The mechanical design of a gas supply and mixing system for the AMS-02 particle detector onboard the international space station

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The Alpha Magnetic Spectrometer 02 is a particle physics experiment that will search for antimatter, dark matter, and measure cosmic rays in space aboard the International space station for 3 years . It is comprised of an array of sub-detectors: Transition Radiation Detector (TRD); Time of Flight detector; Anti-Coincidence Counter; Silicon Tracker; Ring Imaging Cherenkov counter; Electromagnetic Calorimeter and requires the operation of a cryogenic super conducting magnet at its core. It is built by an international collaboration of more than 100 scientists spread all over Europe, USA and the far East. The TRD that is located above the Cryomagnet and Upper Time of Flight, consists of several layers of straw modules interleaved with a fiber fleece material and arranged in a conical octagon structure built out of a carbon fiber/aluminum honeycomb sandwich. A charged particle traversing this detector produces characteristic electromagnetic radiation in each layer that is measured in the gas filled array of straw tubes. From this, the mass and momentum of the particle can be measured provided the tubes are filled with the proper gas mixture. The TRD gas supply stores 50 kg of gas corresponding to 8100 l Xe and 2000 l CO 2 at 1 atm , filters, mixes, recirculates, and purges a daily supply of Xe/CO 2 (80%/20%) gaseous mixture, thus supplying the TRD with clean, mixed gas for the 3-year ISS mission. Designing and building this reliable, weight optimised system to withstand launch loads and the harsh space environment presented a formidable engineering challenge. Adding to the complexity of the system was that a flexible valve/pump arrangement was needed to control mixture ratio, circulation flow and pressure, and purging. These studies are presented in the paper.