Experimental Studies of ITER Demonstration Discharges
Key parts of the ITER scenarios are determined by the capability of the proposed poloidal field coil set. They include the plasma breakdown at low loop voltage, the current rise phase, the performance during the flat top phase, and a ramp down of the plasma. The ITER discharge evolution has been verified in dedicated experiments. New data are obtained from C-Mod, ASDEX Upgrade, DIII-D, JT-60U and JET. Results show that breakdown for Eaxis<0.23-0.33V/m is possible un-assisted (ohmic) for large devices like JET and attainable in devices with a capability of using ECRH assist. For the current ramp up, good control of the plasma inductance is obtained using a full bore plasma shape with early X-point formation. This allows optimisation of the flux usage from the poloidal field set. Additional heating keeps li(3)<0.85 during the ramp up to q95 = 3. A rise phase with an H-mode transition is capable of achieving li(3)<0.7 at the start of the flat top. Operation of the H-mode reference scenario at q95~3 and the hybrid scenario at q95 = 4-4.5 during the flat top phase is documented, providing data for the li(3) evolution after the H-mode transition and the li(3) evolution after a back-transition to L-mode. During the ITER ramp down it is important to remain diverted and to reduce the elongation. The inductance could be kept ≤1.2 during the first half of the current decay, using a slow Ip ramp-down, but still consuming flux from the transformer. Alternatively, the discharges can be kept in H-mode during most of the ramp down, requiring significant amounts of additional heating.