Improving symmetry tuning with I-raum in indirect-drive implosions

Indirectly driven inertial confinement fusion implosions using a three-step- shaped pulse were performed at the 100kJ laser facility. At late time of the pulse, deposition of laser energy and distribution of x-ray radiation were significantly disturbed by motion of gold plasma in the original gas-filled cylindrical hohlraum with gold wall. As a result, the lack of x-ray drive at the equator of the capsule generated an unacceptable oblate implosion. In the I-raum modified from the above cylindrical hohlraum, the initial positions of outer laser spots and gold bubbles were properly shifted to modify the disturbed radiation distribution, due to plasma evolution, resulting in a spherically symmetric drive on the capsule. In implosion shots with almost the same drive pulse, owing to improved symmetry, an spherical hotspot was observed in the new I-raum, and YOS (measured neutron yield over simulated one dimensionally ) was up to 30%, while an oblate hotspot was observed in the cylinder, and YOS was 13%. Both simulation calculations and experimental measurements show that, the I-raum can be used to significantly reduce the impact of gold bubble expansion in the three-step-shaped pulse driven implosion, which helps to tune the drive and implosion symmetry, and to improve its over-all performance.