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In the present work we report recent radial electric field measurementscarried out with the Doppler reflectometry system in the TJ-II stellarator. Thestudy focuses on the fact that, under some conditions, the radial electricfield measured at different points over the same flux surface showssignificantly different values. A numerical analysis is carried out consideringthe contribution arising from the radial dependence of $\Phi_1$ as a possiblecorrection term to the total radial electric field. Here $\Phi_1$ is theneoclassical electrostatic potential variation over the surface. The comparisonshows good agreement in some aspects, like the conditions under which thiscorrection is large (electron-root conditions) or negligible (ion-rootconditions). But it disagrees in others like the sign of the correction. Theresults are discussed together with the underlying reasons of this partialdisagreement. In addition, motivated by the recent installation of the dualDoppler reflectometry system in Wendelstein 7-X (W7-X), $\Phi_1$ estimationsfor W7-X are revisited considering Core-Electron-Root-Plasma (CERC) plasmasfrom its first experimental campaign. The simulations show larger values of$\Phi_1$ under electron-root conditions than under ion root ones. Thecontribution from the kinetic electron response is shown to become important atsome radii. All this results in a potential variation size noticeably largerthan estimated in our previous work in W7-X \cite{Regana_nf_57_056004_2017} forother plasma parameters and another configuration.

On-surface potential and radial electric field variations in electron root stellarator plasmas