In order to maintain a working temperature of 1.8 K for the superconducting cavities, bath cooling with superfluid helium is required. Despite superinsulation of all components and the introduction of an 80 K thermal shield inside the helium cryostat heat loads at 1.8 K total in approx. 113 W, mainly due to the high dissipation energy connected with the cw-regime of the superconducting cavities. Additional 150 W at 80 K are required. The helium plant for the Radiation Source ELBE is based on a modified Claude cycle, specified for 200 W at 1.8 K and 200 W at 80 K respectively.
Compressed helium gas at 1.3 MPa and ambient temperature is cooled down to 4.4 K by passing a combination of counterflow heat exchangers and work extracting expansion turbines. A minor part of the cold helium is branched off and, after passing a further heat exchanger placed directly at the cavity cryostats, expanded to 1.6 kPa (helium vapor pressure corresponding to 1.8 K).
Recompression to atmospheric pressure partially is done by a two stage "cold compression", i. e. the helium gas is partly compressed at low temperatures (approx. 4 K) by means of two centrifugal machines in series. The application of these "cold compressors" implies a lot of sophisticated techniques and therefore up to now was used only in large scale helium plants as e.g. for CERN. Benefits of this solution are savings in size and expenses for the ambient temperature vacuum system and for the low density heat exchangers inside the cold box. For the superconducting cavities are extremely sensitive to microphonics, all components are placed in a separate building with own foundations. The helium cycle compressor is split into a 226 kW and a 60 kW unit thus allowing a flexible adaptation to the actual refrigeration demands (e.g. for stand by / full load mode of the accelerator). Additionally the operation of the plant as liquefier is possible. An outstanding feature of the helium plant for the Radiation Source ELBE is the possibility of a later upgrade to 400 W at 2.1 K in case of increasing refrigeration demand in the future. Even so the cryogenic system represents the fourth in size of this kind all over Germany.
Ch. Haberstroh, H. Quack
Technische Universität Dresden
Institut für Energiemaschinen und Maschinenlabor
Lehrstuhl für Kälte- und Kryotechnik
01062 Dresden, Germany