  It has recently been proposed that transport of energetic deuterium-trition
ions (Knudsen layer), owing to their long mean free path, may deplete the tail
of the velocity distribution in the vicinity of the Gamow peak and thus serve
to greatly reduce the yield of an inertial confinement fusion (ICF) plasma
[Molvig \emph{et al.}, \emph{Phys. Rev. Lett.}, 109:095001 (2012)]. Molecular
dynamics simulations of the non-equilibrium response of a fusion plasma to ion
distribution tail loss show that equilibrium is re-established extremely
rapidly, with ion tail replenishment complete in less than 1 ps for $\rho r >
0.01$. The exceedingly fast relaxation of the fusion plasma dynamics, relative
to the Fokker-Planck collisional operator, can be understood in terms of a
large entropic driving force that pushes the phase space distribution toward a
Maxwellian, and is a characteristic of the Lyupanov instability for the Coulomb
system. The thermal reactivity $\langle \sigma v \rangle$ for the
D(t,n)$\alpha$ fusion reaction is calculated as a function of progress toward
equilibrium for a series of densities relevant to ICF hot spot ignition.
