Analysis of the Variability of the L-H Transition Power Threshold in a Helium-4 Discharge

In this paper, a mechanism about the variability of the L-H transition power thresh- old P _L−H is proposed which is based on the ion orbit losses. Only in the edge where there are enough ion orbit losses and the negative radial electric field E r is high enough can the H-mode be triggered. The ion orbit losses are determined by the ion in the loss region under certain edge conditions. For different mass A and different charge Z, the critical loss energy E ∝ Z ² /A in the loss region. In H and D charges, because the D ⁺ loss region is larger than H ⁺, it can be deduced that the P L−H of H is larger than that of D. In a 4 He discharge, experiment finds there exist a considerable number of ⁴ He ¹⁺ in the plasma edge. The actual ion orbit losses are determined by the mixing ratio of ⁴ He¹⁺ and ⁴ He ²⁺ . The ⁴ He¹⁺ loss region is larger than that of ⁴ He²⁺, and the loss region of D + interposes between ⁴ He ¹⁺ and ⁴ He²⁺ . Different ⁴ He ¹⁺ content can cause the edge ion losses in a ⁴ He discharge to be greater than, less than or equal to that in a D discharge. So a ⁴ He discharge can exhibit multiple experimental phenomena in the P _L−H .