H mode transition threshold power scaling and its relation to the edge neutrals in JT-60U

As a result of an experiment at JT-60U on H mode transition power threshold scaling, which is one of the urgent ITER Physics R&D issues, it has been found that: (a) the derived scaling law of P_th(MW) = 0.18n_e^0.5 (10¹⁹ m^-3) B_T^1.0 (T) R^1.5 (m) with presumed non-dimensional constraints predicts a threshold heating power of ≤ 100 MW at 5 × 10¹⁹ m^-3 for the ITER EDA design, and (b) an increase of the neutral particle density at the plasma edge results in a reduction of the edge ion collisionality just before the transition below unity and thereby an increase of the threshold heating power. It has also been found that both the low density boundary, below which the transition cannot occur, and the apparent density dependence of the threshold power scaling are related to the edge neutrals. It is suggested that a charge exchange process might well be the potential mechanism of the neutral effect.