Answering to a long-standing challenge, a new way to non-intrusively measure weak electric fields in plasmas is presented. Here we show it using an H(2s) atomic probe beam, obtained from the conversion of H+ ions into H(2s) either by collisions with H2 residual gas or in a resonant charge exchange cesium cell. The probe beam is sent between two polarized plates creating an electric field applied within a thermionic plasma. Indeed the interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-alpha line owing to the Stark mixing of the 2s1/2 and 2p1/2 levels. When the field is weak, it can be treated as a perturbation of these two states separated by a small amount of energy called Lamb-shift. It induces a radiation with intensity proportional to the square modulus of the electric field which is measured in a direction perpendicular to the probe beam. Contrary to measurements in vacuum, the Debye sheath formation is clearly observed in the presence of plasma. This contribution reviews: presentation of Stark mixing of the 2s1/2 and 2p1/2 levels and of Lamb shift for hydrogen-like atoms, brief description of the experimental set-up, observation of the formation of a sheath between two polarized plane electrodes in a plasma, either with a probe ion beam, or atom beam.

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