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Description
The ITER Electron Cyclotron Resonance Heating (ECRH) and Current Drive (CD) system will be equipped with four identical Upper Launchers (ULs), each injecting up to eight 170 GHz beams in the plasma. The eight beams can be steered in two groups of four, by an Upper and a Lower Steering Mirror (USM, LSM). The UL optical design and steering range are tailored to its primary task of controlling Neoclassical Tearing Modes (NTMs). To this aim, the UL was designed with beams focused close to the target rational-q surfaces q=3/2 and q=2/1 location, and with the four beams sharing a common SM delivering current profiles as overlapped as possible, so that ECCD occurs over a narrow layer at the NTM location.
Deviations from the design point may impact the UL performance and have the detrimental effect that the power required for NTM control increases. High order modes excitation in the waveguides, alignment tolerances, or thermal deformations of the optical surfaces, can all contribute to a loss of alignment and to distortion of the beams injected in the plasma. At the same time, NTMs can be in different locations for different plasma scenarios, making the aiming more challenging.
In the present study we assess, via a sensitivity analysis, how NTM control is impacted by beam deviations from the nominal optical path, as well as by modifications of the focussing properties. ECCD is characterised with the GRAY code [1], and heuristic criteria [2, 3] are used to estimate the power required for NTM suppression. The purpose is deriving requirements for the UL optical system to maintain NTM control feasible, keeping in mind that beam scattering by density fluctuations at the plasma edge [4] is a further concern that must be considered. Different ITER scenarios are also compared, to provide guidance for the design of plasma scenarios where the EC system can be fully operational.
References
- D. Farina 2007 Fusion Sci. Technol. 52 154-160
- H. Zohm et al 2007 Plasma Phys. Control. Fusion 49 B341
- O. Sauter et al 2010 Plasma Phys. Control. Fusion 52 025002
- A. Snicker et al 2018 Nucl. Fusion 58 016002
