Pulsed Power Produced Cylindrically Convergent Shockwaves in Water at the Mega-Ampere Level

We report on the first experiments using Mega ampere currents to explode wire arrays in water, driving cylindrically convergent shockwaves towards the array's axis and creating extreme pressures in the vicinity of implosion. Our research was performed on the Cepage generator at First Light Fusion with array diameters of 13mm consisting of 80 x 160 µm copper wires. Currents of 1.2 MA were measured through the array - more than twice that of any previous experiments - and the energy deposited into the exploding wires reached ~25 kJ. The shockwaves generated in the water were imaged by laser backlit multiple frame photography, whilst a high-speed streak camera imaged across the array diameter. In spite of non-perfect current contacts and initial asymmetries in wire expansion, the shockwaves `self-healed' becoming ever more symmetric during implosion, and reached significantly higher velocities - 5 kms^-1 at 1mm radius, which is ~1.7 higher than in experiments driven by 550 kA currents. Inside a radius of 1mm, the shockwaves appeared to accelerate to at least 12 kms^-1 likely due to convergence, creating pressures >5 Mbar in the vicinity of implosion axis. With the high level of current available on Cepage we were able to field cylindrical foil liners instead of wire arrays for the first time - these also showed a highly symmetric, high velocity implosions resulting from explosion of the copper foil.