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ITER employs Electron Cyclotron Waves (ECW) for heating of the plasma and to stabilize MHD modes including the neoclassical tearing modes (NTM). To cover all the significant ITER plasma scenarios, the upper port ECW launcher needs a beam steering mechanism to reach the magnetic q-surfaces where NTM instabilities can grow. Two alternative approaches are being studied in Europe under EFDA tasks: Front Steering (FS), which has rotating mirrors situated near the plasma, and Remote Steering (RS), which has the mirror steering mechanism situated in the secondary vacuum. The steering mirrors in the RS system project the mm-wave beam onto a Square Corrugated Waveguide (SCW) of a length such that the input beam profile is imaged at the output, producing a steered output beam. The advantage of RS is that the steering mechanism is shielded from direct plasma exposure and neutron bombardment and it also allows simpler maintenance. One of the drawbacks of RS is that the output steering range is inversely coupled to the focusing. The optimization of the quasi-optical system is done for a short (4.3 m) and a long (9.1 m) SCW. The results are evaluated for their effectiveness in NTM stabilization by calculating the parameter ? which represents the ratio between the EC wave driven current and the bootstrap current, preferably ? should exceed 1.2. The performance is also evaluated in terms of beam focusing properties and power loading on the mirrors. The ? achieved so far meets the requirements in the ITER reference scenarios for q=2/1 and partly for q=3/2. Additional tools under investigation for optimization are: non spherical mirror curvatures, tapered SCWs and the use of general astigmatic beams. This work has been carried out as part of the EFDA work program Task TW6-TPHEECHULB for the development of the ITER ECRH Upper Port Launcher system, in collaboration with the Euratom Associations IFP Stuttgart, IPP Garching, FZK, IFP/CNR and CRPP.

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.

General astigmatism treatment has been included in the description of the resulting beams for the latest design concepts proposed for the Remote Steering (RS) Launcher. The inclusion of general astigmatism is intended to provide the key parameters of the resulting beams and perform realistic beam tracing calculations in the plasma. It is in fact well known that the many requirements foreseen for the ITER ECRH Upper launcher force the beams to be injected in the plasma with a sufficient steering capability aiming at the stabilization of neoclassical tearing modes (NTM) through localized deposition of EC waves. To do this, double curvature mirrors are used in the latest RS quasi-optical systems. The mirrors are oriented at an angle with respect to each other and with respect to the directions of astigmatism of the incoming launched beam. The resulting beams are found to be generally astigmatic beams [1]. The correct description of the beam parameters resulting from this kind of complex launching system can in principle be used to optimize the RS quasi-optical system (in terms of localized heating and current drive efficiency), acting for example on the curvatures of the last mirror. The curvature in the direction determined by the steering plane should not be considered a free parameter, since the necessary output steering range has to be granted, but the curvature in the direction orthogonal to the steering plane could be modified to improve the performances of the launcher. Starting from the design proposed [2], the analysis presented here includes beam optics calculations. This approach was agreed within affiliate institues to optimise the efficiency of the RS Upper launcher design. The work was carried out under EFDA Task TW6-TPHE-ECHULB.

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