Collective Thomson scattering below the electron cyclotron frequency in ITER

The possibility of diagnosing the alpha particle distribution in ITER by means of Collective Thomson Scattering (CTS) using microwaves below the electron cyclotron frequency is investigated. With respect to other schemes, using much shorter wavelengths (e.g., 10 to 200 mu m), this method combines some important advantages, i.e. the availability of the appropriate wave sources, the simplicity of the scattering geometry and the possibility of space resolved measurements. The useful frequency range for this method is limited by refraction effects and by the noise due to spontaneous electron cyclotron emission (ECE). These constraints are evaluated by means of a ray tracing/radiation code, over a wide range of typical ITER parameters. As a result of this general study, a proposal is made for a relatively simple CTS system for ITER, using backscattering of extraordinary waves in the frequency range 60 to 80 GHz. This method allows, in principle, spatially resolved measurements, requiring partial occupation of an equatorial port only.