Helium L-H transition threshold studies in JET-ILW

For ITER, it is important to establish the L-H transition power threshold (P$_{\mathrm{LH}})$, so H-modes can be investigated in its non-active phase, be it in Hydrogen, Helium, or suitable mixtures. In JET-C He and Deuterium appeared to have similar P$_{\mathrm{LH}}$ [1]. In JET-ILW we have shown that adding Helium to a Hydrogen plasma can reduce P$_{\mathrm{LH}}$ [2]. Here we report on recent L-H transition experiments with $^{\mathrm{4}}$Helium plasmas at JET-ILW, heated either with D-NBI or ICRH of H minority, with broad density scans. In all cases n$_{\mathrm{D+H}}$/n$_{\mathrm{e}}$\textless 5{\%} at the time of the L-H transition. The He and H$+$D concentrations were measured spectroscopically using the ratio of $^{\mathrm{4}}$He and D lines in an Optical Penning gauge in the subdivertor. Results at 1.8 T, 1.7 MA and 2.4 T, 2 MA reveal that the electron density at which the power threshold is minimum, n$_{\mathrm{e,min}}$, is 60-70{\%} higher in Helium plasmas than in Deuterium plasmas, while it is only 35{\%} higher for Hydrogen at 1.8 T. [1] D. McDonald, PPCF 46, p. 519 (2004). [2] J. Hillesheim et al, 27$^{\mathrm{th}}$ IAEA Fusion Energy Conference, Ahmedabad, India (2018).