A double-cusp type electrostatic analyzer for high-cadence solar-wind suprathermal ion observations

This paper describes a novel electrostatic analyzer concept to measure suprathermal ions, a Double-Cusp Analyzer for SupraThermals (DCAST) that employs a double-shell cusp structure. Due to the necessity of measuring higher energy levels to cover the suprathermal range, existing ion instruments require greater size and mass. Moreover, observations of potentially low-flux suprathermal ions require a long integration time to fully characterize key ion properties in the plasmas (e.g., anisotropy and energy spectrum) with necessary counting statistics. DCAST covers the suprathermal energy range (2-300 keV/q) spanning heated solar wind and pickup ions; it enables a high cadence, high angular resolution, and wide angle coverage measurement while conserving resources such as mass and size. As a proof-of-concept study, the performance of a prototype DCAST was verified by laboratory measurements (geometric factor, K-factor, and energy resolution), which also involved investigating noise characteristics coming from cross-sector contamination and foreground extreme ultra-violet photons. To understand the specific characteristics of the double-shell type design, the inner and outer sector voltage ratio (R_V) effects were examined in terms of the electro-static analyzer performance.