UV Rejection Design and Performance of the PEACE Electrostatic Analyzers

Abstract

The Plasma Electron and Current Experiment, PEACE, electrostatic analyzers are designed to measure electrons from 0.6 eV to 26,400 eV and will be directly exposed to sunlight in their operational environment. To minimize solar background counts, generated by direct interaction of the solar UV with the microchannel plate detectors or by the generation of photoelectrons, baffles have been incorporated into the analyzer design and the internal surfaces have been coated with a low reflectivity coating based on copper oxide. Baffle designs have been optimized using computer simulations and the performance assessed by laboratory measurements. A copper oxide blackening process has been optimized to achieve a thickness of 12 μm, in order to maintain accurate surface profiles, whilst maintaining adequate adhesion and UV absorption. Photoelectrons are shown to be the dominant source of background and the ratio of background count rate to light flux entering the aperture for Lyman α light is ∼10^-8 for the PEACE analyzer specialising in the low energy measurement. The resulting solar UV caused background of less than 100 counts per complete plasma distribution will have negligible impact on the plasma diagnostics performed by PEACE.