The X pinch, an Extraordinary X-ray Source and High-Energy-Density Plasma

Dr. Marc Mitchell
Laboratory of Plasma Studies
Cornell University, Ithaca, NY

What makes the X-pinch plasma so extraordinary is that we can achieve about a 1-keV, multiply-ionized high-Z, near-solid-density plasma that lasts for 10 to 30 ps and is about 1 µm in size.  These conditions are normally very difficult to achieve, but we can reproduce these conditions by using two (or more) fine wires arranged so that they cross and touch at a single point, in the form of an X, as the load of a pulsed power generator. Very dense, intensely radiating plasmas, commonly called micropinches, consistently form within about 100 µm of the original cross point of the wires, where the currents in the individual wire plasmas merge. High-resolution diagnostics concentrated on that portion of the wires enable a determination of the dynamics of the plasma and its characteristics as an x-ray source with very high spatial and temporal resolution. For example, using an x-ray streak camera, we have determined that X-pinches made from several different wire materials (e.g., Mo and W) produce intense, 3- to 10-keV radiation with pulse durations as short as 10 to 30 ps, and the x-ray spectra recorded by the streak camera indicate that plasma densities can exceed 10²²/cm³ and electron temperatures can reach well into the keV range. From experiments in which slits and thin fibers were imaged, together with detailed wave-optics calculations, we have determined that the (time integrated) size of the 3 to 10-keV x-ray source can be as small as 1 µm for some wire materials (e.g., Mo and Nb). Imaging of one X pinch by another enables us to follow the dynamics of micropinch formation and disassembly with ~ 30-ps time resolution and ~ 1-µm spatial resolution. In addition, laser interferometry from a short pulse (<200ps) laser allows us to observe the coronal plasmas that forms a current carrying sheath around the more dense plasma core.  The methods by which these plasma properties have been determined, along with additional detail on the results, especially the source size measurements, will be discussed. Examples of the capability of an X pinch as an x-ray source for point-projection imaging will also be presented.