active detection

In intense, pulsed active detection, a single, intense pulse of radiation is used to induce photofission in fissionable material, increasing its detectability. The Mercury pulsed-power generator was converted to positive polarity (+3.7 MV, 325 kA, 50-ns FWHM) to drive an intense, pulsed radiation source based on the FIGARO active detection concept. The probing radiation source consisted of an ion-beam diode and a thick PTFE (Teflon) converter where 6–7 MeV γ-rays were produced via the 19F(p,αγ)16O reaction. A suite of radiation detectors was used both to detect the presence of irradiated fissionable material and to characterize the probing radiation source. Four types of detectors were used for the source characterization. Thermoluminescent dosimeters were used to measure the angular distribution of the dose associated with x-rays and γ-rays from the ion-beam diode. Plastic scintillator-photodiode detectors were used to characterize the time dependence of this dose. A plastic scintillator-photomultiplier detector was used to monitor the γ-ray intensity of the probing radiation source and to monitor changes in the production of background neutrons by the diode and PTFE converter. A set of rhodium foil activation counters was used to measure the absolute yield of these background neutrons. Two types of detectors with comparable sensitivities were used to measure delayed neutrons resulting from photofission: 3He proportional counters and a 6Li-loaded-glass-scintillator detector. The neutron detection rate from each detector following the probing radiation pulse was over 100 times higher with depleted uranium present than with lead.