Numerical calculations of beam patterns for the ITER ECRH Upper Launcher

The design of complex, multiple mirrors quasi-optical launchers in ECH&CD systems in fusion grade devices is constrained by severe requirements in terms of high power handling capability, extended steering range and room availability. In these cases, often diffractive effects and aberrations of the beams become important, significant side lobes and beam asymmetries arise [1] and the single mode Gaussian description is not longer sufficient. An efficient way to calculate the beam pattern in vacuum avoiding restrictive assumptions is using a numerical tool in which the detailed characteristics of the reflectors surfaces can be introduced and the resulting field propagation in vacuum can be computed including all the relevant effects. The description of the beam resulting from Physical Optics calculations takes into account the relevant causes of deformation and non gaussianity, such as aberration, beam truncation, thermal deformation of the mirrors and existence of surrounding structures [2]. In this work we discuss the application of the GRASP ® electromagnetic code to the case of the ITER ECRH Upper Launcher. In the process of studying the launcher geometry of the Front Steering option we performed a truncation study to evaluate the beam pattern distortions with respect to mirror dimensions. For the Remote Steering, we simulated the socalled "Dogleg" layout to calculate the beam profiles at deposition location. In these studies, GRASP® has been proved to be a very efficient way to test the subsequent updates in the refinement phase of the model once the basic geometry has been implemented. The work was carried out under EFDA Task TW6-TPHE-ECHULB.