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 bubble explodes, ballistic plasma projectiles form and escape with velocities exceeding 300 km/s. Laser shadowgraphy and interferometry, gated extreme ultraviolet imaging and miniature Bdot probes are used to investigate the magnetohydrodynamics properties of such configurations.

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High energy density (HED) plasmas, produced by diverse techniques, such as lasers or pulsed power generators, can help scientists to better understand extreme states of matter as well as astrophysical phenomena. While fast Z-pinches are the most common approach to generating HED plasmas in the pulsed power community, radial foil configurations can yield plasma pressures on the order of 0.5 Mbar on a 1 MA, 100 ns current rise time generator, similar to wire array configurations producing Z-pinches. In this experimental setup, a thin metallic foil stretched onto a circular anode connects to a very small 'pin' cathode at the center. Radial currents flow in the foil then down the pin cathode, thereby generating an axi-symmetric toroidal magnetic field under the foil. Initial experimental results (Gourdain et al 2010 Phys. Plasmas 17 012706) showed that the foil current interacts with this field and the resulting J × B force lifts the foil upward. Very rapidly the foil plasma turns into a bubbleshaped cavity above the central pin, and that bubble expands at 300 km s −1 , until instabilities destroy the axial symmetry, bursting the bubble open. This paper complements these initial results by using time-integrated x-ray pin-hole cameras and a focusing spectrometer with spatial resolution. In addition to laser interferometry, these new data help to provide a better estimate of the plasma electron density inside the bubble, above 10 20 cm −3 , and of the electron temperature, between 300 and 400 eV inside the central plasma column. We also discovered the presence of 'bright' spots in the plasma, with densities larger than 5 × 10 21 electrons cm −3 and temperatures above 1 keV. Finally laser interferometry gives a precise mapping of the initial plasma jet and bubble areal densities.

Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1 MA, 100 ns rise time pulsed power generator and a 500 kA, 50 ns 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 ͑ϳ10 m/ns͒ coronal plasma, best seen in the laser images, surrounds the ϳ100 m radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. ͓Phys. Plasmas 14, 022705 ͑2007͔͒. 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 rate increases with increasing initial wire diameter from 5.1 m ͑0.1 m/ns͒ to 12.7 m diameter ͑0.3 m/ns͒.