Filterscope Analysis Techniques for Nearby Tungsten Impurity Line Screening

Antennas in fusion reactors generate radio frequency (RF) fields and plasma sheaths that interact with the local plasma which can result in increased erosion of the antenna structure. The RF Plasma Interaction Experiment (RF PIE) at Oak Ridge National Lab (ORNL) is used to simulate this RF antenna structure. RF PIE is an Electron Cyclotron Resonance microwave-based plasma source (2.45 GHz, <5 kW) with an RF biased electrode. RF PIE can provide relatively clean spectral plasmas and a simplified viewing geometry with a large solid angle making spectral measurements easier to capture. The filterscope diagnostic, developed at ORNL, uses band-pass filters and photomultiplier tubes to detect specific spectral emission lines.

Filterscopes are being used to measure the W line emission at 400.88 nm as a function of ion energy on RF PIE. Different filter techniques are being explored and compared. The effectiveness of these techniques to screen out nearby impurity lines, like the Ar II 401.39 nm line expected on the fully-tungsten wall WEST experiment, is being determined by comparing the measurements to a high-resolution 1.0 m Czerny-Turner spectrometer with 40 um spatial resolution and 0.012 nm spectral resolution. The ability to filter out nearby impurity emissions is useful when imaging divertor and antenna guard limiters. Initial results with a helium plasma show little difference between the techniques at higher bias voltages. At lower DC bias voltages, a difference can be seen. Further results will be presented.