Realization of high irradiation uniformity for direct drive ICF at the SG-III prototype laser facility

The direct drive irradiation uniformity during the initial imprinting phase at the SG-III prototype laser facility is analyzed and optimized with different methods. At first, the polar direct drive technique is applied to reduce the root mean square deviation sigma from 16.1% to 6.4%. To further reduce the non-uniformity, we propose a new method by adjusting the intensity distribution of the laser spot. The overlap of laser beams on the capsule surface is studied and a factor is introduced to adjust the intensity of the laser spot for achieving absolute irradiation uniformity while bringing wild intensity change at laser spot edges. Noting that the overlapping region at the capsule surface is symmetrically distributed, the contribution of light from the edge of a spot can be transferred to its own internal. The newly adjusted intensity distributes at two main regions and the intensity varies slowly and continuously in each, further reducing sigma to about 0.35%. Taken into account that the adjusted intensity has very sharp steps, super-Gaussian spatial profiles are used to approximate the required intensity to make it more practicable, which leads sigma to 0.94%. Furthermore, sensitivity analysis to beam errors is performed and results show that this scheme can tolerate a certain amount of uncertainties.