Wire X-pinches (WXPs) have been studied comprehensively as fast (∼1 ns pulse width), small (∼1 μm) x-ray sources, created by twisting two or more fine wires into an “X” to produce a localized region of extreme magnetic pressure at the cross-point. Recently, two alternatives to the traditional WXP have arisen: the hybrid X-pinch (HXP), composed of two conical electrodes bridged by a thin wire or capillary, and the laser-cut foil X-pinch (LCXP), cut from a thin foil using a laser. We present a comparison of copper wire, hybrid, and laser-cut foil X-pinches on a single experimental platform: UC San Diego’s ∼200 kA, 150 ns rise time GenASIS driver. All configurations produced 1–2 ns pulse width, ≤5 μm soft x-ray (Cu L-shell, ∼1 keV) sources (resolutions diagnostically limited) with comparable fluxes. WXP results varied with linear mass and wire count, but consistently showed separate pinch and electron-beam-driven sources. LCXPs produced the brightest (∼1 MW), smallest (≤5 μm) Cu K-shell sources, and spectroscopic data showed both H-like Cu Kα lines indicative of source temperatures ≥2 keV, and cold Kα (∼8050 eV) characteristic of electron beam generated sources, which were not separately resolved on other diagnostics (within 1–2 ns and ≤200 μm). HXPs produced minimal K-shell emission and reliably single, bright, and small L-shell sources after modifications to shape the early current pulse through them. Benefits and drawbacks for each configuration are discussed to provide potential X-pinch users with the information required to choose the configuration best suited to their needs.

In this paper we report on the ability of a compact current driver yielding 250 kA in 150 ns to produce counter-propagating plasma flows. The flows were produced by two vertically-opposed conical wire arrays separated by 1 cm, each comprised of 8 wires. With this array configuration, we were able to produce two supersonic plasma jets with velocities on the order of 100-200 km/s that propagate towards each other and collide. Aluminum wires were tested first; we observed a shock wave forming at the collision region that remained stationary for an extended period of time (~ 50 ns) using optical probing diagnostics and Extreme Ultraviolet imaging. After this period, a bow shock is formed that propagates at 20 km/s towards the cathode of the array, likely due to small differences in the density and/or speed of the jets. The inter-jet ion mean free path was estimated to be larger than the shock scale length for aluminum, indicating that the shock is not mediated by collisions, but possibly by a magnetic field, whose potential sources are also discussed. Radiative cooling and density contrast between the jets were found to be important in the shock wave dynamics. We studied the importance of these effects by colliding jets of two different materials, using aluminum in one and copper in the other. In this configuration, the bow shock was observed to collapse into a thin shell and then to fragment, forming clumpy features. Simultaneously, the tip of the bow shock is seen to narrow as the bow shock moves at a similar speed observed in the Al-Al case. We discuss the similarity criteria for scaling astrophysical objects to the laboratory, finding that the dimensionless numbers are promising.

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 Bz0 ranging from 0 to 0.3 T. The effect of Bz0 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 Bz0=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 Bz0 required for stability, Bz0=0.13 Ipk/R0 (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.

Experiments on a MA-class Dense Plasma Focus (DPF) device have been carried out to investigate changes in neutron production by adding moderate amounts of krypton to a deuterium fill gas. The neutron yield from Z-pinch devices, including DPFs, conventionally scales as the peak current to the fourth power. However, a dramatic drop-off from ∼I4 scaling occurs above 3 MA, which recent modeling [D. T. Offermann et al., Phys. Rev. Lett. 116, 195001 (2016)] attributed to the transition in the predominant neutron production mechanism from beam-target fusion to thermonuclear fusion. Previously, the addition of Kr (and other high-Z) dopants has been shown to enhance beam-target fusion yields at currents below 300 kA, with optimal concentrations at 1%–2% Kr, whereas here we show that the optimal concentration of Kr at the MA level is near 0.1% by volume—elucidating a trend in the optimal Kr doping concentration as a function of the device scale. The neutron time-of-flight data reveal that Kr doping creates shorter and more intense neutron bursts, likely due a tighter but unstable pinch, highlighting a key trade-off for Kr doping.

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.