Inertial fusion research based on pulsed power

PBFA II, with design parameters of 3.5 MJ and 100 TW, is being designed to allow inertial fusion ignition experiments using imploding foils or light ion beams. Flexibility is being retained to implode a cylindrical foil through magnetically insulated power concentration at 30 MA or to drive one or more ion diodes operating in the 10 MV range. In both cases the goal is to deliver 100 TW.cm-2 and 1 MJ to a target in order to investigate ignition and possibly breakeven. Imploding foil data on Proto II have demonstrated that electromagnetic energy can be efficiently converted into foil kinetic energy with the output increasing as the square of the foil current. Experiments at Sandia National Laboratories have reached 60 kJ at 5 MA with the foil imploding in 80 ns and stagnating in 10 ns. This implosion demonstrates adequate stability, and modelling has shown that this behaviour should extrapolate to higher currents if the implosion time is less than 100 ns. To extend the scaling data to 8 MA, Proto II has been modified by extending magnetic insulation to inhibit insulator flashover. One-TW proton diodes have delivered about 2 TW.cm-2 to 3 mm diameter targets using ballistic focusing limited by beam divergences of typically 20 mrad. This divergence is induced by instabilities in the electron-ion flow, non-uniformities in the anode plasma, or a combination of these effects. Further increases in intensity can be achieved with increases in voltage and ion mass, as well as with beam bunching. Analytical models indicate that beam focusing increases with the square of the voltage and that heavier ions such as C+2 or Li+1 are required to obtain the correct stopping power at >10 MV. Correlation with analytical scaling models and electromagnetic code simulations is continuing. PBFA II will be capable of operating over a wide voltage range to accommodate the different options, with scaling experiments on PBFA I providing the basis for the selection. (author)