Design and development of the ITER CTS diagnostic

The Collective Thomson Scattering (CTS) diagnostic will be a primary diagnostic for measuring the dynamics of the confined fusion born alpha particles in ITER and will be the only diagnostic for alphas below 1.7 MeV [1]. The probe beam of the CTS diagnostic comes from a 60 GHz 1 MW gyrotron operated in a ~100 Hz modulation sequence. In the plasma, the probing beam will be scattered off fluctuations primarily due to the dynamics of the ions. Seven fixed receiver mirrors will pick up scattered radiation (the CTS signal) from seven measurement volumes along the probe beam covering the cross section of the plasma. The diagnostic is planned to provide a temporal resolution of ~100 ms and a spatial resolution of ~a/4 in the core and ~a/20 near the plasma edge where a = 2.0 m is the nominal minor radius of ITER. The front-end quasi-optics will be installed in an equatorial port plug (EPP#12). A particular challenge will be to pass the probing beam through the fundamental electron cyclotron resonance, which is located in the port plug (R=10.3 m) for the nominal magnetic field Bt = 5.3 T. Hence, particular mitigation actions against arcing have to be applied. The status of the design and specific challenges will be discussed.