Design of the ITER electron cyclotron wave launcher for NTM control

Neoclassical Tearing modes (NTM) are potentially unstable in plasmas with a positive shear, above a certain marginal beta, and ECCD is the method of choice in ITER for NTM control. A parameter for evaluation of NTM stabilisation efficiency is the ratio η of the driven current density, jcd, to the bootstrap current density, at the mode location. A fit to experimental data provides quantitative criteria that were used for the launcher design evaluation, where sufficient stabilisation is achieved for η > 1.2. Physics analysis for ITER NTM stabilisation was based on 3 selected H-mode scenarios having considerably different q profiles and bootstrap current. Only 3/2 and 2/1 modes were included in the analysis, that was carried out with beam tracing codes calculating jcd and the PEC to achieve η > 1.2. An EC launcher for ITER has to satisfy the functional requirements for NTM stabilisation, the strict environmental constraints, as well as ensuring reliability and availability over the whole lifetime of the ITER device. Concerns about the attainable reliability of a 'standard' front-steering (FS) launcher (steerable mirror in the ITER vacuum) led to the initial choice of a Remote Steering (RS, all moving parts outside the primary vacuum) launcher concept. Although the RS design work showed that a launcher could be integrated into the ITER environment, the analysis also identified intrinsic limitation in the achievable beam focussing in the plasma (i.e low jcd) for the required steering range (∼25-28 deg.), resulting in jcd insufficient to stabilize NTMs over the designated range of ITER plasma scenarios. On the basis of these results, the design of an FS launcher was resumed, achieving very narrow beams in the plasma for all scenarios and high figures of merit, in excess of requirements for NTM stabilisation. Experience gained in the port plug integration of the RS launcher was transferable to the FS design. Detailed analysis has been performed on the steering mechanism design to demonstrate resilience to EM forces, nuclear heating, radiation damage and cycling fatigue. On this basis, FS has been adopted as the reference concept for the ITER upper launcher. The high performance margins achievable with the FS design may open the way to further exploitation of EC waves for ITER, especially sawtooth control. (author)