Using Non-Linear Global Gyrokinetic Simulations to Investigate the Effects of Profile Shaping on Turbulence in the Helically Symmetric eXperiment (HSX)

In this work global gyrokinetic simulations with GENE-3D have been used to investigate changes in turbulent fluctuations in the Helically Symmetric eXperiment (HSX) as a result of profile shaping. We consider two density profiles motivated by density ramp experiments; one that is ‘pedestal-like’ during the density ramp where a flat core density is followed by a strong density gradient to the edge and another where the density peaks in the core after the density ramp. Both cases have peaked temperature profiles. With peaked density profiles, density gradient driven Trapped Electron Modes (TEMs) are known to be dominant [1]. With a pedestal-like profile, linear simulations show higher wave number fluctuations than seen with peaked profiles. Particle flux is consistent with large gradients in both cases, though a larger electromagnetic component of particle flux is seen with pedestal-like profiles, which may correspond to magnetic fluctuation signals seen in experiments. The recent density ramping experiments that motivated these simulations have shown an increase in plasma stored energy and a narrowing of density fluctuation spectra.

[1] B. Faber et al. 2015 Phys. Plasmas 22 072305​