B. R. Kusse

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.

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.

Factors affecting energy deposition and expansion in single wire low current experiments

Single wire experiments were performed on a low current pulse generator at Cornell University. A 220 nF capacitor charged to 15–25 kV was used to drive single wire experiments. The capacitor and wire holder were connected in series through an external variable inductor to control the current rise rate. This external series inductance was adjustable from 0.2 to 2 μH. When coupled with the range of charging voltages this results in current rise rates from 5 to 50 A/ns. The current heated the wire through liquid and vapor phases until plasma formed around the wire. Energy deposition and expansion rates were measured as functions of the current rise rate. These results indicated better energy deposition and higher expansion rates with faster current rise rates. Effects of the wire-electrode connection method and wire polarity were also studied.

Diagnostics on the COBRA pulsed power generator

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...

A simple air wedge shearing interferometer for studying exploding wires

A new type of shearing interferometer using an air wedge is described. This interferometer is based on a beam splitter constructed using two 90-degree prisms. A small air gap, which varies in spacing from top-to-bottom, separates the second prism from the first and forms the air wedge. The single incident laser beam is focused near the gap, and the two primary reflections from the long sides of each prism form the two coherent virtual sources necessary for interferometry. The shift between the two images of the object at the detector, as well as the orientation and frequency of the fringes, can be independently adjusted by altering the air gap thickness and angle, as well as the position of the laser focus in the gap. This interferometry scheme is inexpensive and easily aligned, and has been successfully and reliably used in exploding wire experiments.

Initial experiments using radial foils on the Cornell Beam Research Accelerator pulsed power generator

A novel technique involving radial foil explosions can produce high energy density plasmas. A current flows radially inward in a 5 μm thin aluminum foil from a circular anode, which contacts the foil on its outer rim, to the cathode, which connects to the foil at its geometrical center. When using small “pin” cathodes (∼1 mm in diameter) on a medium size pulsed-current generator such as the Cornell Beam Research Accelerator, the central magnetic field approaches 400 T, yielding magnetic pressures larger than 0.5 Mbar. While the dynamics is similar to radial wire arrays, radial foil discharges have very distinct characteristics. First a plasma jet forms, with densities near 5×1018 cm−3. J×B forces lift the foil upward with velocities of ∼200 km/s. A plasma bubble with electron densities superior to 5×1019 cm−3 then develops, surrounding a central plasma column, carrying most of the cathode current. X-ray bursts coming from the center of this column were recorded at 1 keV photon energy. As the magnetic bubb...

Wire core and coronal plasma expansion in wire-array Z pinches with small numbers of wires

Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1MA, 100ns rise time pulsed power generator and a 500kA, 50ns generator. Two-frame point-projection x-ray imaging and three-frame laser optical imaging and interferometry were the principal diagnostic methods used for these studies. The x-ray images show that dense coronal plasma forms and is maintained close to each dense wire core in the array. A less dense, rapidly expanding (∼10μm∕ns) coronal plasma, best seen in the laser images, surrounds the ∼100μm radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. [Phys. Plasmas 14, 022705 (2007)]. Results are also presented for the dependence of the wire core and coronal plasma expansion rates on the wire diameter, number of wires and current through individual wires and the overall configuration for Al, Cu, and W wire arrays. For example, the W wire dense core expansion ra...

Spectroscopy and implosion dynamics of low wire number nested arrays on the 1 MA COBRA generator

Low wire number nested array Z-pinch experiments have been carried out with wires made of aluminum, stainless steel (uniform), and combinations of these two materials (mixed) on the 1MA COBRA generator at Cornell University [J. D. Douglass, J. B. Greenly, D. A. Hammer et al., in Proceedings of the 15th IEEE International Pulsed Power Conference (IEEE, Piscataway, NJ, 2005)]. The outer array consisted of eight wires, whereas the inner array had four or eight wires. The 10μm Al wires were alloy 5056 and the 6.25μm stainless steel wires were alloy SS304. The diagnostic suite included fast-x-ray and extreme ultraviolet (EUV) detectors, a time-gated x-ray pinhole camera, x-ray spectrometers, and laser shadow imaging. The main focus was made on the spectroscopic study of plasma evolution after the main x-ray burst though the data from photoconducting detector (PCD) and EUV signals over the whole period of current, and in addition laser shadowgraphy images before the main x-ray burst were analyzed. Modeling of t...

Bright spots in 1 MA X pinches as a function of wire number and material

Bright, intense x-ray sources with extreme plasma parameters (micropinch plasmas) have previously been characterized at 0.1–0.4MA, but the scaling of such sources at higher current is poorly understood. The x-ray source size and radiation power of 1MA X pinches were studied as a function of wire material (Al, Ti, Mo, and W) and number (1-, 2-, 8-, 32-, and 64-wire configurations). The smallest bright spots observed were from 32-wire tungsten X pinches, which produced ⩽11–16μm, ∼2J, 1–10GW sources of 3–5keV radiation.