Ablator dopants affect the opacity and density, changing the manner in which energy is absorbed by the ablator.
By adding mid-Z dopants (like bromine, copper, or germanium) to the plastic or beryllium capsule ablator, we can control the density and opacity of the ablator.  This in turn controls the ablation/compression physics in the fuel capsule. The dopants also provide a ‘shield’ against radiation preheat of the fuel, allowing for efficient compression at the lowest entropy possible.