Design of a tangential phase contrast imaging diagnostic for the TCV tokamak

A core density fluctuation imaging diagnostic is being developed for the TCV tokamak, employing a $7cm$ wide $CO2$ laser beam transmitted through the plasma in a near-toroidal direction. The proposed system employs the phase contrast method and can resolve wavelengths ranging from 7 down to $0.1cm$⁠, with a minimum measurable line-averaged density of $3×1015m3∕MHz1∕2$⁠. The broad range of microinstabilities that can be at play in the strongly electron-cyclotron-resonance heated TCV plasmas, from ion to electron spatial scale lengths, widely known as ion-temperature-gradient, trapped-electron-mode, and electron-temperature-gradient modes, would thus be accessible. The use of an imaging technique overcomes the difficulties faced by traditional scattering diagnostics in investigating highly inhomogeneous regions, such as internal transport barriers. Wavelengths and correlation properties can be recovered from the spatial mapping. The tangential configuration, combined with appropriate spatial filtering techniques, provides an excellent spatial resolution, of the order of 1% of the minor radius. In view of the extreme plasma shaping and positioning flexibility of the TCV tokamak, the beam positioning will also be flexible, with translatable mirrors enabling measurements close to the magnetic axis in some configurations.