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

Abstract

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 × 10 19 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.

L–H power threshold studies in JET with Be/W and C wall

“…Three configurations were investigated with the main plasma shape kept fixed (high upper triangularity, d U ~ 0.38) while the divertor configuration was varied by moving the strike points from left to right of the divertor floor (configurations locally known at JET as HT3L, HT3R and HT3, see Figure 10). This change in divertor configuration is obtained by decreasing Finally, we note that, in contrast to the measurements reported here, in earlier JET-C experiments with the MkII-GB SRP geometry as d U was raised from 0.23 to 0.34, with d L and divertor configuration kept constant, no effect on P thr , nor on edge T e and T i at the L-H transition was observed [27]. Thus, it appears that the change in wall composition from JET-C to JET-ILW has resulted in enhanced sensitivity of the H-mode power threshold to variations in main plasma shape.…”

Geometrical properties of a “snowflake” divertor

“…An elegant solution of this type has been recently proposed by M. Kotschenreuther et al [2], in the form of what they call the X-divertor, where the poloidal flux experiences an additional expansion already after entering the divertor zone. In our brief communication, we consider an approach where the null of the poloidal magnetic field in the divertor region is second order, not first order as in the usual X-point divertor (the term X-divertor should not be confused with the X-point divertor; by the latter we mean a standard X-point configuration as used in a number of tokamaks, e.g., [3][4][5][6]). Then, as we show below, the separatrix in the vicinity of the null-point splits the poloidal plane not into four sectors, but into six sectors ( Fig.…”

H-mode access on JET and implications for ITER

“…This study also demonstrated that the increase in δ lower from 0.23 to 0.33 reduced P th by up to 25%. Since the P th is known to be very sensitive to divertor geometry [2], the reduction in P th with increased δ lower could be attributed to the lowering of the X-point height by 6 cm along with the movement of the outer strike point from the vertical to the horizontal target plate. The mechanisms by which changes in the X-point height or strike point position influence the L-H transition are not yet fully understood.…”