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

The pre-conceptual design for a European pulsed fusion reactor (DEMO1) is ongoing and one of the most challenging activities within scenario modeling studies deals with numerical investigations of DEMO1 transient phases. This paper presents an investigation on both ramp-up and ramp-down phases by selected plasma trajectories. Plasma evolution has a clear impact on the engineering design of the reactor, e.g. on auxiliary power systems, current ramp-rates and requirements on the coil system. Studies on ramp-up have been carried out with the fast tokamak simulator METIS to highlight the effects of different ramp-up options in terms of robustness of access to the desired flat-top scenario. Particular attention has been paid to requirements on heating systems and plasma position controllability. A dedicated heating power during ramp-up, in addition to the one required during flat-top, appears to be necessary for reliable access to H-mode. The ramp-down phase poses specific issues on vertical stability due to the complex plasma evolution. Ramp-down trajectories with controllable plasma boundaries generated by the CREATE NL code have been coupled to 1.5D transport simulations by means of the JINTRAC transport suite of codes. The results show the necessity of additional ramp-down heating power to avoid a radiative plasma collapse and to control the current density profile. Reducing the current ramp-rate in the ramp-down phase can help the controllability of the plasma.