Advanced launcher design options for electron cyclotron current drive on ITER based on remote steering

Abstract

Electron cyclotron current drive will become the main scheme on ITER for the stabilization of neoclassical tearing modes (NTMs) and the control of sawtooth oscillations. The effectiveness of this scheme forms the basis for the requirements of the ITER Upper Port Launcher. These requirements include the need for steering the mm-wave beam to the pertinent magnetic flux surfaces where the instability develops. Several concepts have been explored; front steering (FS) where a system of movable mirrors is situated at the plasma-facing end of the launcher; remote steering (RS) where the movable mirror system is situated at the back end of the launcher system. The advantage of the RS concept is that moving parts are placed away from the plasma in a secondary vacuum system, less exposed to plasma neutron flux, thus offering advantages in reliability and safety of the machine. Also, contamination of the beam steering unit by Be and T is less of a problem. However, beam-focusing properties of the RS launcher deteriorate with increased steering angle, which limits the NTM stabilization performance as compared with FS. In this paper, recent improvements in the RS design are presented. The stabilization efficiency calculated reaches values that meet most of the ITER requirements. Improvements include non-spherical front mirrors and a tapered square corrugated waveguide. Rather than steering all mm-wave beams over the entire vertical height range specified, RS launchers with a limited steering range are explored, yielding a performance improvement of 25% with an acceptable mm-wave heat load of 5.7 MW m−2 on the front mirrors. This result leads to a dual launcher concept with one set of beams dedicated to control NTMs at the outward lying q = 2 surface and another set of beams dedicated to control NTMs at the inward lying q = 3/2 surface in combination with sawteeth control at the innermost q = 1 surface.