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Liquid helium cryostat for target cooling
Cryogenics 42 (2002) 147–148 www.elsevier.com/locate/cryogenics
Technical note
Liquid helium cryostat for target cooling B.V. Elkonin
*
Department of Particle Physics, Weizmann Institute of Science, Rehovot 76100, Israel Received 27 September 2001; accepted 20 November 2001
Abstract A liquid helium cryostat for target cooling down to temperature 1.3 K has been developed. The essential technical details and experimental results are presented. A moving target holder is proposed. Ó 2002 Published by Elsevier Science Ltd. Keywords: Liquid helium; Cryostat; Accelerator
1. Introduction The cryostat has been constructed for the project ISOLDE in the European Organization for Nuclear Research (CERN). The outside diameter of the device is 570 mm, its height – 1250 mm. The cryostat is top loading, a form which is simple and fast to assemble. It is completely demountable – every component can be disassembled and reassembled to facilitate maintenance and repair. We have departed from use of superinsulation because its pumping out and cryostat warming to room temperature are time consuming. The aluminium parts inside the cryostat have been mechanically polished and the stainless steel ones were electropolished. The moving target holder design allows several targets to be used without opening the cryostat.
2. Cryostat design and experimental results The construction of the cryostat is shown in Fig. 1. It has an onion-like structure and consists of vacuum vessel (5), liquid nitrogen bath (4), liquid helium tank (6) with superconducting magnet (9), so-called cryogenic rod (15) and top plate (3). The stainless steel vacuum outer vessel has beam port (10) for connection to the accelerator beam line and two aluminium windows (11) for electron detectors. The stainless steel nitrogen cir*
Fax: +972-8-9344-166. E-mail address: [email protected] (B.V. Elkonin).
cular bath (volume 35 dm3 ) has aluminium thermal radiation shield (8) with a penetration to pass the beam. The stainless steel liquid helium tank (volume 17 dm3 ) incorporates long funnel (7) for better use of helium gas enthalpy during the cooling of the superconducting magnet placed in the lower part of the tank. Both the liquid helium tank and the liquid nitrogen bath are suspended from the stainless steel top plate by three stainless steel hollow supporting rods. The two-coil split pair of magnet is manufactured by Cryomagnetics; the current of 31 A flows intially from a power supply through current leads (1) to the superconducting magnet cooled to temperature 4.2 K to provide the magnetic field of 0.25 T. The liquid helium goes from the liquid helium tank through needle valve (14) and three connected in series capillaries to copper cup (13) of the cryogenic rod; the copper cup forms a small circular bath with two stainless steel tubes brazed to it. The second capillary is bent in the spiral form permitting the cup to move up and down with the help of commercial linear motion feedthrough (2); copper target holder (12) is connected to the cup by two screws. Pumping out the liquid helium in the cup through vacuum outlet (16) enables to cool targets down to temperature 1.3 K during 20 min. (Au + 0.07 at.% Fe)– chromel thermocouple and heater of 50 X are attached to the cup for measuring and controlling the temperature. The heater also serves for target vacuum degassing after filling the cryostat by liquid helium; the duration of the degassing is a function of the target type. The levels of the cyrogens in the helium tank and the nitrogen bath were measured using sets of Allen Bradley resistors.
0011-2275/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 1 - 2 2 7 5 ( 0 1 ) 0 0 1 6 1 - 8
148
B.V. Elkonin / Cryogenics 42 (2002) 147–148
Fig. 1. Cryostat design: (1) current leads; (2) linear motion feedthrough; (3) top plate; (4) liquid nitrogen bath; (5) vacuum vessel; (6) liquid helium tank; (7) funnel; (8) thermal radiation shield; (9) superconducting magnet; (10) beam port; (11) aluminium windows; (12) target holder; (13) copper cup; (14) needle valve; (15) cryogenic rod; (16) vacuum outlet.
The cryostat was pumped down to 5:3 10 4 Pa (4 10 6 Torr). The liquid nitrogen bath was filled with liquid. After 24 h we put the liquid helium into the helium tank; the filling time was 22 min. The average heat load measured by liquid helium boil-off is 0.26 W.
3. Conclusions The liquid helium cryostat for target cooling was built and tested. A special design allows use of several targets during an accelerator run without warming and subsequent opening the cryostat.
Technical note
Liquid helium cryostat for target cooling B.V. Elkonin
*
Department of Particle Physics, Weizmann Institute of Science, Rehovot 76100, Israel Received 27 September 2001; accepted 20 November 2001
Abstract A liquid helium cryostat for target cooling down to temperature 1.3 K has been developed. The essential technical details and experimental results are presented. A moving target holder is proposed. Ó 2002 Published by Elsevier Science Ltd. Keywords: Liquid helium; Cryostat; Accelerator
1. Introduction The cryostat has been constructed for the project ISOLDE in the European Organization for Nuclear Research (CERN). The outside diameter of the device is 570 mm, its height – 1250 mm. The cryostat is top loading, a form which is simple and fast to assemble. It is completely demountable – every component can be disassembled and reassembled to facilitate maintenance and repair. We have departed from use of superinsulation because its pumping out and cryostat warming to room temperature are time consuming. The aluminium parts inside the cryostat have been mechanically polished and the stainless steel ones were electropolished. The moving target holder design allows several targets to be used without opening the cryostat.
2. Cryostat design and experimental results The construction of the cryostat is shown in Fig. 1. It has an onion-like structure and consists of vacuum vessel (5), liquid nitrogen bath (4), liquid helium tank (6) with superconducting magnet (9), so-called cryogenic rod (15) and top plate (3). The stainless steel vacuum outer vessel has beam port (10) for connection to the accelerator beam line and two aluminium windows (11) for electron detectors. The stainless steel nitrogen cir*
Fax: +972-8-9344-166. E-mail address: [email protected] (B.V. Elkonin).
cular bath (volume 35 dm3 ) has aluminium thermal radiation shield (8) with a penetration to pass the beam. The stainless steel liquid helium tank (volume 17 dm3 ) incorporates long funnel (7) for better use of helium gas enthalpy during the cooling of the superconducting magnet placed in the lower part of the tank. Both the liquid helium tank and the liquid nitrogen bath are suspended from the stainless steel top plate by three stainless steel hollow supporting rods. The two-coil split pair of magnet is manufactured by Cryomagnetics; the current of 31 A flows intially from a power supply through current leads (1) to the superconducting magnet cooled to temperature 4.2 K to provide the magnetic field of 0.25 T. The liquid helium goes from the liquid helium tank through needle valve (14) and three connected in series capillaries to copper cup (13) of the cryogenic rod; the copper cup forms a small circular bath with two stainless steel tubes brazed to it. The second capillary is bent in the spiral form permitting the cup to move up and down with the help of commercial linear motion feedthrough (2); copper target holder (12) is connected to the cup by two screws. Pumping out the liquid helium in the cup through vacuum outlet (16) enables to cool targets down to temperature 1.3 K during 20 min. (Au + 0.07 at.% Fe)– chromel thermocouple and heater of 50 X are attached to the cup for measuring and controlling the temperature. The heater also serves for target vacuum degassing after filling the cryostat by liquid helium; the duration of the degassing is a function of the target type. The levels of the cyrogens in the helium tank and the nitrogen bath were measured using sets of Allen Bradley resistors.
0011-2275/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 1 - 2 2 7 5 ( 0 1 ) 0 0 1 6 1 - 8
148
B.V. Elkonin / Cryogenics 42 (2002) 147–148
Fig. 1. Cryostat design: (1) current leads; (2) linear motion feedthrough; (3) top plate; (4) liquid nitrogen bath; (5) vacuum vessel; (6) liquid helium tank; (7) funnel; (8) thermal radiation shield; (9) superconducting magnet; (10) beam port; (11) aluminium windows; (12) target holder; (13) copper cup; (14) needle valve; (15) cryogenic rod; (16) vacuum outlet.
The cryostat was pumped down to 5:3 10 4 Pa (4 10 6 Torr). The liquid nitrogen bath was filled with liquid. After 24 h we put the liquid helium into the helium tank; the filling time was 22 min. The average heat load measured by liquid helium boil-off is 0.26 W.
3. Conclusions The liquid helium cryostat for target cooling was built and tested. A special design allows use of several targets during an accelerator run without warming and subsequent opening the cryostat.