  A challenging and fundamental research problem is the better understanding
and control of the turbulent transport of heat in present-day tokamak fusion
experiments. Recent developments in numerical methods along with enormous gains
in computing power have made large-scale simulations an important tool for
improving our understanding of this phenomena. Simulating this highly
non-linear behavior requires solving for the perturbations of the phase space
distribution function in five dimensions. We use a particle-in-cell approach to
solve the equations. The code has been parallelized for a variety of
architectures (C90, CM-5, T3D) using a 1-D domain decomposition along the
toroidal axis, for which the number of particles in each cell remains
approximately constant. The quasi-uniform distribution of particles, which
minimizes load imbalance, coupled with the relatively small movement of
particles across cells, which minimizes communications, makes this problem
ideally suited to massively parallel architectures. We present the performance
of the program for different numbers of processors and problem sizes. In
addition, we discuss some recent scientific results obtained from the code.
