Absolute Beam Monitor: a device to measure the absolute particle flux of a neutral atom beam – prototype development and testing

The Interstellar Mapping and Acceleration Probe (IMAP) mission by NASA, to be launched in 2024, aims at deepening the understanding of the solar heliosphere by verifying and extending the results obtained from the Interstellar Boundary Explorer (IBEX). IMAP-Lo is a neutral atom imaging and analysis instrument to be used to measure heliospheric Energetic Neutral Atoms (ENAs), mainly H, He, O, Ne in the energy range from 10 eV to 1 keV. One key point of improvement of IMAP-Lo compared to IBEX-Lo is having more accurate calibration methods for ENAs at hand. The IMAP-Lo calibration will be carried out in MEFISTO, a calibration facility for ion and neutral particle instruments at the University of Bern. MEFISTO consists of an ion beam source with energies 10 eV/q - 100 keV/q, a removeable beam neutralization stage for neutral atoms from 10 eV to 3 keV, and a large vacuum test chamber.

The beam neutralization process relies on a charge conversion surface and thus results in an energy loss of about 15%, and energy-dependent transmission. It is therefore essential to be able to measure the effective neutral particle flux and beam energies provided at the exit of the neutraliser to improve the calibration process for an ENA instrument, such as IMAP-Lo.

The Absolute Beam Monitor (ABM) is a new laboratory device dedicated to measure the absolute neutral particle flux and coarse energy distribution of a neutral atom beam. The present prototype consists of a tungsten start surface [GJ(1] and two electron multipliers contained in a box of about 1 dm³ volume. By counting the start, stop and coincidence signal rates we infer the effective number of neutral atoms. In addition, the particle energy is determined by a time-of flight measurement.

We present the measurement principle and demonstrate the validity of the concept with the ABM prototype. Neutral H, He, and O beams at different energies and fluxes have been evaluated in MEFISTO with the ABM prototype. The results are compared with IBEX-Lo calibration measurements.