Influence of energetic particle profiles on DIII-D high bootstrap fraction plasmas

Broadening the energetic particle profile in a high bootstrap fraction (~60%), noninductive DIII-D plasma significantly lowers anomalous fast-ion transport and broadens plasma profiles, which is desirable for maintaining the elevated-q_min scenario in steady-state. An upgraded capability allows DIII-D to inject up to half of its neutral beams off-axis, and understanding the stability and transport differences between on-axis injection with a peaked fast-ion profile and 50% off-axis injection with a broad fast-ion profile informs the development of a steady-state burning plasma. Elevated-q_min discharges with only on-axis beams have strong Alfvén eigenmode activity and high anomalous fast-ion transport, whereas steering 50% of the beams off-axis decreases both of these, and results in broader fast-ion profiles, more consistent with classical predictions. Plasmas with off-axis beams also have a broader current profile and higher neutral beam current drive, due to the reduced fast-ion activity, even with less off-axis electron cyclotron current drive. In both cases q_min is slowly decreasing throughout the high-betaN phase from approximately 2 to 1.3, correlated with increasing fast-ion activity and decreasing plasma performance, especially for the on-axis beam case. These two types of plasmas have similar energy confinement time and transport levels; although, the electron density profile switches from highly-peaked to broad with 50% off-axis injection, which may be the source of the other plasma profiles broadening. While modeling shows off-axis beams give higher n=1 ideal-wall kink beta limits, these discharges have stronger and more frequent bursty n=1 and 3 energetic particle modes that often trigger tearing modes. Planned increases in RF power should reduce bursty mode drive and better sustain a high-q_min scenario.



Work supported by US DOE under DE-FC02-04ER54698.