Basic design considerations for a frequency-tunable ECRH&CD system to suppress NTMs in DEMO

To take full advantage of the electron cyclotron resonance heating and current drive (ECRH&CD) system, flexible variation of the resonant (absorption) location is required. Electron cyclotron (EC) waves are used, among other tasks, to drive non-inductive plasma current in the O-point of magnetic islands for neo-classical tearing mode (NTM) stabilization. Typically, the location of absorption is varied by tuning the launching angle of the EC wave beam with movable plasmafacing mirrors. Another possibility to achieve the variation of the absorption position is to tune the frequency of the EC waves, while keeping the wave launching angle constant. Gyrotrons are the high-power microwave sources of any EC system. The oscillation frequency of a frequency-tunable gyrotron is changed in steps over a wide frequency range by changing the operating mode. The minimum frequency step between two neighboring modes and the number of steps define the bandwidth, while the tuning speed of the gyrotron super-conducting magnet is limited. These are the additional parameters to be considered in a frequency-steering EC system. A feasibility study of using frequency-tunable EC wave beams for the NTM suppression in the current EU-DEMO baseline is performed with the transport code ASTRA and the beam tracing code TORBEAM. The basic system parameters and design considerations, in particular, the required frequency bandwidth of the whole EC wave system, the minimum switching speed between two adjacent frequency steps and the needed power of EC waves in absence of turbulence effects, will be discussed in concrete scenarios.