Analysis of the ITER low field side reflectometer employing the Beam Tracing Method

The Beam Tracing Method, which describes electromagnetic wave propagation in the short wavelength limit including diffraction effects, is applied to microwave reflectometry. The torbeam code has been augmented with relativistic corrections to the electron mass – which are necessary for a reliable description in high temperature plasmas such as in ITER, and a beam coupling model for the receiving antenna coupling. The propagation and reception behaviour of reflectometer probe beams in the ITER geometry is computed. The received power is affected by the intensity of the beams, the offset of the beams respectively to the receiver antenna and the angle of incidence. Using a magnetic field derived from a 3D equilibrium it is shown that the effects of toroidal field ripple in ITER on the beam propagation are negligible. Various antenna configurations for the ITER low field side reflectometer are proposed and analyzed, particularly their sensitivity to plasma height variations.

► Formulas for beam-antenna coupling are derived from paraxial beam-tracing method.
► Field ripple has no effect on propagation of reflectometry beams in ITER.
► Options for ITER reflectometer are investigated with respect to antenna alignment.
► Monostatic antenna: good coupling but parasitic reflections; Bistatic: tilt important.
► Combined mono-/bistatic systems are proposed and their merits assessed.