A.S. Chuvatin

Design and Experimental Validation of Two Current Multiplier Configurations on a Microsecond MA Generator

Summary form only given. The current multiplier (CM) concept was proposed to increase the driver-to-load energy transfer efficiency. The suggested CM requires additional volumes with high self-inductance (magnetic flux extruders) connected through vacuum convolutes prior to the load and they extrude the magnetic flux toward the load magnifying the load current. We present the design criteria allowing to achieve high extruder self-inductance at low parasitic inductances added to generator and load in the modified circuit. Two configurations of this new device with one extruder are considered for GIT 12 microsecond MA generator having inductance of ~100 nH. The extruder inductance was either a large vacuum volume or a smaller volume with magnetic core. The discussed design procedure allowed to define optimum coreless and cored CM hardware configurations at conservative values of the AK gaps in CM vacuum lines (15-25 mm). The optimum coreless CM had 80 cm height and 170 cm diameter. Operating on a 8 nH inductive load in experimental tests on GIT 12 it allowed to increase the load current from 4.7 MA @ 1.7 mus without CM to ~6 MA @ 1.5 mus2. A more compact cored CM configuration with 36 cm height and 85 cm diameter operating at a 4.6 nH inductive load allowed further load current increase up to ~8 MA @ 1.7 mus. Further experimental tests with a Ne gas-puff z-pinch load showed that the peak load current at ~1 mus was increased from 3.5 MA (no CM) to 5.2 MA and that the energy-delivered to load at implosion was increased from ~170 kJ to ~330 kJ. No considerable energy losses in the CM vacuum gaps and CM convolute were recorded. Therefore, it is experimentally confirmed that the proposed new device is applicable for improving characteristics of existing and future pulse-power generators.

A composite POS: first proof-of-principle results from GIT-12

Experiments with the microsecond plasma opening switch were carried out on the GIT-12 installation at the conduction current level of 3 MA. In this series an attempt to increase both the critical charge responsible for the conduction phase and the POS voltage was undertaken. The necessity of such improvement is conditioned by the difficulties of application of existing POS technology for construction of a new, multi-MA class IES driver. The experiments were inspired by several phenomenological ideas and theoretical results described as the main background philosophy of this work. Possible improvement of the POS operation is predicted if using the following technique: 1) optimization of the plasma injection length in order to increase the critical charge by keeping the initial plasma density at minimum possible level; 2) use of the initial plasma density gradient in the direction from the load to the generator in order to decouple the plasma densities responsible for the conduction and opening times; 3) use of the controlled plasma motion during the conduction phase for abrupt enhancement of the magnetic field prior to the opening phase. A composite POS satisfying these conditions and consisting of initially prepared MHD and Hall parts was applied. The first, mainly proof-of-principle results from C-POS operation on GIT-12 are reported.

Spectroscopy of composite Z-pinch on ANGARA-5-1 installation

The results of spectroscopy measurements of a composite Z-pinch on ANGARA-5-1 installation are presented. The composite Z-pinch was created with the help of a hollow Ar or methane gas jet which collapsed onto a solid foam cylinder doped with salts of KCl or NaCl. The dependence of the line radiation intensities and plasma parameters on linear mass of the gas jet (3-60 mkg/cm) was investigated.

PPPS-2013: Microsecond ramp compression of a metallic liner driven by a shaped 5 MA current on the SPHINX machine

Summary form only given. SPHINX is a 6MA, 1-μs Linear Transformer Driver operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, is developed using a compact pulse shaping system. This system, a Dynamic Load Current Multiplier (DLCM), is inserted in vacuum between the convolute and the load. We present the overall experimental configuration chosen for these experiments, based on electrical and hydrodynamic simulations. Initial results obtained over a set of experiments on an aluminum cylindrical liner, ramp-compressed to a peak pressure of 230 kbar, are presented and analyzed. Detailed features of the electrical and Photonic Doppler Velocimetry (PDV) setups used to monitor and diagnose the ramp compression experiments are provided. Current profiles measured at various critical locations throughout the system, particularly the load current, are in good agreement with simulated current profiles. They enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. Also, the liner inner free surface velocity measurements agree with the hydrocode results obtained using the measured load current as the input. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development) and optimizing the load dimensions.

Analysis of Al precursor wire array experiments on the 1 MA zebra generator at UNR

Summary form only given. Previous experiments on the 1 MA Zebra generator at UNR studied precursor plasmas with Ni-60 cylindrical wire arrays (CWA). Those precursor plasmas were shown to consistently have electron temperatures > 400 eV1. Continuing research on precursor plasmas at 1 MA on Zebra investigated first other mid-Z wire materials and then alternate arrays using mixed Al/mid-Z CWAs. Results found similar electron temperatures for the mid-Z elements present in the precursor with relatively colder temperatures for Al. A better understanding of the results from the mixed CWAs requires understanding pure Al CWAs. Recent experiments on Zebra using Al CWAs were performed to compare with the mixed Al/mid-Z CWAs. These CWAs consist of 6 wires evenly spaced in a 12 mm diameter, the same configuration as in previous experiments on precursor plasmas. Time-integrated spatially-resolved (TISR) and time-gated spatially-integrated (TGSI) X-ray spectral data, time-integrated and time-gated pinhole X-ray images, shadowgraphy, as well as optical streak camera images were obtained and analyzed. It was found that the Al precursor radiation starts, and stays pronounced until the main X-ray burst. This differs from the mid-Z precursors which show a defined precursor burst with an increase in radiation and decrease to zero before the main x-ray burst. Non-LTE kinetic models of Al have been applied to account for the K-shell radiation from precursor and main X-ray burst plasmas. The resulting plasma parameters from modeling of TGSI and TISR spectra together with analysis of corresponding images allow for the study of precursor plasma formation in time and in space, respectively.

Study of implosion dynamics and radiative mechanisms of planar foil liners in comparison with planar wire arrays at 1.7 MA UNR zebra generator

Summary form only given. Planar foil liners are alternative loads to wire arrays at multi-mega ampere generators as well as a promising object for the investigation of the magnetic energy dissipation mechanisms in Z-pinch plasmas. Experimental comparison of implosion dynamics and radiative mechanisms of Al planar foils and single planar wire arrays (SPWAs) of the same width and linear mass was performed for the 0.9-1.6-MA current region. Foils radiate approximately 80-90% of the total yield and power of SPWAs. The non-LTE code was applied to estimate the average electron temperature in Al planar foils that was found to be 20% higher, than that in SPWAs, and the average electron density in foils that was an order of magnitude lower than for SPWAs. Also, the foils are characterized by smaller axial gradient of electron temperature and density than SPWAs. In addition, anisotropic emission from Al planar foils was observed to be similar to Al SPWAs: the total yield registered orthogonally to the foil plane was 1.3 times higher than that along the plane (compared to 1.5 for SPWAs). The anomalous MHD resistivity consideration suggests that a significant part of foil radiation could be due to formation of strongly-inhomogeneous plasma through instabilities appearing on shadowgraphic images of a symmetric foil as a result of initial sharp edges inhomogeneity. This idea was tested in the recent experiments with modified foils where one edge was initially sharp and the other was folded with smaller initial inhomogeneity. The yield from a foil with a folded edge was 13-15% lower than that with both sharp edges as predicted by MHD modeling. Presented results on radiation from foils suggest them as potentially useful x-ray sources for various HEDP applications due to simpler load foil preparations compared to wire arrays. Preliminary results of the research we started on radiation from double foils in comparison with double planar wire arrays (DPWAs) are also discussed.

Radiation signatures of large sized multi-planar wire arrays

Summary form only given. Experiments on the Zebra generator with LCM (Load Current Multiplier, provides 1.5-1.7 MA) allow for implosions of larger sized wire array loads (including planar wire arrays) than at standard current of 1 MA. Advantages of larger sized planar wire array implosions include enhanced energy coupling to plasmas and better diagnostic access to observable plasma regions. A full set of diagnostics was implemented to study radiation in a broad spectral range from few Å to few hundred Å using PCD, XRD, and EUV detectors, X-ray/EUV spectrometers and X-ray pinhole cameras. In addition, laser shadowgraphy was utilized. In multi-planar wire arrays, two outer wire planes were each 4.9 mm width and made of eight mid-atomic-number (Alumel with 95% of Ni) wires with the inter-row gap increased from 3 or 6 mm (usually used at 1 MA current) up to 9 mm. A central plane located in the middle between the outer planes had empty slots and a few Al wires at the edges. Recently, we have shown that such configuration produces higher linear radiation yield. In the new experiments, the number of empty slots was further increased from 6 up to 10, increasing the gap inside the middle plane from 4.9 to 7.7 mm, respectively. This allows for more independent study of the flows of L-shell Ni plasma (between the outer planes) and K-shell Al plasma (which first fills the gap between the edge wires along the middle plane) and their radiation in space and time. When studying the combined wire arrays before, the time-gated X-ray spectra have always included radiation from both materials, even at early time. In the present work, for the first time we have observed that the K-shell Al radiation was delayed compared to L-shell Ni radiation when the number of empty slots was increased. In addition, the results of another new experiment are presented when a few Al wires on each edge were replaced by a thicker Cu wire to understand their influence on radiation from outer planes.

Enhanced magnetic energy released in solid-state and plasma loads on a nanosecond pulse power generator

The requirements on lossless power transport through vacuum interface and MITLs limit from above the physical volume and hence inductance of the vacuum part of pulse power generators. This in turn limits the generator-to-load energy coupling and hence the magnetic energy available in vacuum loads used in high energy density physics research. We obtained on Zebra generator (1.9 Ohm, 1 MA, 100 ns) an enhanced load magnetic energy corresponding to the load current increase from the nominal 0.95 MA to 1.65 (plusmn0.05) MA. This improvement was achieved without changing the generator architecture, but through better generator-to-load energy coupling using the new Load Current Multipliers (LCM) technique. The average experimental load-to-generator current amplitude ratio in LCM with both a 7 nH constant-inductance load and with z-pinch loads was 1.7plusmn0.2. We report on new generator electrotechnical parameters with LCM and on characterization of the plasma dynamics and radiative properties of planar wire-array z-pinches at the achieved enhanced load magnetic energy level.

Improvement of the Energy Coupling to Low-Impedance Loads using Flux Extruders (Current Multiplier) on Existing Pulse-Power Generators

Summary form only given. The current multiplier (CM) concept was proposed to increase the driver-to-load energy transfer efficiency. The usual pulse-power load may have the inductance substantially lower than that of the generator, The suggested CM requires additional volumes with high self-inductance (magnetic flux extruders). Toroidal flux extruders can be incorporated into the vacuum part of existing pulse-power generator prior to the load and they extrude the magnetic flux toward the load magnifying the load current. Recent experiments with one extruder confirmed the concept at MA load currents with microsecond rise time both in static load inductance and in a dvnamic load (z-pinch). The accumulated experience in practical multiplier designs for high impedance generators suggests experiments to evaluate the performance of this new device on low-impedance nanosecond MA generators used for radiation production with z-pinches and for material studies in isentropic compression experiments (ICE). In this work, we present the optimization procedure for two existing generators, Saturn of Sandia and Zebra of Nevada TF operating with one or two extruders. The procedure allows comparison of the energy transfer efficiencies for different geometrical arrangements of extruders in a CM and design of the corresponding hardware in concrete experiments. Analytical and numerical study performed for these facilities suggest that better generator-to-load energy coupling is possible for both z-pinch and ICE loads. Available experimental results for some realistic CM hardware configurations will be also presented.

Optimization of Single and Double Planar Wire Arrays as a Powerful Radiator for Possible ICF Applicatios

Summary form only given. An enhancement of energy conversion of the pulsed power source into the radiation from the Z-pinch plasma, and shaping of radiation pulses from a compact (in comparison with a cavity dimension) driver is critical for the Z-pinch driven ICF. A planar wire array placed in the center of the Z-pinch chamber was found to be an interesting for these problems resolution. Recently, experiments with single and double planar arrays (SPA and DPA) from Al, Cu, Mo, and W wires have been performed on the 1 MA Zebra generator at the UNR. The diagnostics include X-ray/EUV diodes, bolometer, time-gated and -integrated X-ray spectrometers and pinhole cameras. In the SPA wires were mounted in a one linear row. The DPA includes two parallel rows with an inter-row gap (2-6 mm) smaller than an array width (5-10 mm). The SPA and DPA can be more compact than cylindrical arrays. The DPA more capable than SPA to shaping an X-ray pulse by changing geometry and material. The DPA imploded even with different material in rows (one row-Al and another-Mo). A scaling of arrays performance with width, material, mass, wire numbers, inter-wire and -row gaps was studied. Hot spots play a significant role not only in a final implosion, but also during an early plasma formation. Non-LTE kinetic modeling of X-ray spectra provided time-and spatially-resolved plasma parameters: Te of 1.3 keV and Ne of 1021 cm-3 was observed in hot spots (the Mo SPA.) A comparison with conventional and compact cylindrical arrays is discussed. The total radiation yield (19 kJ and 24 kJ from Mo SPA and DPA, respectively) exceeds the inductive energy change at least by a factor of 4-5. Observed strong small scale plasma inhomogeneity indicates a resistivity of such a plasma as a possible energy coupling mechanism. Wire dynamics model was developed to calculate the implosion dynamics. The 2D (x,y) imploding plasma layer model is used to simulate the radiation yield.