The Effect Of Current Risetime And Load Configuration On Z-pinches Formed From Cd/sub 2/ Fibers

The dynamics, x-ray production and neutron emission of a z-pinch formed from CD₂ fibers in several configurations and driven with different current risetimes has been compared. The effect of current risetime (dI/dt) on a pinch formed from a single 25 to 100 μm diameter fiber has been studied on the POSEIDON Generator (dI/dt = 4.5 kA/nsec, I = 650 kA), the ZFX generator (dI/dt = 1.5 kA/nsec, I = 900 kA) and the Double-EAGLE Generator (dI/dt = 30 kA/nsec, I = 2.0 MA). This work extends the earlier single fiber work performed at dI/dt = 15 kA/nsec, I = 700 kA. Pinches with currents driven at 4.5 kA/nsec were stable at currents up to 650 kA and 100 nsec. This corresponded to more than 40 MHD growth times. Generally these pinches remained grossly stable until the current peaked. Pinches driven at slower rates were stable for currents up to 600 kA and times of 400 nsec, corresponding to about 30 MHD growth times and was well before current peak. At the fastest rate of rise the pinch appeared to be stable at currents up to only 600 kA and times of 30 nsec, which was again well before current peak. Using a rough estimate of the pinch radius, this stable period also corresponds to about 30 MHD growth times. Thus in all cases stability was lost after about a few tens of MHD growth times and at currents of 600 kA. At the point of instability, all these pinches emitted x-rays and neutrons. The neutron yield tended to increase steeply with increasing current and weakly with decreasing fiber diameter. Maximum yield was on the order of 4 x 10¹⁰ neutrons, and was obtained with the fastest dI/dt.