Search

The gas-puff Z-pinch is a well-known, efficient source of X-rays and/or neutrons, in which a cylindrical load is imploded by a pulsed-power driver due to the force arising from an applied axial current interacting with a self-generated azimuthal magnetic field. It is susceptible to the magneto-Rayleigh-Taylor instability (MRTI), which must be mitigated to achieve sufficient energy density, i.e., compression. Density tailoring, axial pre-magnetization, and increasing liner resistivity have been shown independently to be effective approaches. Here, we present 2-D radiation-MHD simulations using the HYDRA code [1] of axially-pre-magnetized multi-shell gas-puffs – one or two annular "liners" of Ne, Ar, or Kr, and a central D jet – that predict the mitigation from these approaches are additive: the axial magnetic field (Bz0) required to stabilize a Ne/D implosion is reduced from 0.7 T to 0.3 T by the addition of a second inner liner, and is further reduced by changing the outer liner to more-resistive Ar or Kr. Furthermore, the results suggest an Ne or Ar inner liner provides greater mitigation via snowplow stabilization than a Kr inner liner, as well as mitigation of ion thermal conduction losses from the fuel, the dominant loss mechanism at the 850-kA level. We conclude with a discussion on the scalability of these phenomena to 20 MA, at which level effects such as radiative cooling and fuel mixing may become more relevant.

The gas puff Z-pinch 1 is a magnetic direct-drive plasma compression scheme with applications as an efficient X-ray or neutron source and for research on the fundamental physics of dense plasmas. Presented here are results of gas puff Z-pinch experiments on the CESZAR driver 2 (500 kA, 160 ns) with an annular liner imploding onto an on-axis target, where one or both species is D2. We investigate the effects of load material (D2, Ne, Ar, or Kr) on implosion dynamics, including stability and energy coupling, and on pinch performance. We show via self-emission and laser imaging that the inclusion of a central target jet improves pinch stability, and we discuss correlations between gas species and stability in liner-on-target implosions. Magneto-hydrodynamic simulations are used to infer values of plasma kinetic and internal energy, which are compared with experimental estimates. Pinch performance is quantified by measuring X-ray and neutron yield. Finally, we show that pinch reproducibility can be improved by pre-ionizing the gas before the main current pulse.

A linear transformer driver (LTD) generator capable of delivering up to 0.9 MA current pulses with 160 ns rise time has been assembled and commissioned at University of California San Diego. The machine is an upgrade of the LTD-III pulser from Sandia National Laboratories, consisting of 40 capacitors and 20 spark gap switches, arranged in a 20-brick configuration. The driver was modified with the addition of a new trigger system, active premagnetization of the inductive cores, a vacuum chamber with multiple diagnostic ports, and a vacuum power feed to couple the driver to plasma loads. The new machine is called compact experimental system for $Z$-pinch and ablation research (CESZAR). The driver has a maximum bipolar charge voltage of $\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }\mathrm{kV}$, but for reliability and testing, and to reduce the risk of damage to components, the machine was operated at $\ifmmode\pm\else\textpm\fi{}60\text{ }\text{ }\mathrm{kV}$, producing 550 kA peak currents with a rise time of 170 ns on a 3.5 nH short circuit. While the peak current is scaled down due to the reduced charge voltage, the pulse shape and circuit parameters are close to the results of the cavity and power feed models but suggest a slightly higher inductance than predicted. The machine was then used to drive wire array $Z$-pinch and gas puff $Z$-pinch experiments as initial dynamic plasma loads. The evolution of the wire array $Z$ pinch is consistent with the general knowledge of this kind of experiment, featuring wire ablation and stagnation of the precursor plasma on axis. The gas puff $Z$ pinches were configured as a single, hollow argon gas shell, in preparation for more structured gas puff targets such as multispecies, multishell implosions. The implosion dynamics agree generally with 1D magnetohydrodynamics simulation results, but large zippering and magneto-Rayleigh-Taylor instabilities are observed. The CESZAR load region was designed to accommodate many load types to be driven by the machine, which makes it a versatile platform for studying $Z$-pinch plasmas. The completion of the CESZAR driver allows plasma experiments on energy coupling from LTD machines to plasma loads, instability mitigation techniques and magnetic field distributions in $Z$ pinches, and interface dynamics in multispecies implosions.

Gas puff Z-pinch experiments with annular Ar and Ne gas shells have been conducted on the Compact Experimental System for Z-pinch and Ablation Research (CESZAR) linear transformer driver (LTD) with 500 kA current and 160 ns rise time. Here, we present results from the first systematic gas puff Z-pinch experiments using a fast ( ≤200 ns) LTD as a driver, in which we show that 7% of the stored energy in the capacitors is coupled to plasma kinetic energy as estimated via self-emission and laser schlieren images. 0D and 1D simulations—which do not allow instability growth and thus reach greater maximum average velocities—using initial conditions inferred from experimental implosion trajectories predict coupling in excess of 10% of the stored energy. The Ar and Ne implosions were comparably massed and thus achieved similar maximum kinetic energies, though the Ne pinches were more stable and the x-ray pulses were longer and produced higher yield: 2–5 ns and 0.21–0.52 J (0.15–0.37 J/cm) of Ar K-shell and 12–25 ns and 2.2–3.9 J (1.6–2.6 J/cm) of Ne K-shell, respectively. The difference in stability is most likely attributed to variations in initial conditions such as density distribution and gas breakdown initiation.

Gas puff Z-pinches are intense sources of X-rays and neutrons but are highly susceptible to the magneto-Rayleigh-Taylor instability (MRTI). MRTI mitigation is critical for optimal and reproducible yields, motivating significant attention toward various potential mitigation mechanisms. One such approach is the external application of an axial magnetic field, which will be discussed here in the context of recent experiments on the Zebra generator (1 MA, 100 ns) at the University of Nevada, Reno. In these experiments, an annular Kr gas liner is imploded onto an on-axis deuterium target with a pre-embedded axial magnetic field B z 0 ranging from 0 to 0.3 T. The effect of B z 0 on the stability of the Kr liner is evaluated with measurements of plasma radius, overall instability amplitude, and dominant instability wavelength at different times obtained from time-gated extreme ultraviolet pinhole images. It was observed that the external axial magnetic field does not affect the implosion velocity significantly and that it reduces the overall instability amplitude and the presence of short-wavelength modes, indicating improved pinch stability and reproducibility. For the highest applied B z 0 = 0.3 T, the stagnation radius measured via visible streak images was found to increase. These findings are consistent with experiments reported in the literature, but here, the B z 0 required for stability, B z 0 = 0.13 I p k / R 0 (where Ipk is the driver peak current and R0 is the initial radius), is lower. This could be attributed to the smaller load geometry, both radially and axially. Consistent with other experiments, the cause of decreased convergence cannot be explained by the additional axial magnetic pressure and remains an open question.

Recent experiments at the Nevada Terawatt Facility at UNR show evidence of uniform compression of a deuterium plasma target compressed by a high-Z, Argon or Krypton gas-puffed liner. Pinch stability is improved by seeding the implosion with a 0.1-0.2 T axial magnetic field. Implosion dynamics and stagnation conditions are also studied computationally with the radiation-MHD code MACH2, using in itial conditions similar to those in the experiment. Simulations show that magnetic field diffuses through the outer shell and piles up at the interface providing narrow profile, high intensity current that Ohm-icly preheats the target. This secondary piston launches a shock wave in the target plasma that heats the several 100 eV. Finally, the preheated target is adiabatically compressed to stagnation. Simulations show: (a) more pronounced preheating with Kr than Ar, (b) the axial magnetic field is compressed only in the shocked target and in the liner plasma, providing greater magneto-Rayleigh-Taylor mitigation during run-in compared to the self-similar model. For typical Ar liner on D target experiments we measured neutron yield up to 2x 109 and for Kr liner, up to 9x 109.

Previous Staged Z-pinch (SZP 1 experiments at the University of California-Irvine demonstrated that gas liners (or gas-puffs) can efficiently couple energy to a target plasma and implode it uniformly. In those experiments, a 1.5 MA, $1 \mu \mathrm {s}$ current driver was used to implode a magnetized, Kr liner onto a D+ target, producing ~1010 neutrons per shot. Time-of-flight data suggested that primary and secondary neutrons were produced. Recent MHD simulations 2 have suggested that liner composition is important for target shock-heating before the main adiabatic implosion and that pre-magnetization is crucial to achieve uniform implosions.A recent series of experiments were carried out to investigate implosion dynamics using Cornell’s 1 MA, 200 ns current driver COBRA with a goal to help us better understand the SZP physics and benchmark MHD codes. We used an optimized gas injector 3 composed of an annular (1 cm radius) high atomic number (e.g., Ar or Kr) gas-puff and an on-axis plasma gun that delivers the ionized hydrogen target. Liner implosion velocity and stability were studied using laser shadowgraphy and interferometry as well as gated visible and XUV imaging. Target temperature and density were measured with Thomson scattering and X-ray spectroscopy. Experimental data is presented and preliminary analysis is discussed.

Recent experiments on the 1 MA, 100 ns Zebra driver at the Nevada Terawatt Facility at the University of Nevada, Reno, investigated the compression of a deuterium target by a high-atomic-number (Ar or Kr) gas-puff liner. Pinch stability improved with axial premagnetization of 1–2 kG observed as a decrease in magneto-Rayleigh-Taylor instability growth. Implosion dynamics and stagnation conditions were studied computationally with the radiation-MHD code MACH2 using initial conditions that approximate those in the experiment. Typical average and peak implosion velocities exceeded 300 and 400 km/s, respectively, which raised the target adiabat by shock heating as the front converges on axis, at which time the target is adiabatically compressed to stagnation. Experimental fusion yields of up to 2 × 109 for Ar liner on D target implosions were measured, while with a Kr liner yields up to 1 × 1010 were measured. Higher yields in Kr compared to Ar were also calculated in 2-D MACH2 simulations. These observations will be further tested with other radiation-MHD codes, and experiments on the 1 MA LTD-III machine at UC San Diego.

A semi-analytic model is presented for the gas-puff Staged Z-pinch, a magneto-inertial fusion concept in which an annular gas-puff liner implodes onto a deuterium or deuterium-tritium target. The one-dimensional model is a modification of the semi-analytic model for MagLIF (SAMM) [R. D. McBride and S. A. Slutz, Phys. Plasmas 22, 052708 (2015)], that addresses the different set of physics inherent to a Staged Z-pinch implosion: azimuthal magnetic field transport, shock heating of the fuel, separate ion and electron energy equations, and a simplified radiation model that approximates the liner transition from optically thin to optically thick. Following the explanation of the model, three sample problems are presented: first, a Staged Z-pinch implosion on the Zebra driver (1 MA, 100 ns) is modeled and compared with the HYDRA simulation results; second, the MagLIF point design is modeled and compared to the original simulation results [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] and results from SAMM; and third, we conduct a simple parameter scan and scaling study for a Staged Z-pinch implosion on the LTD-III driver (0.8 MA, 160 ns). Some agreement with HYDRA and SAMM is obtained, and deuterium-deuterium (DD) neutron yield scaling with current is consistent with other existing models and HYDRA simulations.A semi-analytic model is presented for the gas-puff Staged Z-pinch, a magneto-inertial fusion concept in which an annular gas-puff liner implodes onto a deuterium or deuterium-tritium target. The one-dimensional model is a modification of the semi-analytic model for MagLIF (SAMM) [R. D. McBride and S. A. Slutz, Phys. Plasmas 22, 052708 (2015)], that addresses the different set of physics inherent to a Staged Z-pinch implosion: azimuthal magnetic field transport, shock heating of the fuel, separate ion and electron energy equations, and a simplified radiation model that approximates the liner transition from optically thin to optically thick. Following the explanation of the model, three sample problems are presented: first, a Staged Z-pinch implosion on the Zebra driver (1 MA, 100 ns) is modeled and compared with the HYDRA simulation results; second, the MagLIF point design is modeled and compared to the original simulation results [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] and results from SAM...

We discuss the design and characterization of a compact gas-puff nozzle and plasma gun assembly that will be used in Staged Z-pinch experiments1, where gas liners are imploded onto an on-axis target-plasma column. Previous work on the Staged Z-pinch demonstrated that gas liners can efficiently couple the energy and produce a uniform implosion on a target-plasma.2

Target preheating is an integral component of magnetized inertial fusion in reducing convergence ratio. In the staged Z-pinch concept, it is achieved via one or more shocks. Previous work [Narkis et al., Phys. Plasmas 23, 122706 (2016)] found that shock formation in the target occurred earlier in higher-Z liners due to faster flux transport to the target/liner interface. However, a corresponding increase in magnitude of magnetic pressure was not observed, and target implosion velocity (and therefore shock strength) remained unchanged. To investigate other means of increasing the magnitude of transported flux, a Korteweg-de Vries-Burgers equation from the 1-D single-fluid, resistive magnetohydrodynamic equations is obtained. Solutions to the nondispersive (i.e., Burgers) equation depend on nondimensional coefficients, whose dependence on liner density, temperature, etc., suggests an increase in target implosion velocity, and therefore shock strength, can be obtained by tailoring the mass of a single-liner ...