Proton beam energy deposition in fast ignitionand production of protons on Shenguang II upgraded device

The proton beam energy deposition and the prodution of proton beams in proton fast ignition are investigated with the fluid program, partice-in-cell program and Fokker-Planck program based on the parameters of Shenguang II upgraded device. Firstly, according to the target parameters of fast ignition, the energy depositions of different energy protons are investigated. It is obtained that the higher the incident proton energy, the higher the surface density that the protons go through, accordingly the longer the proton deposition distance in the same background plasma density. On the assumption that the diameter of the compression core is 20-30 mu m, and that the protons deposited in the core give the energy to the background plasma, the energy of the proton required by fast ignition is obtained by Fokker-Planck simulation. Protons with energy of 7- 12 MeV are appropriate for ignition when the background plasma density is 300 g/cm3, while 8-18 MeV protons for 400 g/cm3. The background plasma temperatures are both 5 keV in the two cases. Secondly, we use particle in-cell program to study the proton acceleration with or without preplasma which is given by fluid program with using the laser intensity I = 5.4 x 1019 W/cm2 based on the parameters of Shenguang II upgraded device. The laser has 350 J of enegy, 3 ps of Gaussion pluse width and 10 mu m of spot radius. The curvature of the target which is 10 mu m thick copper coated with 1 mu m thick hydrogen plasma, is 500 mu m. The maximum proton energy obtained with preplama is 22 MeV, however the maximum proton energy obtained without preplasma is 17.5 MeV. The conversion efficiency from laser to protons is 5.12% with preplasma and 4.15% without preplasma. The conversion efficiency with preplasma is 20% higher than that without preplasma. We also study the mechanisms of the acceleration in the two situations. The freely expanding plasma model is used to explain the acceleration mechanism. The simulated electric field is smaller than that calculated by using the freely expanding plasma model, because some protons are accelerated at the time of plasma expansion, which consumes some electric field. The results of proton energy deposition show that the proton beams that are suitable for fast ignition can be obtained by the Shenguang II upgraded device.