A cometary neutral gas simulator for gas dynamic sensor and mass spectrometer calibration

Physics

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Scientific paper

The novel calibration system CASYMIR (CAlibration SYstem for the Mass spectrometer Instrument ROSINA) is used to simulate the neutral gas coma around a cometary nucleus. It was designed to run in two different operational modes: a) in a static neutral gas mode within an extended pressure range from 10-10 to 10-5 mbar and b) in a dynamic neutral gas mode using a supersonic, high intensity molecular beam with intensities in the range of 1010 to 1014 {cm-2s-1}. This facility consists of three major parts: the high vacuum system with a beam diagnostic system and a residual gas analyzer, the instrument positioning system which allows relative movements of the sensors with respect to the molecular beam axis and the gas mixing unit including a water vapor generator. CASYMIR is used to calibrate the three ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) sensors: begin{itemize}
COPS, COmetary Pressure Sensor
DFMS, Double Focusing Mass Spectrometer
RTOF, Reflectron Time Of Flight mass spectrometer The static mode calibration measurements were done within a pressure range of 10-8 to 10-6 mbar for the mass spectrometers and down to 5× 10-10 mbar for the pressure sensor. This measurements were mainly performed with various noble gases and carbon dioxide. The dynamic environment is simulated with a super sonic molecular beam with beam velocities between 300 to 2000 m/s. The beam velocity depends on the nozzle temperature which can be set between 300 and 1000 K. Calibration with the molecular beam yields the sensor sensitivity to the relative orientation of the spacecraft with respect to the comet nucleus. For example, exact calibration of COPS in both modes is a requirement to simultaneously measure the total gas density and the velocity of the coma.

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