Alpha power simulation and burn control in the IBS from ramp up to ramp down

2025 Research Campaign, ITER Integrated Scenarios

Purpose of Experiment

This experiment has two purposes: the first is the full implementation of the alpha power simulation and burn control schemes that were started in FY23, with more ITER-relevant alpha power fraction with NBI, and the interaction with ITER-relevant W radiated fraction values, and the implementation of a reactivity model that better mimics the fractional changes of σ(Ti) in ITER at DIII-D’s temperature (using a Ti multiplier in the polynomial curve instead of a rigid shift). The second goal is to address the challenge of the exit-from-burn, ie the rampdown of the plasma current where the auxiliary heating is low and the alpha heating is mostly uncontrollable. The latter is generally a challenge due to the possible dominance of radiation with lower input power available and the issue of disruptions at the H-L back transition. Especially with the new ITER plans that consider replacing the whole outer wall with W tiles, it is crucial to study how plasmas are affected by the W radiation, especially during the alpha-producing phase of the “burn”.

Experimental Approach

DIII-D is uniquely placed to study this, by exploiting the benign carbon walls, and using radiators that correctly mimic the Tungsten radiative loss rates at the lower DIII-D temperatures (more details on this in the Background section and in Appendix 3). Since it is not possible to predict meaningfully how much W will get into the core plasmas ITER or other reactors, DIII-D can produce scenarios with effectively any W and W-equivalent impurity concentration we want, in the range that is relevant for each machine, exploiting the benign carbon wall and the variety of impurity injection schemes and magnitudes that are available. This experiment makes use of the new PCS algorithm designed and successfully tested in FY23 that includes all the terms necessary to calculate the alpha power based on Ti (from CER) and density (based on the line averaged density from CO2), with a fitted curve for the reaction cross section.