J. Engelbrecht

Measuring 10-20 T magnetic fields in single wire explosions using Zeeman splitting

We have shown that the Zeeman splitting of the sodium (Na) D-lines at 5890 Å and 5896 Å can be used to measure the magnetic field produced by the current flowing in an exploding wire prior to wire explosion. After wire explosion, the lines in question are either not visible in the strong continuum from the exploding wire plasma, or too broad to measure the magnetic field by methods discussed in this paper. We have determined magnetic fields in the range 10-20 T, which lies between the small field and Paschen-Back regimes for the Na D-lines, over a period of about 70 ns on a 10 kA peak current machine. The Na source is evaporated drops of water with a 0.171 M NaCl solution deposited on the wire. The Na desorbs from the wire as it heats up, and the excited vapor atoms are seen in emission lines. The measured magnetic field, determined by the Zeeman splitting of these emission lines, estimates the average radial location of the emitting Na vapor as a function of time under the assumption the current flows only in the wire during the time of the measurement.

Measuring 20-100 T B-fields using Zeeman splitting of sodium emission lines on a 500 kA pulsed power machine

We have shown that Zeeman splitting of the sodium (Na) D-lines at 5890 and 5896 Å can be used to measure the magnetic field (B-field) produced in high current pulsed power experiments. We have measured the B-field next to a return current conductor in a hybrid X-pinch experiment near a peak current of about 500 kA. Na is deposited on the conductor and then is desorbed and excited by radiation from the hybrid X-pinch. The D-line emission spectrum implies B-fields of about 20 T with a return current post of 4 mm diameter or up to 120 T with a return current wire of 0.455 mm diameter. These measurements were consistent or lower than the expected B-field, thereby showing that basic Zeeman splitting can be used to measure the B-field in a pulsed-power-driven high-energy-density (HED) plasma experiment. We hope to extend these measurement techniques using suitable ionized species to measurements within HED plasmas.

Observations of the magneto-Rayleigh-Taylor instability and shock dynamics in gas-puff Z-pinch experiments

We describe a series of experiments performed to study the shock structure generated during the implosion of a gas-puff Z-pinch. The Z-pinch is produced by a double-annular gas-puff with a center jet driven by Cornell Universitys COBRA generator operating with a 1 MA, 200 ns current pulse. Using 532 nm laser interferometry and 100 MHz multi-frame cameras, a shock structure is observed to form early in the implosion. The shock appears to be created by a current layer at the outer radius of the imploding plasma which acts as a piston moving inward at several hundred km s−1. The dynamics of the shock and its radial position ahead of the piston agree well with a simple uniform density model outlined in the study by Potter [Nucl. Fusion 18(6), 813 (1978)]. The outer surface of the current layer is observed to be Magneto-Rayleigh-Taylor unstable. The growth rate of this instability is found to depend on the radial density profile of the material within the layer of high-density fluid between the shock and the piston, as predicted by recent theoretical work [see, e.g., D. Livescu, Phys. Fluids 16(1), 118 (2004)]. Growth rates measured in krypton implosions, where the post-shock material is found to decay quasi-exponentially away from the piston, were more than ten times smaller than those recorded in otherwise identical implosions in argon plasmas, where the material between the shock and the piston was observed to maintain a uniform density.

Enhancing the x-ray output of a single-wire explosion with a gas-puff based plasma opening switch

We present experiments performed on the 1 MA COBRA generator using a low density, annular, gas-puff z-pinch implosion as an opening switch to rapidly transfer a current pulse into a single metal wire on axis. This gas-puff on axial wire configuration was studied for its promise as an opening switch and as a means of enhancing the x-ray output of the wire. We demonstrate that current can be switched from the gas-puff plasma into the wire, and that the timing of the switch can be controlled by the gas-puff plenum backing pressure. X-ray detector measurements indicate that for low plenum pressure Kr or Xe shots with a copper wire, this configuration can offer a significant enhancement in the peak intensity and temporal distribution of radiation in the 1–10 keV range.We present experiments performed on the 1 MA COBRA generator using a low density, annular, gas-puff z-pinch implosion as an opening switch to rapidly transfer a current pulse into a single metal wire on axis. This gas-puff on axial wire configuration was studied for its promise as an opening switch and as a means of enhancing the x-ray output of the wire. We demonstrate that current can be switched from the gas-puff plasma into the wire, and that the timing of the switch can be controlled by the gas-puff plenum backing pressure. X-ray detector measurements indicate that for low plenum pressure Kr or Xe shots with a copper wire, this configuration can offer a significant enhancement in the peak intensity and temporal distribution of radiation in the 1–10 keV range.

X-ray radiation from puff-on-wire implosions on the COBRA generator

Summary form only given. Substantial progress has been made in developing plasma radiation sources from Z-pinch implosions. University pulsed power machines provide a cost effective platform to study alternative mechanisms of producing x-rays in exploratory experiments that may provide guidance in search of further improvements on the larger machines. Radiation from puff-on-wire implosions were studied by Chuvatin et al. [1] and Wessel et al. [2]. We report recent observations and modeling of puff-on-wire implosions using the 1 MA COBRA generator in the long pulse mode. The gas puff used neon, argon, or krypton and the wire material was either copper or manganin 290 (84% Cu, 12% Mn, 4% Ni). The diagnostics include time-integrated pinhole cameras, time-integrated axially resolved spectra, multiple filtered PCDs and Si-diodes, and time-gated XUV cameras. X-ray radiation from the gas puff and K-alpha lines from wire material was detected and the radiation pulse was reproducible. A 1-D multi-zone non-LTE kinetics code with radiation transport will be used to model the radiation to infer the plasma conditions.

Rayleigh-Taylor instability amplification due to radiative losses

Summary form only given. Radiative losses are known to play an important role in the development of hydrodynamic instabilities in many astrophysical (Core-collapse supernovae; HH objects) and laboratory (Inertial confinement fusion) environments. The fielding of triple-annular gas-puff valves on university level pulsed-power generators [1][2] enables study of these instabilities in carefully controlled environments, for inviscid Re >> 1, non-diffusive Pe >> 1 fluids, where radiative losses on dynamically-relevant timescales can be significant. Furthermore, the acceleration of the unstable boundary and radiative cooling rates can both be specified, by variation of nozzle backing-pressures and gas species respectively.A 7 cm outer-diameter, triple-annular nozzle is used to inject gas into the 2.4cm anode-cathode gap of the (1 MA, 200 ns) COBRA generator. An annular current-carrying plasma, formed near the nozzle outer radius, is driven towards the axis of symmetry by the azimuthal magnetic field produced by the machine current. This low-density current-sheath sweeps up injected neutral gas ahead of it into a high-density shell, and the boundary between these layers is unstable to the Rayleigh-Taylor (RT) instability. RT growth is investigated in Argon (Ar) and Krypton (Kr) gas-puffs, initialized with radial mass-density profiles that are determined quantitatively by Planar Laser-Induced Fluorescence. Perturbation wavelengths and amplitudes are imaged at 10ns intervals using two four-frame XUV (10eV <; hν <; 1keV) cameras. Simultaneously, the temperature and velocity of the imploding shell is probed using a (527 nm, 4 GW) Thomson scattering diagnostic. Dominant wavelengths are observed at 1.5mm and 1.7mm for Ar and Kr shells respectively. Amplitude e-folding times of 20ns are recorded in Kr, 20% faster than in Ar under otherwise identical conditions. Ion temperatures of 100eV are recorded in Kr shells, ~40% lower than in Ar, and it appears this cooling is responsible for the observed increase in RT growth rate.

Neon and argon multi-nozzle gas puff z-pinch studies on COBRA

Summary form only given. In the past, we have produced stable z-pinch implosions with an outer Ne shell imploding on an inner Ar (Ne-on-Ar) gas puff load. Using a similar mass density profile to the Ne-on-Ar experiments, we observed that Ar-on-Ne implosions were not stable. In this paper, we present the investigations of the Ne-on-Ar and Ar-on-Ne implosion dynamics with various mass density profiles. The experiments are conducted on the 1-MA, 200-ns COBRA generator. A triple-nozzle is used to produce z-pinch loads with concentric outer and inner annular gas puffs and a center gas puff. Z-pinch loads such as Ne-on-Ar and Ar-on-Ne with or without a center gas jet will be tested. In the experiments, we will use Planar Laser Induced Fluorescence (PLIF) to determine the initial gas puff mass density profile, and a three-frame Laser Shearing Interferometer (LSI) for imploding plasma density profiles. Two four-frame gated XUV cameras capture the implosion plasma structure. A multi-channel, filtered pinhole x-ray camera images the pinch plasma. The Ar and Ne x-ray spectra will be recorded by a spherical crystal x-ray spectrometer. Two spatially resolved UV and visible light spectrometers are used to measure the emission line profiles. The measurements will be compared with the predictions from a 2D multi-material radiation MHD model.