Laboratory calibration of low-energy ENA instruments for space science

Abstract

Imaging space plasmas via Energetic Neutral Atoms (ENA) is a widely used technique to study space plasma population ranging from the Earth’s magnetosphere all the way to the interstellar medium. Laboratory calibration against a known ENA beam source is a key element in the development and testing of spaceborne ENA imaging instruments. For ENA instruments operating at energies below about 1 keV, an ion beam source cannot be used for the instrument calibration, as ion trajectories are distorted through electromagnetic fields inside the ENA instrument whereas neutrals are not. The MEFISTO laboratory test facility at the University of Bern is well suited to carry out such ENA instrument calibrations: It provides neutral atom beams of any species from H to heavy elements at the relevant low-energy range from 3 keV down to 10 eV. MEFISTO is equipped with a large vacuum test chamber and an electron-cyclotron resonance ion source (ECRIS) for the production of a primary ion beam. Ion beam neutralization is performed with a dedicated neutralization stage via grazing scattering surface neutralization. This neutralization method introduces some uncertainty to the neutral beam energy and comes with species and energy dependent throughput.Therefore, neutral beam calibrations have been conducted against a novel Absolute Beam Monitor (ABM), which serves as a primary calibration standard in the low-energy ENA range. Calibrated neutral beam fluxes and energies have been obtained for species of interest to interstellar, heliosphere and planetary science, including H, D, He, O, Ne neutral beams. The recently upgraded 5-axis movable hexapod table in the test chamber allows for precise motion and rotation of instrument under test relative to ENA calibration beam. These measures allow us to carry out thorough ENA instrument calibrations against a well-characterized low-energy neutral atoms beam source.