J.P. Corley

Performance of the hermes-III gamma ray simulator

Hermes III is a 22-MV, 730-kA, 40-ns pulsed power accelerator that drives an electron beam diode/converter to generate an intense pulse of bremsstrahlung radiation. In Hermes III, eighty individual 1.1-MV, 220-kA pulses are generated and added in groups of four to develop twenty 1.1-MV, 730-kA pulses which are then fed through inductively isolated cavities and added in series by a magnetically insulated transmission line (MITL). An extension MITL delivers this surnmed output to the electron beam diode. The construction of Hermes III was completed February 1988 and was followed by an accelerator characterization and performance demonstration phase. Hermes III met all of its performance requirements during this testing period and has gone operational one year ahead or schedule. An overview of the Hermes-III design is presented together with details of the performance obtained for the various subsystems. A discussion of future applications of this technology is also presented.

The ZR refurbishment project

ZR is a refurbished (R) version of Z aiming to improve its overall performance, reliability, precision, pulse shape tailoring and reproducibility. Z, the largest pulsed power machine at Sandia, began in December 1985 as the Particle Beam Fusion Accelerator II (PBFA II). PBFAII was modified in 1996 to a z-pinch driver by incorporating a high-current (20-MA, 2.5-MV) configuration in the inner ∼ 4.5 meter section. Following its remarkable success as z-pinch driver, PBFA II was renamed Z in 1997. Currently Z fires 170 to 180 shots a year with a peak load current of the order of 18–20 MA. The maximum z-pinch output achieved to date is 1.6-MJ, 170-TW radiated energy and power from a single 4-cm diameter, 2-cm tall array, and 215 eV temperature from a dynamic hohlraum. ZR in turn will, operating in double shift, enable 400 shots per year, deliver a peak current of 26 MA into a standard 4cm × 2cm Z-pinch load, and should provide a total radiated x-ray energy and power of 3 MJ and 350 TW, respectively, achieve a maximum hohlraum temperature of 260 eV, and include a pulse-shaping flexibility extending from 100ns to 300ns for equation of state and isentropic compression studies. To achieve this performance ZR will incorporate substantial modifications and upgrades to Marx generator, intermediate store capacitors, gas and water switches, water transmission lines and the laser triggering system. Test beds are already in place, and the new pulsed power components are undergoing extensive evaluation. The Z refurbishment (ZR) will be operational by 2006 and will cost approximately

Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

60M.

Development/tests of 6-MV triggered gas switches at SNL

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has /spl sim/1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below /spl sim/100 kV/cm from positive electrodes or voltages below /spl sim/140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and /spl sim/300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at /spl sim/100 cm//spl mu/s. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching /spl sim/1% of the speed of light when the switch reaches closure.

Performance of the hermes-III laser-triggered gas switches

Gas switch development for application in Z refurbishment (ZR) has continued since PPPS Conference 2001. Several iterations have been tested both in oil and water dielectrics. The first switch tested was an evolved version of the Sandia designed HERMES III switch [G. J. Denison et al., 1985]. The 6 MV ZR baseline switch consists of a self-breakdown (cascade) section in which the discharge current flows in several parallel channels, and a trigger section where the current flows through a single spark channel. The second switch tested is a cantilevered switch. The entire cascade section is cantilevered on a plastic support rod allowing for a single continuous insulator housing but having the same characteristics as the baseline. The third switch tested is a Sandia hybrid switch. The laser triggered section of the hybrid switch includes 5 cascade like electrodes connected in parallel to a triggered gap via a /spl sim/2 /spl mu/H series isolation inductor. The discharge current in the hybrid switch trigger section flows in several parallel channels eliminating the single channel flow as in the baseline switch trigger section. The design iterations of these switches and results of these tests are presented.

ZR laser triggered gas switch requirements and performance

This paper reports the performance of the SF/sub 6/ insulated, multistage, laser-triggered gas switch used in the Hermes-III accelerator.sup 1/ In this accelerator, 20 of these switches are used to transfer energy from intermediate energy storage water dielectric capacitors to the pulse forming lines (PFLs). Approximately 8,000 laser-triggered switch shots have been taken with seven prefires. Nearly 70% of these shots have been at nominal operating parameters. The average first-to last spread in firing times for 20 switches is approximately 8 ns. Removal of systematic differences reduces this spread to /spl tilde/6 ns. This spread implies a one-sigma jitter for a single switch of <2 ns at 70-75% of self-breakdown voltage. Results show that the jitter does not change significantly over an operating range of 70-90% of self-breakdown. In addition, the jitter is insensitive to the factor of two variation in the laser energy delivered to the various switches by the optical system. A detailed summary on the performance, reliability, and maintenance of the switches and optical system is presented. Initial results of a study to investigate the performance of these switches under varying laser trigger conditions is also presented.

Design and code validation of the Jupiter inductive voltage adder (IVA) PRS driver

The Z machine at Sandia National Laboratories is presently undergoing an upgrade, called Z-Refurbishment (ZR) [1], that is aimed at improving capacity, precision, and capability to essentially all of its pulsed power components, including its thirty six laser-triggered gas switches (LTGS). Voltage and current requirements for the ZR LTGS have increased 25% from the onset of the ZR program, with no allowable increase to the physical footprint (or inductance) for the device. Initial design studies indicated that a total machine peak current of 26 MA could be achieved with the each LTGS operating at 5 MV and 600 kA. Increases in the final design inductance in the transition from vertical water transmission lines to horizontal magnetically insulated transmission lines, higher inductance in vacuum from changes in the load position for improved diagnostic access, and conservatism in the vacuum power flow requirements caused the LTGS operational goal to become 5.4 MV and 750 kA for a total machine peak current of 23 MA in 100 ns to a 10 mm radius, 10 mm long wire array. A comprehensive research program was initiated in August 2005 to improve the performance of the ZR gas switch at the 5.4 MV level, and results of that effort to date are presented herein.

A 5-Megavolt, 600-Kiloampere Laser-Triggered Gas Switch for use on Z-R: Comparison of Experiments and Simulations

The proposed Jupiter accelerator is a /spl sim/10 MV, 500 TW system capable of delivering 15 MJ kinetic energy to an imploding plasma radiation source. The accelerator is based on Hermes-III technology and contains 30 identical inductive voltage adder modules connected in parallel. The modules drive a common circular convolute electrode system in the center of which is located an imploding foil. The relatively high voltage of 8-10 MV is required to compensate for the voltage differential generated across the load due primarily to the fast increase in current (L dI/dt) and to lesser extent to the increasing inductance (I dL/dt) and resistive component of the imploding foil. Here, the authors examine the power flow through the device and, in particular, through the voltage adder and long magnetically insulated transmission line (MITL). Analytical models, such as pressure balance and parapotential flow, as well as circuit and PIC codes, were utilized. A new version of the TWOQUICK PIC code, which includes an imploding, cylindrical foil as load, was utilized to compare the power flow calculations done with SCREAMER and TRIFL. The good agreement adds confidence to the Jupiter design. In addition, an experimental validation of the design is under way this year with Hermes III. Long extension MITLs are connected at the end of the voltage adder with inductive and diode loads to benchmark the above design codes. In this paper, the authors outline the accelerators conceptual design with emphasis on the power flow and coupling to the inductive load and include preliminary results of Hermes-III experimental design validation.

The ZR Final Design Pulsed Power Performance Expectations

The Z refurbishment project is designed to increase the peak current to the load on Z to ~26 MA in a 100-ns wide power pulse. This current is achieved by summing the current from 36 independent pulse-power modules. To meet these requirements, we have designed and constructed an SF6-insulated gas switch that can hold off 5.5 MV and conduct a peak current of 600 kA for over a hundred shots. The gas switch is charged by a Marx generator in ~1 microsecond and transfers about 200- kilojoules of energy and 0.25 Coulombs of charge to a pulse-forming line in a ~150-ns-wide power pulse peaking at 2.5 TW. The gas switch consists of a laser- triggered section holding off 15% of the voltage followed by 25 self-breakdown gaps. The self-breaking gaps are designed to provide multiple breakdown arcs in order to lower the overall inductance of the switch. The gas switch is submerged in transformer oil during operation. In this work, we show how simulation and experiment have worked together, first to verify proper operation of the switch, and then to solve problems with the switch design that arose during testing.

Laser Triggered Gas Switches Utilizing Beam Transport Through 1 MΩ-cm Deionized Water

The largest X-ray generating facility in the world (~1.8 MJ), the Z machine at Sandia National Laboratories (SNL), is presently undergoing a major upgrade. Upon completion of its refurbishment, ZR is expected to deliver an output current of 26 MA to a standard 20 mm by 40 mm diameter Z-pinch load with a 100-ns implosion time. In addition to nearly doubling the output energy and providing a useful increase in current capability to the research community, the Project criteria include providing an enhanced precision, improved timing jitter, and advanced pulse shaping flexibility needed for full parameter space assessment for the material science community. Another criteria is to increase the shot capacity by enabling the facility and diagnostics infrastructure to routinely support 400 shots per year while minimizing the impact of implementing improvements on existing experimental programs. The Projects objective of extending the life of Z in a balanced way also served to exercise SNLs pulsed power research and engineering capabilities and resources and retain the expertise that will be required for future endeavors. This paper summarizes the predicted pulsed power performance of the 36-module ZR from the 577-kJ (at 85-kV charge) Marx generators in the oil energy storage section through the water pulse forming section components and the vacuum section to the Z-pinch load. Comparisons show good agreement between the most recent models and experimental results from full-scale, single module systems which are very similar to the final design. Discussions will emphasize the laser triggered gas switches and the self-break water switches which are continuing to be optimized