High-K Mm-Wave Scattering Diagnostic for Measuring Poloidal Wavenumber Electron-Scale Turbulence on MAST-U

Plasma turbulence plays a key, governing role in determining the spatial-temporal evolution of plasmas in astrophysical, geophysical and laboratory contexts. In particular, turbulence on disparate spatial and temporal scales limits the level of confinement achievable in magnetic confinement fusion experiments and therefore limits the viability of sustainable fusion power 1 . The TDoTP project* (Turbulent Dynamics of Tokamak Plasmas) aims to advance understanding of plasma turbulence coupling across a wide range of scale sizes. As part of this effort, a mm-wave based scattering diagnostic is being designed for the MAST-U spherical tokamak, to measure binormal oriented high-k (electron scale) turbulence in the core plasma. We present the results of Gaussian wave optics and beam-tracing calculations 2 that demonstrate the predicted spatial and wavenumber resolution of the diagnostic, as well as estimates for the sensitivity of measurement. Current specifications for the diagnostic include an operating frequency of 260GHz, a turbulence wavenumber measurement range of k ⊥ ρ e = 0.1 -> 0.5 (where k ⊥ is the poloidal turbulence wavenumber and ρ e the electron gyroradius) and a minimum spatial localisation of ~3cm along the primary beam path.