Abstract: | The massively parallel, nonlinear, three-dimensional (3D), toroidal, electrostatic,
gyrokinetic, particle-in-cell (PIC), Cartesian geometry UCAN code, with particle
ions and adiabatic electrons, has been successfully exercised to identify non-diffusive
transport characteristics in present day tokamak discharges. The limitation in applying
UCAN to larger scale discharges is the 1D domain decomposition in the toroidal (or
z-) direction for massively parallel implementation using MPI which has restricted the
calculations to a few hundred ion Larmor radii or gyroradii per plasma minor radius.
To exceed these sizes, we have implemented 2D domain decomposition in UCAN with
the addition of the y-direction to the processor mix. This has been facilitated by use
of relevant components in the P2LIB library of field and particle management routines
developed for UCLA's UPIC Framework of conventional PIC codes. The gyro-averaging
specific to gyrokinetic codes is simplified by the use of replicated arrays
for efficient charge accumulation and force deposition. The 2D domain-decomposed
UCAN2 code reproduces the original 1D domain nonlinear results within round-off.
Benchmarks of UCAN2 on the Cray XC30 Edison at NERSC demonstrate ideal scaling
when problem size is increased along with processor number up to the largest power
of 2 available, namely 131,072 processors. These particle weak scaling benchmarks
also indicate that the 1 nanosecond per particle per time step and 1 TFlops barriers are
easily broken by UCAN2 with 1 billion particles or more and 2000 or more processors. |