Experimental study of current loss and plasma formation in the Z machine post-hole convolute

Abstract

The Z pulsed-power generator at Sandia National Laboratories drives high energy density physics experiments with load currents of up to 26 MA. Z utilizes a double post-hole convolute to combine the current from four parallel magnetically insulated transmission lines into a single transmission line just upstream of the load. Current loss is observed in most experiments and is traditionally attributed to inefficient convolute performance. The apparent loss current varies substantially for z-pinch loads with different inductance histories; however, a similar convolute impedance history is observed for all load types. This paper details direct spectroscopic measurements of plasma density, temperature, and apparent and actual plasma closure velocities within the convolute. Spectral measurements indicate a correlation between impedance collapse and plasma formation in the convolute. Absorption features in the spectra show the convolute plasma consists primarily of hydrogen, which likely forms from desorbed electrode contaminant species such as H 2 O, H 2 , and hydrocarbons. Plasma densities increase from 1 × 10 16 cm −3 (level of detectability) just before peak current to over 1 × 10 17 cm −3 at stagnation (tens of ns later). The density seems to be highest near the cathode surface, with an apparent cathode to anode plasma velocity in the range of 35-50 cm=μs. Similar plasma conditions and convolute impedance histories are observed in experiments with high and low losses, suggesting that losses are driven largely by load dynamics, which determine the voltage on the convolute.

Transmission-line-circuit model of an 85-TW, 25-MA pulsed-power accelerator

“…The large total plasma closure velocity in the convolute is in rough agreement with the vacuum convolute plasma measurements reported in Ref. [36].…”

Ion current losses in the convolute and inner magnetically insulated transmission line of the Z machine

“…There is experimental and simulation evidence [3][4][5][6] of current loss in the vacuum convolute and in the inner MITL of Z [7]. The current losses in the convolute region (CR) itself are measured directly as the difference of the sum of the outer MITL currents entering the convolute, and the current measured by Bdot probes, located at 6 cm from the axis [8].…”

Large time-varying inductance load for studying power flow on the Z machine

“…Present theories for current loss in the load region state that undesirable plasma generation results in some level of current shorting at or within the convolute radius. Possible plasma sources being considered include the aforementioned electron flow current, enhanced SCL ion emission, desorbed and ionized water/contaminants from the MITL surfaces, and/or vaporized metal due to heavy Joule heating [3,19,24]. Beyond the classical shorting that can occur due to these plasmas, in the next section we will present evidence for the presence of plasma invalidating the fundamental MHD assumptions used to model Z machine target dynamics.…”

“…Interest in studying loss mechanisms and overall power flow dynamics on large pulsed power drivers has grown in recent years, with new power flow diagnostics being developed at Sandia National Laboratories for the Z Pulsed Power Facility, commonly known as the Z machine [1][2][3][4][5]. Presently, the only power flow loads that have been studied are cylindrical static or imploding loads that are driven by synchronous short pulse (100 ns rise time).…”

“…Presently, the only power flow loads that have been studied are cylindrical static or imploding loads that are driven by synchronous short pulse (100 ns rise time). This is a logical decision for early phase studies for a number of reasons: (1) The most significant and repeatable current loss is encountered in short pulse experiments; (2) most short pulse experiments have comparable load inductance time histories, allowing for the development of empirical predictive loss models; (3) synchronous pulse experiments impose a simplifying azimuthal symmetry across the entire driver; (4) synchronous short pulse experiments are most desirable in inertial confinement fusion experiments where current loss can prevent access to critical regimes both on existing and proposed future drivers. Though it makes perfect sense to study synchronous short pulse power flow initially (and into the future as diagnostic and modeling techniques mature) there is great value in testing emerging power flow models on targets and current pulse shapes that exist well outside the narrow band of cylindrically imploding synchronous short pulse experiments.…”