  The accumulation of gas atoms in tungsten is a topic of long-standing
interest to the plasma-facing materials community due the metal's use as a
divertor material in some tokamak fusion reactors. The nucleation and growth of
He/H gas bubbles (along with their isotopes) can result from impinging fluxes
of these gases which give rise to damage at the W divertor surface. The
inclusion of He or H in W has been studied extensively by the community,
finding that He bubbles modify the surface through periodic dislocation
punching and bursting mechanisms while H bubbles impact the metal through
plastic-strain induced material failure. However, the mechanisms which are
present during the combined flux of both He and H is not well-studied
atomistically. Motivated by this, an atomistic modeling study is conducted
using molecular dynamics to assess the behavior of mixed concentration He:H
bubbles in W. We find that the introduction of H into a growing He bubble
results in a dramatic change in the nature and presence of dislocation loops
which are typically generated via dislocation punching in over-pressurized He
bubbles. Most notably, at high H concentrations, there is a switchover in
energetic favorability from glissile 1/2$<$111$>$ dislocations to sessile
$<$100$>$ dislocations. This thermodynamic crossover could imply a significant
reduction in W surface morphology changes than with pure He bubbles and,
additionally, we show this to have implications on the trapping of H in the
bubbles and their associated dislocations.
