Beam characteristics including general astigmatism effects in the Remote Steering ITER ECRH Upper Launcher

Abstract. 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.