D. Lojewski

Implosion dynamics and radiative characteristics of a high yield structured gas puff load

A large diameter gas puff nozzle, designed to produce a radial mass profile with a substantial fraction of the injected mass on the axis, has demonstrated an increase in K shell yield by nearly a factor of 2, to 21kJ, in an argon Z pinch at 3.5MA peak current and 205ns implosion time [H. Sze, J. Banister, B. H. Failor, J. S. Levine, N. Qi, A. L. Velikovich, J. Davis, D. Lojewski, and P. Sincerny, Phys. Rev. Lett. 95, 105001 (2005)] and 80kJ at 6MA and 227ns implosion time. The initial gas distribution produced by this nozzle has been determined and related to measured plasma dynamics during the implosion run-in phase. The role of two gas shells and the center jet are elucidated by the inclusion of a tracer element sequentially into each of the three independent plenums and by evacuating each plenum. The implosion dynamics and radiative characteristics of the Z pinches are presented.

Two-dimensional gas density and velocity distributions of a 12-cm-diameter, triple-nozzle argon Z-pinch load

We have developed a 12-cm-diameter Ar gas Z-pinch load, which produces two annular gas shells and a center gas jet. The two-dimensional (2-D) gas density profiles of the load, in r-/spl theta/ and r-z planes, were measured with submillimeter spatial resolutions using the planar-laser-induced fluorescence (PLIF) method, for conditions used in Z-pinch experiments. Due to interactions between the shells, the net gas density profile differs from the superposition of the individual shell profiles. Narrow density peaks are observed both at smaller and larger radii than the radius where the shells come in contact with each other. Two-dimensional flow velocity distributions are determined from the displacements between the fluorescence and later time phosphorescence images. The measured stream velocities of argon gas puffs are 650 /spl plusmn/ 20 m/s, higher than the ideal gas velocity due to the formation of clusters in the supersonic gas flow. Indeed, clusters were observed in earlier Rayleigh scattering experiments. The gas measurements of the initial phase using the PLIF will be combined with other density measurements of the implosion and pinch phases to better understand the implosion dynamics and to provide initial conditions for simulation codes.

Magnetic Rayleigh-Taylor instability mitigation in large-diameter gas puff Z-pinch implosions

Recently, a new approach for efficiently generating K-shell x-rays in large-diameter, long-implosion time, structured argon gas Z-pinches has been demonstrated based on a “pusher-stabilizer-radiator” model. In this paper, direct observations of the Rayleigh–Taylor instability mitigation of a 12-cm diameter, 200-ns implosion time argon Z-pinch using a laser shearing interferometer (LSI) and a laser wavefront analyzer (LWA) are presented. Using a zero-dimensional snowplow model, the imploding plasma trajectories are calculated with the driver current waveforms and the initial mass distributions measured using the planar laser induced fluorescence method. From the LSI and LWA images, the plasma density and trajectory during the implosion are measured. The measured trajectory agrees with the snowplow calculations. The suppression of hydromagnetic instabilities in the “pusher-stabilizer-radiator” structured loads, leading to a high-compression ratio, high-yield Z-pinch, is discussed. For comparison, the LSI an...

K-shell and extreme ultraviolet spectroscopic signatures of structured Ar puff Z-pinch loads with high K-shell x-ray yield

Structured 12-cm-diam Ar gas-puff loads have recently produced Z-pinch implosions with reduced Rayleigh-Taylor instability growth and increased K-shell x-ray yield [H. Sze, J. Banister, B. H. Failor, J. S. Levine, N. Qi, A. L. Velikovich, J. Davis, D. Lojewski, and P. Sincerny, Phys. Rev. Lett. 95, 105001 (2005)]. To better understand the dynamics of these loads, we have measured the extreme ultraviolet (XUV) emission resolved radially, spectrally, and axially. Radial measurements indicated a compressed diameter of ≈3mm, consistent with the observed load inductance change and an imploded-mass consisting of a ≈1.5-mm-diam, hot, K-shell-emitting core and a cooler surrounding blanket. Spectral measurements indicate that, if the load is insufficiently heated, then radiation from the core will rapidly photoheat the outer blanket, producing a strong increase in XUV emission. Also, adding a massive center jet (⩾20% of load mass) increases the rise and fall times of the XUV emission to ⩾40ns, consistent with a mo...

X-ray framing camera for pulsed, high current, electron beam x-ray sources

High power x-ray sources built for nuclear weapons effects testing are evolving toward larger overall diameters and smaller anode cathode gaps. We describe a framing camera developed to measure the time-evolution of these 20-50 ns pulsed x-ray sources produced by currents in the 1.5-2.5 MA range and endpoint voltages between 0.2 and 1.5 MV. The camera has up to 4 frames with 5 ns gate widths; the frames are separated by 5 ns. The image data are recorded electronically with a gated intensified CCD camera and the data are available immediately following a shot. A fast plastic scintillator (2.1 ns decay time) converts the x-rays to visible light and, for high sensitivity, a fiber optic imaging bundle carries the light to the CCD input. Examples of image data are shown.

Proof-of-Principle Time-Resolved Bremsstrahlung Spectral Measurement for Intense, Low-Endpoint (<300 keV) Pulsed Sources

In a proof-of-principle test, we measured how the L-3 Pulse Sciences MBS (~200 keV peak voltage) Bremsstrahlung spectrum changed in time using a pair of filter-fluorescer channels, one at 41 ± 18 keV and another at 87 ± 27 keV. This demonstrates an approach for measuring a time-resolved bremsstrahlung spectrum for SGEMP applications, albeit with coarse energy resolution (30–45%). Using filter fluorescers and detecting the resultant X-rays with plastic scintillator PMTs, there is adequate sensitivity and dynamic range to correlate spectra from two sources, MBS and PITHON, which differ by a factor of up to ~100× in brightness (from 1 to 100 rad CaF2 at the spectrometer entrance). For such a correlation, no hardware changes are required—only the PMT gains need to be changed. The time resolution of the measurement is expected to be 2 ns.

High Yield Argon Z‐pinch Results with a Large Diameter Nozzle

We modified our original 12 cm diameter double‐shell gas puff nozzle to include an on‐axis jet with a large diameter throat and an independent plenum to allow a large fraction of the total mass to be contained in the central region (r=0–1.5 cm). By judicious selection of pressures for the jet and the two shells, we were able to double the Argon K‐shell yield from ∼10 kJ to > 20 kJ with a 3.5 MA current drive and implosion time of ∼205 ns, equivalent to the yield produced at 100 ns implosion time, but with half the pulse‐width, for radiated K‐shell power up to 2 TW.The radiation produced by gas originating in each of the three plenums was distinguished by the use of a chlorine tracer introduced sequentially into each plenum. We thereby deduce that 65% of the K‐shell radiation is produced by gas originating in the jet, 30% from gas originating in the inner shell and only 5% from gas originating in the outer shell.The flexibility of the hardware was further exercised by selectively evacuating one of the thre...

Investigation of plasma evolution in a high power electron-beam diode using a two-frame laser shearing interferometer

Summary form only given. A two-frame laser shearing interferometer (LSI) was used to study the plasma evolution in a 140-kV, 220-kA electron-beam diode with a current pulse rise time of 40 ns. LSI used a 150-ps, 100-mJ, 532-nm, 10-cm-diameter, linearly-polarized laser beam. The p- and s-polarized components of the laser were split and re-combined with the s-component delayed by a 7.5-ns optical path length. The LSI imaged a ~0.5 cm by ~10 cm diode region using two cylindrical and two spherical lenses, so that the diode diameter was de-magnified 1:3 and the diode gap was magnified 2:1. The overall spatial resolution of the LSI images across the anode-cathode (AK) gap was about 10 mum. The cathode consisted of six 2.5-cm hexagonal segmented rods evenly spaced on a 10-cm diameter circle. The diode electrodes were assembled using a systematic procedure to assure the reproducibility of the 2.54plusmn0.05 mm AK gap spacing. The LSI images showed that dense plasmas were generated from both anode and cathode surface when the current reached its peak. Expansion of the electrode plasmas into the AK gap (gap closure) resulted in a collapse of the diode impedance. From the two-frame LSI images (7.5 ns apart) the typical plasma expansion velocity is measured to be ap2 cm/mus. Two kinds of plasmas are observed during the current pulse: an arc discharge plasma and a dense jet plasma. The arc plasma typically initiates over relatively large areas on both cathode and anode surfaces. When the arc plasma is present, the anode plate typically shows significant melting at the diode location. However, the 1-mm-diameter dense jet plasma (>1018 cm-3) is produced only from the cathode and is not reproducible. There is no melting pattern at the corresponding surface of the anode plate and much less discharge erosion on the cathode, indicating less current and/or plasma at the jet location. Plasma formation from different anode materials such as aluminum, stainless steel, tantalum, and thin electroplated gold layer were also studied. We found that the Al and Ta anodes produced the least and most plasma respectively. The ap1 mum gold layer was not effective in reducing the electrode plasma production.

Status of Fast Marx Energy Storage Development

Summary form only given. Design studies have been completed to investigate the impact of improvements in fast energy storage systems on the designs of: larger future simulators, simulator upgrades of operational machines, and for very compact, smaller simulators. The fast energy storage system that is under development is a fast Marx generator (FMG) with (LC)1/2 = 300 ns. This new fast Marx energy storage system uses newly developed, low-inductance rail switches and low-inductance capacitors. These components are configured in a low-inductance FMG stage and then stacked in series to form a unit for the voltage required and a number of units in parallel for the required system inductance and stored energy. This new FMG technology will provide the capability to build X-ray machines in a significantly more compact configuration. The new FMG technology minimizes or eliminates the need for storing the energy in a large water transfer capacitor. A four-stage fast Marx prototype has been demonstrated with a total of 60 kJ energy stored and an output voltage of 680 kV. We will report the progress on the maintainable six-stage fast Marx.

K and XUV signatures of high K-yieldstructured AR puff loads

Summary form only given. Structured 12 cm diameter Ar gas puff loads have produced Rayleigh-Taylor stable implosions and K-yields close to 1D hydrocode predictions. In order to optimize the designs of future loads, it is important to identify the reasons for these stable implosions. Diagnostics of the K-shell (3 keV 15 nsec and rises to a peak relatively slowly (~15 nsec). At locations where the XUV emission rises abruptly (les5 nsec), close in time to when the K-emission initiates, the K-emission is weaker. This observation is consistent with calculations that predict improved K-emission when shock preheating of the load mass is followed by quasiadiabatic compression