GPEC: A Real-Time–Capable Tokamak Equilibrium Code

A new parallel equilibrium reconstruction code for tokamak plasmas—the Garching Parallel Equilibrium Code (GPEC)—is presented. GPEC allows one to compute equilibrium flux distributions sufficiently accurate to derive parameters for plasma control within 1 ms of run time, which enables real-time applications at the ASDEX Upgrade (AUG) experiment and other machines with a control cycle of at least this size. The underlying algorithms are based on the well-established off-line–analysis code CLISTE, following the classical concept of iteratively solving the Grad-Shafranov equation and feeding in diagnostic signals from the experiment. The new code adopts a hybrid parallelization scheme for computing the equilibrium flux distribution and extends the fast, shared-memory-parallel Poisson solver that we have described previously by a distributed computation of the individual Poisson problems corresponding to different basis functions. The code is based entirely on open-source software components and runs on standard server hardware and software environments. The real-time capability of GPEC is demonstrated by performing an off-line computation of a sequence of 1000 flux distributions that are taken from 1 s of operation of a typical AUG discharge and deriving the relevant control parameters with a time resolution of 1 ms. On the current server hardware, the new code allows employing a grid size of 32 × 64 zones for the spatial discretization and up to 15 basis functions. It takes into account about 90 diagnostic signals while using up to four equilibrium iterations and computing more than 20 plasma-control parameters, including the computationally expensive safety factor q on at least four different levels of the normalized flux.