Measurements of electric fields in a plasma by Stark mixing induced Lyman-α radiation

Abstract: This paper treats a non-intrusive method of measuring electric fields in plasmas and other sensitive or hostile environments. The method is based on the use of an atomic hydrogen beam prepared in the metastable fine structure quantum state 2s1/2. Interaction with the field that is to be measured causes Stark mixing with the closely lying 2p1/2, whose spontaneous decay rate is much higher than that of 2s1/2. As a result, the total transition rate to the ground state and consequently the intensity of the Lyman-α line (121.6nm) is increased. Observations of emitted radiation from a region in which the interaction takes place are used to draw conclusions about the electric field, effectively providing a way to measure it. In the first section, the theory behind the method is described, using time dependent perturbation theory and taking into account both Lamb shift and hyperfine structure. A description of the set-up that we have used to test the theoretical predictions follows and practical aspects related to the operation of the experiment are briefly addressed. Measurements of the dependence of the Lyman-α intensity on both electric field frequency and amplitude are presented and shown to be in agreement with theory. These measurements have been performed in vacuum and in an argon plasma, both for static and RF fields. Two mechanisms, labeled oscillatory and geometrical saturation, that decrease the emitted intensity for strong fields are identified and described, and both are of importance for the future implementation of the studied diagnostic in a fusion device or other plasma experiment. Studies of the field profiles between a pair of electrically polarized plates have been carried out and algorithms for relating measured data to actual values of electric field strength have been developed. For static fields in vacuum, collected data is compensated for geometrical saturation and the resulting profiles are compared to those calculated with a finite element method. Good correspondence is seen in many cases, and where it is not, the discrepancies are explained. Static profile measurements in a plasma show the formation of a sheath whose thickness has been studied while varying discharge current, pressure and plasma frequency. The qualitative dependence of the sheath thickness on these parameters is in accordance with well established theory. When it comes to RF fields, a possible standing wave pattern is detected in the plasma despite problems with low signal to noise ratio. In order to optimize the working conditions of the set-up, effects of charge accumulation due to ions present in the hydrogen beam have been studied as well as errors due to residual particle fluxes during the off-phase when pulsing the beam. A conceptual design suggestion for implementing the method in the edge plasma of a tokamak or another similar device, based on the collected information, is also given.