Energy Released During the H-L Back Transition*

Prompt energy loss ($\Delta$W) at the H-L transition, as a fraction of total stored energy before the transition, is about 30$\%$ and is insensitive to density in ITER-similar DIII-D plasmas. Occasionally, some ELMs will appear before the transition and reduce total energy, thus reducing $\Delta$W across the following transition. Other results (not in the ITER-similar shape) have shown that ELMs can be triggered in low powered H-modes, prior to H-L transitions, when the plasma is stable to ideal P-B modes (these are not typical type-I ELMs, despite superficial similarities) and $E\times B$ shear is strong. These are indeed ELMs occurring in H-mode and not part of a dithering transition. Finally, ELM $\Delta$W is sensitive to edge toroidal rotation and becomes smaller than uncertainty ($<$ 5 kJ) at low rotation ($\omega_{tor}<$ 5 krad/s). These results point to a strategy where $\Delta$W for the H-L transition may be reduced by the presence of (not type-I) ELMs before the transition, and $\Delta$W for the ELMs may be reduced by controlling rotation.