Intermediate-k Density and Magnetic Field Fluctuations During Inter-ELM Pedestal Evolution in MAST

Measurements of local density and magnetic field fluctuations near the pedestal top, conditionally-averaged over the ELM cycle, have been made in MAST. A Doppler backscattering (DBS) system installed at MAST was used to measure intermediate-k (k⊥ρi ≈ 3 − 4) density fluctuations at the top of the pedestal. A novel diagnostic technique combining DBS with crosspolarization scattering (CP-DBS) enabled magnetic field fluctuations to also be locally measured at similar wavenumbers. Polarization isolation and other effects for CP-DBS are discussed. Both measurements were used in a series of high-beta (βn ≈ 4.0 − 4.5) MAST plasmas with large type I ELMs, with a ∼ 8− 9 ms period, where microtearing modes (MTM) had been predicted unstable in similar conditions [Dickinson Phys. Rev. Lett 108, 135002 (2012)]. The measured density fluctuation level increased by a factor of about 4 between 2-4 ms after the ELM, which was correlated with the recovery of the density profile, while the temperature pedestal height continued to increase slowly. Magnetic field fluctuations showed different temporal behavior, slowly increasing throughout the ELM cycle as the local β increased. Linear GS2 calculations show both MTM and electron temperature gradient (ETG) modes unstable at similar wavenumbers as the measurements (although with more overlap between ETG wavenumbers and diagnostic spectral resolution), at the top of the pedestal, along with kinetic ballooning modes unstable lower in the pedestal (at larger wavelengths). The inferred ratio of fluctuation levels from experiment was (δB/B)/(δn/n) ≈ 1/20. The comparable ratios from GS2 were (δB/B)/(δn/n) ≈ 0.4 for MTM and (δB/B)/(δn/n) ≈ 0.02 for ETG. Both the experimental wavenumber range and fluctuation ratio are more similar to the linear characteristics of ETG than MTM. These results imply that intermediate-k fluctuations due to ETG play a role in inter-ELM pedestal evolution. ∗ [email protected]