Detection of Neoclassical Tearing Modes in DEMO using the Electron Cyclotron Emission

Content: Tokamak plasmas, in low safety factor scenarios, are prone to magnetohydrodynamic (MHD) low m,n instabilities which may affect the energy and particle confinement time and possibly lead to disruptive plasma termination. In presently operating tokamaks high space resolution (~2cm) and high time resolution (0.01-0.1ms) Electron Cyclotron Emission (ECE) diagnostics are embedded in the control loop finalized to MHD control, often in synergy with pick-up coils sensitive to the magnetic fluctuations. Microwave diagnostics have plasma-facing components that are electrically passive, have metal body and are mechanically fixed. Such characteristics of robustness and reliability are promising features but it has not been proved yet that the ECE diagnostic performances are good enough for this task in the DEMO reactor. Moreover, the same kind of solution used today plant might be beyond reach in a fusion power, given the much higher neutron fluence (15-20 times of ITER) which makes unlikely the regular operation of detectors close to the vessel wall like pick-up coils. One specific task that the ECE diagnostics should accomplish in DEMO is then the prompt detection of Neoclassical Tearing Modes without the auxiliary detection capabilities of fast magnetic diagnostics. An assessment of this capability can be performed simulating the ECE temperature signals [1] associated with NTM perturbation [2] and then processing them with a detection algorithm [3] without using any other diagnostic signal, also taking into account noise sources. The results of such assessment referred to the EU-DEMO1-2015 scenario is reported in this paper, showing that extraordinary mode ECE in 2ndnd harmonics seems to have enough space resolution in the region interested by 3/2 and 2/1 NTMs. [1] D. Farina et al, AIP Conference Proceedings 988 128 [2] H. van den Brand et al, Nuclear Fusion 53 013005 [3] J. Berrino et al., Nucl. Fusion 45 1350