Observations of the dynamics and stability of Z pinches
James Michael Bayley · Spiral (Imperial College London) · 1991
A C T This thesis presents studies of the compressional Z pinch and the occurrence of hotspots in an argon doped plasma focus.The design and testing of a new compressional pinch driven by a 3 H water filled transmission line is described.The compressional pinch is formed in a quartz tube filled with either hydrogen or argon at pressures of about 1 mbar.The circuit current rose to a maximum of 150 kA in 70 ns and remained constant for 100 ns before it began to fall but it was found that the pinch current fell at a lower rate.The hydrogen plasma produced had the following parameters I = 150 kA, line density Ne = 2.0 x 1019 m-1 , Lunquist number S = 1.6 x 103 , WiTa = 7 and ai,arTnor/ apinch = 0.18.The observations we report here are the first detailed set of measurements of a collisionless compressional pinch where finite Larmor radius (FLR) effects may be present.The pinch dynamics were measured using optical framing and streak photography.To measure the electron density and investigate stability a new side-on holographic interferometric technique was developed.This detected an m = 0 (sausage) instability which formed during the flat top of the current pulse.However its growth rate was less than the ideal mhd value and it saturated at approximately 30%.The instability did not cause the immediate disruption of the pinch.The SPEED2 plasma focus Z-pinch was shown to be capable of generating argon plasmas with very high temperatures and densities (1.5 keV, 1028 m -3 ).Argon hotspots were created by injecting a stream of argon gas in front of the collapsing deuterium sheath.The hotspots were measured using many diagnostics including an X-ray streak camera and an X-ray crystal spectrometer.These pro vided the first spectral resolution of hotspots on a 100 ps timescale.The hotspots were shown to be formed during the expansion of the pinch and have a duration of only 1 ns.The helium-like emission from highly ionised atoms was present for only 250 ps.These measurements showed that the formation mechanisms of hotspots are much faster than had been previously reported.Each pinch generated about 7 hotspots which were about 250 pm diameter and very close to the axis.The pinch formed a point-like source of 700 pm diameter and emitted 1 J of soft Xrays (A < 2.0 nm) into 47r over 30 ns.Such a high power point source may prove exploitable for X-ray lithography and flash X-ray microscopy.