Abstract: Laser and plasma interaction (LPI) and Rayleigh-Taylor instability (RTI) are two main ingredients affecting the success of ignition. The gas fill near the Au wall along the inner laser cone is the main region where SRS instabilities occur. At this region, pressure balance and energy balance between the inside and the outside of inner laser cone path are obtained. A plasma scaling model in ignition hohlraum of ICF is developed. Considering the scaling formula in capsule, an index is proposed which describes the stability performance. Designing of ignition targets is directed by using this index to obtain more margin for LPI and RTI.