New extreme ultraviolet transmission measurements of two thin-film filters for imaging of plasmaspheric cold ions

We present extreme ultraviolet (EUV) transmission measurements of two thin-film filters designed to obtain improved images of plasmaspheric He+, and the first global images of O+/O++ in the dense oxygen torus. Compared to previous He+ 30.4 nm imaging that used an Al filter, we show that a combined Al+C filter achieves superior rejection of 58.4 nm background from neutral helium (He I). We show that an indium filter provides both the required transmission at 83.4 nm and adequate rejection of brighter He I (58.4 nm) and H I (121.6 nm) background emissions. We find that the In transmission at 58.4 nm is a factor of ∼16 lower than predicted based on optical constants that rely on interpolations at λ<68 nm. We show that the observed lower 58.4 nm transmission is consistent with alternative optical constants derived from previous lab measurements.