N. Le Galloudec

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.

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.

SNL-Z wire-array initiation experiments at NTF

Summary form only given. Plasma formation from SNL-Z-style single and nested W wire arrays is being investigated by driving such arrays with the Zebra pulsed power generator. The load current reaches about 1 MA with 70 ns (10%-90%) rise-time and 10/sup 13/ A/s current rise rate a current waveform similar to the Z machine prepulse. This offers a range of wire array initiation parameters relevant for benchmarking complex simulation codes and improving the predictive capabilities, required for developing larger scale high power X-ray generators. In these experiments the ratio of the current flowing through the inner array to the total load current is measured as a function of time. Gated optical imaging, X-pinch backlighting, streaked optical spectroscopy, UV and EUV spectroscopy, and a variety of laser diagnostics are in different stages of development and fielding on Zebra. The main objective of the NTF wire array experiments is the study of SNL-Z standard single and nested wire arrays initiation to 1 MA current intensity. This includes but is not limited to the individual wires explosion, the plasma formation and early dynamics, the onset and early growth of instabilities, the current division among nested arrays. The early spectrum of perturbations and the diffusion of magnetic field are required inputs for 2D radial-axial MHD computer simulations of wire-array implosions. The experiments also address more complex questions such as the mechanism leading to the inner array heating, and the interaction process between two nested arrays. In an attempt to get information relevant for larger-scale experiments, a wide range of parameters will be explored by modifying in principal the wires nature and conditioning, the array geometry, and the current pulse shape.

Dense Z-pinch at the Nevada terawatt facility

Summary form only given, as follows. A 2-TW z-pinch (Zebra from LANL: 2 MV, 1.2 MA, 100 ns, 200 kJ, 1.9 ohm) and a 200-MW z-pinch (100 kV, 2 kA, 50 ns) are used to investigate the early-time evolution of a current-driven load. We studied the heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z in isolated-wire and SNL-Z wire-array geometries. Diagnostics in the X-ray domain include time-resolved imaging, imaging spectroscopy, and X-pinch backlighting. Laser and optical diagnostics include imaging, schlieren, laser-absorption, interferometry, and imaging spectroscopy, obtained with streak cameras and a Nd:glass laser. Other diagnostics X-ray interferometry, polarization X-ray imaging spectroscopy, laser-induced fluorescence, laser polarimetry, and collective Thomson scattering are coming online. 2D and 3D MHD and PIC modelling of Z-pinches has been initiated, initial instability development will be compared with the linear growth rates of m=0 sausage modes, with and without axial sheared flow, axial magnetic field, and the Hall term.

Dense Z-pinch research at the Nevada Terawatt Facility

Summary form only given, as follows. A high-repetition-rate, 2-terawatt Z-pinch (HDZP-II from LANL: 2 MV, 1 Mg 100 ns, 200 kJ, 1.9 ohm) has been reassembled 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. The heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z is being examined first. Optical, laser, and radiographic measurements of prepulse-driven exploding wires will be compared with the modeling results of Reisman et al. (LLNL). SNL-Z wires are exploded by an independent pulse generator (100 kV, 2 kA, 50 ns). Plasma self-emission, laser-schlieren, laser-absorption, and interferometric images (10 micron, 0.1 ns resolution) are obtained with streak cameras (S-l and S-20) and an Nd:glass laser (30 ns, 1064 or 532 nm). Multiframe point-projection radiography (few-micron, sub-ns resolution) is acheived by driving several X-pinch backlighters with HDZP-II. In addition, laser-induced-fluorescence imaging and X-ray absorption spectroscopy are being developed for this prepulse experiment, while laser polarimetry and collective Thomson scattering, and a suite of X-ray diagnostics, are being developed for future high-current experiments.