Evolution of He bubbles in Au has been followed with in situ electron microscopy during 400 keV Ar ion irradiation at 500 K. He bubbles were produced by implantation at 500 K of 3 keV He ions into prethinned Au samples that were then annealed to 670 K. During Ar irradiation, bubbles undergo athermal migration, coalescence, and disintegration. The bubbles after annealing were underpressurized, but Ar irradiation brought the bubbles to equilibrium pressure after a dose of 0.6 displacements per atom (dpa). Bubble behavior can be understood on the basis of their interactions with adjacent cascades. We propose that a discrete bubble jump is induced by the melt zone formed during the thermal spike phase of a contiguous cascade. The rate of bubble migration is modeled with the assumption that the jump distance is determined by the bubble and cascade volumes.