Fusion Science and Technology / Volume 56 / Number 1 / July 2009 / Pages 384-390
IFE Target Design / Eighteenth Topical Meeting on the Technology of Fusion Energy (Part 1)
Block ignition was proposed recently as a possible alternate approach to fast ignition for ICF fusion. This approach uses a modified petawatt-picosecond (PW-ps) laser pulse shape where the prepulse is strongly suppressed. This results in highly directed plasma blocks due to nonlinear (ponderomotive) force acceleration with space charge neutral ion current densities above 1011 Amp/cm2. This allows ignition of deuterium-tritium targets at densities somewhat above solid state density. However, a key issue has been the need to reduce the extremely high thresholds for the high energy flux densities of the blocks as pointed out in a related theory by Bobin and Chu in 1972. Here we show how the threshold can be reduced by a factor up to 20 by two effects. An important contribution comes from the inhibition factor for thermal conductivity due to electric double layers created in the block process. The second effect is the reduction to the stopping length, giving increased heating by the fusion product alpha due to collective interactions in the blocks. Results from including these effects in a hydrodynamic analysis are presented. The advantage of this approach for an ICF fusion reactor is the relaxed pre-compression requirement for high gain.