The role of blob-filament turbulence on biased H-mode transition thresholds in HBT-EP

Blob-filament structures are a type of turbulence known to dominate the scrape-off layer (SOL) region of plasmas by radially ejecting turbulent structures with well-defined probability-distribution functions [1]. These structures have been postulated to affect L- to H-mode transitions through enhancing plasma-wall interactions and acting as a momentum, energy, and particle sink [2]. It was recently shown in the HBT-EP tokamak that the probability distribution functions of ion saturation and SOL currents closely resemble one another and indicate the presence of blob-filament turbulence [2]. This poster further characterizes blob-filament turbulence via a new set of biasable high-field side SOL current detectors to compare turbulent structures in regions of good and bad curvature. This greater understanding of blob-filament turbulence is applied to analyzing the critical voltages and currents necessary to achieve a biased H-mode on HBT-EP, which is controlled by using a high-power amplifier to vary the voltage on a bias probe inserted into the plasma. Measurements of the timescales and biasing requirements of L- to H-mode transitions with respect to the production of blob-filaments are presented through controlling the ramping rate of the bias voltage.

[1] G. Grenfell et al 2018 Nuclear Fusion 59, 016018.

[2] I. Stewart et al 2021 Phys. Plasmas 28, 052506.