  Massive stars that end their lives with helium cores in the range of 35 to 65
Msun are known to produce repeated thermonuclear outbursts due to a recurring
pair-instability. In some of these events, solar masses of material are ejected
in repeated outbursts of several times 10$^{50}$ erg each. Collisions between
these shells can sometimes produce very luminous transients that are visible
from the edge of the observable universe. Previous 1D studies of these events
produce thin, high-density shells as one ejection plows into another. Here, in
the first multidimensional simulations of these collisions, we show that the
development of a Rayleigh-Taylor instability truncates the growth of the high
density spike and drives mixing between the shells. The progenitor is a 110
Msun solar-metallicity star that was shown in earlier work to produce a
superluminous supernova. The light curve of this more realistic model has a
peak luminosity and duration that are similar to those of 1D models but a
structure that is smoother.
