EU DEMO transient phases: Main constraints and heating mix studies for ramp-up and ramp-down

Content: EU DEMO studies for pulsed (DEMO1) and steady-state (DEMO2) concepts are currently in the pre-conceptual phase [1]. DEMO1 aims at producing about 2GW of fusion power with a burn time of approximately 2 hours. Within EUROfusion Power Plant Physics and Technology department, DEMO scenario modelling is carried out as part of the validation of feasibility and performance of DEMO designs. One of the most challenging activities deals with numerical investigations of DEMO1 transient phases including ramp-up and ramp-down, which exhibit peculiar issues with respect to existing devices. Studies on ramp-up have been carried out to highlight the effects of different ramp-up options in terms of robustness of the access to the desired flattop scenario. A heating power during ramp-up, additional to the one required during flattop, appears to be necessary for plasma burn initiation and access to H-mode, with Paux,RU50MW depending on the uncertainties on L-H transition scaling. Current ramp-rate and heating power influence also plasma position controllability, and results are presented in terms of the achieved li(3) and bp. Additional power requirements and integration of different systems which are relevant for DEMO heating mix assessment are here discussed. Ramp-down phase in DEMO poses specific issues on vertical stability given the distance of control actuators from the plasma. Ramp-down trajectories with controllable plasma boundaries have been coupled to transport studies showing the necessity of additional ramp-down heating power to avoid radiative plasma collapses. Off-axis power deposition helps plasma controllability, together with a current ramp-rate100kA/s. Plasma radiation also dominates the H-L transition, which is investigated and appears to be a critical step in terms of plasma control. DEMO performance is strongly linked to the maximum plasma elongation, which has to be assessed comparing different ramp-down trajectories. [1] G. Federici et. al, Fus. Eng. Des. 89, 882-889 (2014)