Comparison of a ∇B drift effect model with measured H-mode power thresholds

The H-mode power threshold has a weak but positive BT dependence when the ion ∇B drift is away from the X-point, in contrast to the nearly linear BT dependence when the ion ∇B drift is toward the X-point. This indicates that geometry plays an important role in the H-mode power threshold scaling. A simple model of the ∇B drift effect failed to predict this behaviour, but successfully predicted the sign change of gas puffing and low X-point height on the power threshold. The difference between the threshold power required for sawtooth and non-sawtooth-triggered transitions can be substantial. This effect may contribute to the observed B scaling of the H-mode power threshold. (author)