Performance Evaluation of the Remote- Steering Option for the ITER EC Upper Launcher

One of the main goals of the ITER Electron-Cyclotron (EC) Upper Launcher is the stabilisation of Neoclassical Tearing Modes (NTMs). The fulfilment of this objective is connected with the possibility of generating a current inside the island by EC waves, thus replacing the missing bootstrap current driving the mode. To achieve this, the EC current profile should be as narrow as possible, to increase the current density in the island region, thus reducing the total injected power necessary to suppress the mode. A design option for the ITER Upper Launcher is based on the so-called Remote Steering (RS) concept [1], in which the steering mechanism is placed behind the waveguides, thus avoiding direct neutron bombardment from the plasma. In this paper, the performance of the latest RS layout is evaluated. Generally astigmatic beams [2] are employed to minimise the beam broadening in the plasma due to diffraction, which has a negative impact on the width of the current profile. The stabilisation efficiency is estimated in terms of the figure of merit ?NTM=JCD/Jbs (JCD being the EC driven current and Jbs the equilibrium bootstrap current at a given surface), for (2,1) and (3,2) modes for three reference ITER scenarios. The criterium for complete mode suppression is given as ?NTM >1.2. The EC current profile is calculated by means of the TORBEAM code [3], which allows a straightforward modelling of generally astigmatic beams. The performance of a "long waveguide" design of the RS launcher is also discussed.