A.S. Safronova

Radiation sources with planar wire arrays and planar foils for inertial confinement fusion and high energy density physics research

This article reports on the joint success of two independent lines of research, each of them being a multi-year international effort. One of these is the development of innovative sources, such as planar wire arrays (PWAs). PWAs turned out to be a prolific radiator, which act mainly as a resistor, even though the physical mechanism of efficient magnetic energy conversion into radiation still remains unclear. We review the results of our extensive studies of PWAs. We also report the new results of the experimental comparison PWAs with planar foil liners (another promising alternative to wire array loads at multi-mega-ampere generators). Pioneered at UNR, the PWA Z-pinch loads have later been tested at the Sandia National Laboratories (SNL) on the Saturn generator, on GIT-12 machine in Russia, and on the QiangGuang-1 generator in China, always successfully. Another of these is the drastic improvement in energy efficiency of pulsed-power systems, which started in early 1980s with Zuckers experiments at Nava...

Radiation from Ag high energy density Z-pinch plasmas and applications to lasing

Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8u2009keV) observed on the Zebra generator so far and upwards of 30u2009kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.5–5.0u2009A. With this, L-shell Ag as well as cold Lα and Lβ Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8u2009keV). Along with PCD signals, other signals, such as filtered XRD (>0.2u2009keV) and Si-diodes (SiD) (>9u2009keV), are analyzed covering a broad range of energies from a few eV to greater than 53u2009keV. The observation and analysis of cold Lα and Lβ lines show possible correlations w...

Compact wire array sources: power scaling and implosion physics.

A series of ten shots were performed on the Saturn generator in short pulse mode in order to study planar and small-diameter cylindrical tungsten wire arrays at {approx}5 MA current levels and 50-60 ns implosion times as candidates for compact z-pinch radiation sources. A new vacuum hohlraum configuration has been proposed in which multiple z pinches are driven in parallel by a pulsed power generator. Each pinch resides in a separate return current cage, serving also as a primary hohlraum. A collection of such radiation sources surround a compact secondary hohlraum, which may potentially provide an attractive Planckian radiation source or house an inertial confinement fusion fuel capsule. Prior to studying this concept experimentally or numerically, advanced compact wire array loads must be developed and their scaling behavior understood. The 2008 Saturn planar array experiments extend the data set presented in Ref. [1], which studied planar arrays at {approx}3 MA, 100 ns in Saturn long pulse mode. Planar wire array power and yield scaling studies now include current levels directly applicable to multi-pinch experiments that could be performed on the 25 MA Z machine. A maximum total x-ray power of 15 TW (250 kJ in the main pulse, 330 kJmorexa0» total yield) was observed with a 12-mm-wide planar array at 5.3 MA, 52 ns. The full data set indicates power scaling that is sub-quadratic with load current, while total and main pulse yields are closer to quadratic; these trends are similar to observations of compact cylindrical tungsten arrays on Z. We continue the investigation of energy coupling in these short pulse Saturn experiments using zero-dimensional-type implosion modeling and pinhole imaging, indicating 16 cm/?s implosion velocity in a 12-mm-wide array. The same phenomena of significant trailing mass and evidence for resistive heating are observed at 5 MA as at 3 MA. 17 kJ of Al K-shell radiation was obtained in one Al planar array fielded at 5.5 MA, 57 ns and we compare this to cylindrical array results in the context of a K-shell yield scaling model. We have also performed an initial study of compact 3 mm diameter cylindrical wire arrays, which are alternate candidates for a multi-pinch vacuum hohlraum concept. These massive 3.4 and 6 mg/cm loads may have been impacted by opacity, producing a maximum x-ray power of 7 TW at 4.5 MA, 45 ns. Future research directions in compact x-ray sources are discussed.«xa0less

Planar wire array performance scaling at multi-MA levels on the Saturn generator.

A series of twelve shots were performed on the Saturn generator in order to conduct an initial evaluation of the planar wire array z-pinch concept at multi-MA current levels. Planar wire arrays, in which all wires lie in a single plane, could offer advantages over standard cylindrical wire arrays for driving hohlraums for inertial confinement fusion studies as the surface area of the electrodes in the load region (which serve as hohlraum walls) may be substantially reduced. In these experiments, mass and array width scans were performed using tungsten wires. A maximum total radiated x-ray power of 10 {+-} 2 TW was observed with 20 mm wide arrays imploding in {approx}100 ns at a load current of {approx}3 MA, limited by the high inductance. Decreased power in the 4-6 TW range was observed at the smallest width studied (8 mm). 10 kJ of Al K-shell x-rays were obtained in one Al planar array fielded. This report will discuss the zero-dimensional calculations used to design the loads, the results of the experiments, and potential future research to determine if planar wire arrays will continue to scale favorably at current levels typical of the Z machine. Implosion dynamics will be discussed, includingmorexa0» x-ray self-emission imaging used to infer the velocity of the implosion front and the potential role of trailing mass. Resistive heating has been previously cited as the cause for enhanced yields observed in excess of jxB-coupled energy. The analysis presented in this report suggests that jxB-coupled energy may explain as much as the energy in the first x-ray pulse but not the total yield, which is similar to our present understanding of cylindrical wire array behavior.«xa0less

Radiation from mid-atomic-number X-pinches at 1.5–1.7 MA

Recently, the first X-pinch experiments were performed at enhanced current on the Zebra generator using the Load Current Multiplier (LCM). Previously, X-pinches were found to achieve the highest K-shell electron temperatures at 1 MA on Zebra and these new experiments were performed to determine how the increased current will affect the radiative properties of the X-pinches. A comparison of the linear radiation yields suggests an increase of around 50% for the LCM experiments (∼10u2009kJ/cm at 1 MA, ∼16u2009kJ/cm with LCM). These experiments used Cu or Ti alloy (6% Al, 4% V) wires for a first look at X-pinches at 1.5–1.7 MA at the University of Nevada, Reno. For Cu X-pinches, intense L-shell Cu radiation with electron temperatures >300u2009eV was recorded by both time gated and time integrated spectrometers. The time gated spectra show an evolution of line intensities from the high Rydberg states. For Ti alloy X-pinches, many interesting results from time gated spectra recorded during the Ti experiments were found suc...