Characterization of oscillations observed in reduced physics SOLPS simulations

As part of a scoping study for an ITER‐sized tokamak, more than 50,000 simulations have been performed of the edge plasma with SOLPS5.0‐B2 using aggressive charge state bundling, fluid neutrals, and constant‐in‐time boundary conditions. These simulations have 40, 80, 100, 125, and 250 MW crossing into the simulation domain from the core region, and a range of D/T (deuterium/tritium) and impurity gas puffs giving a variation of electron density and Zeff. Most of the simulations are steady‐state, but about 10% show oscillations where the range of peak power flux densities to the outer target exceed 1% of the average value, and about 1% where the normalized range exceeds 10%. These oscillating cases present a challenge in determining whether a particular case has converged, or needs to be continued. The oscillations seem to have a physical origin because: the frequency of oscillations changed by less than a factor of two despite the time‐step for one case being varied by a factor 100; doubling the grid‐resolution resulted in similar oscillations; at least in some cases, a physically plausible limit‐cycle is present.