THE WIRE ARRAY Z-PINCH: AN EFFICIENT X-RAY SOURCE FOR ICF AND A NEW ION HEATING MECHANISM

The Z-pinch provides an efficient x-ray source for driving a hohlraum for inertial confinement fusion. The basic physics of wire-array implosions is reviewed. It can be understood in several sequential stages. First, the wires heat and form a surrounding vapor which ionizes, causing the current to transfer to this lower resistance. The (J) under bar x (B) under bar global force leads to ejection of this plasma towards the axis to form a precursor plasma. The wire cores continue to ablate due to the heat flux from the Joule-heated nearby plasma. The cooling of this plasma by the wire-cores leads to a low magnetic Reynolds number so that the precursor plasma carries little or no current. When gaps appear in the liquid/vapor cores the plasma temperature and Reynolds' number rise and this plasma accelerates in towards the axis carrying the current. This is the main implosion, and it sweeps up earlier ablated plasma, which acts to reduce Rayleigh-Taylor growth. At stagnation the ion kinetic energy is thermalised and equipartition beats the electrons, which then radiate in a 5ns pulse. In some conditions the energy radiated as soft x-rays exceeds the ion kinetic energy by a factor of 3 or 4. A theory has been developed to explain this in which fine-scale, fast growing m=0 MHD instabilities grow to saturation, viscous dissipation of which leads to ion heating, followed by equipartition. World record ion temperatures of 2 to 3 billion Kelvin were predicted, and measured at Sandia National Laboratory. Lastly progress in capsule implosions and in application to inertial fusion energy is reported.