Terawatt fiber pinch experiments

Fiber pinches have been found to be less prone to disruptive instabilities (m = o type) than predicted by MHD theory (pinch life time > 100 MHD growth times). Comparing different experiments, this remarkable macroscopical stability seems to scale with driver power. Fiber experiments performed at the pulseline KALIF (1,6 TW, 2 MV, 2Ω) show no pinch disruptions during the whole current pulse (> 120 ns) if the initial fiber radius exceeds a critical limit (> 10 μm). From optical framing pictures it is realized that local pinch column neckings develop when the ratio of the pinch current to the initial fiber radius exceeds 15 kA/μm. This leads to local radiative collapses (micropinches) near the cathode first and up to 20 ns later near the anode as seen on X-ray streak pictures. A few ns after these short-lived (∼ ns) micropinches a more or less axially inhomogeneous hot (Te < 1 keV) pinch plasma appears that lives for more than 50 ns. In cases of disruptions (if the fiber radius is < 10 μm and the pinch current > 300 kA) intensive hard X-ray emission starts immediately after the micropinches and no hot pinch plasma develops. Depending on power input and fiber radius the fiber ablation and ionization process lasts for more than 20 ns. It has been found that reduced power input per electron-ion pair during fiber ablation and ionization reduces initial discharge expansion whereas high power input per plasma particle during pinch phase improves the hot pinch plasma (axial homogeneity and confinement). From these contradictory power requirements it is concluded that a solid fiber is not the best start condition for these Z-pinches that need initial pinch conductivity and compressibility in order to further improve axial homogeneity and confinement