CASYMIR Performance

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.

 

CASYMIR characteristics

Parameter Comet Churyumov-Gerasimenko CASYMIR
Pressure (mbar) 10-11 -10-5 >10-10
Flow velocity (m/s) 300 - 800 300 - 4000
Flux (cm-2 s-1) 1010- 1017 1012 - 1015
Speed ratio 1-10 1-20

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.