JET divertor geometry and plasma shape effects on the L–H transition threshold

Results from recent experiments to study the effects of divertor geometry and increased plasma shaping on the L–H transition threshold on JET are reported. Equivalent septum configurations run with the new septum replacement plate (SRP) in the MkII Gas Box divertor have shown that the presence of the septum lowers the L–H transition power threshold, P_th, by 20%. For X-point to virtual septum top distances of less than 6 cm, the SRP plasmas also demonstrate a significant decrease of the L–H P_th and pedestal electron temperature, T_e with reduced X-point height. Although, the SRP plasma's P_th remains above that with the septum, there is no difference in the pedestal T_e at the L–H transition. The influence of plasma shaping on the L–H transition has also been investigated for the first time on JET in a series of density scans at I_p/B_t of 2.5 MA/2.7 T. While keeping the lower triangularity, δ_lower, and divertor geometry constant, the upper triangularity, δ_upper, has been increased from 0.23 to 0.34 with no effect on P_th or the pedestal T_e or T_i. In a separate edge n_e scan, two configurations with different δ_upper/δ_lower values of 0.23/0.23 and 0.43/0.33 have been compared. A large difference in the transition threshold is observed at values of edge n_e above 1.8 × 10¹⁹ m⁻³, with the higher δ plasmas characterized by P_th of up to 25% lower at the highest densities scanned. This is thought to be the result of lowered X-point and outer strike point heights with increased δ.