Ion-scale turbulence measurements in L-mode plasmas with beam emission spectroscopy (BES) on MAST Upgrade*

Experimental characterisation of turbulence is of importance due to its role in limiting confinement of tokamak devices. In MAST-U, the BES diagnostic [1] is used to measure ion-scale (k_⊥ρ_i<1) turbulent density fluctuations. In this work, we utilise its 8×8 detector array [2] and high sampling frequency (Δx≈1.8cm, f_s=4MHz) in a series of repeatable L-mode discharges to obtain measurements of turbulence over almost the entire LFS minor radius, 0.15<Ψ_N<1.1.

We record relative plasma fluctuation amplitudes of O(0.1%) in the plasma core which increases to O(10%) at the plasma edge. A minimum in the skewness of the fluctuation signal is seen ≈10cm inside the separatrix, similar to measurements on MAST [3]. At lower radii, conditional wavenumber-frequency spectra show features propagating in opposite poloidal directions, but the k_z<0 propagating feature is absent at larger radii. Measurements of velocity and plasma-frame decorrelation time show a peak in the radial profiles at the minimum skewness location, with poloidal correlation lengths slightly displaced to a larger radius. Decorrelation times increase and turbulent structure tilt angles decrease moving towards the plasma edge. Our results are compared with results from MAST [3,4].

References

[1] D. Dunai et al., Rev. Sci. Instrum., In progress

[2] D. Dunai et al., Rev. Sci. Instrum., 81, 103503 (2010)

[3] A. Sladkomedova et al., J. Plasma Phys., 89(6), 905890614 (2023)

[4] A. R. Field et al., Plasma Phys. Control. Fusion 56, 025012 (2014)