Collective high-k adjustable-radius scattering Instrument for electron scale turbulence measurement on MAST-U

Plasma turbulence on disparate spatial and temporal scales and associated cross-field particle / heat transport plays a key role in limiting the level of confinement achievable in tokamaks. The development of reduced numerical models that accurately predict cross-scale turbulent interactions is essential for understanding and maximising confinement. Such models require experimental turbulence data at both electron and ion scales to inform development. In this paper, we propose a novel, mm-wave based collective scattering diagnostic for measuring normal and binormal high-k (electron-scale) turbulence in the core and edge plasma of MAST-U. This will complement the existing ion-scale BES (beam emission spectroscopy) diagnostic, yielding core and edge measurements at both electron and ion scales whilst providing full spatial coverage under all operating conditions. We present detailed hardware specifications along with beam-tracing calculations predicting the spatial and wavenumber resolution of measurement. We also perform analysis of the instrument selectivity function computing the localisation and sensitivity of measurement accounting for both magnetic pitch rotation with radius and spatial overlap of the incident and scattered Gaussian beams. A synthetic diagnostic framework is presented combining CGYRO predictions of ETG turbulence for a sample equilibrium with beam tracing data, mapping the instrumental wavenumbers to field-aligned coordinates and predicting the scattered power spectrum. Baseline specifications of the diagnostic include an operating frequency of 376 GHz, a source power of ~100mW and a normalised turbulence wavenumber measurement range of k_⊥ρ_e = 0.1 – 0.5 where k_⊥ is the binormal turbulence wavenumber and ρ_e the electron gyroradius.