Introduction

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.