S. Batie

Advanced x-ray and extreme ultraviolet diagnostics and first applications to x-pinch plasma experiments at the Nevada Terawatt Facility

A wide variety of x-ray and extreme ultraviolet (EUV) diagnostics are being developed to study z-pinch plasmas at the Nevada Terawatt Facility at the University of Nevada, Reno. Time-resolved x-ray/EUV imaging and spectroscopy, x-ray polarization spectroscopy, and backlighting will be employed to measure profiles of plasma temperature, density, flow, and charge state, and to investigate electron distribution functions and magnetic fields. The instruments are state-of-the-art applications of glass capillary converters (GCC), multilayer mirrors (MLM), and crystals. New devices include: a novel GCC-based two-dimensional imaging spectrometer, a six-channel crystal/MLM spectrometer (“polychromator”) with a transmission grating spectrometer, and two sets of x-ray/EUV polarimeters/spectrometers. An x-pinch backlighter is under development. X-ray polarimeter/spectrometer, a survey spectrometer, a multichannel time-gated x-ray pinhole camera, and filtered fast x-ray diodes have observed the structure of Ti and Fe ...

Operation regimes of magnetically insulated transmission lines

Magnetically insulated transmission lines (MITLs) are commonly used for efficient power transport in the vacuum section of pulsed power devices. Plasma forming from metal surfaces limits the power transmitted to a load through MITLs. It eventually shunts the load, producing so-called MITL closure. Fundamental experiments are being performed on high intensity power transmission through coaxial cylindrical vacuum transmission lines. A current that rises to 1 MA in 100 ns is driven through the MITLs by a 2-MV, 2-/spl Omega/ pulse generator (Zebra). The condition of the MITL surfaces is carefully controlled and characterized before each shot. Differential B-dot probes measure the current before and after the MITL, to determine the time of gap closure. Optical imaging and laser diagnostics observe the plasma evolution in the gap with time and space resolution. The radial gap of the cylindrical vacuum transmission line has been systematically varied, and the time of MITL closure measured. They increase with the radial gap size in a discontinuous manner. Critical transitions (discontinuous jumps in closure time) appear to separate distinct MITL operation regimes. This is the first experiment and data set of this kind known to the authors. Electromagnetic-particle-in-cell and radiation-magnetohydrodynamic computer modeling assist the experiment, being used to refine the experimental design and to interpret the results.

Implosion dynamics and Spectroscopy of X-pinches and Wire arrays with doped Al wires on the UNR 1MA Z-pinch generator

The study of implosion dynamics and spectroscopy of X‐pinches and wire arrays with Al wires alloyed or coated with other near‐Z or higher‐Z materials is discussed. In particular, X‐pinches from two combined Al 5056 and Mo wires and composed from four identical Al 5056 (5%Mg) wires and Cu clad Al (90% Al and 10%Cu) are studied. In addition, wire arrays with Alumel wires (95% Ni and 5% Al) and with Al 5056 wires (uncoated) and coated with 5% NaF are investigated. Spatially‐resolved and integrated x‐ray spectral data and time integrated and time‐gated pinhole x‐ray images accumulated in these X‐pinch and wire array experiments on the UNR 1MA Zebra generator are analyzed. Modeling of K‐shell radiation from Mg provides K‐shell plasma parameters for all Al 5056 wire experiments, whereas modeling of L‐shell radiation from Ni, Cu, and Mo provide parameters for L‐shell plasmas. The importance of using different materials or dopants for understanding of implosion dynamics of different wire materials is illustrated.

X-ray temporal, spatial and spectral study of 0.9 MA X-pinch Ti, Fe, Mo, W and Pt radiation sources. Energetic electron beam and hard X-ray generation

The X-ray emission of Ti, Fe, Mo, W, and Pt X-pinches currently being studied at the Nevada Terawatt Facility (0.9-1.0 MA, 100 ns). New X-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in X-pinch experiments. Total X-ray/EUV yield was more than 10 kJ. The minimum X-ray pulse duration was 1.1 ns. For Ti, Mo and W it was observed that X-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. Most of the compact emitting region has been observed for a planar-loop Mo X-pinch. Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of the X-pinch includes energetic electron beams directed toward the anode and along wires. A pulse anisotropic hard X-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV (Mo). The size of the source was smaller than 1 mm.

Powerful microfocus x-ray and hard x-ray 1 MA x-pinch plasma source for imaging, spectroscopy, and polarimetry

The x-ray emission of Ti, Fe, Mo, W and Pt x-pinches are currently bieng studied at the Nevada Terawatt Facility z- pinch machine (0.9-1.0 MA, 100 ns). New x-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in x-pinch experiments. The total x- ray/EUV yield was more than 10 kJ. The minimum x-ray pulse duration was 1.1 ns (Mo, W, Pt). For Ti, Mo and W pinches x-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. The most compact emitting region has been observed for a planar-loop Mo x-pinch (the number of hot spots ranging from 1-5 with a minimum size smaller than 30 micrometers at (lambda) <1.5-2 Angstoms). Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of an x-pinch includes energetic electron beams directed toward the anode and along wires. The total beam energy increases from Ti to W. A pulse of hard x-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV(Mo). The size of this source was smaller than 1 mm. The measurements of temperature and density of x-pinch plasmas were based on theoretical modeling of K-shell Ti and L-shell Mo spectra (Te=1.5 keV for Ti, 0.8 keV for Mo, Ne up to 2- 3x1022 cm-3 with 1-10% of hot electrons).

Grazing incidence extreme ultraviolet spectrometer fielded with time resolution in a hostile Z-pinch environment

This recently developed diagnostic was designed to allow for time-gated spectroscopic study of the EUV radiation (4 nm < λ < 15 nm) present during harsh wire array z-pinch implosions. The spectrometer utilizes a 25 μm slit, an array of 3 spherical blazed gratings at grazing incidence, and a microchannel plate (MCP) detector placed in an off-Rowland position. Each grating is positioned such that its diffracted radiation is cast over two of the six total independently timed frames of the MCP. The off-Rowland configuration allows for a much greater spectral density on the imaging plate but only focuses at one wavelength per grating. The focal wavelengths are chosen for their diagnostic significance. Testing was conducted at the Zebra pulsed-power generator (1 MA, 100 ns risetime) at the University of Nevada, Reno on a series of wire array z-pinch loads. Within this harsh z-pinch environment, radiation yields routinely exceed 20 kJ in the EUV and soft x-ray. There are also strong mechanical shocks, high velocity debris, sudden vacuum changes during operation, energic ion beams, and hard x-ray radiation in excess of 50 keV. The spectra obtained from the precursor plasma of an Al double planar wire array contained lines of Al IX and AlX ions indicating a temperature near 60 eV during precursor formation. Detailed results will be presented showing the fielding specifications and the techniques used to extract important plasma parameters using this spectrometer.

New results on planar wire array implosions and their comparison with cylindrical wire arrays on the 1 MA zebra generator

Summary form only given. Planar wire arrays were shown to produce the strong power and yield in EUV/X-ray region. The new results of the total radiation yield Et, time-resolved sub-keV and keV outputs, X-ray spectra and images from ten wire planar arrays from low-to high-Za materials were collected recently on Zebra at UNR. Data were compared with the similar mass cylindrical wire arrays results. Planar arrays were characterized by a short rise-time of a single radiation peak: up to 8 ns for sub-keV radiation and near 2 ns in keV and harder X-ray regions, and much higher peak power compared to cylindrical arrays usually having precursor and main peak with a fastest rise-time >10-15 ns. The largest Et>18 kJ was found for Cu and Mo planar arrays (up to 25% from an energy delivered to a load) that is higher than a maximum Etap15-16 kJ for Cu or 10 kJ for Alumel (Ni) cylindrical arrays. The radiating imploding plasma is strongly inhomogeneous. The plasma (few-mm thick in the EUV/sub-keV region) has within itself hundred-mum scale structures on the axis (in several keV quanta). The electron temperatures up to 800-1000 eV were estimated for Mo planar arrays that are much higher than for cylindrical arrays and comparable with the values for X-pinches. The implosion dynamics of wire arrays were studied using spectral and imaging techniques. The results were compared with radiation and MHD modeling

Radiative Properties, Structure, and Dynamics of Asymmetric and Symmetric, Uniform and Combined X-Pinches on 1MA Zebra Generator

Experimental results of studies of the 1MA symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X‐pinches are presented. Implosions of X‐pinches are studied by spatially‐resolved and time‐gated x‐ray imaging, spectroscopy, and laser probing diagnostics. New data for the total radiation yield are obtained. The planar‐shape structures of X‐pinch plasma and the corresponding electron beam is observed for the most of X‐pinches. The effects of generation of hot spots along original wires positions that are cooler than that from the cross‐wire region and arc structures from hot spots between wires are found for X‐pinches composed from Al, Cu and W wires.

Two-terawatt Zebra Z-pinch at the Nevada terawatt facility

A high-repetition-rate, 2-TW Z-pinch (Zebra or HDZP-II from LANL: 2 MV, 1.2 MA, 100 ns, 200 kJ, 1.9 ohm) has been assembled to investigate the early-time evolution of a current-driven wire, the plasma turbulence around and between wires, the acceleration of a plasma current sheet by a magnetic field, and the suppression or reduction of plasma instabilities, and to generate radiation for applications. The heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z is being examined in isolated wires and soon in SNL-Z wire arrays. 290 trillion watts of X-rays can now be generated by a few cubic millimeters of plasma. The source of this plasma is the Z-pinch. This plasma confinement device drives a giant current through a tiny load, compressing and heating it with extreme current-produced magnetic fields. The Z-pinch suffers from plasma instabilities that limit its performance. The ultimate performance limit of the Z-pinch is unknown: another order of magnitude increase in X-ray power levels may be possible. Such an improvement would open up new applications. Understanding the dense Z-pinch is vital to the search to ameliorate it. This article describes the activation of the 2-TW Zebra Z-pinch, the development of diagnostics, and an initial single-wire experiment.

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.