The SPAN-A module has two ESAs to measure ions and electrons from the ram direction and nadir. SPAN-B consists of a single ESA to measure electrons from the anti-ram direction. SPAN-A is located on the ram direction side of PSP and SPAN-B is on the anti-ram side. Significant savings in mass are realized by combining the electron and ion ESAs, which was a lesson learned from FAST and THEMIS. Electrostatic deflectors extend the narrow planar intrinsic angular field of view, FOV, of each ESA to 240°?120°. Together the SPAN electron sensors provide a nearly 4? sr FOV for electrons only excluding the region of the sky blocked by the heat shield.
The SPAN sensors utilize the classic top-hat hemispherical ESA design developed by UCB (Carlson et al. 1983) that affords a uniform response over a planar 360° FOV. Particles entering the analyzer are selected for energy per charge, E/q, by a voltage applied to the inner hemisphere. This voltage is swept from near zero to several kV to measure ion and electron energies as low as a few eV/q to as high as 30 keV/q thus providing excellent energy coverage and resolution. Angular resolution is provided in one plane by discrete segmented anodes and out of that plane by electrostatic deflectors, resulting in a broad instrumental FOV appropriate for a non-spinning spacecraft like PSP.
Both ion and electron sensors use microchannel plate, MCP, detectors for particle detection, and discrete anodes for MCP charge collection. The electron sensor uses chevron-pair detectors.Pulse-counting electronics for the electron sensors utilize a multi-channel preamplifier ASIC developed by researchers at the Laboratoire de Physique du Plasmas, LPP, for the Solar Orbiter mission.
SPAN-B measures electrons only and is a near duplicate of the SPAN-A e-analyzer as only the anode patterns are different. SPAN-B is mounted in an orthogonal orientation to SPAN-A and it is on the opposite side of the spacecraft. The ability of SWEAP to use the same design for both SPAN-A and SPAN-B electron sensors provides significant savings in design time and analysis.