Driver coupling to quasi static z-pinches

Quasistatic Z-pinches formed “on axis” and magnetically confined by a current larger than a critical current (> 1.5 MA for deuterium) undergo radiative collapse providing high density thermal plasmas. Z-pinches created from solid fibers do not well couple to the driver due to lacking initial conductivity and compressibility resulting in fast plasma expansion and current leaks. Especially fiber experiments performed with high power drivers (necessary to rapidly force radiative collapse) tend to escape close coupling. Therefore experiments starting from plasmajets (ne = 10²² − 10²³ m⁻³, Te = 0.5 − 1 eV) have been carried out at the pulseline KALIF (2 MV, 900 kA) and at the fast condensor bank SPEED 1 (200 kv: 900 kA). All plasmajet experiments show a better initial coupling than fiber experiments. Unfortunately, the KALIF driver is decoupled from the load by rapid plasma erosion taking place typically 50 ns after ignition. This is due to high values of the reduced electric field (E/n > 10⁻¹⁵ Vm²) causing runaway electrons and preventing sheath formation. Since this decoupling was avoided in SPEED 1 experiments (E/n < 10⁻¹⁷ Vm²) high power experiments need plasmajets with densities in the order of ne = 10²⁴ m⁻³.