Frequency step-tunable ECRH&CD from mid-plane port plug for the NTM stabilization in DEMO

Electron cyclotron resonance heating and current drive (ECRH&CD) is able to deposit megawatt power in a localized plasma volume. ITER will use the front steering to adjust the position of the localized EC wave absorption. To avoid using movable plasma facing mirrors with actuators in a harsh environ¬ment close to the plasma in a fusion reactor, the remote steering can be applied, as demonstrated in the Wendelstein-7X stellarator. However, the engineering for the latter steering method is not easy, due to the fact that it needs a very large focusing front mirror. Another method, the so-called mid-steering option is under development for DEMO, in which the actuator and the first mirror sit behind the breeding blanket. Nonetheless high neutron fluxes are expected, which would limit the lifetime of the system. On the other hand, the frequency steering, which involves discrete frequencies in steps of 2 to 3 GHz for a total frequency bandwidth in the order of ± 10 GHz, could also steer the position of the localized current drive, avoiding large movable or fixed front mirrors and actuators, and thus being a lifetime component for DEMO. A preliminary study was performed to check the possibility and steering range with the assumed frequency bands for the purpose of neo-classical tearing mode (NTM) stabilization in DEMO. The results will be discussed in this presentation and an outlook of the entire study will be given. The entire study will consider the whole ECRH&CD system, including (i) the launching of the RF wave to the plasma, (ii) the radio frequency (RF) transmission line system (with the torus dielectric vacuum window, polarizer, evacuated waveguide transmission line with miter bends or evacuated quasi-optical transmission system), and (iii) the feasibility of frequency step-tunable gyrotrons being the RF source. Especially, the time scale for the frequency shift between two steps will be calculated based on carefully examined assumptions.