CASYMIR - Calibration System for the Mass Spectrometer Instrument Rosina

The scope of the CASYMIR project (Calibration System for the mass spectrometer instrument Rosina) was the development of a calibration chamber with the aim of testing and calibrating for the two ROSINA space mass spectrometers. The two mass spectrometers (DFMS and RTOF), which are the main part of the ROSINA instrument package, will analyze the elemental, isotopic and molecular composition of the neutral and ionizes atmosphere of the comet Churyumov-Gerasimenko. In order to simulate these conditions of the comet surroundings, CASYMIR consists of a vacuum system with several stages leading to a high vacuum, and a supersonic molecular beam. CASYMIR has been designed and set-up at the Space Research & Planetary Sciences research division at the Physikalisches Institut of the University of Bern, Switzerland.

The mass spectrometers will be calibrated in a static as well as in a dynamic mode. In the static calibration mode, a static atmosphere is introduced into chamber V1 and V0 in a pressure range of 10-6 to 10-10 mbar.

In the dynamic calibration mode, a neutral molecular beam will be used to calibrate the spectrometer. Therefore, the gas will be introduced into the nozzle-chamber V3 via a heated 40-100 µm nozzle to form a neutral molecular beam. In order to reduce the overall pressure in the main chamber V1 and the docking chamber V0, the beam has to pass a skimmer (400-1000 µm) that will be mounted between V3 and V2 as well as a collimator mounted between V2 and V1.

Since the comets atmosphere consists mainly of water, special efforts were made to allow the calibration with water in a high vacuum environment. Other gases existing in the comets tail are CO, CO2, N2, H2CO, CH3OH (all in the 10% range), CH4, C2H2, C2H6 and NH4 (in the 1% range) and HCN, CH3CN, H2S, CS2 and OCS in the 1‰ range.

The gas mixture will be supplied from a special designed Gas-Mixing-Unit (GMU) that allows mixtures of the above mentioned gases.

Molecular Beam Characteristics

CASYMIR should be able to simulate the space environment in the vincinity of a comet. From previous flights to other comets it is expected that the major components of the released gas will be water vapour (H2O) with CO, CO2, N2 and H2CO as minor species in the percent range. CASYMIR can be run in three modes, a static and two dynamic neutral gas modes. The heart of the dynamic modes is a supersonic, high intensity molecular beam with beam intensities between 109 and 1015cm-2s-1.

Measurement of the Velocity Distribution and Beam Intensity

Two sensors are used to detect the molecular beam. One of them is a Fast Ionization Gauge (FIG). The advantage of this gauge is that it is commercial available and therefore easy to handle, the disadvantage is a lower resolution compared with other detection system. In addition to the FIG we use a cross beam deflector ionisizer with a channeltron/faraday cup. Furthermore we want to derive the beam intensity from the measured velocity distribution. In order to get absolut intensity values we measure the mass flow rates entering and exiting V1 and fit the relative beam intensity measurements to these values.

relative intensity
Two dimensional profile of the N2 beam perpendicular to its direction. The plot shows the normalized beam intensity versus its vertical (z) and horizontal (y) position, whichis indicated here as an arbitrary position. The width of the beam, which can be varied by means of the iris diaphragm, is about 10mm. This corresponds to a beam width of 20mm at the instrument interface docking section.
Beam width vs. Iris diaphragm width.
Deconvolution of a beam profuile. Both the experimental data and the model show the 3 mm opening diameter of the molecular beam detector as the horizontal difference between the base and the top line of the profile.