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Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

The Z pulsed power driver at Sandia National Laboratories is used for a wide range of high energy density physics experiments in areas such as inertial confinement fusion, radiation effects, and dynamic material properties. Experimental demands are pushing for the highest energy attainable with more reliability and precision in timing and pulse compression. A previous version of the laser-triggered gas switch had been made reliable at voltages up to 5.7 MV, allowing 5 nanosecond load accuracy. The desire for higher energy and higher precision dictated a new laser-triggered switch design. In Z, 36 DC-charged Marx generators pulse-charge water-insulated capacitors in 1.5 microseconds. The laser-triggered gas switch commutes the energy stored in the water-insulated capacitor to subsequent pulse compression stages that utilize self-closing water switches. The laser-triggered switch is the last command triggered switch in the chain, and largely determines the temporal accuracy of the total load current. Both switches consist of a laser triggered section and a self-closing cascade section. The previous design required a trigger plate to provide mechanical support for the cascade section. With fixed laser energy, it was impossible to increase the triggered fraction of the switch. Because of the trigger support plate that affects the field distribution after triggering, establishing an operating pressure that provides a reliable balance between low pre-fire rate and low jitter becomes difficult, and more so at higher voltage. The new switch uses a cantilevered design that increases the electric stress in the self-closing section after triggering, even with a slightly-reduced triggered gap. It was required that the new design work within the same operating space and infrastructure as the previous. We will show details of the design and features necessary for reliable operation in the extreme electrical and mechanical environment presented by daily operation on Z.

Here, we have developed conceptual designs of two petawatt-class pulsed-power accelerators: Z 300 and Z 800. The designs are based on an accelerator architecture that is founded on two concepts: single-stage electrical-pulse compression and impedance matching [Phys. Rev. ST Accel. Beams 10, 030401 (2007)]. The prime power source of each machine consists of 90 linear-transformer-driver (LTD) modules. Each module comprises LTD cavities connected electrically in series, each of which is powered by 5-GW LTD bricks connected electrically in parallel. (A brick comprises a single switch and two capacitors in series.) Six water-insulated radial-transmission-line impedance transformers transport the power generated by the modules to a six-level vacuum-insulator stack. The stack serves as the accelerator’s water-vacuum interface. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs), which are joined in parallel at a 10-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator’s physics-package load. Z 300 is 35 m in diameter and stores 48 MJ of electrical energy in its LTD capacitors. The acceleratormore » generates 320 TW of electrical power at the output of the LTD system, and delivers 48 MA in 154 ns to a magnetized-liner inertial-fusion (MagLIF) target [Phys. Plasmas 17, 056303 (2010)]. The peak electrical power at the MagLIF target is 870 TW, which is the highest power throughout the accelerator. Power amplification is accomplished by the centrally located vacuum section, which serves as an intermediate inductive-energy-storage device. The principal goal of Z 300 is to achieve thermonuclear ignition; i.e., a fusion yield that exceeds the energy transmitted by the accelerator to the liner. 2D magnetohydrodynamic (MHD) simulations suggest Z 300 will deliver 4.3 MJ to the liner, and achieve a yield on the order of 18 MJ. Z 800 is 52 m in diameter and stores 130 MJ. This accelerator generates 890 TW at the output of its LTD system, and delivers 65 MA in 113 ns to a MagLIF target. The peak electrical power at the MagLIF liner is 2500 TW. The principal goal of Z 800 is to achieve high-yield thermonuclear fusion; i.e., a yield that exceeds the energy initially stored by the accelerator’s capacitors. 2D MHD simulations suggest Z 800 will deliver 8.0 MJ to the liner, and achieve a yield on the order of 440 MJ. Z 300 and Z 800, or variations of these accelerators, will allow the international high-energy-density-physics community to conduct advanced inertial-confinement-fusion, radiation-physics, material-physics, and laboratory-astrophysics experiments over heretofore-inaccessible parameter regimes.« less

We study the physics of water switching at 4 to 5 MV as part of Sandia's Z-Refurbishment program. We present time histories of the light emitted by two sets of water switches and correlate the light emission to the accelerator power pulse. Framing camera images demonstrate how the initial "bushy" streamers collapse to narrow current channels at late time. Spectroscopy of the water switches indicates that the early-time streamers are already hot-dense blackbodies with temperatures between 7000 K and 9000 K. Late time hydrogen emission lines allow estimates of electron densities at that time ranging from 0.2 to 2 times 1018/cm3.

Sandia in collaborations with a number of other institutions, is developing a new paradigm in pulsed power technology: the Linear Transformer Driver (LTD) technology. This approach can provide very compact devices that can deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. We have extensively tested several induction LTD flat doughnut-shaped cavities which are 3m in diameter and have a 1.8 m bore and only ∼20 cm thickness, but they can deliver 0.1 TW, 70 ns rise time pulses to a matched ∼ 0.1 Ohm load. Experiments with a 1 TW Inductive Voltage Adder (IVA), which includes 10, 1-MA LTD cavities connected in series, are in preparation at MYKONOS LTD Laboratory in Sandia National Laboratories, Albuquerque, NM. This MYKONOS voltage adder will be the first ever IVA built with a transmission line insulated with de-ionized water. We present the design and experimental results of the first two-cavity water isolated voltage adder with a liquid salt solution resistive load. This is the first step towards the assembly of a ten cavity voltage adder.

The Linear Transformer Driver (LTD) technology can provide very compact devices that deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of large number of high current pulses can be produced with variable inter-pulse separation from nanoseconds to microseconds. Most importantly these devices can be rep-rated to frequencies only limited by the capacitor specifications that usually is 10Hz. To date we have completed the experimental evaluation of two (LTD I, LTD II) 500-kA cavities in both single and rep-rated modes. A larger 1-MA cavity was tested in a single shot mode and in a voltage adder configuration. The first built inductively isolated 1-MA LTD Voltage Adder (IVA) was composed of five cavity connected in series with a vacuum insulated transmission line. It was tested with both resistive and electron diode loads. The experimental results are in excellent agreement with numerical simulations. Experiments with a 1 TW Inductive Voltage Adder (IVA), which includes ten, 1-MA LTD cavities connected in series, are in preparation at Mykonos LTD Laboratory in Sandia National Laboratories, Albuquerque, NM. This time around the Mykonos voltage adder will be built with a transmission line insulated with deionized water. In this experimental work we aim to test the advantages of water insulation as compared to self Magnetic Insulated Transmission Line transport (MITL). It is hoped that the vacuum sheath electron current losses will be avoided without any new difficulties caused by the deionized water. This voltage adder will be rep-rated at 6 shots per minute. In the present paper we briefly present the LTD cavity architecture, describe the two LTD cavity types built and experimentally tested in single and rep-rated modes and present the design of the first two-cavity water insulated voltage adder with a liquid salt solution resistive load. This is the first step towards the assembly of a ten-cavity voltage adder of which most of the components are already built and/or fully designed.

A 6.1-MV, 0.79-MA laser-triggered gas switch (LTGS) is used to synchronize the 36 modules of the $Z$ machine at Sandia National Laboratories. Each module includes one switch, which serves as the last command-fired switch of the module, and hence is used to determine the time at which each module electrically closes relative to the other modules. The switch is $\ensuremath{\sim}81\mathrm{\text{\ensuremath{-}}}\mathrm{cm}$ in length, $\ensuremath{\sim}45\mathrm{\text{\ensuremath{-}}}\mathrm{cm}$ in diameter, and is immersed in mineral oil. The outer switch envelope consists of six corrugated monomer-cast acrylic insulators and five contoured stainless-steel rings. The trigger electrodes are fabricated from copper-infused tungsten. The switch is pressurized with several atmospheres of sulfur hexafluoride (${\mathrm{SF}}_{6}$), which is turbulently purged within 2 seconds after every shot. Each switch is powered from a 6-MV, 0.78-MJ Marx generator which pulse charges a 24-nF intermediate-store water capacitor in $1.4\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{s}$. Closure of the switch allows power to flow into pulse-forming transmission lines. The power pulse is subsequently compressed by water switches, which results in a total accelerator output power in excess of 70-TW. A previous version of the LTGS performed exceptionally at a 5.4-MV, 0.7-MA level on an engineering test module used for switch development. It exhibited a $1\mathrm{\text{\ensuremath{-}}}\ensuremath{\sigma}$ jitter of $\ensuremath{\sim}5\text{ }\text{ }\mathrm{ns}$, a prefire and flashover rate less than 0.1%, and a lifetime in excess of 150 shots. When installed on the $Z$ accelerator, however, the switch exhibited a prefire probability of $\ensuremath{\sim}3%$, a flashover probability of $\ensuremath{\sim}7%$, and a 15-ns jitter. The difference in performance is attributed to several factors such as higher total charge transfer, exposure to more debris, and more stressful dynamic mechanical loading upon machine discharge. Under these conditions, the replacement lifetime was less than ten shots. Since refurbishment of $Z$ in October 2007, there have been three LTGS design iterations to improve the performance at 6.1-MV. The most recent design exhibits a prefire rate of less than 0.1%, a flashover rate of $\ensuremath{\sim}0.2%$, a single switch jitter of $\ensuremath{\sim}6\mathrm{\text{\ensuremath{-}}}\mathrm{ns}$, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.

Laser triggered, megavolt, megampere gas switches are frequently utilized to synchronize multiple pulsed power driver modules for inertial-confinement fusion, isentropic compression, and radiation physics experiments. The device developed to synchronize the 36 modules of the refurbished Z accelerator at Sandia National Laboratories is a 5.4 MV, 700 kA, sulfur-hexafluoride (SF_{6}) filled, laser triggered gas switch. At this operating level, switch jitter is 5 ns, the prefire rate is less than 0.1%, the average optic lifetime is greater than 200 shots, and the flashover rate is less than 1%. Over 1000 shots on a single-module test facility were conducted while iterating several potential design improvements, including utilizing low-erosion electrode material, varying SF_{6} pressure, and modifying internal switch geometry all while keeping the basic switch architecture and footprint constant. Results of this development effort are presented herein.

The Z machine at Sandia National Laboratories was recently refurbished to improve capacity, precision, and capability of nearly all of its pulsed power components in each of its thirty-six modules. The goal of this upgrade was to increase the total machine current by 30%, from ~20 MA to 26 MA in 120 ns into a wire-array z-pinch load. Initial design calculations indicated the operating point for an individual gas switch would be 5 MV and 600 kA to meet these design goals. This manuscript discusses design improvements that were required to achieve reliable performance at this level, the highest requirement for an LTGS in history. Significant factors include the electric potential grading along the switch length, the materials used in the switch, and the insulator housing design. It also discusses design criteria for robust SF6 gas switch design. Several design iterations are discussed that address performance improvements for each failure mode.

Summary form only given. The Z pulsed power driver at Sandia National Laboratories is used to develop high energy density Z-pinch X-ray sources for inertial confinement fusion research and radiation effects testing, and drive megabar pressures in material samples for equation of state studies. The pulsed power system is in the process of being replaced, improving reliability and increasing energy delivered to the load. The upgraded pulsed power system will deliver more than nine megajoules of forward wave energy in the first one hundred nanoseconds of its pulse. The system is comprised of thirty-six nominally identical modules, each producing a 3.3-terawatt pulse in 6Omega water-insulated transmission lines. The peak forward-going voltage is about 5 MV. The pulse rise time is -75 ns; the full width at half maximum is -190 ns. The thirty-six modules are combined in parallel and drive twenty to twenty-five MA into the single load. In such a system, reliable insulation and precise switching are primary concerns. We show key components of the system and results from a test module. We also show performance results from the energy storage, triggering, and pulse-forming systems. We also show the differing constraints of power flow from the 175 kA from each Marx generator, to currents in excess of 24 MA in the final feed to the load.

We are studying several competing gas switch geometries for Linear Transformer Drivers in an attempt to design an optimum switch for these applications as well as to increase our knowledge of the physics of the switching process. In addition to standard electrical diagnostics (V, I), we are studying the switches with a variety of optical diagnostics including fast photodiodes, a framing camera and a time-resolved spectroscopy system. In our test system, 20-nF capacitors on top and bottom of the switch are charged to voltages up to ±100 kV. Then the switch is triggered and current flows through the switch and a load resistor in a geometry that resembles the LTD ‘brick’ architecture. We will present results using the multistage gas switches designed at the High Current Electronics Institute [1] and compare them to results with a low inductance switch designed at Sandia that provides a 30% higher peak current through the LTD ‘brick’.

Summary form only given. The ZR gas switch is a multistage gas switch designed to hold off a voltage of ~6 MV and then transmit a 600-kA current pulse to the water section of ZR. The first stage of the switch, which holds off approximately 900 kV, is laser triggered by the focused beam from a frequency-quadrupled ND:YAG laser operating at 266 nm. The breakdown of the laser-triggered section causes a wave of breakdowns in the remaining 22 gaps in the switch, starting near the laser-triggered section and cascading to the other end of the switch. The gaps in this "cascade" section normally break down in multiple arc channels, which lowers the overall inductance of the switch. We are observing the details of normal and abnormal switch operation with fast framing and streak cameras, observing the time-history of the light emitted by the switch with fast photo-detectors and taking time-integrated photographs of the switch. The diagnostics indicate that the laser-triggered gap normally begins conducting current 10-to-15 ns after the laser arrives in the gap, with the entire switch breaking down 45-to 55-ns after the laser beam arrives in the switch. Details of the experiments and the operation of different switches are presented.

Summary form only given. The Z machine at Sandia National Laboratories is presently undergoing an upgrade, called Z-Refurbishment (ZR), that is aimed at improving capacity, precision, and capability to nearly 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 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 How requirements caused the LTGS operations goals to become 6.25 MV and 750 kA for a total machine peak current of 26 MA. Tests of the proposed switch design at 5 MV with a resistive load demonstrated jitter (

Summary form only given. The Z machine at Sandia National Laboratories is a thirty six module pulsed power driver utilized for the study of inertial confinement fusion, isentropic compression experiments, and high density physics. Currently it is undergoing an upgrade, called Z-Refurbishment (ZR). The upgraded Z pulsed power driver requires thirty six gas switches to be capable of low jitter high voltage switching, to deliver energy to the load. The final command-triggered switch in the Z driver is an SF6-insulated laser triggered gas switch. The laser triggered gas switch test program developed from an effort to improve gas switch performance and lifetime. The switches must remain open as voltage rises in ~one microsecond, then close with a few nanosecond jitter upon arrival of the laser pulse. Switch performance is directly related to component materials since switches must routinely withstand a 6.25 MV, 750 kA pulsed power environment and perform reliably upon each command fire. Switch lifetime is primarily influenced by insulator flashover and electrode degradation. Early in the program the most high profile problem was flashing of the insulator housing. To address this, the program investigated insulator housing materials, surface conditions and cleaning procedures. Electrode materials were studied in an attempt to optimize switch lifetime with respect to erosion rate and to reduce degradation of laser optics that are in close proximity to the switch. Theories on electrode ablation have contributed it to enhancing fields in the trigger section and flashing of the cascade housing. Electrode materials investigated included, tungsten-copper, stainless steel, molybdenum, tantalum and brass. SEM imaging was utilized to examine effects of arc damage for different materials. SEM imaging is also being used in attempts to understand preconditioning of electrodes and early shot switch performance.

Summary form only given. The Z pulsed power driver at Sandia National Laboratories is in the process of being refurbished as part of the ZR project to improve reliability and increase the energy delivered to the load. The new ZR gas switches currently close at a peak voltage of 6.2 MV to generate a projected 26 MA load current as compared to 4.6 MV and ~20 MA for the old switches on Z. The Laser Trigger System (LTS) has been redesigned to meet requirements that it trigger this higher voltage switch with comparable optic lifetime (>100 shots) and jitter (1sigma < 5ns) to the old Z switches. We will discuss critical design features of the LTS and show performance results from the Z20 test bed which was used to study the operation of a single ZR module.

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 fundamentalmore » 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.« less