Lyman-α radiation of a probe atomic beam to explore the electric field in plasma sheaths

An innovative way to non-intrusively measure weak electric fields in plasmas is presented. The electric field is produced between two planar plates which can be immersed in a thermionic plasma. An atomic probe beam, obtained from the conversion of ions into H(2s) in a resonant charge exchange cesium vapor cell, is sent between the two plates. Emission of the Lyman-? line is induced by the external electric field. It originates in the Stark mixing of the two and levels that are separated in energy by the so-called Lamb shift. The light is collected in a direction perpendicular to the probe beam. Theory predicts that its intensity is proportional to the square modulus of the electric field below a critical value. With a proper calibration through measurements in vacuum, the amplitude of the electric field is obtained. Creating an electric field within a plasma, Debye shielding is thus clearly observed. Such measurement can also be done in transparent diluted media. The presented method answers to a long-standing challenge in experimental physics. This paper is dedicated to late Professor R. A. Stern who was the inspirer of this research