Er 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 (E_r) 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 E_r profile. At low densities, the E_r 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 E_r peak diminishes quickly and the E_r well deepens until a Greenwald fraction of f_GW ≈ 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 E_r 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 f_GW ≈ 0.35. A steep edge density gradient persists up to f_GW ≈ 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 f_GW ≈ 0.5.