An isochronous poloidal geometry time-of-flight ion mass spectrometer for energetic space plasmas

Space missions to the outer planets of the solar system require advanced mass-spectroscopic measurements that cover an unprecedented range of particle phase space. We present here a discussion of satellite-borne mass-spectrometer design considerations and put forward a design for an isochronous time-of-flight mass spectrometer based on poloidal ion trajectories in a set of three tandem toroidal electrostatic analyzers. The proposed instrument focuses a wide range of ion energies (ΔE/E = 0.10) and entrance angles (± 5deg) over a field of view subtending 360deg with a mass resolution M/ΔM > 20. In order to create such a large field of view, the device accepts a broad range of entrance angles in the nondispersive (azimuthal) direction of the toroids. Isochronous focusing takes place in the polar plane of the toroid, thus the optics of this device are poloidal rather than toroidal. Because of the lack of an analytical theory up to now, development has relied primarily on 3D ray tracing to find triple-focusing (angle-energy-displacement) solutions that are isochronous. The proposed device has the useful feature that it discriminates molecular species from atomic species of the same mass (e.g. CH4+ from O+ or CH2+ from N+). (orig.)