UC San Diego

Experiments are reported on that examine sources for investigating low density plasma (around 10̂17 electrons/ cm³). Quantitative measurements of the coronal plasma density are recovered using interferometry simultaneously with laser shadowography of the late time diode gap formation in 2- and 4-wire tungsten x-pinches using an 80 kA, 50ns current pulse are presented. Axial expansion of the gap occurs at ̃ 10⁶ cm/s for both 2- and 4-wires systems and is likely to be driven by an axial JxB force resulting from radial current flow in the plasma mini- diode 'electrodes'. Radial density profiles suggest re- pinching of the low density plasma occurs after the main pinch resulting in secondary x-ray emission peak >10ns after the first, which is recorded with a pair of pin diodes. We also report on the first investigations of the use of carbon fibers in an x-pinch load using a 250 kA linear transformer driver (LTD). Multi-frame laser shadowography is used to examine the evolution of the pinch, and shows that carbon loads demonstrate wire expansion, cross-point pinch and gap formation as observed in x-pinches constructed from high Z materials. Radiographs using the carbon x-pinch as the source and time-resolved x-ray emission measurements show that the carbon x-pinch has sufficient emission at distances >10 cm from a small hotspot in the 500 eV < hv < 3 keV range to show that the carbon x-pinch has potential as a radiographic source for diagnosing low density plasmas