Geant4 simulation of secondary photons generated by laser–solid interaction in the presence of external magnetic field

Abstract

A parametric study of the average energy and scattering angle distribution of Bremsstrahlung x-ray photons generated in over-dense fuel were investigated using the Geant4 simulation toolkit in fast ignition concept. In our simulation, over-dense fuel with the mass ρc∼(300−1000)g.cm−3, and fast ignitor with the intensity I=(1021,1023)W.cm−2 and wavelength λ=(0.35,0.53)μm were considered in the presence of an external magnetic field with the strength Bext=(0−10)kT. Further, for the produced MeV electrons in pre-plasma, two types of energy distribution functions, i.e., exponential and quasi two-temperature, were considered. The simulation results show that by applying an external magnetic field strength Bext∼(1−2)kT, a smooth increase was observed in the x-ray photon average energy and a considerable increase in scattering angle and photon counts in over-dense plasma obtained, so that the peak values mostly obtained for Bext∼1kT. Meanwhile, between the two energy distribution functions for electrons, the optimal average energy and scattering angle values were obtained by considering the quasi-two-temperature energy distribution function and for the fast igniter intensity and wavelength, I=1021W.cm−2,λ=0.35μm respectively, and the fuel density ρc∼300g.cm−3. Finally, regarding to the obtained optimal average photon energy (i.e., Ebph<0.4MeV), which could be considered as minimum photon energy loss and maximum electron penetration toward the fuel, the minimum thickness of lead and concrete shields required to reduce the flux of photons to its one-tenth value were found to be 1.5 cm and 30.7 cm, respectively.