Indirect-drive ICF Capsule Implosion:
compression and heating via ablation
The X-rays that fill the hohlraum bathe the fuel capsule at its center, depositing energy on the outside of the capule and heating the outer capsule layers (referred to as the ablator).  This hot plasma rushes away from the capsule into the relative vacuum of the hohlraum.  Conservation of momentum causes compression of the interior of the capsule (the rocket effect), with an inward moving shockwave converging on the fuel in the capsule’s center. 
shock wave
radiation wave
A hydrodynamical simulation of the ablation and compression of a solid plastic sample by thermal X-rays. The X-rays are incident from the left.  Hot, tenuous material expands rapidly off the (previously) solid surface toward the left, while a shockwave moving toward the right compresses the remaining material.