Implosion dynamics of wire array Z-pinches: experiments at Imperial College

We report on experiments on the MAGPIE generator (1 MA, 250 ns) to study the physics of plasma formation in wire array Z-pinches. It is shown that the gradual redistribution of the array mass by the precursor plasma flow from wire cores plays a very important role in implosion dynamics. It is found that the rate of wire ablation depends on the magnitude of the global (collective) magnetic field of the array and increases with the field. Due to this dependence, the azimuthal variations of the global magnetic field affect the implosion dynamics. The existence of the modulation of the ablation rate along the wires leads to the presence of a 'trailing' mass left behind by the imploding current sheath. The trailing mass provides an alternative path for the current, reducing the force available for compression of the pinch at stagnation. The observed dependence of the ablation rate on inter-wire separation suggests an explanation for the existence of the optimal wire number maximizing the x-ray power.