Publications

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The Saturn X-ray generator is a 2.5 megavolt, 10 megampere electrical driver at Sandia National Laboratories. Saturn has been in operation for more than 30 years. Work is underway to identify key areas of the machine, improvement of which would benefit operational efficiency and reproducibility of the system. Saturn is used to create high-dose, short-pulse intense ionizing radiation environments for testing electronic and mechanical systems. Saturn has 36 identical modules driving a common electron beam bremsstrahlung load. Each module utilizes a microsecond Marx generator, a megavolt gas switch, and untriggered water switches in a largely conventional pulse-forming system. Achieving predictable and reliable radiation exposure is critical for users of the facility. Saturn has endured decades of continual use with minimal opportunities for research, improvements, or significant preventive maintenance. Because of degradation in components and limited attention to electrical performance, the facility has declined both in the number of useful tests per year and their repeatability. The Saturn system resides in a cylindrical tank 33m in diameter, and stores 5.6 MJ at the nominal operating Marx charge voltage. The system today is essentially identical to that described by Bloomquist in 1987. [1] Advances in technology for large pulsed power systems affords opportunities to improve the performance and more efficiently utilize the energy stored. Increased efficiency can improve reliability and reduce maintenance. The goals for the Saturn improvement work are increased shot rate, reduced X-ray dose shot-To-shot dose fluctuation, and reduced required maintenance. Major redesign with alternate pulsed power technology is considered outside the scope of this effort. More X-ray dose, larger exposure area, and lower X-ray endpoint energy are also important considerations but also deemed outside the scope of the present project due to schedule and resource constraints. The first considerations, described here, are improving the present design with better components.

Calculation of the power flow from the 36 pulse forming lines to the vacuum region of Saturn has always been complicated by the three-dimensional structure of the rod and bottle connections to the vacuum insulator stack. Recently we have completed a 3-D calculation of the bottle configuration and found a large error in previous impedance estimates. We have used this calculation to determine impedance and to construct a 2-D model of each of the 36 bottles of each level of the insulator using the Transmission Line Matrix (TLM) technique. These TLM models are then used in a 2-D model for each of the three levels of the insulator. Each model starts at a measured forward-going pulse in the water tri-plate and ends at the Brehmstrahlung load at the center of the machine. Because of long transmission line lengths and short pulse lengths, each level can be considered independent of the others. A combination of the three models then represents a quasi-3-D model of the load region of the machine. The results of these calculations agree well with measurement and thereby provide confidence in simulation predictions for those areas where measurements are not possible. Details of the 3-D bottle calculation, the TLM model, and results of the load region simulations using this model are given.

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Two versions of a current driver for single-turn, single-use 1-cm diameter magnetic field coils have been built and tested at the Sandia National Laboratories for use with cluster fusion experiments at the University of Texas in Austin. These coils are used to provide axial magnetic fields to slow radial loss of electrons from laser-produced deuterium plasmas. Typical peak field strength achievable for the two-capacitor system is 50 T, and 200 T for the ten-capacitor system. Current rise time for both systems is about 1.7 {mu}s, with peak current of 500 kA and 2 MA, respectively. Because the coil must be brought to the laser, the driver needs to be portable and drive currents in vacuum. The drivers are complete but laser-plasma experiments are still in progress. Therefore, in this report, we focus on system design, initial tests, and performance characteristics of the two-capacitor and ten-capacitors systems. The questions of whether a 200 T magnetic field can retard the breakup of a cluster-fusion plasma, and whether this field can enhance neutron production have not yet been answered. However, tools have been developed that will enable producing the magnetic fields needed to answer these questions. These are a two-capacitor, 400-kA system that was delivered to the University of Texas in 2010, and a 2-MA ten-capacitor system delivered this year. The first system allowed initial testing, and the second system will be able to produce the 200 T magnetic fields needed for cluster fusion experiments with a petawatt laser. The prototype 400-kA magnetic field driver system was designed and built to test the design concept for the system, and to verify that a portable driver system could be built that delivers current to a magnetic field coil in vacuum. This system was built copying a design from a fixed-facility, high-field machine at LANL, but made to be portable and to use a Z-machine-like vacuum insulator and vacuum transmission line. This system was sent to the University of Texas in Austin where magnetic fields up to 50 T have been produced in vacuum. Peak charge voltage and current for this system have been 100 kV and 490 kA. It was used this last year to verify injection of deuterium and surrogate clusters into these small, single-turn coils without shorting the coil. Initial test confirmed the need to insulate the inner surface of the coil, which requires that the clusters must be injected through small holes in an insulator. Tests with a low power laser confirmed that it is possible to inject clusters into the magnetic field coils through these holes without destroying the clusters. The university team also learned the necessity of maintaining good vacuum to avoid insulator, transmission line, and coil shorting. A 200-T, 2 MA system was also constructed using the experience from the first design to make the pulsed-power system more robust. This machine is a copy of the prototype design, but with ten 100-kV capacitors versus the two used in the prototype. It has additional inductance in the switch/capacitor unit to avoid breakdown seen in the prototype design. It also has slightly more inductance at the cable connection to the vacuum chamber. With this design we have been able to demonstrate 1 MA current into a 1 cm diameter coil with the vacuum chamber at air pressure. Circuit code simulations, including the additional inductance with the new design, agree well with the measured current at a charge voltage of 40 kV with a short circuit load, and at 50 kV with a coil. The code also predicts that with a charge voltage of 97 kV we will be able to get 2 MA into a 1 cm diameter coil, which will be sufficient for 200 T fields. Smaller diameter or multiple-turn coils will be able to achieve even higher fields, or be able to achieve 200-T fields with lower charge voltage. Work is now proceeding at the university under separate funding to verify operation at the 2-MA level, and to address issues of debris mitigation, measurement of the magnetic field, and operation in vacuum. We anticipate operation at full current with single-turn, magnetic field coils this fall, with 200 T experiments on the Texas Petawatt laser in the spring of 2012.

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Sandia National Laboratories, Albuquerque, N.M., USA, in collaboration with the High Current Electronic Institute (HCEI), Tomsk, Russia, is developing a new paradigm in pulsed power technology: the Linear Transformer Driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high current and high voltage pulses straight out of the cavity with out any complicated pulse forming and pulse compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. The load may be a vacuum electron diode, a z-pinch wire array, a gas puff, a liner, an isentropic compression load (ICE) to study material behavior under very high magnetic fields, or a fusion energy (IFE) target. This is because the output pulse rise time and width can be easily tailored to the specific application needs. In this paper we briefly summarize the developmental work done in Sandia and HCEI during the last few years, and describe our new MYKONOS Sandia High Current LTD Laboratory. An extensive evaluation of the LTD technology is being performed at SNL and the High Current Electronic Institute (HCEI) in Tomsk Russia. Two types of High Current LTD cavities (LTD I-II, and 1-MA LTD) were constructed and tested individually and in a voltage adder configuration (1-MA cavity only). All cavities performed remarkably well and the experimental results are in full agreement with analytical and numerical calculation predictions. A two-cavity voltage adder is been assembled and currently undergoes evaluation. This is the first step towards the completion of the 10-cavity, 1-TW module. This MYKONOS voltage adder will be the first ever IVA built with a transmission line insulated with deionized water. The LTD II cavity renamed LTD III will serve as a test bed for evaluating a number of different types of switches, resistors, alternative capacitor configurations, cores and other cavity components. Experimental results will be presented at the Conference and in future publications.

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In addressing the issue of the determining the hazard categorization of the Z Accelerator of doing Special Nuclear Material (SNM) experiments the question arose as to whether the machine could be fired with its central vacuum chamber open, thus providing a path for airborne release of SNM materials. In this report we summarize calculations that show that we could only expect a maximum current of 460 kA into such a load in a long-pulse mode, which will be used for the SNM experiments, and 750 kA in a short-pulse mode, which is not useful for these experiments. We also investigated the effect of the current for both cases and found that for neither case is the current high enough to either melt or vaporize these loads, with a melt threshold of 1.6 MA. Therefore, a necessary condition to melt, vaporize, or otherwise disperse SNM material is that a vacuum must exist in the Z vacuum chamber. Thus the vacuum chamber serves as a passive feature that prevents any airborne release during the shot, regardless of whatever containment may be in place.

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Sandia National Laboratories, Albuquerque, N.M., USA, in collaboration with the High Current Electronic Institute (HCEI), Tomsk, Russia, is developing a new paradigm in pulsed power technology: the Linear Transformer Driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high current and high voltage pulses straight out of the cavity with out any complicated pulse forming and pulse compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. The load may be a vacuum electron diode, a z-pinch wire array, a gas puff, a liner, an isentropic compression load (ICE) to study material behavior under very high magnetic fields, or a fusion energy (IFE) target. This is because the output pulse rise time and width can be easily tailored to the specific application needs. In this paper we briefly summarize the developmental work done in Sandia and HCEI during the last few years, and describe our new MYKONOS Sandia High Current LTD Laboratory.

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Post-hole convolutes are used in high-power transmission line systems and join several individual transmission lines in parallel, transferring the combined currents to a single transmission line attached to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due, in part, to the formation of magnetic nulls, resulting in current losses. At very high-power operating levels, the formation of electrode plasmas is considered likely which can lead to additional losses. A recent computational analysis of the Sandia Z accelerator suggested that modest plasma desorption rates in the convolute region could explain measured current losses [1]. The recently completed Sandia ZR accelerator has utilized new convolute designs to accommodate changes to the parallel-plate transmission lines on ZR. Detailed particle-in-cell simulations that are fully electromagnetic and relativistic, and include plasma desorption from electrode surfaces in the post-hole convolutes, are carried out to assess the measured current losses on ZR. We find that the plasma desorption rate used to model the Z convolute also applies to three different ZR convolute designs that have been fielded. Based on these findings, the simulation model is being used to develop newer convolute designs with the goal of reducing the current losses, particularly for higher-impedance loads. ©2009 IEEE.

There is a need to generate magnetic fields both above and below 1 megagauss (100 T) with compact generators for laser-plasma experiments in the Beamlet and Petawatt test chambers for focused research on fundamental properties of high energy density magnetic plasmas. Some of the important topics that could be addressed with such a capability are magnetic field diffusion, particle confinement, plasma instabilities, spectroscopic diagnostic development, material properties, flux compression, and alternate confinement schemes, all of which could directly support experiments on Z. This report summarizes a two-month study to develop preliminary designs of magnetic field generators for three design regimes. These are, (1) a design for a relatively low-field (10 to 50 T), compact generator for modest volumes (1 to 10 cm3), (2) a high-field (50 to 200 T) design for smaller volumes (10 to 100 mm3), and (3) an extreme field (greater than 600 T) design that uses flux compression. These designs rely on existing Sandia pulsed-power expertise and equipment, and address issues of magnetic field scaling with capacitor bank design and field inductance, vacuum interface, and trade-offs between inductance and coil designs.

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The Z Refurbishment Project was completed in September 2007. Prior to the shutdown of the Z facility in July 2006 to install the new hardware, it provided currents of {le} 20 MA to produce energetic, intense X-ray sources ({approx} 1.6 MJ, > 200 TW) for performing high energy density science experiments and to produce high magnetic fields and pressures for performing dynamic material property experiments. The refurbishment project doubled the stored energy within the existing tank structure and replaced older components with modern, conventional technology and systems that were designed to drive both short-pulse Z-pinch implosions and long-pulse dynamic material property experiments. The project goals were to increase the delivered current for additional performance capability, improve overall precision and pulse shape flexibility for better reproducibility and data quality, and provide the capacity to perform more shots. Experiments over the past year have been devoted to bringing the facility up to full operating capabilities and implementing a refurbished suite of diagnostics. In addition, we have enhanced our X-ray backlighting diagnostics through the addition of a two-frame capability to the Z-Beamlet system and the addition of a high power laser (Z-Petawatt). In this paper, we will summarize the changes made to the Z facility, highlight the new capabilities, and discuss the results of some of the early experiments.

A fully three-dimensional electromagnetic model of the major pulsed power components of the 26-MA ZR accelerator is presented. This large-scale simulation model tracks the evolution of electromagnetic waves through the intermediate storage capacitors, laser-triggered gas switches, pulse-forming lines, water switches, tri-plate transmission lines, and water convolute to the vacuum insulator stack. The plates at the insulator stack are coupled to a transmission line circuit model of the four-level magnetically-insulated transmission line section and post-hole convolutes. The vacuum section circuit model is terminated by either a short-circuit load or dynamic models of imploding z-pinch loads. The simulations results are compared with electrical measurements made throughout the ZR accelerator and good agreement is found, especially for times before and up to peak load power. This modeling effort represents new opportunities for modeling existing and future large-scale pulsed power systems used in a variety of high energy density physics and radiographic applications.

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Experiment demonstrates the first direct transformation of a tungsten wire core to the plasma state by Joule heating during nanosecond electrical explosion in vacuum. Energy of ∼130 eV/atom was deposited into the 12 μm W wire coated by 2 μm polyimide during the first ∼10 ns. All the metal rapidly transformed to highly ionized plasma, while the surrounding polyimide coating remained primarily in a gaseous state. This coating totally suppressed corona formation. The expansion velocity of the wire was ∼12-18 km/s, the average wire ionization at 50 ns reached ∼67% with corresponding LTE temperature of ∼1.2 eV. Explosion of bare W wire demonstrated earlier termination of the wire core heating due to shunting corona generation. Magnetohydrodynamic (MHD) simulation reproduces the main features of coated and uncoated W wire explosion. © 2008 The American Physical Society.

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During the last few years Sandia is actively pursuing the development of new accelerators based on the novel technology of Linear Transformer Driver (LTD). This effort is done in close collaboration with the High Current Electronic Institute (HCEI) in Tomsk, Russia, where the LTD idea was first conceived and developed. LTD based drivers are currently considered for many applications including future very high current Z-pinch drivers like ZX and IFE (Inertial Fusion Energy), medium current drivers with adjustable pulse length for ICE (Isentropic Compression Experiments), and finally relatively lower current accelerators for radiography and x-pinch. Currently we have in operation the following devices: One 500-kA, 100-kV LTD cavity, a 1-MVvoltage adder composed of seven smaller LTD cavities for radiography, and one 1-MA, 100-kV cavity. The first two are in Sandia while the latter one is still in Tomsk. In addition a number of stackable 1-MA cavities are under construction to be utilized as building blocks for a 1-MA, 1-MV voltage adder module. This module will serve as a prototype for longer, higher voltage modules, a number of which, connected in parallel, could become the driver of an IFE fusion reactor or a high current Z-pinch driver (ZX). The IFE requirements are more demanding since the driver must operate in rep-rated mode with a frequency of 0.1 Hz. In this paper we mainly concentrate on the higher current LTDs: We briefly outline the principles of operation and architecture and present a first cut design of an IFE, LTD z-pinch driver. © 2005 IEEE.

The University of Missouri Terawatt Test Stand (MUTTS) has conducted many untriggered experiments on a Rimfire gas switch scaled to 2.5 MV. The focus of these experiments was to evaluate what methods may be used to control the distribution of cascade arcs. The untriggered data indicates that the rise time of switch current does not statistically improve, as expected, as the number of cascade arcs per gap increased beyond two channels. For the same data, the number of arcs in the cascade section more dramatically affects the output current period. This indicates that in late time increased multichanneling has a more pronounced effect than in early time. The switch is triggered with a frequency quadrupled Nd:YAG laser at 30 mJ with a 3-5 ns pulse width. Since the focused laser does not ionize the full length of the trigger section, there is little effect on current rise time when compared to untriggered data, but more channels form in the cascade section for an air filled switch. The cascade section was shorted and data are presented describing the contribution of the single channeling trigger section to overall switch impedance. The electrical effects of multichanneling using a laser trigger, the formation of arc channels in the cascade section, and the implications the results have on the future design of fast gas switches are discussed. © 2005 IEEE.

The ZR gas switch, located between the Intermediate Store capacitor (I-Store) and the Pulsed Forming Line (PFL), requires a laser pulse for its triggering. There are several routes for the beam to reach the gas switch but all of them cross over the high voltage regions. The Z laser tube crosses over the outer to inner PFL electrodes with a voltage difference no larger than 3.5 MV. The ZR gas switch was designed to be in oil, given the higher operational voltages, as a consequence the laser tube is in the oil side of the PFL interface. The ZR laser tube is required to hold in excess of 5 MV across it using high pressure SF6 gas, the ID is 2.5″ to accommodate the laser beam, mechanically should tolerate the non-axial shock loading during the water switches firing. After a couple of iterations it was decided to use Polyurethane, it provided most of the desired mechanical properties, except that it outgases ether and ether based compounds. The effect of just a few ppm of ether on SF6 is a significant reduction on the HV hold off especially surface tracking or flashover. As a consequence the final design is such that the electric field distribution on the tube is as conservative as it was possible due to space constrains. We present the basic design, the field distribution, its relationship with available SF6 breakdown data and the present performance. © 2005 IEEE.

We have developed a system of differential-output monitors that diagnose current and voltage in the vacuum section of a 20-MA 3-MV pulsed-power accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuum-insulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The inner-MITL monitors are located 6 cm from the axis of the load. Each of the stack and outer-MITL current monitors comprises two separate B-dot sensors, each of which consists of four 3-mm-diameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100-ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the B-dot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Common-mode-noise rejection is achieved by combining these signals in a 50-{Omega} balun. The signal cables that connect the B-dot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50-{omega} cable transmits the output signal of each balun to a double-wall screen room, where the signals are attenuated, digitized (0.5-ns/sample), numerically compensated for cable losses, and numerically integrated. By contrast, each inner-MITL current monitor contains only a single B-dot sensor. These monitors are fielded in opposite-polarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for common-mode-noise rejection after digitization. All the current monitors are calibrated on a 76-cm-diameter axisymmetric radial transmission line that is driven by a 10-kA current pulse. The reference current is measured by a current-viewing resistor (CVR). The stack voltage monitors are also differential-output gauges, consisting of one 1.8-cm-diameter D-dot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MV on dedicated accelerator shots with a short-circuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outer-MITL B-dot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA to a low-impedance z-pinch load, the peak lineal current densities at the stack, outer-MITL, and inner-MITL monitor locations are 0.5, 1, and 58 MA/m, respectively. On such shots the peak currents measured at these three locations agree to within 1%.

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In October 2005, an intensive three-year Laser Triggered Gas Switch (LTGS) development program was initiated to investigate and solve observed performance and reliability issues with the LTGS for ZR. The approach taken has been one of mission-focused research: to revisit and reassess the design, to establish a fundamental understanding of LTGS operation and failure modes, and to test evolving operational hypotheses. This effort is aimed toward deploying an initial switch for ZR in 2007, on supporting rolling upgrades to ZR as the technology can be developed, and to prepare with scientific understanding for the even higher voltage switches anticipated needed for future high-yield accelerators. The ZR LTGS was identified as a potential area of concern quite early, but since initial assessments performed on a simplified Switch Test Bed (STB) at 5 MV showed 300-shot lifetimes on multiple switch builds, this component was judged acceptable. When the Z{sub 20} engineering module was brought online in October 2003 frequent flashovers of the plastic switch envelope were observed at the increased stresses required to compensate for the programmatically increased ZR load inductance. As of October 2006, there have been 1423 Z{sub 20} shots assessing a variety of LTGS designs. Numerous incremental and fundamental switch design modifications have been investigated. As we continue to investigate the LTGS, the basic science of plastic surface tracking, laser triggering, cascade breakdown, and optics degradation remain high-priority mission-focused research topics. Significant progress has been made and, while the switch does not yet achieve design requirements, we are on the path to develop successively better switches for rolling upgrade improvements to ZR. This report summarizes the work performed in FY 2006 by the large team. A high-level summary is followed by detailed individual topical reports.

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The purpose of this work was to develop a conceptual design for the Saturn accelerator using the modular Liner-Transformer Driver (LTD) technology to identify risks and to focus development and research for this new technology. We present a reference design for a Saturn class driver based on a number of linear inductive voltage adders connected in parallel. This design is very similar to a design reported five years ago [1]. However, with the design reported here we use 1-MA, 100-kV LTD cavities as building blocks. These cavities have already been built and are currently in operation at the HCEI in Tomsk, Russia [2]. Therefore, this new design integrates already-proven individual components into a full system design.

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Measurements within 10% accuracy of the dynamic dipole polarizabilities α (λ) for five nonrefractory metal atoms (Mg, Ag, Al, Cu, and Au) at laser wavelengths of λ=532 and 1064 nm are presented using electrical explosion of thin wires in vacuum and a novel laser probing integrated-phase technique. The technique is based on single-wavelength interferometry and does not require axial symmetry of the tested object. Theoretical prediction of α (λ) for wavelengths λ=355, 532, and 1064 nm, as well as the static dipole polarizabilities αst, are also presented. An agreement within 20% was obtained between calculated data, recommended static polarizabilities αst, and the measured dynamic polarizabilities α (532 nm) and α (1064 nm). © 2006 The American Physical Society.

Summary from only given. The capabilities of the Z accelerator will be significantly enhanced by the Z Refurbishment (ZR) project [McDaniel DH, 2002]. The performance of a single ZR module is currently being characterized in the pre-production engineering evaluation test bed, Z20 [Lehr, JM, 2003]. Z20 is thoroughly diagnosed so that electrical performance of the module can be established. Circuit models of Z20 have been developed and validated in both Screamer [1985] and Bertha [1989] circuit codes. For the purposes of predicting ZR performance, a full ZR circuit model has also been developed in Bertha. The full ZR model (using operating parameters demonstrated on Z20) indicates that the required 26 MA, 100 ns implosion time, output load current pulse will be achieved on ZR. In this paper, the electrical characterization of Z20 and development of the single module circuit models will be discussed in detail. The full ZR model will also be discussed and the results of several system studies conducted to predict ZR performance will be presented.

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Many accelerators at Sandia National Laboratories utilize the Rimfire gas switch for high-voltage, high-power switching. Future accelerators will have increased performance requirements for switching elements. When designing improved versions of the Rimfire switch, there is a need for quick and accurate simulation of the electrical effects of geometry changes. This paper presents an advanced circuit model of the Rimfire switch that can be used for these simulations. The development of the model is shown along with comparisons to past models and experimental results.

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A quasi-spherical z-pinch may directly compress foam or deuterium and tritium in three dimensions as opposed to a cylindrical z-pinch, which compresses an internal load in two dimensions only. Because of compression in three dimensions the quasi-spherical z-pinch is more efficient at doing pdV work on an internal fluid than a cylindrical pinch. Designs of quasi-spherical z-pinch loads for the 28 MA 100 ns driver ZR, results from zero-dimensional (0D) circuit models of quasi-spherical implosions, and results from 1D hydrodynamic simulations of quasi-spherical implosions heating internal fluids will be presented. Applications of the quasi-spherical z-pinch implosions include a high radiation temperature source for radiation driven experiments, a source of neutrons for treating radioactive waste, and a source of fusion energy for a power generator.

The experimental and computational investigations of nanosecond electrical explosion of thin Al wire in vacuum are presented. We have demonstrated that increasing the current rate leads to increased energy deposited before voltage collapse. Laser shadowgrams of the overheated Al core exhibit axial stratification with a {approx}100 {micro}m period. The experimental evidence for synchronization of the wire expansion and light emission with voltage collapse is presented. Two-wavelength interferometry shows an expanding Al core in a low-ionized gas condition with increasing ionization toward the periphery. Hydrocarbons are indicated in optical spectra and their influence on breakdown physics is discussed. The radial velocity of low-density plasma reaches a value of {approx}100 km/s. The possibility of an overcritical phase transition due to high pressure is discussed. 1D MHD simulation shows good agreement with experimental data. MHD simulation demonstrates separation of the exploding wire into a high-density cold core and a low-density hot corona as well as fast rejection of the current from the wire core to the corona during voltage collapse. Important features of the dynamics for wire core and corona follow from the MHD simulation and are discussed.

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A new laser trigger system (LTS) has been installed on Z that benefits the experimenter with reduced temporal jitter on the x-ray output, the confidence to use command triggers for time sensitive diagnostics and the ability to shape the current pulse at the load. This paper presents work on the pulse shaping aspects of the new LTS. Pulse shaping is possible because the trigger system is based on 36 individual lasers, one per each pulsed power module, instead of a single laser for the entire machine. The firing time of each module can be individually controlled to create an overall waveform that is the linear superposition of all 36 modules. In addition, each module can be set to a long- or short-pulse mode for added flexibility. The current waveform has been stretched from ∼100 ns to ∼250 ns. A circuit model has been developed with BERTHA Code, which contains the independent timing feature of the new LTS to predict and design pulse shapes. The ability to pulse-shape directly benefits isentropic compression experiments (ICE) and equation of state measurements (EOS) for the shock physics programs at Sandia National Laboratories. With the new LTS, the maximum isentropic loading applied to Cu samples 750 um thick has been doubled to 3.2 Mb without generating a shockwave. Macroscopically thick sample of Al, 1.5 mm, have been isentropically compressed to 1.7 Mb. Also, shockless Ti flyer-plates have been launched to 21 km·s-1, remaining in the solid state until impact.

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Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling ("hohlraum energetics") is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)]. © 2001 American Institute of Physics.

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Saturn is a dual-purpose accelerator. It can be operated as a large-area flash x-ray source for simulation testing or as a Z-pinch driver especially for K-line x-ray production. In the first mode, the accelerator is fitted with three concentric-ring 2-MV electron diodes, while in the Z-pinch mode the current of all the modules is combined via a post-hole convolute arrangement and driven through a cylindrical array of very fine wires. We present here a point design for a new Saturn class driver based on a number of linear inductive voltage adders connected in parallel. A technology recently implemented at the Institute of High Current Electronics in Tomsk (Russia) is being utilized. In the present design we eliminate Marx generators and pulse-forming networks. Each inductive voltage adder cavity is directly fed by a number of fast 100-kV small-size capacitors arranged in a circular array around each accelerating gap. The number of capacitors connected in parallel to each cavity defines the total maximum current. By selecting low inductance switches, voltage pulses as short as 30-50-ns FWHM can be directly achieved. The voltage of each stage is low (100-200 kv). Many stages are required to achieve multi-megavolt accelerator output. However, since the length of each stage is very short (4-10 cm), accelerating gradients of higher than 1 MV/m can easily be obtained. The proposed new driver will be capable of delivering pulses of 15-MA, 36-TW, 1.2-MJ to the diode load, with a peak voltage of {minus}2.2 MV and FWHM of 40-ns. And although its performance will exceed the presently utilized driver, its size and cost could be much smaller ({approximately}1/3). In addition, no liquid dielectrics like oil or deionized water will be required. Even elimination of ferromagnetic material (by using air-core cavities) is a possibility.

A z-pinch radiation source has been developed that generates 60 {+-} 20 KJ of x-rays with a peak power of 13 {+-} 4 TW through a 4-mm diameter axial aperture on the Z facility. The source has heated NIF (National Ignition Facility)-scale (6-mm diameter by 7-mm high) hohlraums to 122 {+-} 6 eV and reduced-scale (4-mm diameter by 4-mm high) hohlraums to 155 {+-} 8 eV -- providing environments suitable for indirect-drive ICF (Inertial Confinement Fusion) studies. Eulerian-RMHC (radiation-hydrodynamics code) simulations that take into account the development of the Rayleigh-Taylor instability in the r-z plane provide integrated calculations of the implosion, x-ray generation, and hohlraum heating, as well as estimates of wall motion and plasma fill within the hohlraums. Lagrangian-RMHC simulations suggest that the addition of a 6 mg/cm{sup 3} CH{sub 2} fill in the reduced-scale hohlraum decreases hohlraum inner-wall velocity by {approximately}40% with only a 3--5% decrease in peak temperature, in agreement with measurements.