E r measurements in JET L-mode plasmas for a wide range of densities—from the low-recycling regime up to the density limit

This study investigates the dependence of the radial electric field (Er) on the line-averaged density in JET L-mode plasmas, utilizing Doppler backscattering measurements. Density ramp discharges up to the density limit are analyzed to investigate the physical processes that determine the edge Er profile. At low densities, the Er profile at the midplane exhibits a pronounced peak in the near scrape-off layer (SOL) and a shallow well inside the separatrix. As density increases, the SOL Er peak diminishes quickly and the Er well deepens until a Greenwald fraction of fGW ≈ 0.8, followed by a slight reduction near the density limit. Our findings indicate that no collapse of edge flow shear occurs prior to the density limit onset, within a time scale of 10 ms. The Er at the divertor target does not appear to play a significant role in the density limit as it is only significant in the low recycling regime, not changing appreciably above fGW ≈ 0.35. A steep edge density gradient persists up to fGW ≈ 0.95 with the density limit disruption onset coinciding roughly with a reduction in the pedestal top density. The edge E × B shear appears to be sufficient to maintain a steep density gradient region near the density limit. Finally, it is shown that the density limit is not due to a reduction in the shear induced by oscillating flows, as the amplitude of the geodesic acoustic modes vanishes around fGW ≈ 0.5.