We report on a series of Magnetized Liner Inertial Fusion (MagLIF) experiments conducted on the Z pulsed power facility that utilized high aspect ratio (ratio of outer radius to wall thickness) liners with dielectric coatings and low-mix laser preheat configurations. The liners consisted of an aspect ratio of 10.6 beryllium tube coated with 75 μm of epoxy on the outside that have been shown to maintain a better implosion stability than uncoated beryllium and have demonstrated consistent stagnation performances in previous experiments [Ampleford et al., Phys. Plasmas 31, 022703 (2024)]. Two-dimensional HYDRA simulations were used to design three different “co-injection” laser configurations, whereby a second laser is used to provide an early prepulse before the main pulse, to reduce LEH foil mix while increasing the fuel density and coupled energy. The laser preheat energy for each configuration was constrained using dedicated laser experiments before being applied to the integrated MagLIF experiments on Z. The DD neutron yield for experiments using co-injection preheat configurations is found to increase with the specific preheat energy in line with simulations. The highest neutron yield achieved in this study of 1.1 × 1013 matches the highest reported in a MagLIF experiment to date and is a factor 3.5 times higher than similar experiments using preheat with no phase plate smoothing. We attempt to assess the effects of mix and morphology to explain the improved performance; however, neither factor is found to be conclusive within the uncertainty of the measurements.

The Z machine is a current driver producing up to 30 MA in 100 ns that utilizes a wide range of diagnostics to assess accelerator performance and target behavior conduct experiments that use the Z target as a source of radiation or high pressures. We review the existing suite of diagnostic systems, including their locations and primary configurations. The diagnostics are grouped in the following categories: pulsed power diagnostics, x-ray power and energy, x-ray spectroscopy, x-ray imaging (including backlighting, power flow, and velocimetry), and nuclear detectors (including neutron activation). We will also briefly summarize the primary imaging detectors we use at Z: image plates, x-ray and visible film, microchannel plates, and the ultrafast x-ray imager. The Z shot produces a harsh environment that interferes with diagnostic operation and data retrieval. We term these detrimental processes "threats " of which only partial quantifications and precise sources are known. We summarize the threats and describe techniques utilized in many of the systems to reduce noise and backgrounds.

A line VISAR (Velocity Interferometer System for Any Reflector) has been designed and commissioned at the Sandia National Laboratory's Z-machine. The instrument consists of an F/2 collection system, beam transport, and an interferometer table that contains two Mach-Zehnder type interferometers and an eight channel Gated Optical Imaging (GOI) system. The VISAR probe laser operates at the 532 nm wavelength, and the GOI bandpass is 540-600 nm. The output of each interferometer is passed to an optical streak camera with four selectable sweep speeds. The system is designed with three interchangeable optics modules to select a full field of view of 1 mm, 2 mm, or 4 mm. The optical beam transport system connects the target image plane to the interferometers and the gated optical imagers. The target is integrated into a sacrificial final optics assembly that is integral to the transport beamline.

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 22MJ of energy stored in its capacitor banks into a current pulse that rises in 100ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15years 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 Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17–18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10–1000 MJ) should be possible with pulsed power machines producing larger drive currents (45–60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.

Optical spectroscopic measurements in pulsed power diodes have been performed to estimate plasma parameters and local magnetic fields. Initial experiments on the Self-Magnetic Pinch diode at the RITS-6 accelerator (5-11 MV, 100-200 kA) at Sandia Labs used lens coupled fiber arrays that were imaged onto a high resolution gated spectrometer. Local magnetic fields (3-4 T) were estimated from Zeeman split C IV lines, which yielded current profiles at the anode surface. Additionally, electron density profiles were estimated from the Stark widths of several ion and neutral spectral lines.

Sandia National Laboratories is pursuing a variation of Magneto-Inertial Fusion called Magnetized Liner Inertial Fusion, or MagLIF. The MagLIF approach requires magnetization of the deuterium fuel, which is accomplished by an initial external B-Field and laser-driven pre-heat. While magnetization is crucial to the concept, it is challenging to couple sufficient energy to the fuel, since laser-plasma instabilities exist, and a compromise between laser spot size, laser entrance window thickness, and fuel density must be found. Nonlinear processes in laser plasma interaction, or laser-plasma instabilities (LPI), complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation and beam-spray. Key LPI processes are determined, and mitigation methods are discussed. Results with and without improvement measures are presented.

Eighty years ago, it was proposed that solid hydrogen would becomemetallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.

Sandia's Z Machine uses its high current to magnetically implode targets relevant to inertial confinement fusion. Since target performance is highly dependent on the applied drive field, measuring magnetic field at the target is essential for accurate simulations. Recently, the magnetic field at the target was measured through splitting of the sodium 3s-3p doublet at 5890 and 5896 Å. Spectroscopic dopants were applied to the exterior of the target, and spectral lines were observed in absorption. Magnetic fields in excess of 200 T were measured, corresponding to drive currents of approximately 5 MA early in the pulse.

Summary form only given. The goal of streaked visible pyrometry experiments on Z is to determine the temperature of dynamic material samples using the Plank equation dL/dλ = ε 2hc2/λ5 (ehc/λkT-1) (1). The spectral irradiance dL/dl describes the amount of power emitted per unit area per unit solid angle per unit wavelength. Ideally, an absolute calibration is used to convert the measured signal such as CCD counts or film exposure to units of spectral irradiance (W/mm2/steradian/nm). Relative calibration, where measurements at different wavelengths are proportional to radiance by the same scaling factor can be useful for determining a “color” temperature. However, even at lower temperatures a relative measurement does not constrain the temperature as accurately as an absolute measurement. At higher temperature where there is little difference in the spectral irradiance at visible wavelengths, a relative calibration fails to constrain temperature at all. We have been unable to find a broadband calibration source that is bright enough to adequately expose our streaked pyrometer in a single step. Instead, this paper describes a multistage process for absolute calibration, taking care to link each stage into a complete calibration with minimum errors.

are potential candidates for laser-switch integration will indeed trigger the PCSS switch, and (2) that high gain GaAs switches can be used to trigger optical Q-switches in lasers such as the lasers to be used in the X-1 Advanced Radiation Source and the laser used for direct optical initiation (DOI) of explosives. The technology developed with this LDRD is now the prime candidate for triggering the Q switch in the multiple lasers in the laser trigger system of the X-1 Advanced Radiation Source and may be utilized in other accelerators. As part of the LDRD we developed a commercial supplier. To study laser/switch integration we tested triggering the high gain GaAs switches with: edge emitting laser diodes, vertical cavity surface emitting lasers (VCSELs), and transverse junction stripe (TJS) lasers. The first two types of lasers (edge emitting and VCSELs) did activate the PCSS but are harder to integrate with the PCSS for a compact package. The US lasers, while easier to integrate with the switch, did not trigger the PCSS at the US laser power levels we used. The PCSS was used to activate the Q-switch of the compact laser to be used in the X-1 Advanced Radiation Source.

For gas puff Z-pinches, the K-shell x-ray yield is maximized with the use of a multi-shell nozzle. Optimization of the yield, verification of hydrodynamic models of the nozzle flows, and plausible MHD code modeling of the implosions require data on the radial and axial (R,Z) distribution of mass in the nozzle's flow field. Interferometry is a well-established technique for acquiring such data. We describe the development and use of a two-dimensional interferometer with emphasis on the required data reduction methods. We also show that the instrument can derive the flow from each individual nozzle in a multi-shell system.

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 similar to 81-cm in length, similar to 45-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 (SF6), 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-mu 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-sigma jitter of similar to 5 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 similar to 3%, a flashover probability of similar to 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 similar to 0.2%, a single switch jitter of similar to 6-ns, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.

A 6.1-MV, 790-kA laser triggered gas switch (LTGS) is utilized to synchronize the 36-modules of the Z-machine at Sandia National Laboratories. The switch is ~81-cm in length, 45-cm in diameter, and is immersed in transformer oil. The switch is pulse-charged from a 780-kJ, 6-MV Marx generator in 1.4-?s. Closure of the switch allows energy stored in a 24-nF intermediate-store water capacitor to flow into subsequent pulse-forming stage. The entire system (36-modules) generates a ~70-TW at the present system operating level, but after near term improvements are complete, this level will increase to ~100-TW. The initial design of the LTGS exhibited exceptional performance at a 5.4-MV, 700-kA level (1-? jitter of ~5 ns, prefire and flashover rate less than 0.1%, lifetime in excess of 150 shots) on an engineering test-module utilized for component development. When initially implementing the same design on the 36-module Z-machine, the switch exhibited a prefire probability of ~3%, a flashover probability of ~7%, and a jitter increase of a factor of 3. The difference in performance is attributed to several factors such as higher Coulomb transfer, exposure to a more debris prone environment, and more stressful dynamic mechanical loading when the machine discharges, compared to that of the engineering test module. Under these conditions the replacement lifetime was less than 10 shots, which is unacceptably frequent. Since refurbishment of Z in October 2007, there have been three major LTGS design iterations to improve the performance at a 6.1-MV operating point. The most recent design iteration includes modifications to the switch geometry and electrode and insulating materials that reduces the random prefire rate to less than 0.1%, reduces the flashover rate to -0.2%, reduces single switch jitter to ~6-ns, while increasing the average switch life to greater than 50 shots. Modifications and performance improvements are detailed in this manuscript.

The refurbished Z pulsed power driver has been operational since October of 2007 delivering a peak current of similar to 26 MA to the load. A critical component of the refurbished Z accelerator was the Laser Triggered Gas Switch (LTGS) with a maximum proven operating point of 6.5 MV, 820 kA and an overall 1-sigma timing jitter of similar to 6 ns. We have identified a feature in the V-dot monitor on the Pulse Forming Line (PFL) downstream of the LTGS which is indicative of the closure of the trigger section of the switch. The PFL "squiggle" feature allows us to independently measure the runtime of the cascade and trigger sections and identify problems associated with the laser triggering of the switch, such as poor alignment or degrading transmission of the focusing lens. The squiggle also helps characterize the effect of changes in operating conditions and switch design. For the most recent design version of the LTGS, the trigger and cascade section runtimes with +/-1-sigma jitter are 0.8 +/- 1.3 ns and 46 +/- 5.3 ns respectively. The trigger and cascade section runtimes are not correlated suggesting that the trigger and cascade sections operate independently of each other.

The effect of a short circuit across the final anode-cathode (A-K) gap of the powerful Z-Accelerator could hamper effective power delivery to z-pinch plasmas. The objective of this work is to develop an extreme ultraviolet (EUV) diagnostic technique for diagnosis of the low-temperature plasmas created in the final transmission line (A-K gap near the load) of the Z-Accelerator at the Sandia National Laboratories (SNL). The purpose of this effort is to help in understanding and mitigating this potentially serious problem. This work includes developing EUV grazing incidence spectrometers, investigation of the EUV spectra of highly charged ions in well diagnosed laser-produced plasmas, and the comparison of these laser plasma spectra with the spectra of plasmas created in the inner transmission line. Spectra of highly-charged iron (Fe) ions were investigated using EUV spectroscopy methods in a spectral range of 2 to 80 nm. Experiments at SNL have shown that the most stripped ion observed in the spectra is FeXVII. Comparison of the experimental spectra of FeIII through FeXVII ions with theoretical calculations gives an electron temperature Te of ∼ 200 eV.