J.E. Rauch

Recent ACE 4 Z-pinch experiments: Long implosion time argon loads, uniform fill versus annular shell distributions and the Rayleigh-Taylor instability problem

Hammer (1996) and Velikovich (1996) have discussed ways to mitigate the growth of the magneto-Rayleigh-Taylor (MRT) instability in z-pinch (PRS) implosions. They predict that initial mass distributions more complex than a simple annular shell will reduce instability development. Sanford (1996) reported experimental data showing a benefit for a uniform mass distribution compared to a shell; those tests used “conventional” load radii of 2.25 and 1.25 cm respectively, and implosion times under 100 ns. However, the instability problem is expected to grow exponentially as the implosion time, or alternatively the initial radius, increases. Thus we made a comparison of a uniform fill load with a shell but at larger radii, 3.6 and 2.5 cm respectively, and at implosion times well above 100 ns. We see nearly a factor of 10X improvement in peak K-shell power and 2X increase in K-shell yield for the uniform mass load. Hence it appears that suitable tailoring of the imploding mass distribution can significantly limit ...

Sub‐kilovolt X‐ray Emission from Imploding Wire Plasmas

We present measurements of the sub‐kilovolt X‐ray emission from imploding wire arrays on the 1 TW BLACKJACK 3 pulsed power generator. The plasma is created by driving a 1 MA, 100 ns current pulse through a cylindrical array of 12 fine wires. The wires form individual plasmas which then implode to become a single plasma on the axis of the array; this hot, dense plasma is an intense source of soft X‐rays. Calorimeter measurements show that maximum soft radiation yield is produced from arrays having a linear density of 100 to 200 μg/cm; for such arrays the implosion occurs within 20 ns of the current peak. The dependence of the total yield on mass is largely independent of wire material. The radiation pulsewidth measured with an unfiltered X‐ray diode increases monotonically with array mass from a 25 ns FWHM observed for 50 μg/cm arrays.The soft X‐ray spectra radiated by the imploding plasmas approach a blackbody spectrum with increasing wire mass and atomic number. Tungsten and silver plasmas radiate contin...

DECADE Quad monolithic POS development on ACE 4 [pulsed power plasma opening switch]

The DECADE Quad (DQ) is a Defense Threat Reduction Agencys radiation effects simulator consisting of four individual DECADE Modules (DMs). When operated as a large area Bremsstrahlung source, each of the DMs utilize separate plasma opening switches (POSs) and e-beam loads. A monolithic POS for DQ is being developed for small area Bremsstrahlung source applications requiring a single e-beam diode. Power flow from the four DMs will be converged upstream of the monolithic POS which will drive a nominal 0.2 /spl Omega/, small area Bremsstrahlung source. The baseline approach for the development of a monolithic switch for DQ is a POS currently being tested on ACE 4. The applicability of ACE 4 as a test bed is based on a conduction current time(I/sub c/*T/sub c/) scaling argument applied to the conduction phase. The key concept for I/sub c/*T/sub c/ scaling, the preservation of the opening phase initial conditions, has both experiment and theoretical support. The DQ prototype coaxial POS uses a 9 cm radius cathode, a 3 cm A-K gap, and has a nominal 20 cm length. A set of 15 Teflon(R) substrate flashboards supply the necessary POS plasma, injected through a 60% transparent anode mesh. Issues being addressed in this development besides I/sub c/*T/sub c/ scaling include an understanding of current losses due to ion currents in the POS region and PQS-to-load coupling issues related to POS opening and plasma transport into the load region. In this paper, the authors report on the characterization of several POS-to-load coupling geometries on ACE 4.

Integrated, interactive modeling of plasma opening switches, plasma radiation sources and bremsstrahlung diodes with commercial circuit analysis software tools

The authors describe their use of analog behavioral modeling techniques to develop physical system models of plasma opening switches, plasma radiation sources and bremsstrahlung diode radiation sources. A commercially available electronic circuit analysis software program is used as the programming environment for the models, and as the calculation engine for the system. The models are created as macros, which are saved in a component library. Macros may be selected like any other component and inserted into the electrical schematic/circuit model of an associated pulse power generator. The output is a realistic simulation of electrical system response and predicts radiation yield. These features, combined with a simple drag and drop graphical user interface, allow rapid interactive optimization of electrical pulse generator and radiation source design parameters. The use of standardized, modern code serves as a framework for collaboration and allows the incorporation of special devices. The technique has been used as an aid to the design of a compact X-ray simulator, and is presently being used to assist in studies of new machine technology.

Conduction time/current limitation on the Defense Special Weapons Agency DECADE Module 1

The Defense Special Weapons Agencys DECADE radiation effects simulator design consists of 16 separate pulsed power modules each driving individual bremsstrahlung diodes. DECADE is required to produce a dose in a near field test plane of 20 krad (Si) with 2:1 uniformity over 10000 cm/sup 2/, as well as meeting several other specifications. Each DECADE module utilizes a plasma opening switch (POS) in the final stage of pulse compression. DECADE Module-1 (DM 1), the first full scale module tested, is capable of delivering up to 1.8 MA in 300 ns to the POS. Power delivery to the load is dependent on the POS and load coupling performance. During testing the DMI performance level was found to be nearly a factor of two below that required to meet the DECADE dose specification. A DECADE Assessment Program was initiated to evaluate and assess the likelihood of achieving the original design specification based on the performance of a single module. The Assessment Program, with emphasis on diagnostics, was coordinated by scientists from Maxwell Technologies, the Naval Research Laboratory, and Primer/Physics International and included involvement by other collaborators throughout the pulsed power community. This paper describes the results of a study addressing the observed degradation in dose with increased POS conduction current and/or conduction time.

Improved radiation output from a DECADE module

The DECADE generator consists of individual modules operated in parallel. Each module consists of a Marx bank, a water transfer capacitor, a passive water line, a vacuum line, a plasma opening switch (POS), and a bremsstrahlung diode load. Initial results from single module tests on DECADE Module 1 resulted in less radiation than originally specified. A subsequent, multi-laboratory effort demonstrated that the bremsstrahlung from this module can be increased to the levels originally specified for the multi-module system. The increased radiation was largely the result of two changes: discharge cleaning in the POS-load region with an argon-oxygen mixture (instead of pure argon), and a change in the converter package. Evidence exists for other processes that limit the radiation. If these limiting processes are circumvented, the radiation output could increase by an additional factor of two.

Initial results for tandem puff X-ray loads on ACE 4

Summary form only given, as follows. Thompson et al. (1989) reported on a new Z-pinch load termed a tandem puff. In this geometry, current from a Marx bank is initially carried by an annular plasma column. Magneto-hydrodynamic forces push the current sheath axially. At the end of the conduction region, the current is transferred onto an annular gas puff without an intervening vacuum power flow volume. The puff then implodes much like a standard Z-pinch. The advantages of this geometry are (1) it uses a simple, compact and relatively inexpensive Marx bank, (2) a separate plasma opening switch (POS) is not needed between the source and the load, and (3) Z-pinch parameters like mass loading and radius need not be tuned to match the Marx banks current time history. We present here initial tests of this tandem puff configuration using the 4 MJ ACE 4 inductive energy storage (IES) machine. Neon and a few krypton puffs from an 11 centimeter diameter fast gas valve gave significant (>10 kJ) soft X-ray (/spl sim/1 keV) yields. Load currents exceeded 3 megamps, a level 3 times higher than our earlier experiments. We observed implosion times of over 400 nanoseconds. Our diagnostics included calorimeters, time-resolved sensors (PCDs, XRDs, and silicon PIN diodes), and time-integrated images (pinhole and KAP spectra). Only a limited number of shots were made, hence this configuration is far from optimized. However, our results suggest that the tandem puff geometry is very promising as a long implosion time X-ray source.

Measurements of the K Continuum for Argon Z-Pinches

Summary form only given. While the characteristic resonance lines dominate the K-shell X-ray emission from Z pinches, the energy in the K continuum (free-bound and free-free transitions) can be significant. A simple model implies that the continuum could supply well over 10% of the K-shell yield if the plasma is warm enough and dense enough. We present here new measurements of the continuum strength for argon implosions. The data were taken at peak currents of 3.5 MA with implosion times exceeding 200 ns. Heavily filtered GaAs photoconductive detectors (PCD) were used as direct measures of the argon continuum above ~4 keV. Other major diagnostics included X-ray power sensors, calorimeters, time-integrated CCD spectra and CCD pinhole images. By combining the data from all of these instruments, we have derived the strength and temperature of the continuum as well as the yield in the K-lines using a self-consistent method. As implosion conditions changed, the changes seen in electron temperature (derived from K-line ratios) were not as large as the changes seen in the continuum temperature (derived from the PCDs). The yield in the continuum was close to a linear function of the K yield. The relative strength of the continuum does appear to depend on the detailed mass distributions produced by the 12 cm diameter nozzles used for these tests

GaAs - A Versatile Photoconductive Material for the Measurement of X-Rays in Pulsed Power Applications

We developed four types of GaAs PCDs: bremsstrahlung (two types), soft X-ray, and gamma for pulsed reactors. GaAs PCDs are advantageous in applications that require fast response, high dose rate, and/or neutron insensitivity. Desired detector sensitivity and response were obtained through modification of carrier lifetime by neutron irradiation, size of the sensing element, and orientation of the sensing element. The GaAs PCDs were used to characterize pulsed radiation sources in over 10 different tests and the measured data from these tests are presented. GaAs PCDs (E7 - E11 rad(Si)/s) for use with pulsed bremsstrahlung sources were tested at DTRA Radiation sources, Linac (Idaho Accelerator Center), Linac (White Sands), hybrid radiation source (NRL), GAMBLE II (NRL), and HERMES III (Sandia). GaAs PCDs for gamma characterization in pulsed reactors (E6 - E7 rad(Si)/s) were calibrated with a Linac (White Sands), checked for high dose rate (E12 rad(Si)/s) response at HERMES III, checked for low dose rate response (5-500 rad(Si)/s) and stability at a Co-60 source (Sandia), and checked for stability, sensitivity, and residual radioactivity at the ACCR reactor (Sandia). Soft X-ray (1 to 15 keV) GaAs PCDs (E2-E6 Watts/cm2) were used to measure the argon plasma free-bound continuum temperature from a plasma radiation source (PRS) and compared with the plasma temperature obtained from line ratio measurements.

Parallel biased GaAs photoconductive detector

The response of a transverse biased photoconductive detector (PCD) may be modeled as a continuously variable parallel resistor through the depth of the detector. Since the bias is perpendicular to the dose gradient the electric field is constant and the mean conductivity is proportional to the mean dose rate. The response of a parallel biased PCD can be modeled as a continuously variable series resistor through the depth of the detector with the bias electric field dependent on the resistivity at the given depth in the detector. Hence, even though the front region the PCD becomes highly conductive, the PCD will not respond unless free carriers are generated through the entire depth of the PCD between the bias electrodes. Experimental measurements on parallel biased gallium arsenide, GaAs, PCDs using the MBS (Modular Bremsstrahlung Source) pulse x-ray source at AEDC (Arnold Engineering Development Center) are given that substantiate the response model. Agreement in the response between PCDs of varying x-ray filtration were observed with the effective dose model given in the paper as opposed to a 40% discrepancy when the mean dose is used.