Effect of boron powder injection on the density limit in the Large Helical Device

The density limit in stellarators is described in first approximation by the empirical Sudo limit [1], n_Sudo=√[(P_inB)/(r²R)], believed to be determined by the balance between the input power and the power radiated by the plasma. Densities above the Sudo density limit are sometimes observed in stellarators, e.g. in LHD [2]. In particular, further studies in LHD and W7-X indicated that the density limit in stellarators might be determined by the power radiated in the edge of the plasma by impurities [2,3]. On the other hand, recent theory development showed that the density limit in tokamaks might be determined by a transition in edge turbulence [4]. The same mechanism could be at play in stellarators as well.

In this contribution, we report on recent experiments in the Large Helical Device, where the injection of boron powder into plasma density ramps increased the maximum attainable density by up to 30%, reaching n_e ~ 1.1n_Sudo. Both the concentration of intrinsic impurities (C,O) measured by spectroscopic means, and the turbulence level measured by phase contrast imaging, are decreased during boron powder injection, with respect to e a reference discharge. This is in line with previous boron powder injection experiments in LHD [5]. Possible mechanisms responsible for the improvement of the density limit are therefore the reduction of intrinsic impurities, as well as reduction of edge turbulence, both driven by powder injection. Further experiments to disentangle the two contributions are necessary, and are planned for the next LHD experimental campaign.





[1] S. Sudo et al.,, 1990 Nucl. Fusion 30 11

[2J. Miyazawa et al., 2008 Nucl. Fusion 48 015003

[3] G. Fuchert et al., 2020 Nucl. Fusion 60 036020

[4] M. Giacomin et al., Phys. Rev. Lett. 128 (2022) 185003

[5] F. Nespoli et al., Nucl. Fusion 63 (2023) 076001 (15pp)