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A horizontal dilution refrigerator with high cooling power for a spin frozen target
Nuclear Instruments and Methods 171 (1980) 269.-274 © Nort h-I Iolland t'ublish ing ('o mpany
A HORIZONTAL DILUTION REFRIGERATOR WITH HIGH COOLING POWER FOR A SPIN FROZEN TARGET Shigeru ISIIIMOTO, Shigeru ISAGAWA, Akira MASAIKE and Kimio MORIMOTO A"ational I.aborarorv /or tligh Energy l'hysics. Oho-machi. gsukuha-gun, lbaraki, 305, Japan Received 27 July 1979
A horizontal dilution refrigerator was constructed for a spin frozen proton and deuteron target at KEK (National Laboratory for lligh I'nergy Physics). It was designed to obtain very high cooling capacity over the wide temperature range, esf,ecially at rather high temperatures (50 mK ~ 500 inK). It has a high gas flow rate (typically 4.5 X 10-3 nlol s-I ), a powerful 4|{e precooling system and an adequate heat exchange efficiency between the incoming stream and outgoing solution. It has double-tube heat exchangers, whose lower temperatvre parts are filled with sintered copper. The mixing chamber is made of PFA which is joined with a brass llange by a simple demountablc seal. "Iypical cooling power was about 40 mW and 5 mW at 500inK and 200 mK respectively and the lowest temperature was about 20 inK.
as DRI. It had a hmg stainless steel double-tube heat exchanger (about 5 m hmg) and two coaxial type heat exchangers filled with sitatered copper between the still and the mixing chamber. The cooling power o1" tile refrigerator was 3.7 mW and 50/2W at 400 mK and 50 mK respectively', with the illaximunl gas circulation rate being 5 X 10 -4 tool s -l. It was sufficient for a target mass of about 5 g. Larger target, however, needs higher cooling power between 200 mK and 500 mK to overcome microwave heating. This is achieved by use of a dilution refrigerator which has a higher 3He circulation rate and a better heat exchange efficiency between incoming and outgoing streatns. In tiffs paper we will report on a horizontal dilution refrigerator with much higher cooling power than DRi. Although the basic principle of the design of the new refrigerator is similar to DRI, the still, the heat exchangers, the mixing chamber and the 311e pumping tube are larger than those of DRI. The cooling power was about 4 0 r o w and 5 mW at 5 0 0 m K and 200 inK. respectively. The typical gas circulatkm rate was about 4.5 X 10 -3 tool s -1 at 400 inK.
1. Introduction The application of a dilution refrigerator to a polarized target is quite helpful in the measurement of the several spin dependent parameters of complicated reactions in high energy physics. One of the advantages of the polarized target using a dilution refrigerator is the long nuclear spin--lanice relaxation time realized at quite low temperatures. The hmg relaxation time allows us to use the polarized target in a lower and less homogeneous field than the polarizing one [I,2[. It makes it easy to have a larger access angle for the incident and scattered particles. We can obtain higher polarization in a dilution refrigerator "than in a 3He cryostat because of the lower temperature. It plays a remarkable role with the deuteron polarized target. Deuteron polarization of more than 407c can be obtained by use of the dilution refrigerator [2,3]. Furthermore high polarization of protons is very useful as a proton filter to polarize slow neutrons [4], For tile polarized proton filter very high polarization must be kept quite stable for a long time. Such stability and high polarization of protons can be realized with a dilution refrigerator. Several types of the dilution refrigerators were developed at KEK for these purposes (Nationa.l Laboratory for Iligh Energy Physics) and at CERN [1]. Previously we reported a horizontal dilution refrigerator for a polarized target [5]. It will be referred to
2. Apparatus A schematic view of the dilution refrigerator is shown in fig. 1. The design principle of the refrigerator was to obtain high cooling capacity over a wide temperature 269
S. lshimoto et al. / A horizontal dilution refrigerator
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range, especially at rather high tempe,atures (50 500 inK). It requires high gas tqc+w rate, a powerful 4|-Ie precolling system and an adequate heat exchange efficiency between the incoming concentrated stream and the outgoing dilute solution. A cone st, r,-ounding the refrigerator was used as a high 3He pumping tube for minimizing the flow impedance of the gas. "lhe '~lle system was inside tile cone. The refrigerator had a big still for efficient evaporation of 31te gas and a large space for heat exchangers between the dilute and concentrated streams. Tile flow impedance of the heat exchangers was designed to be quite low and the surface area o f the wall between two streams was large enough to minimize the effect of the Kapitza resistance.
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"file 4He precooling system was located inside tile 3lie pumping tube. Inlet 3He gas was precooled and condensed in tile 41te precooling system. The radiation shields surrounding the cryostat and tile baffles in tile pumping line of 311e gas were also cooled by tile 4tte system. Tile liquid helium was introduced from a container into a 4He gas separator through an " k - t y p e " transfer tube. The separated gas was pumped out through a long tube of 6 mm diameter, cooling the radiation shields in tile insulation vacuum. The liquid 4lie passed through a needle valve and a J - T heat exchanger, and then flowed into a 4He evaporator. The 4He evaporator was located at the end of tile 4lie pumpung tube which had a 15 nlnl diameter pumping orifice. Evaporated gas was pumped out through tile baffles, cooling incoming 31-1e gas. The heat exchanger in the evaporator was made of stainless steel. Its dimensions were 1.3 mm internal diameter and 180 cm long. The flow rate of liquid 3lie was controlled by a capillary tube and a needle valve which were connected in parallel and located between the 4He precooling system and the still• The impedance of the capillary was rather high (1 X 10 I° cm -3) and additional flow was adjusted by the needle valve.
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The still is shown schematically in fig. 2. It was an annular box, the volume of which was about 300 cm 3. It had a cylindrical hole ahmg the central axis for pumping tile vacuum space around the mixing chamber. Evaporated gas in the still was pumped out
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and 0.2 111111wall thickness. The total length of tile wound inner tube was 100cm. Impedance of the tube was ttelpful for recondensation of vapour. The low tentperature part consisted of six coaxial doublc-tt, be heat exchangers filled wilh sinlered copper {second ~ s e v e n t h heal exchangersL Six pieces had almost lhe same sltape. The inner tubes were made of oxygen free pure copper with 3.4 mm inside dianteter, 4 mm outside diameter and 17 cm long. The sintered copper was made as tbllows. The resides of copper tubes were filled with copper powder of
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tltrough a tube which was inserted in the still. The tube had a pumping orifice of 7 trim in diameter on the upper side. The still heat exchanger was made of a copper tube filled and surrounded with sintered copper. 1"he process of making sintered copper will be described below. A coil of manganin wire was put in the still to control the still temperature.
4 0 ~ m , and the outsides of tt,bes were surrounded with 325 ntesh powder in a graphite hoMer whose internal diameter was about 5 ram. l h e y were sintered for 30 rain at 875°C in tl2 gas attnost~here. Powder sizes were cttosen so as to get a good fit to the copper wall. After sintering, the inside of the tube was only half filled with sintered copper because of the shrinkage during the sintering process. "fhe outer tubes were cupro-nickel of 7 111111 inside diameter, 8 111111 outside diameter and 190 cm long. Six pieces of suclt a heat exchanger were cotmected in series with coaxial cupro-nickel tubes.
2.3. Ih'at exchanger 2.4. Mtving chamher A schematic view of heat exchangers between tile still and the mixing chamber is shown in fig. 3. It was carefully designed to allow sufficient 3tie flow. The high tentperature part of ttte heat exchanger (first heat exchanger) consisted of a rather short stainless steel tube of large diameter, in which a wound stainless steel tube was inserted. "fhe diluted stream flowed in a space between the wound tube and the outer tube. The dintensions of outer tubes were 5.5 tnm internal diameter. 6 mm outside diameter and I0 ctn hmg. Ttle concentrated stream flowed in the inner tube which had 0.6 mtn inside diameter
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A target holder ntade of PFA {perfluoroalcoxy "'teflon") was used as a mixing chamber in actual target. "lhe size of tile target bolder was 1.6 X 2 X 6.5 cm 3. It was joined with a brass flange by a simple demtmntable seal * as shown in fig. 4. The beads of target material were put into the targel bolder in a liquid nitrogen bath. Microwave radiation was exposed from outside of the mixing ch~nber. l h e space inside the vacuum jacket serves as a microwave cavity.
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A cylindrical mixing chamber of 60 cm 3 w~lume made of brass was used for the cooling power test. The mixing chamber was sealed by an indium gasket. A calibrated Speer carbon resistor (~~0 -~-~ ~). ,7 I W } was pt,t in it to measure the temperature. Wires from the resistt)r were sealed by Stycast 2850 G I . A manganin wire was wound otttside the chamber as a beater of cooling
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Fig. 3. Schematic view of the heat exchangers bet~vccn the still and the mixing chamber.
2.5. Vacuwn and precooling part Tile mixing chamber and tile heat exchangers were covered wittt a copper vacuum jacket which was * Details of the joint will be given elsewhere.
S. lshimoto et al. / A horizontal dilution refrigerator
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l'ig. 4. "lhe PI.A (tel]on) target holder joined with the brass flange. I.or the sake of convenience the right hand part is shown iJ1 the side view and the let'! hand part is shown in the top view. 1. wavcguide: 2. aluminized mylar (tot microwave shielding); 3. Stycast 2850 (.,'1"seal; 4. NMR coil: 5.PFA {tet'ton) target holder 10.3 mm thickncss):6, tapcred joint ~.ith grease; 7. brass cap screw: 8. concentrated stream inlet tttbe; 9. heater; 10. dilute stream outlet lube.
sealed by an indium gaskel, l h e inside of the jacket was pumped oul through a central tube of the refrigerator, ill which wires for temperature nleasurements, NMR cables and a waveguide were situated and thermally grounded to tile 4He precooling systenl and the still. Ill order to nlininlize the cooling time from 77 K to about lO K. tile still and tile vacul, nljacket of the mixing chanlber were cooled down by an auxiliary 4[le pot wllich was soldered on the flange of tile still. Liquid helit, nl llowed from tile separator to the pot through a needle valve and then into the 4He evaporator. 2.6. P t t m p i n g .~rslem
Tile ~lle pumping system consisted of two-stage Rools punlps (Leybold WS 1000 and WS 500) backed with a rotary pump (Edwards ISC-~)001. 3lie gas was pumped out through an exit t)f 15 Clll in diameter, which was connected to a pumping duct of 20 cm in diameter. The total lengtll of tile pumping line was about 4 m. Tile 4He pumping system consisted of a Roots pump (Shinko-Seiki SMB-1500) backed with a rotary pu nap (Edwards I:.'S-400011E).
3. Operation
"He gas separator. A t'cw To,rs of 4He gas were intr()duced inside tile vacuum jacket as exchange gas. Then alle-411e mixing gas was circulated. File 4He evaporator and tile still were cooled down separately by direct heliunl flows from the "lie gas separator through precooling pipes. Tile flow rates were controlled by needle valves in file middle of the pipes. [t took about 2 h to cool the vacuum jacket and inner parts from 77--10 K. After the needle valves in tile precooling lines were closed, tile excha,lge gas in tile vacuum jacket was pumped out and tile 3He-4lie gas circulation was stopped at this stage, keeping the still temperature at about IOK. It took about 1 h to pump out tile exchange gas completely. Then aHe-4He gas was circulated again. The temperature of the mixing chain-
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Tile refrigerator was first cooled down from the room temperature by liquid nitrogen. After the nitrogen was pumped out, liquid 4lie was supplied into the
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Fig. 5. Tile typical cooling time from 400-60 inK.
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Fig. 6. The cooling power of tim refrigerator. Solid dots show tile results using the pumping system of Leybolt WS I000 and WS 500 backed with a rotary pump of Edwards IS('-900 for 311c 4lie pumping. Open circles shows the results usiug tile pumping systenl of Roots punlps ot" Alcatel MIV 3000 and MIV 350 backed v.'ith a rotary pump of Alcate1206011.
ber went down front l0 K to 500 mK within I I1. Tile typical cooling time from 400--60 mK was about 18 rain as shown in fig. 5. In this case, the power of the heater in the still was constant and the needle valve which controlled 3He-'*He gas flow was completely opened. Tile measured cooling power is given in fig. 6 as a function of the temperature of the mixing chamber. The cooling power of the refrigerator in the case when we used the pumping system of Roots pumps of Alcatel MIV 3000 and MIV 350 backed with a rotary pump of Alcatel 2060t-I for the 3 t i e '*He system, is also shown in fig. 6. The flow rate of 3tie was controlled by the heater in the still and the 31te needle valve. The optimal flow rate was dependent on the temperature of the mixing chamber. The needle valve was closed under 100 inK. The temperalure of about 20 mK was obtained without heat load
in the mixing chamber. In t h i s case. 3lie circulation rate was 7.4 X 10 -4 tool s -1. Typical flow rate and the still temperature are shown in fig. 7. The calculated cooling power Qm in W with an ideal heat exchanger and the measured flow rate iz,~ in tool s -1 is given by, Q ~ = 83fi.~T~Z~ , where 7"m is the temperature in K of tile mixing chamber. Tile measured cooling power at a temperalure higher than 150 mK is about a half of the calculated one. rhis is attributed to the actual heal exchange efficiency, the viscous heating, and other heat leaks to the ntixing chamber. The expected `*lie contamination due to superflt, id flow from the still surface in the circulating stream caused further inefficient cooling of the refrigerator. Consumption of liquid helium was about 4 1 h-I i n normal operation at 200 inK. The performance described above is satisfactory for a spin frozen proton and deuteron target of about 2 0 g of organic material. The cooling power of 100 /aW at 50 mK is large enough to hold the high nuclear polarization for a few days. The refrigerator was first used for the polarization experiment of K*nt -*K*n, K°p reactions at KEK. The authors wish to express their sincere thanks to Dr. S. Hiramatsu for the construction of the NMR system and Dr. T. Nakajima o f ' l o h o k u University for calibrating a carbon resistor. They are much indebted
274
S. lshimoto et al. / A horizontal dilution re.frigerator
to Prof. H. Shigi, Dr. T. K o d a m a o f Osaka ('ity University and Dr. T. Niinikoski o f C I ' R N for their valuable advice. T h e y are also much indebted to all the members o f "KN-group'" at KEK for their co-operation and help. T h e y wish to thank Prof. S. Suwa for his e n c o u r a g e m e n t t h r o u g h o u t this work.
References [1IT. Niinikoski and 1.'. t;do, Nucl. Instr. and Melh. 134 (19761 219. I21 S. Iliramatsu, S. Isagav, a, S. Ishimoto. A. Masaike, K. Morimoto, in preparation. [3] W. de Boer, M. Borghini, K. Morimoto, T. Niinikoski and I. Udo, J. Low Temp. Phys. 15 (,1974) 249. I4I S. Iliramatsu, S. l,~Leawa, S. Ishili)oto, A. Masaike. K. Morimolo, S. I'unahashi, Y. tlanla,euchJ, N. Minakawa and Y. Yamaguchi, J. Phys. Soc. Japan 45 (1978) 949. [51 S. lsagawa, S. Ishimoto, A. Masaike and K. Morimoto, Nucl. Instr. and Melh. 154 (1978)213.
A HORIZONTAL DILUTION REFRIGERATOR WITH HIGH COOLING POWER FOR A SPIN FROZEN TARGET Shigeru ISIIIMOTO, Shigeru ISAGAWA, Akira MASAIKE and Kimio MORIMOTO A"ational I.aborarorv /or tligh Energy l'hysics. Oho-machi. gsukuha-gun, lbaraki, 305, Japan Received 27 July 1979
A horizontal dilution refrigerator was constructed for a spin frozen proton and deuteron target at KEK (National Laboratory for lligh I'nergy Physics). It was designed to obtain very high cooling capacity over the wide temperature range, esf,ecially at rather high temperatures (50 mK ~ 500 inK). It has a high gas flow rate (typically 4.5 X 10-3 nlol s-I ), a powerful 4|{e precooling system and an adequate heat exchange efficiency between the incoming stream and outgoing solution. It has double-tube heat exchangers, whose lower temperatvre parts are filled with sintered copper. The mixing chamber is made of PFA which is joined with a brass llange by a simple demountablc seal. "Iypical cooling power was about 40 mW and 5 mW at 500inK and 200 mK respectively and the lowest temperature was about 20 inK.
as DRI. It had a hmg stainless steel double-tube heat exchanger (about 5 m hmg) and two coaxial type heat exchangers filled with sitatered copper between the still and the mixing chamber. The cooling power o1" tile refrigerator was 3.7 mW and 50/2W at 400 mK and 50 mK respectively', with the illaximunl gas circulation rate being 5 X 10 -4 tool s -l. It was sufficient for a target mass of about 5 g. Larger target, however, needs higher cooling power between 200 mK and 500 mK to overcome microwave heating. This is achieved by use of a dilution refrigerator which has a higher 3He circulation rate and a better heat exchange efficiency between incoming and outgoing streatns. In tiffs paper we will report on a horizontal dilution refrigerator with much higher cooling power than DRi. Although the basic principle of the design of the new refrigerator is similar to DRI, the still, the heat exchangers, the mixing chamber and the 311e pumping tube are larger than those of DRI. The cooling power was about 4 0 r o w and 5 mW at 5 0 0 m K and 200 inK. respectively. The typical gas circulatkm rate was about 4.5 X 10 -3 tool s -1 at 400 inK.
1. Introduction The application of a dilution refrigerator to a polarized target is quite helpful in the measurement of the several spin dependent parameters of complicated reactions in high energy physics. One of the advantages of the polarized target using a dilution refrigerator is the long nuclear spin--lanice relaxation time realized at quite low temperatures. The hmg relaxation time allows us to use the polarized target in a lower and less homogeneous field than the polarizing one [I,2[. It makes it easy to have a larger access angle for the incident and scattered particles. We can obtain higher polarization in a dilution refrigerator "than in a 3He cryostat because of the lower temperature. It plays a remarkable role with the deuteron polarized target. Deuteron polarization of more than 407c can be obtained by use of the dilution refrigerator [2,3]. Furthermore high polarization of protons is very useful as a proton filter to polarize slow neutrons [4], For tile polarized proton filter very high polarization must be kept quite stable for a long time. Such stability and high polarization of protons can be realized with a dilution refrigerator. Several types of the dilution refrigerators were developed at KEK for these purposes (Nationa.l Laboratory for Iligh Energy Physics) and at CERN [1]. Previously we reported a horizontal dilution refrigerator for a polarized target [5]. It will be referred to
2. Apparatus A schematic view of the dilution refrigerator is shown in fig. 1. The design principle of the refrigerator was to obtain high cooling capacity over a wide temperature 269
S. lshimoto et al. / A horizontal dilution refrigerator
270
_o
~ "-'F-
L'20
..~
range, especially at rather high tempe,atures (50 500 inK). It requires high gas tqc+w rate, a powerful 4|-Ie precolling system and an adequate heat exchange efficiency between the incoming concentrated stream and the outgoing dilute solution. A cone st, r,-ounding the refrigerator was used as a high 3He pumping tube for minimizing the flow impedance of the gas. "lhe '~lle system was inside tile cone. The refrigerator had a big still for efficient evaporation of 31te gas and a large space for heat exchangers between the dilute and concentrated streams. Tile flow impedance of the heat exchangers was designed to be quite low and the surface area o f the wall between two streams was large enough to minimize the effect of the Kapitza resistance.
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"file 4He precooling system was located inside tile 3lie pumping tube. Inlet 3He gas was precooled and condensed in tile 41te precooling system. The radiation shields surrounding the cryostat and tile baffles in tile pumping line of 311e gas were also cooled by tile 4tte system. Tile liquid helium was introduced from a container into a 4He gas separator through an " k - t y p e " transfer tube. The separated gas was pumped out through a long tube of 6 mm diameter, cooling the radiation shields in tile insulation vacuum. The liquid 4lie passed through a needle valve and a J - T heat exchanger, and then flowed into a 4He evaporator. The 4He evaporator was located at the end of tile 4lie pumpung tube which had a 15 nlnl diameter pumping orifice. Evaporated gas was pumped out through tile baffles, cooling incoming 31-1e gas. The heat exchanger in the evaporator was made of stainless steel. Its dimensions were 1.3 mm internal diameter and 180 cm long. The flow rate of liquid 3lie was controlled by a capillary tube and a needle valve which were connected in parallel and located between the 4He precooling system and the still• The impedance of the capillary was rather high (1 X 10 I° cm -3) and additional flow was adjusted by the needle valve.
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The still is shown schematically in fig. 2. It was an annular box, the volume of which was about 300 cm 3. It had a cylindrical hole ahmg the central axis for pumping tile vacuum space around the mixing chamber. Evaporated gas in the still was pumped out
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and 0.2 111111wall thickness. The total length of tile wound inner tube was 100cm. Impedance of the tube was ttelpful for recondensation of vapour. The low tentperature part consisted of six coaxial doublc-tt, be heat exchangers filled wilh sinlered copper {second ~ s e v e n t h heal exchangersL Six pieces had almost lhe same sltape. The inner tubes were made of oxygen free pure copper with 3.4 mm inside dianteter, 4 mm outside diameter and 17 cm long. The sintered copper was made as tbllows. The resides of copper tubes were filled with copper powder of
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tltrough a tube which was inserted in the still. The tube had a pumping orifice of 7 trim in diameter on the upper side. The still heat exchanger was made of a copper tube filled and surrounded with sintered copper. 1"he process of making sintered copper will be described below. A coil of manganin wire was put in the still to control the still temperature.
4 0 ~ m , and the outsides of tt,bes were surrounded with 325 ntesh powder in a graphite hoMer whose internal diameter was about 5 ram. l h e y were sintered for 30 rain at 875°C in tl2 gas attnost~here. Powder sizes were cttosen so as to get a good fit to the copper wall. After sintering, the inside of the tube was only half filled with sintered copper because of the shrinkage during the sintering process. "fhe outer tubes were cupro-nickel of 7 111111 inside diameter, 8 111111 outside diameter and 190 cm long. Six pieces of suclt a heat exchanger were cotmected in series with coaxial cupro-nickel tubes.
2.3. Ih'at exchanger 2.4. Mtving chamher A schematic view of heat exchangers between tile still and the mixing chamber is shown in fig. 3. It was carefully designed to allow sufficient 3tie flow. The high tentperature part of ttte heat exchanger (first heat exchanger) consisted of a rather short stainless steel tube of large diameter, in which a wound stainless steel tube was inserted. "fhe diluted stream flowed in a space between the wound tube and the outer tube. The dintensions of outer tubes were 5.5 tnm internal diameter. 6 mm outside diameter and I0 ctn hmg. Ttle concentrated stream flowed in the inner tube which had 0.6 mtn inside diameter
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A target holder ntade of PFA {perfluoroalcoxy "'teflon") was used as a mixing chamber in actual target. "lhe size of tile target bolder was 1.6 X 2 X 6.5 cm 3. It was joined with a brass flange by a simple demtmntable seal * as shown in fig. 4. The beads of target material were put into the targel bolder in a liquid nitrogen bath. Microwave radiation was exposed from outside of the mixing ch~nber. l h e space inside the vacuum jacket serves as a microwave cavity.
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A cylindrical mixing chamber of 60 cm 3 w~lume made of brass was used for the cooling power test. The mixing chamber was sealed by an indium gasket. A calibrated Speer carbon resistor (~~0 -~-~ ~). ,7 I W } was pt,t in it to measure the temperature. Wires from the resistt)r were sealed by Stycast 2850 G I . A manganin wire was wound otttside the chamber as a beater of cooling
FXC'~.,EQ;
Fig. 3. Schematic view of the heat exchangers bet~vccn the still and the mixing chamber.
2.5. Vacuwn and precooling part Tile mixing chamber and tile heat exchangers were covered wittt a copper vacuum jacket which was * Details of the joint will be given elsewhere.
S. lshimoto et al. / A horizontal dilution refrigerator
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l'ig. 4. "lhe PI.A (tel]on) target holder joined with the brass flange. I.or the sake of convenience the right hand part is shown iJ1 the side view and the let'! hand part is shown in the top view. 1. wavcguide: 2. aluminized mylar (tot microwave shielding); 3. Stycast 2850 (.,'1"seal; 4. NMR coil: 5.PFA {tet'ton) target holder 10.3 mm thickncss):6, tapcred joint ~.ith grease; 7. brass cap screw: 8. concentrated stream inlet tttbe; 9. heater; 10. dilute stream outlet lube.
sealed by an indium gaskel, l h e inside of the jacket was pumped oul through a central tube of the refrigerator, ill which wires for temperature nleasurements, NMR cables and a waveguide were situated and thermally grounded to tile 4He precooling systenl and the still. Ill order to nlininlize the cooling time from 77 K to about lO K. tile still and tile vacul, nljacket of the mixing chanlber were cooled down by an auxiliary 4[le pot wllich was soldered on the flange of tile still. Liquid helit, nl llowed from tile separator to the pot through a needle valve and then into the 4He evaporator. 2.6. P t t m p i n g .~rslem
Tile ~lle pumping system consisted of two-stage Rools punlps (Leybold WS 1000 and WS 500) backed with a rotary pump (Edwards ISC-~)001. 3lie gas was pumped out through an exit t)f 15 Clll in diameter, which was connected to a pumping duct of 20 cm in diameter. The total lengtll of tile pumping line was about 4 m. Tile 4He pumping system consisted of a Roots pump (Shinko-Seiki SMB-1500) backed with a rotary pu nap (Edwards I:.'S-400011E).
3. Operation
"He gas separator. A t'cw To,rs of 4He gas were intr()duced inside tile vacuum jacket as exchange gas. Then alle-411e mixing gas was circulated. File 4He evaporator and tile still were cooled down separately by direct heliunl flows from the "lie gas separator through precooling pipes. Tile flow rates were controlled by needle valves in file middle of the pipes. [t took about 2 h to cool the vacuum jacket and inner parts from 77--10 K. After the needle valves in tile precooling lines were closed, tile excha,lge gas in tile vacuum jacket was pumped out and tile 3He-4lie gas circulation was stopped at this stage, keeping the still temperature at about IOK. It took about 1 h to pump out tile exchange gas completely. Then aHe-4He gas was circulated again. The temperature of the mixing chain-
I
.
.
.
.
.
.
.
.
of the refrigerator
Tile refrigerator was first cooled down from the room temperature by liquid nitrogen. After the nitrogen was pumped out, liquid 4lie was supplied into the
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Fig. 5. Tile typical cooling time from 400-60 inK.
S. Ishimoto et al. /,4 horizontal dilution refrigerator
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.
.
.
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Fig. 7. A typical 311e-4lie llov., rate and the still temperature as a function of the temperalt, re c,f tile mixing chamber.
~20mK
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30
400
50(;
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Fig. 6. The cooling power of tim refrigerator. Solid dots show tile results using the pumping system of Leybolt WS I000 and WS 500 backed with a rotary pump of Edwards IS('-900 for 311c 4lie pumping. Open circles shows the results usiug tile pumping systenl of Roots punlps ot" Alcatel MIV 3000 and MIV 350 backed v.'ith a rotary pump of Alcate1206011.
ber went down front l0 K to 500 mK within I I1. Tile typical cooling time from 400--60 mK was about 18 rain as shown in fig. 5. In this case, the power of the heater in the still was constant and the needle valve which controlled 3He-'*He gas flow was completely opened. Tile measured cooling power is given in fig. 6 as a function of the temperature of the mixing chamber. The cooling power of the refrigerator in the case when we used the pumping system of Roots pumps of Alcatel MIV 3000 and MIV 350 backed with a rotary pump of Alcatel 2060t-I for the 3 t i e '*He system, is also shown in fig. 6. The flow rate of 3tie was controlled by the heater in the still and the 31te needle valve. The optimal flow rate was dependent on the temperature of the mixing chamber. The needle valve was closed under 100 inK. The temperalure of about 20 mK was obtained without heat load
in the mixing chamber. In t h i s case. 3lie circulation rate was 7.4 X 10 -4 tool s -1. Typical flow rate and the still temperature are shown in fig. 7. The calculated cooling power Qm in W with an ideal heat exchanger and the measured flow rate iz,~ in tool s -1 is given by, Q ~ = 83fi.~T~Z~ , where 7"m is the temperature in K of tile mixing chamber. Tile measured cooling power at a temperalure higher than 150 mK is about a half of the calculated one. rhis is attributed to the actual heal exchange efficiency, the viscous heating, and other heat leaks to the ntixing chamber. The expected `*lie contamination due to superflt, id flow from the still surface in the circulating stream caused further inefficient cooling of the refrigerator. Consumption of liquid helium was about 4 1 h-I i n normal operation at 200 inK. The performance described above is satisfactory for a spin frozen proton and deuteron target of about 2 0 g of organic material. The cooling power of 100 /aW at 50 mK is large enough to hold the high nuclear polarization for a few days. The refrigerator was first used for the polarization experiment of K*nt -*K*n, K°p reactions at KEK. The authors wish to express their sincere thanks to Dr. S. Hiramatsu for the construction of the NMR system and Dr. T. Nakajima o f ' l o h o k u University for calibrating a carbon resistor. They are much indebted
274
S. lshimoto et al. / A horizontal dilution re.frigerator
to Prof. H. Shigi, Dr. T. K o d a m a o f Osaka ('ity University and Dr. T. Niinikoski o f C I ' R N for their valuable advice. T h e y are also much indebted to all the members o f "KN-group'" at KEK for their co-operation and help. T h e y wish to thank Prof. S. Suwa for his e n c o u r a g e m e n t t h r o u g h o u t this work.
References [1IT. Niinikoski and 1.'. t;do, Nucl. Instr. and Melh. 134 (19761 219. I21 S. Iliramatsu, S. Isagav, a, S. Ishimoto. A. Masaike, K. Morimoto, in preparation. [3] W. de Boer, M. Borghini, K. Morimoto, T. Niinikoski and I. Udo, J. Low Temp. Phys. 15 (,1974) 249. I4I S. Iliramatsu, S. l,~Leawa, S. Ishili)oto, A. Masaike. K. Morimolo, S. I'unahashi, Y. tlanla,euchJ, N. Minakawa and Y. Yamaguchi, J. Phys. Soc. Japan 45 (1978) 949. [51 S. lsagawa, S. Ishimoto, A. Masaike and K. Morimoto, Nucl. Instr. and Melh. 154 (1978)213.