B. Le Galloudec

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 ...

Investigation of regimes of wire array implosion on the 1 MA Zebra accelerator

Implosion of wire arrays was investigated at the 1MA Zebra accelerator by multiframe laser probing and gated x-ray self-emission diagnostics. Different regimes of implosion were observed in Al and Cu wire arrays. Implosion of Al loads with masses of 33–37μg∕cm produces a dense pinch 1–1.5mm in diameter. Strong instabilities are observed in the Z pinch at the time of stagnation. Implosion of “overmassed” loads produces a plasma column 3–4mm in diameter with a core. The plasma column does not collapse during the x-ray pulse. The core of the plasma column is not subjected to the kink instability and transforms to a chain of dense spots in the later stage. Different regimes of implosion were observed in Al 8×15μm loads presumably due to variations in the current pulse and load conditions. Observed regimes are compared to three-dimensional hybrid simulation of ideal and nonideal magnetohydrodynamics modes of implosion.

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.

Plasma Formation and Evolution from an Aluminum Surface Driven by a MG Field

Summary form only given. Applying a magnetic field of several megagauss to a surface drives an interesting interplay of magnetic diffusion, hydrodynamics, and radiative energy transfer. This physics is important in wire-array Z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. To investigate such plasmas experimentally, 1 MA was driven through a 1 -mm-diameter cylindrical aluminum rod, using the UNR Zebra generator. The 70-ns current rise was sufficiently short that the current skin depth was a small fraction of the conductor radius. Diagnostics included optical imaging to a time-gated intensified CCD camera and a streak camera, magnetic field probes, photodiodes, photomultipliers, and laser shadowgraphy, schlieren, interferometry, and Faraday rotation. These yielded information on the threshold for plasma formation, the expansion of the aluminum, the temperature at the transition between optically thick and optically thin matter, and the growth of the unstable m=0 mode driven by the curvature of the magnetic field. Plasma formation due to ohmic heating was distinguished from plasma formation due to high electric fields or electrical contacts by comparing shots with wire loads vs. loads machined from a solid aluminum cylinder to have a 1-mm-diameter central length but large-diameter contacts. Time-gated images show markedly more uniform light from the machined load than from the wire load. The relatively simple experimental setup was chosen in the hope of providing a benchmark with which to test and improve radiation-magnetohydrodynamics modeling. Measurements have been compared with the results of RAVEN and MHRDR computer simulations, using various assumptions for equation of state, electrical conductivity, and radiation. The simulations yield observed quantities such as luminosity, laser shadowgraphs, and m=0 mode growth. They also yield many additional interesting details, such as the propagation of a compression wave from the surface to the axis and back, with a resultant rapid radial expansion of the surface after peak current.

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.

Thermal ionization of an aluminum surface by pulsed megagauss field

When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The question of the conductivity of a metal surface under pulsed megagauss magnetic field has been posed since at least 1959, when Fowler et al.1 produced fields above 10 MG. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with well-characterized experiments2,3 and detailed 1-D and 2-D numerical modeling4–6. Aluminum rods with radii larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. A novel mechanical connection eliminates nonthermal precursor plasma, which in earlier experiments was produced by electric-field-driven electron avalanche and arcing electrical contacts. The surface was examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, in agreement with recent theoretical results4. Measurement of the surface temperature, expansion velocity, and ionization state, as a function of applied field, constrains the choice of models used in the radiation-magnetohydrodynamic simulations, which include the Eulerian MHRDR code and the Lagrangian RAVEN and UP codes. Numerical predictions can vary by orders of magnitude, but, for MHRDR modeling, the computed times of plasma formation agree well with observations if a standard SESAME Maxwell-construct EOS is used in conjunction with a VNIIEF resistivity model. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.

Analysis of Plasma Formation in an Experiment with Pulsed Megagauss Field on 1.0-mm Diameter Aluminum Rods

Summary form only given. The physics of the interaction between large magnetic field and conducting media is important to wire-array z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. In an experiment on the 1 MA UNR Zebra Marx generator, megagauss magnetic field was pulsed on the surface of 1.0-mm-diameter aluminum rods. This rod diameter is large enough to confine current to a skin layer, so that the effects of magnetic diffusion are important, yet small enough to enable magnetic field in the range of a few megagauss; a regime where the formation of plasma on conducting surfaces is expected. Furthermore, to obtain experimental results with a one dimensional character to benchmark Radiation-MHD codes, loads were designed so that the growth of instability leaves the wire approximately axially uniform throughout the current rise. Rods with 1.0-mm-diameter fit this condition for the Zebra bank. An effort was made to distinguish plasma formation due to ohmic heating from plasma formation due to high electric fields or electrical contacts. Standard 1.0-mm-diameter wire loads were compared to loads machined from a solid aluminum cylinder to form a 1 -mm-diameter central length which transitioned smoothly to large-diameter contacts. Diagnostics included V-dot and B-dot probes, streak and time-gated intensified CCD cameras, photodiodes and photomultipliers, and laser shadowgraphy, schlieren, and interferometry. Filtered photodiodes measured radiation from the heated surface of the load. Assuming blackbody emission yields a surface temperature of order 10-eV near the time of peak current. Images from a time-gated intensified CCD camera and a streak camera give snapshots of complex surface phenomena, a time history of the expansion of the rod, and potentially uncover wave propagation speeds in the compressed aluminum. Images obtained support our expectation of a slowly expanding, highly axially symmetric surface during current rise, followed by fast expansion as the field strength diminishes. Laser diagnostics give evidence of plasma formation, as m=0 perturbation growth is observed after peak current. V-dot and B-dot data are being analyzed to obtain insight into the total energy deposition and the dynamic impedance of the load.

Magnetized Laser-Plasma Interactions to Create Solid Density Warm Matter

Summary form only given. Collisional particle-in-cell simulations predict that solid density matter irradiated with a short pulse high intensity laser can be heated to keV temperatures by applying an external magnetic field. The role of the magnetic field is to restrict the radial diffusion of the hot electrons accelerated by the laser field. The confinement can be effective if the gvro-period is less than the collision time. This reduces the radial diffusion of the hot electrons long enough so that they can couple to the cold electrons which in turn couple to the ions. To test these predictions, an experiment is being developed that takes advantage of the coupled Tomcat/Leopard -Zebra facility. According to simulations performed for achievable values of the parameters, with a laser intensity higher than 1017 W/cm2 and a magnetic field of the order of 1 MG material volumes of 105 mum can be heated for several ps to temperatures of several hundred eV. These parameters make this technique extremely appealing for fusion and opacity studies with numerous applications that include modeling the radiation transport in the interiors of stars. In preparation for the integrated experiment, magnetic fields higher than 1 MG were produced in vacuum with the pulsed power generator Zebra (0.6 MA, 200 ns) using horseshoe shaped coils. In the configuration used, no plasma was created on the surface of a CM laser target placed inside the coil. To date, the best parameters measured for the Tomcat compressed laser pulse are: energy 4 J, duration 0.8 ps, and focal spot FWHM 30 mum (measured with the unamplified beam), resulting in an irradiance on target around 1018 W/cm2. Higher irradiance will be soon available using the 100 TW laser Leopard, the pulse compression of which is currently under way. The jitter of Zebra was reduced to less than 15 ns rms assuring successful synchronization with the lasers. The goal of the experiment is to demonstrate enhanced heating of a solid target irradiated by an intense, short pulse laser in the presence of an external magnetic field. Several types of targets including homogeneous Si and CD targets, as well as layered targets CD-Si-CD will be used, and their heating compared. The electron temperature and ionization balance will be inferred from X-ray spectra. A von Hamos KAP crystal spectrograph was built and used to record single shot Al and Si spectra from laser irradiated targets. Neutron yield measurements with scintillator-photomultiplier detectors will be used to determine the deuteron temperature.

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.

Investigation of Magnetic Fields in Wire Arrays and X-Pinches at the 1-MA Zebra Accelerator

Summary form only given. A Faraday rotation diagnostic was developed at the Nevada Terawatt Facility (NTF) for investigation of magnetic fields in plasma from 1-MA wire arrays and X-pinches. Optical diagnostics at the NTF includes a four-channel polaro-interferometer and four-frame shadowgraphy. The polaro-interferometer consists of identical shadow and Faraday channels, shearing air-wedge interferometer, and schlieren channel. Different regimes of the wire array implosion were observed in Al and Cu wire arrays. A current in the plasma column of the Al wire array was found by the Faraday rotation diagnostic. Bubble-like magneto-active objects were observed for the first time in the plasma column of the wire array. Interferometry showed that the plasma column consists of strongly turbulent plasma. Shadowgraphy shows imprints of streams on the surface of the precursor. The five shadow frames per shot presents details of plasma evolution in the wire array. Transfer of current to the axis was investigated in Al, W and Cu wire arrays. In the X-pinch the Faraday rotation effect shows the current in the waist and alternative paths of current through the peripheral plasma. The multi frame shadowgraphy presents the dynamics of X-pinch plasma