Abstract
METIS, the Mid-infrared E-ELT Imager and Spectrograph, is one of the proposed instruments for the European Extremely Large Telescope (E-ELT) that will cover the thermal/mid-infrared wavelength range from 3-14 m. Its detectors and optics require cryogenic cooling at four temperature levels, 8 K for the N-band detectors, 25 K for the N-band imager, 40 K for the L/M-band detectors and 70 K for the optics. To provide cooling below 70 K, a vibration-free cooling technology based on sorption coolers is developed at the University of Twente in collaboration with Airbus Defence and Space Netherlands B.V. (former Dutch Space B.V.). We propose a sorption-based cooler with three cascaded Joule-Thomson (JT) coolers of which the sorption compressors are all heat sunk at the 70 K platform. A helium-operated cooler is used to obtain the 8 K level with a cooling power of 0.4 W. Here, three pre-cooling stages are used at 40 K, 25 K and 15 K. The latter two levels are provided by a hydrogen-based cooler, whereas the 40 K level is realized by a neon-based sorption cooler. To validate the designs, three demonstrators were built and tested: 1. Full-scale 8 K helium JT cold stage; 2. Scaled helium sorption compressor; 3. Scaled 40 K neon sorption JT cooler. In this paper, we present the design of these demos. We discuss the experiment results obtained so far, the lessons that were learned from these demos and the future development towards a real METIS cooler.
Highlights
The European Extremely Large Telescope (E-ELT) is European Southern Observatorys revolutionary new concept of a ground-based telescope for optical/near-infrared range
Three pre-cooling stages are used at 40 K, 25 K and 15 K. The latter two levels are provided by a hydrogen-based cooler, whereas the 40 K level is realized by a neon-based sorption cooler
Full-scale 8 K helium JT cold stage This demonstration setup focuses on the 15-8 K part of the helium stage in the METIS cooler chain, including the 15-8 K counter flow heat exchanger (CFHX), the 8 K cold heat exchanger (CHX) and the JT restriction
Summary
The European Extremely Large Telescope (E-ELT) is European Southern Observatorys revolutionary new concept of a ground-based telescope for optical/near-infrared range. With its 39-metre primary mirror on the sky, E-ELT will gather 15 times more light than the largest optical telescopes operating at the time of its development and vastly advance astrophysical knowledge. It will allow for detailed observations of among others the first objects in the universe and planets in other star systems [1]. By heating and cooling in a cyclic manner, the adsorbent material adsorbs and desorbs the working fluid It can produce a steady DC flow for the JT cold stage with passive valves regulating the flow direction. Apart from a few passive check valves, sorption JT coolers have no moving parts, which is attractive for a number of reasons: they are vibration-free, EMI-free and have the potential of a long lifetime.
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