First measurements of ion-scale turbulence with the upgraded beam emission spectroscopy diagnostic in MAST Upgrade

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Abstract
Beam emission spectroscopy (BES) is used to diagnose ion-scale ( k ⊥ ρ i < 1 ) plasma turbulence in mega ampere spherical tokamak MAST Upgrade (MAST-U) by analysing the Doppler shifted D α light emitted from the on-axis neutral deuterium heating beam. These are the first results of ion-scale turbulence measured in MAST-U, and the first results from the upgraded BES diagnostic system which has an 8 × 8 array of avalanche photo-diodes in the R − z plane, increased from 4 poloidal channels in the old BES system on MAST, spaced ≈ 2 cm apart at up to 4 MHz sampling frequency. A series of three repeatable L-mode discharges are analysed, with the BES sampling different locations in each shot, to obtain measurements over almost the entire low-field side minor radius, 0.15 ⩽ Ψ N ⩽ 1.1 , giving radial profiles of key statistical quantities. The 2D coverage of the BES diagnostic ( R × z ≈ 13 × 15 cm) is exploited to give simultaneous measurements of radial and poloidal correlation lengths and wavenumbers, as well as tilt angles of turbulent eddies. Compared with the system’s predecessor, the increased poloidal coverage allows for significant improvements in the inference of poloidal quantities (correlation lengths and wavenumbers), as well as improved turbulence decorrelation time and poloidal velocity inferences. Local maxima are seen mid-radius ( R ≈ 1.3 m), which corresponds closely with a local maximum in the plasma rotation velocity, and a flattening of the electron density gradient. The apparent z -velocity from the BES is used in tandem with other plasma profiles (i.e. electron temperature and density, ion temperature, plasma rotation velocity, and magnetic field components) to infer the E × B velocity profile. These are compared with incomplete E × B velocity estimates from the charge exchange diagnostic. Some features of the velocity profiles are present in both inferences, giving good qualitative agreement, but the absolute values do not agree. As there is no poloidal velocity measurement from charge exchange in MAST-U, we assume it is small in our inference of the radial electric field, E r . For MAST plasmas, however, it has been shown that the poloidal flow contribution to E r can be large, and we suggest this may be the cause of the quantitative difference in our profiles.
Funding Information
  • Engineering and Physical Sciences Research Council (EP/R034737/1)

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