New high-resolution electrostatic ion mass analyzer using time of flight

Abstract

We have developed a high-resolution mass analyzer that utilizes a measurement of an ion’s time of flight (TOF) in an electrostatic field that is configured to produce a harmonic potential. The TOF corresponds to one-half period of a harmonic oscillator and is independent of ion energy, in analogy to the amplitude independence of a harmonic oscillator’s period, and is proportional to the square root of the ion’s mass per charge ratio. The TOF is determined from a start pulse from secondary electrons produced when the ion passes through a thin carbon foil (1–3 μg/cm2) at the entrance of the TOF region and a stop pulse from the ion striking a microchannel plate (MCP) upon exiting the region. The energy independence of the TOF implies there is no degradation of resolution from energy straggling in the foil. We have achieved a mass resolution M/ΔM (FWHM) of greater than 100 with a laboratory prototype. Three versions of this analyzer for solar wind measurements are included on upcoming space missions and will be flown on the GGS/WIND spacecraft, the SOHO spacecraft, and the ACE spacecraft.