  After decades of research, recent laser-driven inertial fusion experiments
have demonstrated rapid progress toward achieving thermonuclear ignition using
capsule designs with cryogenic fuel layers. The ignition physics for these
layered capsules involves a complex interplay between the dynamically forming
hot spot and the dense surrounding fuel. Using analytic theory and numerical
simulations, we demonstrate that the mass ablation rate into the hot spot
depends sensitively upon the temperature of the dense fuel, resulting in
ablative inflows up to ~4x faster than previous estimates. This produces an
enthalpy flux into the hot spot that plays a critical role in controlling the
hot spot temperature, the ignition threshold, and the subsequent burn
propagation. The net influence of mass ablation on the ignition threshold is
regulated by a dimensionless parameter that depends upon the temperature of the
dense fuel. As a consequence, the ignition threshold is sensitive to any
mechanism that heats the dense fuel, such as neutrons or radiation emitted from
the hot spot. These predictions are confirmed using radiation hydrodynamic
simulations for a series of capsules near ignition conditions. This analysis
may have relevance for understanding the variable performance of recent
experiments and for guiding new capsule designs toward higher fusion yields.
