The Dynamics of Radiation Driven Gap Closure Across Megagauss Fields on Z

Summary form only given. We present experimental observations of the closure of the power feed gap on the Z machine during the implosion of a wire array load. The cathode surface and wire array edge were imaged by time-resolved laser shadowgraphy and X-ray backlighting. During the run in phase of the wire array which lasts through maximum current, ~20 MA, the radial and axial power-flow surfaces of the cathode expanded < frac14 mm. In contrast, after peak X-ray emission, the radial power-flow surface expanded at a velocity of 4.8 times 10⁴ m/s as observed by laser shadowgraphy. Assuming both anode and cathode power-flow surfaces expand with similar velocities, the extrapolated time to close a 4 mm gap is ~40 ns. Previous gap closure experiments indicated closure times less than this. Therefore a low density, low gradient plasma must be responsible for shorting the gap at earlier times. The high density plasma serves as a moving surface and source of ions.