Small ELM high qmin experiment utilizing stronger shaping with the SVR divertor
2024 Research Campaign, Steady State and Pulsed Fusion Core
Purpose of Experiment
FPP steady-state designs such as CAT-DEMO propose operation at qmin > 2 and q95 ~ 5–6.5, but at higher βN (~3.5–4.5) than has been achieved in high qmin DIII-D plasmas. This experiment aims to utilize the larger machine volume enabled by the recent installation of the Shape and Volume Rise (SVR) stage 1 divertor, wherein predictions allow for high qmin scenario operation at higher values of βN, H-factor, and non-inductive fraction. This stronger shaping is also favorable for achieving small/grassy ELMs, whose character will be assessed during this experiment.
Experimental Approach
This experiment will adapt reference high qmin plasmas (qmin > 2, βN ~ 3, and q95 ~ 6) into a larger shape compatible with the new SVR and lower hybrid limiting surfaces, while ensuring that sufficient density control for external current drive is maintained. This experiment will also leverage the newly available six gyrotrons (~4 MW) for increased off-axis current drive. Once a new SVR high qmin reference discharge is obtained in the first phase of the experiment, the focus will turn to studying ELMs. Either the low or high collisionality small ELM branch may be accessed through the stronger shaping alone, and scans of separatrix density, plasma current, and lower shaping are planned to further probe ELM behavior in the (more likely accessible) high collisionality small ELM branch. The best performing, most stable shot(s) achieved in these two first experiment phases will be used as the basis for the third, which concerns finding the highest stable βN high qmin plasma possible. All available EC and NBI power will be employed, possibly in tandem with an increase in density to prevent deleterious tearing modes that often plague high qmin plasmas at βN ≳ 3.
See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).