The Construction of a Neutral Molecular Beam Facility and its Application to Mass Spectrometers Carried on Space Vehicles.
The design and implementation of a neutral molecular beam system intended for the simulation of the motion of planetary probes is discussed. The beam system employed the technique of isentropic expansion of gas through a free jet. Beam velocities of up to 6.3 Km/sec were obtained by raising the temperature of the gas behind the nozzle orifice to 2400 K and by 'seeding' a small percentage of the gas of interest into a light mass carrier gas, i.e., hydrogen. This molecular beam was used as a tool to investigate the response of open source mass spectrometers to neutral gas beams. A method of calibrating the response of open source mass spectrometers against an idealized closed source is demonstrated. Measurements of this calibration constant for mass spectrometers similar to those employed on several earth satellites and on several planetary entry probes show that correction factors of up to 30% are required for some instruments. The response of the spectrometer to angles of attack up to twenty degrees is plotted. A thin sheet beam scanned across the front of the ion source showed the gas contributions from the various ion source surfaces to the total signal. A relatively new mode of ion source operation, the flythrough mode, which utilizes the energy of the incoming molecules to differentiate between beam molecules and molecules which have scattered from surfaces, is extensively investigated. The response of the mass spectrometer when operated in this mode is plotted with respect to velocity, angle of attack, beams in the presence of high background gas pressures, and thin sheet beams. Variations on the basic flythrough mode source which permit wide angular response, low velocity operation, or electronic angle compensation, are presented. Data is presented showing the usefulness of the flythrough mode for beam velocities as low as 620 m/sec.
- Pub Date:
- June 1981
- Physics: Atmospheric Science