Session KO2 - Z-Pinch.
ORAL session, Wednesday morning, November 13
Salon 9-10, Rosen Centre Hotel

[KO2.001] Spall Experiments Using the Atlas Z-Pinch Facility

The spall experimental series conducted on the twenty-four megajoule capacitor bank, Atlas, is described. These experiments are intended to demonstrate the proof-of-principle of the technique of using a Z-pinch to investigate spall, and to determine whether there are qualitative differences between spall phenomena in convergent and planar geometries. Spall, which arises as a result of intersecting release waves putting a material into tension, can be a complicated function of shock amplitude, shock profile, and shock duration. However, quantifying differences attributable to these factors is difficult. Often the best one can do is obtain the “spall strength” of the material. This constitutive parameter is inferred from laser interferometry measurements (VISAR) of the “pull back” velocity of the free surface of a shocked sample. In this talk the experimental technique is described, and the results of the first experiments discussed, including measured pull back velocities and radiographic data of the spalled aluminum target. Simulations of the liner target interaction will also be presented along with a brief discussion of the Atlas facility.

[KO2.002] Analysis of 0.9-1.0 MA x-pinch experiments on x-ray spectropolarimetry.

Previously reported results on Ti K-shell spectropolarimetry indicated the polarization of resonance and satellite lines caused by the presence in x-pinch plasma the electron beams with at least two energies: 3-5 keV and more than 30 keV. In the present work, the analysis of the x-pinch x-ray spectropolarimetry is complemented with the new results on the L-shell Mo study. Polarization-sensitive Ti K-shell and Mo L-shell spectra of 0.9-1.0 MA x-pinch shots at the Nevada Terawatt Facility are analyzed. The polarization of x-ray lines was studied with a polarimeter that includes a pair of identical convex crystal spectrometers, specifically LiF crystals with 2d=4.027 Åfor Ti K-shell and \alpha-quartz crystals with 2d=6.687 Åfor Mo L-shell radiation. The difference in K-and L-shell polarization-dependent spectra related to polarization states parallel and perpendicular to the electron beam is used to diagnose the EDF in plasmas. In addition, a developed x-ray diagnostic complex provides time-gated and spatial-resolved spectra, time-gated images and fast x-ray (from sub-keV up to sub-MeV) signals. The differences and similarities in polarization of K-shell and L-shell radiation from x-pinches are discussed.

[KO2.003] Radiation Transport Models for Ti Z-Pinches

Radiation transport, atomic level kinetics, and the dynamics of an imploding Z-pinch comprise the essential physics for understanding plasma radiation sources. One dimensional simulations of titanium (Ti) wire array Z pinches on the Z generator are compared with experimental data using four models of radiation transport: single and multi-group diffusive transport, tabulated and non-local collisional radiative equilibrium (CRE) transport. While the multi-group diffusion model can reasonably predict the total and K-shell radiative yields, there are significant discrepancies in the plasma properties at implosion between the diffusion approach and data. The present CRE models for Ti match the K-shell yield and the plasma properties, but the total yield increases with the number of emission lines transported.

[KO2.004] Analysis of Enhanced K-Shell Power and Mass Participation from Al:Mg Nested Wire Array Implosions on the Z Generator

The first shots on Z (70-72) that used loads of Al:Mg alloy wires employed single arrays with diameters of 30-50 mm and mass loads of 1.3-3.6 mg/cm. Maximum K-shell x-ray power ranged from 3 TW for the high mass, 30 mm load to 8 TW for the low mass, 50 mm diameter load. Typically, about 15less of the load participated in the K-shell emission. Since these early single-array experiments, enhancements of x-ray power have been demonstrated for higher atomic number elements by using nested-array loads. Recent Z shots with nested Al:Mg arrays also show power enhancement. A peak K-shell power of 29 TW was observed on shot 811, which employed a total mass load of 1.5 mg/cm: 0.5 mg/cm at a diameter of 20 mm, and 1.0 mg/cm at 40 mm. We present a comparative spectroscopic analysis of nested vs. single array shots. For example, shot 811 produced a cooler pinch plasma than shot 70, but the K-shell mass participation of shot 811 approached 50three times that of shot 70.

[KO2.005] Radiation ablation gap closure calculations for Z and ZR

Wire array z-pinches on the Z accelerator provide the most intense laboratory source of soft X rays in the world. Various hohlraum configurations are being investigated to utilize tailored z-pinch X-ray pulses to drive ICF capsules for high yield. The vacuum A-K gap feeding electrical power to the z-pinch implosion is subject to radiation ablation closure, in a competition between the radiation ablation pressure and the containing magnetic pressure corresponding to current flowing in the electrode walls. We report here on our 1D RMHD (multi-group radiation diffusion approximation) ALEGRA Lagrangian simulations of gap closure, in which we consider an electrode wall of a given material(Au for instance) exposed to Planckian radiation whose temperature is prescribed as a function of time. The electrode current diffusing into the electrode is applied as a function of time, and we use experimentally obtained traces. The vacuum radiation temperature is constructed from various experimental observations. As part of the ALEGRA validation we compare with previous calculations, done at zero electrode current, using LASNEX [1]. No direct electrode plasma expansion measurements are available at present, but we evaluate our results by comparing with what is inferred from power flow experiments in Z [2]. We then scale both current and radiation temperature to ZR conditions with the purpose to elucidate gap closure issues for ZR. This work generalizes previously performed calculations by including the self-consistent effects of magnetic diffusion to the radiation driven electrode plasma expansion.

[1] M. E. Cuneo et al, Laser and Particle Beams (2001), 19, 481-495. [2] W. A. Stygar et al, submitted to Phys. Rev. Letters

[KO2.006] Unexpected Up/Down asymmetry measured in axial radiation exiting high-temperature dynamic-hohlraum x-ray source

A ~10 TW radiation source has been developed on the 20-MA Z facility that produces a high-temperature (~215 eV) x-ray pulse. The pulse is generated in the positive z-direction through a REH (radiation exit hole), primarily from the interior of a collapsing dynamic-hohlraum (DH) centered within a z-pinch [1]. By adding an identical REH at the bottom of the hohlraum, radiation generated in the negative z direction through the bottom offers the possibility of doubling the utility of the hohlraum. Because of the up/down symmetry of the DH about the mid-plane of the z-axis (aside from the power feed), a radiation pulse similar to that exiting the top-REH (anode) is expected leaving the bottom-REH (cathode). Measurements indicate, however, that the peak radiated power exiting the top-REH is 2±0.2 times that exiting the bottom-REH. In contrast, the total energy radiated from either REH is about the same. Detailed measurements of this unexpected asymmetry, apparently dependent on polarity, together with potential origins of the asymmetry are discussed. [1] T. W. L. Sanford, et al, in press, Phys. Plasmas 9, (Aug. 2002). *Sandia is a multi-program laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE under Contract No. DE-AC04-94AL85000.

[KO2.007] Investigation of Implosion Dynamics and Stagnation of Tungsten Wire Arrays With Varied Wire Number and Constant Total Mass*

We are currently performing an experimental campaign to study the initiation, implosion dynamics and radiation yield of tungsten wire arrays. The wire array dimensions and mass are those of interest for the z-pinch driven ICF program. An optimization study of the x-ray emitted peak power, rise time and FWHM is being effectuated by varying the wire number while keeping the total array mass constant and equal to ~ 5.8mg. The driver utilized is the 20-MA Z accelerator in its usual short pulse mode of 100ns. In the first series of experimentation we imploded arrays with 50, 120, 194, and 500 wires while in the second series the arrays contained 30, 50, 90, 300 and 450 wires. A third series of shots are planned utilizing the LBZ back lighter for better evaluating the implosion dynamics and wire location at specific times before the pinch. In the first series only radial line-of-sight diagnostics were used, including XRDs, PCDs, bolometers, calorimeters, spectrometers, and x-ray imaging time resolved pinhole cameras. In the second series axial diagnostics were also added. Most of the arrays studied had 20-mm diameter. Experimental results will be presented, analyzed and compared with numerical simulations.

[KO2.008] The origin of 3D perturbations in wire array Z-pinches and their affect on global implosion symmetry and X-ray production

At early times, the plasma structure in wire array Z-pinches consists of low density coronal plasma surrounding high density wire cores. The streams of coronal plasma emanating from each wire exhibit highly periodic axial modulations which are very different to the m=0 instabilities seen in single wire experiments. The potential origins of these periodic structures are discussed. Mechanisms for their evolution are studied using a 3D resistive magneto-hydrodynamic code. The transition from uncorrelated modulations on each wire to a 3D correlated global instability structure during the implosion phase is examined. The influence of these structures on the final peak X-ray power radiated by the pinch is explored using 3D simulations.

[KO2.009] Ablation of wire cores in wire array Z pinch experiments

The process of plasma formation and the gradual ablation of dense wire cores plays an important role in the implosion dynamics of wire array Z-pinches [1-3]. In this paper we will present experiments on the MAGPIE generator (1MA, 250ns), designed to obtain quantitative information on the scaling of the ablation rate of the wires as a function of different parameters of a wire array. The experiments were performed using different materials of the wires (e.g. Al, W, Cu), different wire diameters and different array diameters. Diagnostics included laser probing, soft x-ray imaging, x-ray radiography with an X-pinch, and PCD and XRD x-ray detectors. The use of wire arrays of different diameters (8mm, 16mm and 36mm), but with fixed wire number and wire diameter, allows a study of the effect of the global magnetic field on the rate of plasma formation and on the dynamics of the precursor plasma flow. It is found that the time of complete ablation of the wire cores, leading to the formation of gaps in some axial positions of the wires, decreases with a decrease of the array diameter. This suggests that the ablation rate is sensitive to the magnitude of the global magnetic field of the array, increasing with the global magnetic field. Experimental data on the plasma formation in conical wire arrays and in twisted wire arrays also indicate that a higher magnitude of the global magnetic field results in a higher rate of wire core ablation. 1. S.V. Lebedev et al., Phys. Plasmas 8, 3734 (2001) 2. V.V. Aleksandrov et al., Plasma Phys. Rep. 27, 89 (2001) 3. S.V. Lebedev et al., Phys. Plasmas 9, 2293 (2002)

[KO2.010] Plasma dynamics in multi-wire z-pinch experiments

An important issue associated with cylindrical wire array z-pinch experiments is to understand the effects that plasma formation and dynamics have on the final implosion results. Parallel wire arrays were studied using Cornell¡¯s XP pulser. A global magnetic field was provided by the return current path (2.5mm above the array). Al and W arrays (4-8 wires) were studied. Effects of wire material, diameter, number, and inter-wire gaps are discussed. Two channels of shearing interferometry (\lambda = 532nm, 5ns FWHM) separated by 20ns were employed for observing the plasma dynamics driven by the global field. Pushing of plasma right at the start of the current was observed. The pushing velocities were measured. Al x-pinches direct backlighters were used to measure the mass distributions as a function of time, how fast the wire materials were ablated, and the wire merging toward the array center. Plasma merging was observed to experience a delay of 30-50ns relative to the start of current. The current distribution between the wires and the corresponding magnetic field distribution were calculated. Effects of magnetic field on the experimental results will be discussed.

[KO2.011] Energy Deposition and Condition of the Metal Core in Exploding Wire Experiments

Measurements of the Joule energy deposition into exploding wire and its relation with condition of the expanding wire core are presented. Wires of nine different metals with diameters of 10-30 microns, have been exploded by fast ~150A/ns and slow ~20A/ns pulses, in vacuum and in air. It has been shown by interferometry and light emission that expanding wire core has different conditions. The substances with small atomization enthalpy (Ag, Al, Cu, Au) demonstrate full vaporization of the wire core. The refractory metals (Ti, Pt, Mo, W) demonstrates that core consists from vapor and small and hot microparticles. In this case we observe “firework effect” when large radiation from the wire exceed the energy deposition time in a three order of magnitude. For non-refractory metals radiation dropping fast in 100 ns time scale due to effective adiabatic cooling. It is possible if main part of the metal core was vaporized. The interferometrical investigation of the refraction coefficient of expanding metal core is proof this conclusion. It has been shown that energy deposition before surface breakdown dependent strongly from current rate, surface coatings, environment, wire diameter and radial electric field. The regime of wire explosion in vacuum without shunting plasma shell has been realized for fast exploding mode. In this case we observe anomaly high energy deposition in to the wire core exceeding regular value in almost 20 times. The experimental results for Al wire have been compared with ALEGRA 2D MHD simulations. *Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL8500.

[KO2.012] Observation of Skeletal Filamentary Structures in Plasma of a Fast Z-Pinch

The results are presented of analyzing the plasma images taken at facility C-300 under condition of 1-3 MA electric currents for current growth front of ~ 100 ns duration [1]. The load was a tailored cylinder (8 mm long, 3-5 mm in diameter, with a neck of ~ 1 mm diameter), made of agar-agar with various heavy-element filling. The analyzed images were taken with the help of electronic optical converters (exposure 3 ns, interframe interval 15 ns, spatial resolution 50-100 micrometers). Three successive images were taken in each discharge in the visible light and soft x-rays, with covering a part of vacuum ultraviolet spectral range. This enabled us to (i) observe skeletal structures (SSs) in plasma, (ii) reveal the continuity of SS in the core and periphery, (iii) roughly trace their dynamics during the entire discharge and thus show the longevity of SS, (iv) resolve the fine structure of SS. The analysis carried out allows to extend the formerly identified phenomenon of long-lived SSs in a straight Z-pinch plasma [2] of microsecond discharge duration to the case of a fast Z-pinch. [1] Proc. 29-th EPS PPCF, Montreux, 2002, P4_015 (http://elise.epfl.ch/pdf/P4_015.pdf). [2] Proc. 27-th EPS PPCF, Budapest, 2000, http://epsppd.epfl.ch/Buda/pdf/p2_051.pdf.

Part K of program listing