Implementing and diagnosing magnetic flux compression on the Z pulsed power accelerator

Abstract

We report on the progress made to date for a Laboratory Directed Research and Development (LDRD) project aimed at diagnosing magnetic flux compression on the Z pulsed-power accelerator (0-20 MA in 100 ns). Each experiment consisted of an initially solid Be or Al liner (cylindrical tube), which was imploded using the Z accelerator's drive current (0-20 MA in 100 ns). The imploding liner compresses a 10-T axial seed field, B z (0), supplied by an independently driven Helmholtz coil pair. Assuming perfect flux conservation, the axial field amplification should be well described by B z (t) = B z (0) × [R(0)/R(t)] 2 , where R is the liner's inner surface radius. With perfect flux conservation, B z (t) and dB z /dt values exceeding 10 4 T and 10 12 T/s, respectively, are expected. These large values, the diminishing liner volume, and the harsh environment on Z, make it particularly challenging to measure these fields. We report on our latest efforts to do so using three primary techniques: (1) micro B-dot probes to measure the fringe fields associated with flux compression, (2) streaked visible Zeeman absorption spectroscopy, and (3) fiber-based Faraday rotation. We also mention two new techniques that make use of the neutron diagnostics suite on Z. These techniques were not developed under this LDRD, but they could influence how we prioritize our efforts to diagnose magnetic flux compression on Z in the future. The first technique is based on the yield ratio of secondary DT to primary DD reactions. The second technique makes use of the secondary DT neutron time-of-flight energy spectra. Both of these techniques have been used successfully to infer the degree of magnetization at stagnation in fully integrated Magnetized Liner Inertial Fusion (MagLIF) experiments on Z [

A Primer on Pulsed Power and Linear Transformer Drivers for High Energy Density Physics Applications

“…This principle is also used in MagLIF, where a flux compressed axial magnetic field is used to keep the hot fusion fuel thermally insulated from the cold liner wall that surrounds the fuel-for more information on magnetic flux compression in MagLIF, see Refs. [7], [69].…”

Controlling Rayleigh-Taylor Instabilities in Magnetically Driven Solid Metal Shells by Means of a Dynamic Screw Pinch

“…Figure 2(a) shows the liner trajectory and drive pressure history for the "null" case with straight return current posts, calculated using a realistic circuit model for Z [60]. Like MagLIF, an initial axial field B z = 10 T exists outside and inside the shell, but here its only dynamical significance is to facilitate stagnation via a brief (≈ 1 ns) surge in magnetic back-pressure at the liner's inner surface, r = r g (t), due to flux compression [61]. During the acceleration-MRTI phase, B z is too weak to provide any shear stabilization.…”