DIII-D Overview - Research Toward Resolving Key Issues for ITER and Steady-State Tokamaks

Full text: The DIII-D Research Program has made significant advances in the physics understanding of key ITER issues and operating regimes important for ITER and future steady-state fusion tokamaks. Edge localized mode (ELM) suppression with resonant magnetic perturbations (RMP) has been now been demonstrated in the ITER baseline scenario at q95 = 3.1 by controlling the poloidal mode spectrum of n = 3 RMP. Temporal modulation of the n = 2 and n = 3 RMP toroidal phase reveals a complex plasma response that includes an island-like modulation in Te consistent with recent theory that predicts such island formation can inhibit the pedestal expansion. Pellet pacing experiments with injection geometry similar to that planned for ITER produced a ten-fold increase in the ELM frequency and a strong reduction in ELM divertor energy deposition. Disruption experiments producing reproducible runaway electron beams (IRE ∼ 300 kA with 300 ms lifetimes) reveal RE dissipation rates ∼ 2x faster than expected and demonstrate the possibility of full RE ramp down with feedback control. Long-duration ELM-free QH-mode discharges have been produced with co-current NBI by using n = 3 coils to generate sufficient counter-IP torque. With electron cyclotron heating, ITER baseline discharges at βN = 2 and scaled neutral beam injection torque have been maintained in stationary conditions for more than 4 resistive times. Successful modification of a neutral beam line to provide 5 MW of adjustable off-axis injection has enabled sustained operation at βN ∼ 3 with minimum safety factors well above 2 accompanied by broader current and pressure profiles than previously observed. With qmin above 1.5, stationary discharges with βN = 3.5 have been extended to 2τR, limited only by available beam energy (power and pulse length). (author)