Collective High-k Adjustable-radius Scattering Instrument (CHASI) for measuring electron scale turbulence on MAST-U

Plasma turbulence on disparate spatial and temporal scales plays a key role in defining the level of confinement achievable in tokamaks, with the development of reduced numerical models for cross-scale turbulence informed by experimental measurements essential for understanding and maximising confinement. MAST-U is a well-equipped facility having instruments to measure ion and electron scale turbulence at the plasma edge. However, measurement of core electron scale turbulence is challenging, especially in H mode. We therefore propose a mm-wave based scattering diagnostic for measuring binormal oriented high-k (electron scale) turbulence in the MAST-U core and edge plasma. We present detailed hardware specifications along with Gaussian wave optics and beam-tracing calculations predicting the spatial and wavenumber resolution of measurements. We conduct an analysis of the instrument selectivity function, factoring in both theta-z and r-z pitch rotation of the binormal wavevector and compare simulated measurement specifications with CGYRO predictions of ETG turbulence by remapping the instrumental wavenumbers to field-aligned coordinates for a sample equilibrium. Baseline specifications of the diagnostic include an operating frequency of 376 GHz and a turbulence wavenumber measurement range of k_⊥ρ_e = 0.1 -> 0.43 (for a MAST-U high-beta sample equilibrium), where k_⊥ is the binormal turbulence wavenumber and ρ_e the electron gyroradius.