The structure, evolution and role of the radial edge electric field in H-mode and L-mode on MAST

The first measurements of the structure of the edge radial electric field, E_r, in a spherical tokamak (MAST) are presented. Using active Doppler spectroscopy on He⁺ with 120 lines of sight E_r profiles are calculated from the leading terms of the radial momentum balance. A spatial resolution up to Δr ≈ 1.5 mm with a typical time resolution of Δt = 5 ms can be achieved. In L-mode the field is largely determined by the diamagnetic term of the force balance, and fields of only a few kV/m are observed. The measured impurity flow is mostly parallel to B, and is greatly affected by MHD, such as sawteeth or mode locking of tearing modes, or error fields. In H-mode a strong perpendicular flow evolves with poloidal and toroidal velocities up to v^He+_ϕθ ≈ -20 km/s, and a deep negative electric field well E_r^min ≳ -15 kV/m develops. The profile form is dominated by the diamagnetic term.