Description
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 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 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 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 plasma turbulence with beam tracing data for a sample equilibrium, mapping the instrumental wavenumbers to field-aligned coordinates and predicting the scattered power spectrum. All optics and mm-wave electronics will be mounted ex-vessel, with low-loss fused silica windows used for injection and egress of the probe and scattered beams. Precision measurements have been conducted on the dielectric properties of suitable fused silica glasses from 140 – 750GHz, using a novel Mason’s gain formulation to compute the Fabry-Perot transmission characteristics and minimise losses for the probe and scattered beams. 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 – 0.6 where k_⊥ is the binormal turbulence wavenumber and ρe the electron gyroradius.