Study of the EFILE diagnostic under radiofrequency electric field

The electric field is a parameter of special relevance in plasma physics. Its fluctuations are responsible for various macroscopic phenomena, such as anomalous transport in fusion plasmas or plasma-wall interactions, and its measure is therefore required. At present, most diagnostic tools perturb the field (like Langmuir probes) or require multiple simultaneous measures to achieve a good spatial resolution (like spectroscopy and tomography). The EFILE diagnostic (Electric Field Induced Lyman-alpha Emission) is currently under development at PIIM laboratory at Aix-Marseille Université (France) and aims to provide a non intrusive and precise measurement of the electric field in the plasma edge region, using a beam of hydrogen atoms prepared in the metastable 2s state. The metastable particles are obtained by means of a proton beam extracted from a hydrogen plasma source, and neutralised by interaction with vaporised caesium. When a 2s atom enters a region where an electric field is present, it undergoes a transition to the 2p state (Stark mixing). It then quickly decays to the ground level, emitting Lyman-alpha radiation, which is collected by a photomultiplier. The 2s->2p transition rate is proportional to the square of the magnitude of the electric field, and depends on the field oscillation frequency (with peaks around 1 GHz). By measuring the intensity of the Lyman-alpha radiation emitted by the beam it is possible to determine the magnitude of the field in a defined region. In this work, an analysis of the behaviour of the diagnostic under static or radiofrequency electric field is presented. Electric field simulations obtained with a finite element solver of Maxwell equations, combined with theoretical calculations of the Stark mixing transition rate, are used to develop a model for the interpretation of photomultiplier data. This method shows good agreement with experimental results for the static field case, and allows to measure the field magnitude for the oscillating case. Issues linked to the experimental set-up and limits of the diagnostic itself are shown and possible design implementations are given.