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

Abstract

Abstract. Electron Cyclotron Current Drive (ECCD) will become the main scheme on ITER for the stabilization of neoclassical tearing modes (NTM) and the control of sawtooth oscillations. The effectiveness of this scheme forms the basis for the requirements of the ITER Upper Port Launcher (UPL). 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 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 Remote Steering 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 stabilisation performance as compared with Front Steering. In this paper, recent improvements in the RS design are presented. The stabilisation 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 saw teeth control at the innermost q=1 surface.

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

“…In the early development stage of the launcher, it was found that a design based on remote steering [4], although attractive because movable parts would have been placed in a secondary vacuum away from the plasma, was unable to deliver sufficiently focused beams over the whole steering range. The adoption of a front steering concept [5] has allowed a larger flexibility in the design of the beams, which in turn has led to an optimization of the distribution of the tasks between EL and UL [6].…”

Design activities on helical DEMO reactor FFHR-d1

“…Those RS system and installation of reflection mirrors have a benefit to separate the moving parts from the source region of the plasma neutrons, and it contributes to increase the reliability and to reduce the damage of the ECH system. Longpulse operation of normal electromagnetic mode has been verified in the LHD experiment [50][51][52]. The EBW heating with so-called the slow X-B and the O-X-B methods have been tested in overdens plasmas confined in the LHD [53,54].…”

A remotely steered millimetre wave launcher for electron cyclotron heating and current drive on ITER

“…Engineering requirements include a safely handled peak power load on the mirrors well below 10 MW/m 2 , millimetre wave stray power below 1%, and operation under ITER environmental conditions (magnetic field 6 T, ultra-high vacuum, vibration from cooling and shock from plasma disruptions). Various options of the RS concept have been explored and optimised in terms of effectiveness in NTM suppression [7]. Optimisation of the quasi-optical design has been carried out by [8].…”