Overview of Recent Results from Alcator C-Mod including Applications to ITER Scenarios

Full text: Alcator C-Mod combines high magnetic field, advanced shaping and divertor configurations, and the ability to operate with solid all-metal plasma-facing components. C-Mod accesses regimes of extreme edge power density with SOL power widths of order of a few mm implying mid-plane parallel power flows > 1 GW/m2, surpassing the design for ITER. We have significantly extended the I-mode regime to high power and plasma performance. I-mode yields strong edge ion and electron temperature barriers, excellent energy confinement (HITER-98 up to 1.2), and low collisionality. The I-mode regime has no need for ELMs to maintain density and impurity control. Experiments to simulate ITER-like plasma evolution during startup and rampdown have been carried out on C-Mod using the ITER shape and magnetic field, with comparable safety factor, normalized pressure and energy confinement scaling. During ramp-up, with early divert times and transition to H-mode, significant loop-voltage savings are realized, as predicted for ITER from TSC simulations. Detailed studies of ICRF-induced flow drive on C-Mod reveal that the efficiency depends strongly on He3 concentration in the D(He3) mode conversion regime, with driven core toroidal rotation up to 110 km/s (M ∼ 0.3). Experimental and theoretical studies of intrinsic rotation show that central toroidal rotation, observed in the absence of external momentum input, scales with edge temperature gradient, and the relationship to fluctuation-induced residual stress is under investigation. For ne > 1 x 1020 m-3 LHCD efficiency drops off more rapidly than expected theoretically, and mechanisms of anomalous absorption in the edge plasma are under investigation. Results from a new, advanced Lower Hybrid launcher, aimed at low-loss and high power density (∼ 100 MW/m2) will be reported. Lower Hybrid waves have been used to produce a seed population of non-thermal electrons (E> 100 keV), which can be accelerated during the thermal quench (TQ) phase of disruptions up to ∼ 20 MeV. Modeling using the 3D NIMROD code shows that, in these conditions, when massive gas puffing is applied for disruption mitigation, the strong MHD activity which grows during the TQ causes a nearly complete stochasticization of the magnetic field, in turn causing loss of the runaway electrons during the TQ. (author)