  This work starts an in situ processing capability to study a certain
diffusion process in magnetic confinement fusion. This diffusion process
involves plasma particles that are likely to escape confinement. Such particles
carry a significant amount of energy from the burning plasma inside the tokamak
to the diverter and damaging the diverter plate. This study requires in situ
processing because of the fast changing nature of the particle diffusion
process. However, the in situ processing approach is challenging because the
amount of data to be retained for the diffusion calculations increases over
time, unlike in other in situ processing cases where the amount of data to be
processed is constant over time. Here we report our preliminary efforts to
control the memory usage while ensuring the necessary analysis tasks are
completed in a timely manner. Compared with an earlier naive attempt to
directly computing the same diffusion displacements in the simulation code,
this in situ version reduces the memory usage from particle information by
nearly 60% and computation time by about 20%.
