  Collapsars may be a source for the long Gamma Ray Bursts (GRBs) in the BATSE
catalog. Collapsars may radiate gamma rays anisotropically by beamed jet
emission close to the observer's line of sight. These jets must penetrate the
radiation-dominated medium of their collapsar progenitor and break through its
atmosphere in order to produce a GRB. We present a study of previously
published, axisymmetric, collapsar jet simulations. Here we use the linearized,
relativistic fluid equations to find the Kelvin-Helmholtz modes that are
triggered by recollimation shocks within the jet. The modes will grow as they
propagate with the jet. These are of interest since the light curves of GRBs
show evidence of a variable flow from the GRB engine. We also evaluate effects
of grid scaling in the numerical simulation and show that short wavelength
modes are suppressed by grid scaling before the jet breaks out of the Helium
shell.
