Experimental investigation of the collision of a magnetized plasma jet with a gas target cloud to simulate the compression in magnetized target fusion*

We report detailed experimental results and interpretation of an experiment investigating the physics underlying compression of a magnetized plasma jet. This investigation exploits a change of reference frame such that an MHD-driven plasma jet and a gas target cloud impacted by the jet correspond respectively to the compressed magnetized plasma and to the liner in an actual magnetized target fusion configuration. Measurements of the jet-cloud impact are made using several diagnostics including an axially translatable interferometer, Thomson scattering, a magnetic probe array, a spectrometer, and a radiated power detector. These measurements indicate that the impact causes both compression and heating of the jet plasma as well as compression of the magnetic field frozen into the plasma. The temperature initially increases in a manner consistent with adiabatic scaling, but then rapidly drops. Analysis reveals that this temperature drop results from UV radiation emitted by neutral hydrogen atoms which are spontaneously formed during the compression by the process of threebody recombination. A criterion for how fast compression must be to outrun radiative loss is discussed for a wide range of plasma parameters including the fusion regime.