B. Moosman

Valve and nozzle design for injecting a shell-on-shell gas puff load into a z pinch

We have developed a dual-plenum gas valve coupled to a double shell nozzle for the generation of “shell-on-shell” gas loads in z-pinch plasma radiation source experiments. The gas density profiles of the nozzles have been characterized with laser interferometry. This valve/nozzle combination has been successfully fielded on the Double-EAGLE and Saturn pulsed-power generators. The design and characterization of the shell-on-shell valve/nozzle are presented in this article.

Long time implosion experiments with double gas puffs

Long time implosion experiments with argon double gas puffs have been conducted on the GIT-12 [S. P. Bugaev et al., Izv. Vyssh. Uchebn. Zaved., Fiz. 40, 38 (1997)] generator at the current level of 2.2–2.4 MA. A double gas puff was used as one of the alternative ways to improve implosion stability at implosion times from 230 to 340 ns. The results of these experiments were compared with two-dimensional snowplow simulations. The experiments and the simulations show that the final pinch is sufficiently stable when the inner-to-outer shell mass ratio is greater than 1. The maximum argon K-shell yield obtained in the experiments is 740 J/cm with 220 GW/cm radiation power. At the long implosion times, the K-shell yield obtained in the double gas puff implosion is twice the K-shell yield of a 4-cm-radius single gas puff, with more than an order of magnitude increase in radiation power.

Measurements of gas preionization for plasma radiation sources

Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh–Taylor instabilities or other nonuniformities affecting the final implosion stage of the pinch, leading to an improvement in K-shell x-ray yield. We report on measurements of a flashover UV photoionization scheme, capable of ionizing the periphery of argon gas puffs to 1%–10% of the initial gas density. Measurements are made with a two-color, (1064 and 532 nm), high sensitivity (∼10−5λ), Mach–Zehnder type interferometer. Two methods of measuring preionization are investigated. The first uses a single laser wavelength, 1064 nm, to probe a chord of the cylindrical gas puff. The gas density is measured first without preionization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, de...

Long-implosion plasma radiation sources using “solid-fill” nozzles

Solid-fill nozzles for long-implosion Z-pinch experiments to produce argon K-shell x rays (photon energy >3.1 keV) have been developed. With a 7 cm diam nozzle, which is appropriate for a 200 ns driver, stable implosions at 180 ns and 4 MA have produced peak argon K-shell yields exceeding 15 kJ. As previously seen with short (∼100 ns) implosion times, the K-shell yield scales as the fourth power of peak current, I4. Limited testing with a 10 cm nozzle, which is appropriate for a >250 ns driver, has also achieved a stable implosion.

Differential phase shift interferometer for measuring axisymmetric gas distributions for high-power Z-pinch research

A sensitive interferometer [B. V. Weber and S. F. Fulghum, Rev. Sci. Instrum. 68, 1227 (1997)] with 10−5-wave phase resolution is used to measure gas distributions from axisymmetric, supersonic nozzles employed in high power Z-pinch experiments. The line-integrated gas density, N≡∫ndx, is measured as a function of time at one distance, z, from the nozzle and one distance, y, from the axis. The (y,z) region of interest is scanned shot to shot. The N(y) data are analyzed to find radial density distributions via Abel inversion: n(r)=−(1/π)∫r∞[(dN/dy)/y2−r2]dy. Error propagation based on the uncertainty in N results in larger relative uncertainties in n, especially near the axis. The precision may be improved using a modified version of this interferometer, where the reference beam is moved into the gas, a small distance (1–2 mm) from the scene beam. The phase difference is then a direct measurement of dN/dy, reducing the error introduced by numerically differentiating N. These two techniques are compared usi...

Characterization of the conduction phase of a plasma opening switch using a hydrogen plasma

Plasma opening switch (POS) experiments were conducted on the Hawk generator using an inverse pinch plasma source to inject a hydrogen plasma. Using a combination of interferometry, current measurements, and spectroscopic observations, it is shown that the conduction phase is characterized by the propagation of a current channel through the switch region that pushes a significant fraction of the plasma mass downstream, past the load edge of the switch. The data indicate that the current channel arrives at the load edge of the switch ≈550ns into the 950-ns-long conduction phase, in agreement with calculations based on J×B displacement. Previously published POS experiments, using multispecies plasmas, observed that a relatively small fraction of the injected plasma mass propagated downstream and that the conduction phase ended soon after the current channel reached the load edge of the switch. It is suggested that the observed differences between these two types of switches involves the separation of ionic ...

Gas puff implosion measurements and modeling

Two-dimensional interferometric images obtained in gas-puff z-pinch experiments on the Hawk generator are compared with snowplow calculations using the measured gas distribution for various times during the 250-ns implosion. To match the calculated implosion time with the observed implosion time, 88% to 101% of the measured cathode current, adjusted for each shot, is required to implode the snowplow. The snowplow model reproduces the (r,z) shape of the measured 2-D plasma shell at different times during the implosion. The inferred current loss between the measurement location (17-cm upstream of the nozzle) and the z-pinch may be due to vacuum flow resulting from the plasma-opening switch.

Comparison of POS-load dynamics using different plasma sources on Hawk

Hydrogen Plasma Opening Switch (POS) experiments on Hawk show potential for increasing bremsstrahlung radiation on DECADE. Interferometry indicates that the density of POS plasma is greatly reduced over a large fraction of the interelectrode region (>1 cm out of 2.2 cm) at the time of opening. This low-density region is much larger for the hydrogen POS than for flashover-source POSs used previously on Hawk (and DECADE), and may result in a significantly larger effective vacuum gap, improving the efficiency of energy coupling to loads. Species separation may be responsible for the difference in plasma dynamics between single- and multispecies POSs.

High-sensitivity interferometry of plasmas and gases in pulsed power experiments

Accurate measurements of plasmas and gases in pulsed power devices are needed so they can be used (or avoided) as desired. This paper reviews measurements of plasmas and gases using the high-sensitivity, two-color interferometer in use at the Naval Research Laboratory (NRL) since 1993. The instrument uses two cw Nd:YAG lasers at 1.064 and 0.532 /spl mu/m wavelengths that can be aligned along the same line-of-sight to simultaneously measure electron and neutral densities. This interferometer can measure optical path changes (phase shifts) as small as 10/sup -5/ waves. The interferometers capabilities are illustrated by several examples, including the first measurement of electrons co-moving in vacuum with an intense proton beam, detection of gas turbulence, preionization of gas from a supersonic nozzle, and gas desorption from a metal surface exposed to intense UV.

Gas pre-ionization system for DECADE Module 2 PRS experiments

DECADE Module 2 (DM2) implodes argon gas-puffs in 200-300 ns from initial radii of 2.5-5 cm to generate K-line radiation. An important issue for these experiments is the lower-than-expected K-line radiation associated with such large radius implosions. Preionization may improve the implosion quality by improving the symmetry of the initial current flow. A preionizer is described based on photoionization by ultraviolet (UV) illumination from a flashover source. The preionization is diagnosed using a high-sensitivity interferometer. This UV source produces 1-10% ionization of the outer periphery of the argon gas distribution prior to the arrival of the flashboard plasma. DM2 experiments indicate increased, more reproducible K-shell yields when this preionization system is incorporated.