Session PO5: Pinch Physics

Novel wire ablation configurations for basic physics studies

We present the status of investigations of novel wire ablation configurations on the COBRA 1 MA pulser. Arrays of wires heavy enough to serve as continuous sources of ablation streams for times of 100-200ns are being employed to create configurations suited to study of a variety of physical effects, including shock structures, instabilities and the conversion of magnetic energy to kinetic energy by magnetic reconnection. A promising configuration being developed is a linear array of wires, or a section of foil, with return current and other current-carrying structures located so as to form a nearly uniform sheet of flowing plasma away from the wire plane, which can be directed at obstacles for shock and wake studies, or at another similar plasma sheet for reconnection studies. Simulations with the PERSEUS two-fluid code are being used intensively both to guide design and to interpret the results of the experiments.   

Instability studies in radial foil configurations on the COBRA generator

Radial foil configurations prove to be a very simple experimental set up to study high energy density plasmas. A 5-micron thin metallic foil lies flat over a stretcher which is connected to the anode of a pulsed power generator such as COBRA (1MA, 100 ns current rise time). The cathode contacts the foil at its geometrical center using a hollow stainless steel pin. As the foil ablates, JxB forces lift the foil leading to the formation a plasma bubble surrounding a central plasma column, which is a z-pinch. Force densities on this column should increase considerably as the initial pin diameter is diminished and we expect plasma properties to change accordingly. Based only on pin diameter considerations, radial foil explosions could produce magnetic pressures ranging from 160 kbar (for 2-mm pins) to 2.5 Mbar (for 0.5-mm pins). However, as the cathode diameter diminishes, instabilities appear earlier in the discharge, preventing the z-pinch implosion to occur at maximum current, de facto limiting plasma parameters. We investigate the cause of these instabilities, the possible means to reduce plasma instabilities and to improve plasma performances.   

Electrical Contact Resistance with Dissimilar Materials

Electrical contact is important to wire-array Z pinches [1], high power microwave sources, and metal-insulator-vacuum junction, etc. It is also an important issue to ITER. Because of the surface roughness on a microscopic scale, true contact between two pieces of conductors occurs only at the asperities of the contacting surfaces, leading to contact resistance. In this paper, Holm's basic a-spot theory [2] for contact resistance is vastly extended to higher dimensions with dissimilar materials [3]. Scaling laws are constructed for both Cartesian and cylindrical channels. The theory was validated by simulations and experiments in several limiting cases [4]. [1] D. A. Chalenski et al., Phys. Plasmas 16, 082707 (2009); M. R. Gomez et al., Rev. Sci. Inst. 79, 093512 (2008). [2] R. Holm, Electric Contact (Springer-Verlag, 1967). [3] P. Zhang and Y. Y. Lau, J. Appl. Phys. (July 2010). [4] M. R. Gomez et al., Appl. Phys. Lett. 95, 072103 (2009).   

Absorption spectroscopy of wire-array plasma at the non-radiative stage

Absorption spectroscopy was applied to 1 MA wire-array Z-pinches. The 50 TW Leopard laser was coupled with the Zebra generator for x-ray backlighting of wire arrays. Wire-array plasmas were investigated at the ablation and implosion stages. Broadband x-ray radiation from a laser produced Sm plasma was used to backlight Al star wire arrays in the range of 7-9 {\AA}. Two time-integrated x-ray conical spectrometers recorded reference and main spectra. The backlighting radiation was separated from the powerful Z-pinch x-ray burst by collimators. A comparison of the backlighting radiation spectra that passed through the plasma with reference spectra indicates absorption lines in the range of 8.2-8.4 {\AA}. A plasma density profile was simulated with a 3D resistive MHD code. Simulations with atomic kinetics models derived an electron temperature of Al wire-array plasma.   

First Analysis of Radiative Properties of Moderate-atomic-number Planar Wire Arrays on Zebra at UNR at Higher Current of 1.7 MA*

The analysis of implosions of Cu and Ag planar wire array (PWA) loads recently performed at the enhanced 1.7 MA Zebra generator at UNR is presented. Experiments were performed with a Load Current Multiplier with a 1cm anode-cathode gap (twice shorter than in a standard 1 MA mode). A full diagnostic set included more than ten different beam-lines with the major focus on time-gated and time-integrated x-ray imaging and spectra, total radiation yields, and fast, filtered x-ray detector data. In particular, the experimental results for a double PWA load consisting of twelve 10$\mu $m Cu wires in each row (total mass M $\sim $ 175 $\mu $g) and a much heavier single PWA load consisting of ten 30$\mu $m Ag wires (M $\sim $ 750 $\mu $g) were analyzed using a set of theoretical codes. The effects of both a decreased a-c gap and an increased current on radiative properties of these loads are discussed. * This work was supported by NNSA/DOE Coop. Agr. DE-FC52-06NA27588, 27586, and 27616. Sandia is a multi-program laboratory operated by Sandia Co., a LMC, for the US DOE under Contract DE-AC04-94AL85000.   

Innovative Approach for Enhancing Shaped X-ray Production in Z-pinches*

Among z-pinch loads tested at 1.7 MA Zebra generator, planar wire arrays (PWAs) were found to be the best x-ray radiators. PWAs were recently highlighted (PRL 104, 125001, 2010) as potential sources for a new ICF multisource compact hohlraum setup at multi-MA generators. Recent performance optimization of PWA, which exhibits a large resistive energy/power gain and a small, mm-scale size, is reviewed. The anisotropy of radiation yields from single- and double-PWAs that might be caused by opacity effects was observed: higher yield was orthogonally to an array plane or along wire rows in single- and double-PWA (DPWA), respectively. Skewed DPWA implosions, which produce an axial magnetic field to reduce instabilities, generate higher Te/Ne compared to a standard DPWA. Feasible x-ray pulse shaping was demonstrated with DPWA and triple-PWA by varying array composition and parameters. The studies were supported by non-LTE kinetic, WADM, and MHD simulations. * This work was supported by NNSA under DOE Cooperative Agreements DE-FC52-06NA27586, DE-FC52-06NA27588, and in part by DE-FC52-06NA27616.   

Experimental investigation of the inverse wire array current switch

A current switch geometry in which an inverse wire array (where the wires hang down from a central current conductor, acting as a return current cage) switches current into a cylindrical wire array have been fielded on the MAGPIE generator (1.4MA, 240ns). The inverse wire array has been found to be very effective as a current switch switching the equivalent of up to 1.4MA in 95ns into the cylindrical array with little current beforehand. As well as rapidly switching current the setup drives a prepulse over 100ns before the current switch into the cylindrical array which delivers enough energy to begin boiling the wires. When current switches into the vapourised wires the whole wire mass is observed to ionise and participate in the implosion which accelerates towards the axis leaving no trailing mass and with ablation suppressed. This paper will focus on the effectiveness of the current switch.   

Evolution of the 3D structure of the magneto Rayleigh-Taylor instability in imploding liner Z-pinch fusion schemes

Imploding aluminum or beryllium Z-pinch liners provide a method of directly compressing fusion fuel to high densities and temperatures, with excellent energy coupling efficiency. The presence of large magnetic fields also provides a method of suppressing thermal conduction losses and increasing alpha particle confinement, significantly reducing the rho-R criterion required for ignition. A critical requirement is the final integrity of the inside surface of the liner which is used to compress the fusion fuel and which would be significantly degraded should the Rayleigh-Taylor instability grow to large amplitude. Recent experiments at Sandia National Laboratory have provided high quality radiography data which can be used to test the ability of MHD codes to model the Rayleigh-Taylor instability in such liners. We present three dimensional resistive magneto-hydrodynamic simulation results which examine the characteristic Rayleigh-Taylor wavelengths and growth rates and the sensitivity of these results to assumptions of the model. The differences between the Rayleigh-Taylor growth in 2D and 3D are highlighted. Departures from azimuthal symmetry are found to be increasingly important as the implosion approaches the axis and play a pivotal role in determining the peak energy density of the fuel.   

Evaluation of Nested Wire Array Dynamics with Mixed Wire Array Z Pinches

A series of experiments at the Z Accelerator was performed with 40mm and 50mm diameter nested wire arrays to investigate the interaction of the arrays and assess radiative characteristics. These arrays were fielded with one array as Al:Mg (either the inner or the outer array) and the other array as Ni-clad Ti (the outer or inner array, with respect to location of the Al:Mg). In all the arrays, the mass and radius ratio of the outer:inner was 2:1. The wire number ratio was also 2:1 in some cases, but the Al:Mg wire number was increased in some loads. This presentation will focus on analysis of the emitted radiation (in multiple photon energy bins) and measured plasma conditions (as inferred from x-ray spectra). A discussion on what these results indicate about nested array dynamics will also be presented.   

Total X-Ray Power Improvement on Recent Wire Array Experiments on the Z Machine

Recent experiments on the refurbished Z-machine were conducted using large diameter stainless steel arrays which produced x-ray powers of 260 TW.\footnote{D.J. Ampleford, International Conference on Plasma Science, 2009} Follow-up experiments were then conducted utilizing tungsten wires with approximately the same total mass with the hypothesis that the total x-ray power would increase. On the large diameter tungsten experiments, the x-ray power averaged over 300 TW and the total x-ray energy was greater than 2MJ. Different analysis techniques for inferring the x-ray power will be described in detail.   

Beryllium liner z-pinch implosions for equation of state studies on Z

We are investigating the feasibility of determining equation of state data from a temporal sequence of x-ray radiography images of an imploding z-pinch liner on the Z accelerator. Time and space dependent density profiles of the imploding liner are obtained via Abel inverting the x-ray images. The density profiles are then used to calculate pressure on either the Hugoniot or isentrope of the material depending on the form of the drive magnetic pressure. We present experimental and computational results for beryllium (Be) liners shock and quasi-isentropically compressed to pressures of 3 Mb, and discuss techniques for unfolding pressure using the x-ray images. We have captured multiple images of a shock moving through a Be liner and, by shaping the 20 MA current pulse on Z, have successfully imploded a Be liner quasi-isentropically and captured multiple images of the dynamic compression.   

ALEGRA Modeling of Gas Puff Z-Pinch Experiments at the ZR Facility*

Gas puff z-pinch experiments have been proposed for the refurbished Z (ZR) facility for CY2011. Previous gas puff experiments [Coverdale et. al., Phys. Plasmas 14, 056309, 2007] on pre-refurbishment Z established a world record for laboratory fusion neutron yield. New experiments would establish ZR gas puff capability for x-ray and neutron production and could surpass previous yields. We present validation of ALEGRA simulations against previous Z experiments including X-ray and neutron yield, modeling of gas puff implosion dynamics for new gas puff nozzle designs, and predictions of X-ray and neutron yields for the proposed gas puff experiments. $*$Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Mbar pressure generation using converging strong shock waves

The results of underwater electrical wire explosions in microsecond and nanosecond time scales are reported. The main purpose of this research is investigation of parameters of strong shock waves generated by explosion of cylindrical wire array. It was shown that up to $\sim $24{\%} of the deposited energy is transferred into the water flow mechanical energy. A high uniformity of the generated cylindrical shock waves was revealed. Using cylindrical wire array underwater explosion it was found that converging shock waves can be used to achieve pressure, density and temperature of $\sim $1.3Mbar, 3.4 g/cm$^{3}$ and 5000 K, respectively at the vicinity of the axis of implosion with the energy of only 4kJ stored in pulsed power generator. Hydrodynamic simulations showed that using relatively moderate pulsed power generators with stored energy of several hundreds of kJ, the pressure of several Mbar can be achieved at the axis of implosion.   

Neutron spectroscopy of D(d,n)3He Z-pinch experiments

The neutron time-of-flight (TOF) diagnostics of hot dense Z- pinch fusion plasma is the spectroscopy method with the best energy resolution. The diagnostic setup consists of several neutron TOF detectors placed in one line. The principle of the reconstruction of time-resolved neutron spectra when the duration of neutron production is not negligible will be described. The possibilities and limitations of this method: (i) reconstruction from mutually opposite directions of the neutron detection (ii) influence of the scattered neutrons and (iii) energy resolution will be also discussed. In addition to that, the procedure for determination of average kinetic energy of the reacting deuterons will be presented. Finally, the design of the appropriate neutron TOF diagnostics setup will be described.   

Multiple neutron peaks from D(d,n)$^{3}$He reactions in Z-pinches and plasma foci

Neutron production from D(d,n)$^{3}$He reactions has been studied in compressional Z-pinches, plasma foci and deuterium gas puffs since the 1950. Recently, neutron measurements have been carried out on the S-300 Z-pinch and the PF-1000 plasma focus at 2 MA current. When the experimental results from these generators were compared, some similarities were found. The characteristic feature was the neutron emission which consisted of multiple peaks. The main neutron emission is usually explained by the disruptive development of necks of $m$=0 instabilities. On the contrary to that, neutron diagnostics and laser interferometry at the PF-1000 showed that up to 10$^{11}$ neutrons were produced at the stagnation, i.e. during the so-called quiescent phase of the discharge. The plasma dynamics observed during this phase will be described and the results from neutron TOF diagnostics will be presented.