J. B. Greenly

Experiments measuring the initial energy deposition, expansion rates and morphology of exploding wires with about 1 kA/wire

Wire-array Z-pinch implosion experiments begin with wire heating, explosion, and plasma formation phases that are driven by an initial 50–100 ns, 0–1 kA/wire portion of the current pulse. This paper presents expansion rates for the dense, exploding wire cores for several wire materials under these conditions, with and without insulating coatings, and shows that these rates are related to the energy deposition prior to plasma formation around the wire. The most rapid and uniform expansion occurs for wires in which the initial energy deposition is a substantial fraction of the energy required to completely vaporize the wire. Conversely, wire materials with less energy deposition relative to the vaporization energy show complex internal structure and the slowest, most nonuniform expansion. This paper also presents calibrated radial density profiles for some Ag wire explosions, and structural details present in some wire explosions, such as foam-like appearance, stratified layers and gaps.

The effect of insulating coatings on exploding wire plasma formation

Substantial increases are reported in the expansion rates of exploding, dense wire cores under conditions simulating the prepulse phase of wire array z-pinch experiments [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] using wires with insulating coatings. The insulation apparently allows additional wire heating by delaying the formation of plasma around the wires. Once plasma is formed it terminates significant current flow in the residual wire cores. This effect is demonstrated for 25-μm diameter W and 25-μm diameter Ag wires.

Magnetically insulated ion diode with a gas‐breakdown plasma anode

An active anode plasma source has been developed for use in a magnetically insulated ion diode operated on a 1010‐W pulsed power generator. This source uses an inductive voltage from a single turn coil to beak down an annular gas puff produced by a supersonic nozzle. The resulting plasma is magnetically driven toward the radial insulating magnetic field in the diode accelerating gap and stagnates at a well‐defined surface after about 300 ns to form a plasma anode layer defined by magnetic flux surfaces. An ion beam is then extracted from this plasma layer by applying a 150‐kV, 1‐μs pulse to the accelerating gap. Optimization of the timing of the gas puff, the plasma production discharge, and the high voltage pulse has resulted in 1‐μs duration 75–150‐keV ion beam pulses with >100‐A/cm2 peak ion current density over an area of about 400 cm2. Up to 5 J/cm2 has been collected by a 4‐cm2 calorimeter. The diode impedance history can be varied so that rising, flat, and falling voltage pulse waveforms can be pro...

A 1 MA, variable risetime pulse generator for high energy density plasma research.

COBRA is a 0.5 Omega pulse generator driving loads of order 10 nH inductance to >1 MA current. The design is based on independently timed, laser-triggered switching of four water pulse-forming lines whose outputs are added in parallel to drive the load current pulse. The detailed design and operation of the switching to give a wide variety of current pulse shapes and rise times from 95 to 230 ns is described. The design and operation of a simple inductive load voltage monitor are described which allows good accounting of load impedance and energy dissipation. A method of eliminating gas bubbles on the underside of nearly horizontal insulator surfaces in water was required for reliable operation of COBRA; a novel and effective solution to this problem is described.

Beryllium liner implosion experiments on the Z accelerator in preparation for magnetized liner inertial fusion

Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200 ns (200 ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liners inner surface in the images. These images allow us to assess the stability of the liners inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liners inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerators pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-B” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liners inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.

Multiwire

The rebuilt COBRA pulsed-power generator, which has a variable current-pulse waveform and amplitude (95-150-ns rise time and ~1-MA peak current), has extended the range of the current-pulse parameters that can be used to study the X-pinches. X-pinches with 4-12 wires of several different wire materials (from Al to W) with diameters from 25 to 75 mum have been studied. The influence of the rate of the rise of current and the X-pinch wire mass on the X-pinch plasma formation and pinch implosion dynamics has been studied using a set of diagnostics with spatial and/or temporal resolution. Multiwire X-pinches were placed in the diode center, and/or two four-wire X-pinches were placed in one of the four parallel return-current circuits of the diode. The experiments showed that the most important factor determining the first X-ray burst timing is the linear mass density of the X-pinch wires, and an optimal value for obtaining a single high-energy X-ray burst was found. Radiographic images of the different test objects, wires in wire-array Z-pinches, and the X-pinches, themselves, were obtained using a micrometer-scale spatial resolution

X

X-ray backlighter images (radiographs) of current-induced explosions of 12.7–25 μm diam Al wires have been used to determine the expansion rate and internal structure of the dense wire cores. The current rises to 1–4.5 kA per wire in 350 ns, but voltage and current measurements show that the energy driving the explosion is deposited resistively during the first 40–50 ns, when the current is only a few hundred amperes per wire. A voltage collapse then occurs as a result of plasma formation around the wire, effectively terminating the energy deposition in the wire core. High-resolution radiographs obtained over the next 150–200 ns show the expanding wire cores to have significant axial stratification and foamlike structures with ∼10 μm scale lengths over most of the wire length before they disappear in the expansion process. The expansion rate of the portion of the wire cores that is dense enough to be detected by radiography is 1.4–2 μm/ns commencing approximately 25 ns after the moment of the voltage coll...

-Pinches at 1-MA Current on the COBRA Pulsed-Power Generator

The nonlinear transport of magnetic field in collisionless plasmas, as present in the plasma opening switch (POS), using the implicit multifluid simulation code anthem [J. Comput. Phys. 71, 429 (1987)] is studied. The focus is on early time behavior in the electron–magnetohydrodynamic (EMHD) limit, with the ions fixed, and the electrons streaming as a fluid under the influence of ve×B Hall forces. Through simulation, magnetic penetration and magnetic exclusion waves are characterized, due to the Hall effect in the presence of transverse density gradients, and the interaction of these Hall waves with nonlinear diffusive disturbances from electron velocity advection, (ve⋅∇)ve, is studied. It is shown how these mechanisms give rise to the anode magnetic insulation layer, central diffusion, and cathode potential hill structures seen in earlier opening switch plasmas studies.

Exploding aluminum wire expansion rate with 1-4.5 kA per wire

Surface treatment experiments using intense pulsed ion beams have demonstrated new capabilities for materials surface treatment. These experiments have confirmed corrosion resistance, surface hardening, amorphous layer and nanocrystalline grain size formation, metal surface polishing, controlled melt of ceramic surfaces, surface cleaning and oxide layer removal by rapid melting and resolidification. Deposition of beam energy in a thin surface layer allows melting of the layer with relatively small energies (1-10 J/cm 2 ) and allows rapid cooling (10 9 -10 10 K/sec) and resolidification of the melted layer by thermal diffusion into the underlying substrate. At higher intensities (≥20 J/cm 2 ), this technology can provide rapid ablation of material from targets followed by rapid, congruent deposition of polycrystalline thin films on substrates. This technology uses high energy pulsed (40–400 ns) ion beams to directly deposit energy in the top 2–20 micrometers of the surface of materials.

Nonlinear magnetic field transport in opening switch plasmas

The COBRA pulsed power generator has a variable current pulse wave form and amplitude (95–180ns rise time, up to 1MA peak current). It was designed to study wire array Z pinches and X pinches, including plasma formation, pinch implosion dynamics, and pinch plasma parameters as a function of current rise time. These loads have been studied using an extensive set of diagnostics with spatial and/or temporal resolution. The set of electrical diagnostics on the COBRA generator includes Rogowski coils to monitor the total load current and the current through individual return current posts, and there is also an inductive voltage monitor. A set of extreme ultraviolet and x-ray detectors is used to study the load radiation. Wire array and X pinch plasma formation and dynamics are studied using two-frame, point projection X-pinch x-ray imaging as well as with multiframe laser probing. Flat potassium acid phtalate crystal (KAP), convex, extreme luminosity imaging conical spectrograph, and focusing spectrograph with...