Patent Publication Number: US-8991658-B2

Title: Liquid supply system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2012/050738, filed Jan. 16, 2012, which claims the benefit and priority of Japanese Patent Application No. 2011-056426, filed Mar. 15, 2011 and Japanese Patent Application No. 2011-216621, filed Sep. 30, 2011. The entire disclosures of each of the above applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a liquid supply system for supplying ultracold liquid such as liquid nitrogen and liquid helium. 
     Conventionally, there is a known technique for supplying ultracold liquid such as liquid nitrogen into a vessel, in which a superconducting coil or the like is housed, in order to maintain the superconducting coil or the like in an ultracold state (see Patent Literature 1). With reference to  FIG. 7 , a prior-art liquid supply system as disclosed in Japanese Patent Application Laid-Open No. 2008-215640 will be described.  FIG. 7  is a schematic block diagram showing a state of use of the prior-art liquid supply system. 
     The prior-art liquid supply system  500  constantly supplies ultracold liquid L into a resin vessel  310  in order to maintain a superconducting coil  320  in a superconductive state in a cooled device  300  including the superconducting coil  320  in the vessel  310 . 
     The liquid supply system  500  includes a first vessel  510  for housing the ultracold liquid L, a second vessel  520  disposed in the liquid L housed in the first vessel  510 , and a bellows  530  disposed to enter the second vessel  520 . An area in the second vessel  520  and outside the bellows  530  forms a pump chamber P. The second vessel  520  is provided with an intake port  521  for taking the liquid L into the pump chamber P and a delivery port  522  for delivering the taken-in liquid L from inside the pump chamber P into a supply passage K 1  communicating with an outside of the system. The intake port  521  and the delivery port  522  are respectively provided with one-way valves  521   a  and  522   a.    
     A shaft  550  which is caused to reciprocate by a driving source  540  enters the bellows  530  from outside the first vessel  510  and a tip end of the shaft  550  is fixed to a tip end of the bellows  530 . In this way, when the shaft  550  reciprocates, the bellows  530  expands and contracts. 
     With the above-described structure, when the bellows  530  contracts, a volume of the pump chamber P increases and the liquid L in the first vessel  510  is taken into the pump chamber P through the intake port  521 . When the bellows  530  expands, the volume of the pump chamber P reduces and the liquid in the pump chamber P is delivered into the supply passage K 1  through the delivery port  522 . In this manner, by repetition of expansion and contraction of the bellows  530 , the liquid L is supplied to the cooled device  300  through the supply passage K 1 . A return passage K 2  connecting the liquid supply system  500  and the cooled device  300  is provided as well and the same amount of liquid L as that supplied to the cooled device  300  is returned to the first vessel  510  of the liquid supply system  500 . A cooling device  200  for cooling the liquid L into the ultracold state is provided at a position of the supply passage K 1 . With this structure, the liquid L cooled to an ultracold temperature by the cooling device  200  circulates between the liquid supply system  500  and the cooled device  300 . 
     In the liquid supply system  500  formed as described above, by expansion and contraction of the bellows  530 , the liquid L is supplied intermittently to the cooled device  300  through the supply passage K 1 . In other words, liquid pressure in the supply passage K 1  alternately becomes high and low, which causes what is called pulsations. Therefore, if the resin vessel  310  is formed by bonding two resin molded products together by using an adhesive, a load of pressure due to the pulsations may cause a low-temperature brittle fracture. To cope with this, variation in the pressure is suppressed by providing a damper  600  to the supply passage K 1  in the prior art. 
     However, because the damper  600  is provided to the supply passage K 1  connecting the liquid supply system  500  and the cooled device  300  in the prior art, an extra installation space is required and also heat exchange is carried out at the damper  600  to reduce cooling efficiency. 
     SUMMARY 
     Technical Problem 
     It is an object of the present disclosure to provide a space-saving liquid supply system with increased cooling efficiency. 
     Solution to Problem 
     The present disclosure employs the following means to achieve the above-described object. 
     Specifically, according to the present disclosure, there is provided a liquid supply system including: a first vessel in which ultracold liquid is housed; a second vessel disposed in the liquid housed in the first vessel to take in the liquid and to deliver the taken-in liquid into a supply passage communicating with an outside of the system; a bellows disposed to enter the second vessel; and a shaft formed to be reciprocated by a driving source to cause the bellows to expand and contract, wherein an outside of the bellows in the second vessel serves as a first pump chamber provided with a first intake port for taking the liquid in the first vessel into the first pump chamber and a first delivery port for delivering the taken-in liquid from inside the first pump chamber into the supply passage, and an inside of the bellows serves as a second pump chamber formed by a sealed space and provided with a second intake port for taking the liquid in the first vessel into the second pump chamber and a second delivery port for delivering the taken-in liquid from inside the second pump chamber into the supply passage. 
     According to the present disclosure, the liquid is delivered from inside the second pump chamber into the supply passage and the liquid is taken into the first pump chamber when the bellows contracts while the liquid is taken into the second pump chamber and the liquid is delivered from the first pump chamber into the supply passage when the bellows expands. Therefore, it is possible to double an amount of liquid supplied by the expansion and contraction of the bellows as compared with the case in which the pump function is performed only by the first pump chamber. Moreover, while the liquid is intermittently supplied when the pump function is performed only by the first pump chamber, the liquid is supplied both when the bellows contracts and expands in the invention. Therefore, the liquid is supplied continuously, which suppresses pulsations themselves. As a result, a damper need not be provided outside the system, which saves space as compared with the case in which the damper is provided outside the system and increases cooling efficiency. 
     A sealed space through which the shaft extending from outside the first vessel to reach the bellows is inserted and an inside of which is filled with gas may be formed. 
     In this way, the sealed space filled with the gas exerts heat insulating effect, which suppresses vaporization of the liquid due to heating in the first pump chamber and the second pump chamber. Therefore, it is possible to suppress deterioration of the pump function. 
     A sealed space through which the shaft extending from outside the first vessel to reach the bellows is inserted and an inside of which is evacuated may be formed. 
     In this way, the evacuated sealed space exerts the heat insulating effect, which suppresses vaporization of the liquid due to heating in the first pump chamber and the second pump chamber. Therefore, it is possible to suppress deterioration of the pump function. The evacuated sealed space has more heat insulating effect than the sealed space filled with the gas. 
     A sealed space through which the shaft extending from outside the first vessel to reach the bellows is inserted is formed, a layer of the liquid and a layer of gas are formed in the sealed space, and a branch passage branching off the supply passage is connected to the sealed space to form a buffer structure for buffering pressure variation of the liquid supplied through the supply passage. 
     According to the present disclosure, the buffer structure for buffering the pressure variation (pulsations) of the liquid supplied through the supply passage is provided in the system. Therefore, while saving space and increasing the cooling efficiency, it is possible to suppress the pulsations in cooperation with the above-described suppression of the pulsations themselves in a synergistic manner. Even if transfer of heat from a driving source or the atmosphere to the shaft due to reduction of a liquid level in the first vessel causes vaporization of the inside liquid, it merely increases a thickness of the layer of the gas for performing the buffering function (the function as a gas damper) in the above-described sealed space and vaporization in the pump chamber is suppressed. Therefore, the pump function is not deteriorated. 
     The buffer structure may be provided with a safety valve for allowing internal pressure to escape to the outside when the pressure in the sealed space through which the shaft is inserted becomes equal to or higher than predetermined pressure. 
     In this way, even if the pressure in the sealed space becomes abnormally high due to increase of an amount of the vaporized gas or the like in the sealed space, it is possible to allow the pressure to escape. Therefore, it is possible to suppress breakage or the like of respective members due to abnormally high internal pressure. 
     The sealed space through which the shaft is inserted and the second pump chamber may be separated by a small bellows, the sealed space and an outside space are separated by a small bellows, and both the bellows expand and contract as the shaft reciprocates and have smaller outer diameters than the bellows. 
     In this way, it is possible to form the sealed space through which the shaft is inserted without forming sliding portions, which avoids generation of heat caused by frictional resistance due to sliding. 
     A heater for adjusting a temperature may be provided near the small bellows separating the sealed space and the outside space from each other. 
     In this way, it is possible to suppress (prevent) adhesion of frost and lumps of ice to the small bellows to suppress breakage the small bellows. Moreover, it is possible to adjust thicknesses of the layers of the liquid and the gas in the structure in which the layer of the liquid and the layer of the gas are formed in the sealed space as described above. In this way, it is possible to adjust the thicknesses of the respective layers according to the pulsations which would occur if the damper was not provided to effectively suppress the variation (pulsations) of the pressure. 
     A shaft member and a bearing of the shaft member may be provided below the bellows. 
     In this way, it is possible to suppress displacement of axes of the shaft and the bellows in reciprocation of the shaft. 
     A bottom side of the second vessel and the bellows may be connected by a small bellows which communicates with the inside of the first vessel, expands and contracts as the shaft reciprocates, and has a smaller outer diameter than the bellows. 
     In this way, it is possible to reduce a pump rate of the first pump chamber to reduce a difference from a pump rate of the second pump chamber. Therefore, it is possible to further suppress the pulsations. 
     The above-described respective structures can be employed in combination wherever possible. 
     Advantageous Effects of the Present Disclosure 
     As described above, with the present disclosure, it is possible to increase the cooling efficiency while saving space. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic block diagram showing a state of use of a liquid supply system according to Embodiment 1 of the present disclosure. 
         FIG. 2  is a schematic block diagram showing a state of use of a liquid supply system according to Embodiment 2 of the present disclosure. 
         FIG. 3  is a schematic block diagram showing a state of use of a liquid supply system according to Embodiment 3 of the present disclosure. 
         FIG. 4  is a schematic block diagram showing a state of use of a liquid supply system according to Embodiment 4 of the present disclosure. 
         FIG. 5  is a diagrammatic sectional view of the liquid supply system according to Embodiment 4 of the present disclosure. 
         FIG. 6  is a graph showing pressure variation. 
         FIG. 7  is a schematic block diagram showing a state of use of the prior-art liquid supply system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for carrying out the present disclosure will be specifically described below based on embodiments with reference to the drawings. However, dimensions, materials, shapes, and relative positions of component parts described in the embodiments are not intended to restrict a scope of the invention to only themselves unless otherwise specified. 
     Embodiment 1 
     With reference to  FIG. 1 , a liquid supply system according to Embodiment 1 of the disclosure will be described. 
     &lt;Liquid Supply System&gt; 
     With reference to  FIG. 1 , an overall structure and how to use the liquid supply system  100  according to Embodiment 1 of the present disclosure will be described. In the liquid supply system  100  according to the invention, as in the prior art, supply of ultracold liquid L to a cooled device  300  including a superconducting coil  320  in a rein vessel  310  will be described as an example. Specific examples of the ultracold liquid L are liquid nitrogen and liquid helium. 
     The liquid supply system  100  includes a first vessel  110  for housing the ultracold liquid L, a second vessel  120  disposed in the liquid L housed in the first vessel  110 , and a bellows  130  disposed to enter the second vessel  120 . An area in the second vessel  120  and outside the bellows  130  forms a first pump chamber P 1 . An inside of the bellows  130  is a sealed space and the sealed space serves as a second pump chamber P 2 . The second vessel  120  is provided with a first intake port  121  for taking the liquid L in the first vessel  110  into the first pump chamber P 1  and a first delivery port  122  for delivering the taken-in liquid L from inside the first pump chamber P 1  into a supply passage (supply pipe) K 1  communicating with an outside of the system. The second vessel  120  is also provided with a second intake port  123  for taking the liquid L in the first vessel  110  into the second pump chamber P 2  and a second delivery port  124  for delivering the taken-in liquid L from inside the second pump chamber P 2  into a supply passage K 1 . The first intake port  121  and the second intake port  123  are respectively provided with one-way valves  121   a  and  123   a  and the first delivery port  122  and the second delivery port  124  are respectively provided with one-way valves  122   a  and  124   a.    
     A shaft  150  which is reciprocated by a linear actuator  140  as a driving source enters the bellows  130  from outside the first vessel  110  and a tip end of the shaft  150  is fixed to a tip end of the bellows  130 . In this way, when the shaft  150  reciprocates, the bellows  130  expands and contracts. 
     In the present embodiment, a sealed space R 1  filled with gas is formed around the shaft  150 . The sealed space R 1  is formed by a cylindrical (preferably circular cylindrical) pipe portion  161  through which the shaft  150  extending from outside the first vessel  110  to reach the bellows  130  is inserted and small bellows  162  and  163  respectively provided to a lower end portion and an upper end portion of the pipe portion  161 . The small bellows  162  separating the sealed space R 1  and the second pump chamber P 2  from each other and the small bellows  163  separating the sealed space R 1  and an outside space from each other respectively have tip ends fixed to the shaft  150  and expand and contract as the shaft  150  reciprocates. The small bellows  162  and  163  respectively have smaller outer diameters than the bellows  130 . 
     In the embodiment, the small bellows  162  is provided on the upper end side of the bellows  130  as described above to form the inside of the bellows  130  as the sealed space and this sealed space serves as the second pump chamber P 2  as described above. 
     With the above structure, if the bellows  130  contracts, the liquid L is delivered from inside the second pump chamber P 2  into the supply passage K 1  through the second delivery port  124  and the liquid L is taken into the first pump chamber P 1  through the first intake port  121 . If the bellows  130  expands, the liquid L is taken into the second pump chamber P 2  through the second intake port  123  and the liquid L is delivered from inside the first pump chamber P 1  into the supply passage K 1  through the first delivery port  122 . In this manner, the liquid L is delivered into the supply passage K 1  both when the bellows  130  contracts and expands. 
     As described above, in the liquid supply system  100  according to the embodiment, by repetition of expansion and contraction of the bellows  130 , the liquid L is supplied to the cooled device  300  through the supply passage K 1 . Moreover, a return passage (return pipe) K 2  connecting the liquid supply system  100  and the cooled device  300  is provided as well and the same amount of liquid L as that supplied to the cooled device  300  is returned to the liquid supply system  100 . A cooling device  200  for cooling the liquid L into the ultracold state is provided at a position of the supply passage K 1 . With this structure, the liquid L cooled to an ultracold temperature by the cooling device  200  circulates between the liquid supply system  100  and the cooled device  300 . 
     Advantages of the Liquid Supply System According to the Embodiment 
     As described above, in the liquid supply system  100  according to the embodiment, the inside of the bellows  130  is formed as the sealed space which serves as the second pump chamber P 2 . In this way, the liquid L is delivered into the supply passage K 1  both when the bellows  130  contracts and expands, which doubles the amount of liquid supplied by the expansion and contraction of the bellows  130  as compared with the case in which the pump function is performed only by the first pump chamber P 1 . As a result, it is possible to reduce the amount of liquid supplied at one time by half as compared with the case in which the pump function is performed only by the first pump chamber P 1 , which reduces the maximum pressure of the liquid in the supply passage K 1  by about half. Therefore, it is possible to suppress an adverse influence by pressure variation (pulsations) of the supplied liquid. 
     Moreover, while the liquid L is intermittently supplied when the pump function is performed only by the first pump chamber P 1 , the liquid L is supplied both when the bellows  130  contracts and expands in the embodiment. Therefore, the liquid L is supplied continuously, which suppresses the pulsations themselves. As a result, it is possible to save space as compared with the case in which a damper is provided outside the system, which reduces the portion where the heat exchange is carried out to increase the cooling efficiency. 
     Furthermore, in the embodiment, the inside of the cylindrical pipe portion  161  through which the shaft  150  is inserted is formed as the sealed space R 1  and the sealed space R 1  is filled with the gas. Because the sealed space R 1  filled with the gas performs a function of preventing heat transfer, it is possible to suppress transfer of heat generated in the linear actuator  140  and atmospheric heat to the liquid L. Even if the heat is transferred to the liquid L to vaporize the liquid L, new liquid L is constantly supplied to exert cooling effect, which suppresses increase the temperature of the liquid L in the pump chamber to such a temperature that the liquid L is vaporized. Therefore, deterioration of the pump function can be prevented. 
     Moreover, even if the heat transfer from the shaft  150  or the like causes vaporization of the liquid L in the bellows  130  to generate gas and deteriorates the pump function by the second pump chamber P 2 , the pump function by the first pump chamber P 1  can be performed stably. Furthermore, as compared with the prior art in which the gas (which is compressible fluid) exists inside the bellows  530 , the liquid L (which is incompressible fluid) exists both inside and outside the bellows  130  in the embodiment and therefore it is possible to suppress whirling and buckling of the bellows  130  when the bellows  130  expands and contracts. 
     In the embodiment, the sealed space R 1  is formed by the pipe portion  161  and the pair of small bellows  162  and  163 . Both of the small bellows  162  and  163  have the tip ends fixed to the shaft  150  and expand and contract as the shaft  150  reciprocates. Therefore, the sealed space R 1  is formed without forming sliding portions, which avoids generation of heat caused by frictional resistance due to sliding. 
     Although the sealed space R 1  is filled with the gas in the above-described embodiment, the inside of the sealed space R 1  may be evacuated. By evacuating the inside of the sealed space R 1 , it is possible to further increase heat insulating effect. 
     Embodiment 2 
       FIG. 2  shows Embodiment 2 of the present disclosure. In the present embodiment, a structure in which a small bellows is provided below a bellows will be described. The other structures and operations are the same as those in Embodiment 1 and therefore the same components will be provided with the same reference numerals and will not be described. 
     In the embodiment, a bottom side of a second vessel  120  and a bellows  130  are connected by the small bellows  125  which communicates with an inside of a first vessel  110 , expands and contracts as a shaft  150  reciprocates, and has a smaller outer diameter than the bellows  130 . 
     If the structure shown in Embodiment 1 described above is employed, a pump rate (discharge rate) of the first pump chamber P 1  is greater than a pump rate of the second pump chamber P 2 . For smaller pressure variation (pulsations), it is preferable that a difference between the pump rates is small. 
     Here, a pressure receiving area of an effective diameter of the bellows  130  is represented by S1 and a pressure receiving area of an effective diameter of the small bellows  162  is represented by S2 in Embodiment 1 and Embodiment 2. A pressure receiving area of an effective diameter of the small bellows  125  is represented by S3 in Embodiment 2. And a moving distance of the shaft is represented by L. If the effective diameter of the bellows  130  is represented by D1, the effective diameter of the small bellows  162  is represented by D2, and the effective diameter of the small bellows  125  is represented by D3,
 
 S 1=π×( D 1) 2 ÷4 ,S 2=π×( D 2) 2 ÷4,and  S 3=π×( D 3) 2 ÷4.
 
     In Embodiment 1, the pump rate of the first pump chamber P 1  is S1×L and the pump rate of the second pump chamber P 2  is (S1−S2)×L. 
     In Embodiment 2, on the other hand, the pump rate of the first pump chamber P 1  is (S1−S3)×L and the pump rate of the second pump chamber P 2  is (S1−S2)×L. 
     Therefore, by providing the small bellows  125 , it is possible to reduce the difference between the pump rate of the first pump chamber P 1  and the pump rate of the second pump chamber P 2 . By equalizing S2 and S3 with each other, it is theoretically possible to equalize the pump rate of the first pump chamber P 1  and the pump rate of the second pump chamber P 2  with each other, which further effectively suppresses the pulsations. 
     Embodiment 3 
       FIG. 3  shows Embodiment 3 of the invention. In the embodiment, a case in which a structure for suppressing displacement of axes is provided below a bellows will be described. The other structures and operations are the same as those in Embodiment 1 and therefore the same components will be provided with the same reference numerals and will not be described. 
     In the embodiment, a shaft member  181  is provided to a lower send portion of the bellows  130  and a bearing  182  of the shaft member  181  is provided to a bottom of a second vessel  120 . The bearing  182  is formed by an annular member and a bearing member  182   a  is provided to an inner peripheral portion of a tip end of the bearing  182 . The other structures are the same as those in Embodiment 1 and therefore will not be described. Through holes are preferably provided in a side face of the bearing  182  to allow the liquid L to freely flow into and out of the bearing  182 . In this way, it is possible to suppress obstruction of reciprocation of the shaft  150 . 
     With the above-described structure, in the embodiment, it is possible to suppress displacement of axes of the shaft  150  and the bellows  130 . In this way, it is possible to suppress displacement of the bellows  130  in a radial direction to suppress damage to the bellows  130 . Moreover, it is possible to suppress contact of the shaft  150  with small bellows  162  and  163  to suppress impairment of buffering functions. 
     Because the shaft  150  protrudes below a bottom of the bellows  130 , part of the shaft  150  can function as the shaft member  181 . As shown in an encircled part in  FIG. 3 , a shaft member  181   a  may be formed by permanent magnets and the bearing member  182   a  provided to the tip end of the bearing  182  may be formed by a permanent magnet so that the shaft member  181   a  and the bearing member  182   a  repel each other with magnetic forces. In this way, it is possible to suppress contact between the shaft member  181   a  and the bearing member  182   a  to further suppress the displacement of the axes. Although the shaft member is provided on the bellows  130  side and the bearing is provided to the bottom of the second vessel  120  in the embodiment, the shaft member may be provided to the bottom of the second vessel  120  and the bearing may be provided on the bellows  130  side. Arrangements and the number of shaft members and bearings can be set arbitrarily. For example, the structure shown in the embodiment may be employed in the structure shown in Embodiment 2 described above. In this case, the shaft member and the bearing need to be disposed at positions displaced from a center of the bellows  130  unlike in  FIG. 3  in which the shaft member and the bearing are positioned near the center. 
     Embodiment 4 
     With reference to  FIGS. 4 and 5 , a liquid supply system according to Embodiment 4 of the invention will be described. While the sealed space through which the shaft is inserted is filled with the gas or evacuated in Embodiment 1 described above, a layer of liquid and a layer of gas are formed in the sealed space to function as a gas damper in the embodiment. The other structures and operations are the same as those in Embodiment 1 and therefore the same components will be provided with the same reference numerals and will not be described. 
     In the present embodiment, a buffer structure  160  for buffering variation (pulsations) of pressure of liquid L supplied through the supply passage K 1  is provided around the shaft  150 . The buffer structure  160  includes a cylindrical (preferably circular cylindrical) pipe portion  161  through which a shaft  150  extending from outside a first vessel  110  to reach a bellows  130  is inserted and small bellows  162  and  163  respectively provided to a lower end portion and an upper end portion of the pipe portion  161 . The pipe portion  161  and the pair of small bellows  162  and  163  form a sealed space R 2  inside themselves. The small bellows  162  separating the sealed space R 2  and a second pump chamber P 2  from each other and the small bellows  163  separating the sealed space R 2  and an outside space from each other respectively have tip ends fixed to the shaft  150  and expand and contract as the shaft  150  reciprocates. The small bellows  162  and  163  respectively have smaller outer diameters than the bellows  130 . 
     In the sealed space R 2 , the layer of the liquid L and the layer of the gas G formed by vaporization of the liquid L are formed. In  FIG. 4 , a graph shows a temperature gradient in the sealed space R 2  (X in the drawing). As shown in this graph, a lower portion in the sealed space R 2  is stable at temperature T 1  (about 70 K in a case of liquid nitrogen) and the temperature increases toward an upper portion which is exposed to the outside air. Near a saturation temperature T 0  (about 78 K in the case of liquid nitrogen), an interface between the layer of the liquid L and the layer of the gas G is formed. 
     A branch passage K 3  branching off the supply passage K 1  is connected to the sealed space R 2 . As a result, pressure of the liquid L supplied through the supply passage K 1  is also applied to an inside of the sealed space R 2  and therefore the gas in the sealed space R 2  functions as the damper to buffer the variation (pulsations) of the pressure of the liquid L supplied through the supply passage K 1 . 
     In the buffer structure  160  according to the embodiment, a safety valve  164  for allowing internal pressure to escape to the outside when the pressure in the sealed space R 2  becomes equal to or higher than predetermined pressure is provided near the small bellows  163 . In this way, even if the pressure in the sealed space R 2  becomes abnormally high due to increase of an amount of the vaporized gas G or the like in the sealed space R 2 , it is possible to allow the pressure to escape. Therefore, it is possible to suppress breakage of the pipe portion  161  and the small bellows  162  and  163  due to abnormally high internal pressure. 
     With reference to  FIG. 5 , a more concrete example of the liquid supply system  100  according to the embodiment will be described.  FIG. 5  is a diagrammatic sectional view of the liquid supply system  100  according to the embodiment of the invention disclosure and taken along an axis of the shaft  150 . In the sectional view in  FIG. 5 , a return passage (return pipe) K 2  is not shown. 
     In the example shown in  FIG. 5 , a hollow shaft is employed as the shaft  150 . In this way, it is possible to reduce the shaft  150  in weight. Moreover, because a sectional area is reduced, it is possible to suppress transfer of atmospheric heat to the inside by the shaft  150 . The shaft  150  is provided with a relief hole  151  connecting the inner hollow portion and the outside of the shaft  150 . Therefore, it is possible to suppress breakage of the shaft  150  caused by a sudden rise in the internal pressure due to vaporization of the liquid entering the hollow inside through a crack or the like. 
     In the example shown in  FIG. 5 , heaters  171  and  172  are provided near the small bellows  163  (specifically, in the hollow inside of the shaft  150  and on an outer periphery side near an end portion of the shaft  150  on an atmosphere side). In this way, temperature in the sealed space R 2  can be adjusted and it is possible to suppress (prevent) adhesion of frost and lumps of ice to the small bellows  163  during operation. 
     As described above, according to the liquid supply system  100  in the embodiment, the buffer structure  160  for buffering the variation (pulsations) of the pressure of the liquid L supplied through the supply passage (supply pipe) K 1  is provided in the system. Therefore, as compared with the above-described respective embodiments, it is possible to further suppress the pulsations. 
     In the embodiment, as the buffer structure  160 , the inside of the cylindrical pipe portion  161  through which the shaft  150  is inserted is formed as the sealed space R 2  and the layer of the liquid L and the layer of the gas G are formed in the sealed space R 2 . As a result, the layer of the gas G performs the function of preventing heat transfer and therefore it is possible to suppress transfer of the heat generated in the linear actuator  140  and atmospheric heat to the liquid L. Even if the heat is transferred to the liquid L to vaporize the liquid L, new liquid L is constantly supplied to exert cooling effect, which only results in increase in a thickness of the layer of the gas G for performing the buffering function (the function as the gas damper) in the sealed space R 2 . Therefore, it is possible to suppress increase of the temperature of the liquid L in the pump chamber to such a temperature that the liquid L is vaporized in the pump chamber and deterioration of a pump function can be prevented. In the prior art, if the heat is transferred by the shaft to vaporize the liquid in the second vessel  520 , the generated gas is pushed out or the gas portion is compressed in a compression process of the bellows, thereby leading to reduction in pump efficiency, while this problem does not occur in the embodiment. 
     Furthermore, in the example shown in  FIG. 5 , the heaters  171  and  172  capable of adjusting the temperature in the sealed space R 2  in the pipe portion  161  are provided. Therefore, it is possible to adjust thicknesses of the layer of the liquid L and the layer of the gas G according to the pulsations that would occur if the damper was not provided to effectively suppress the variation (pulsations) of the pressure. 
     In the embodiment, if the small bellows  125  is provided below the bellows  130  as shown in Embodiment 2 described above, it is possible to further suppress the pulsation. Moreover, if the structure for suppressing the displacement of the axes is provided as shown in Embodiment 3 described above, it is possible to suppress the displacement of the axes to allow the damper function to be performed stably. 
     &lt;Amount of Gas in Gas Damper&gt; 
     Here, in the embodiment, an amount of the gas required to cause the inside of the sealed space R 2  to effectively function as the gas damper will be described briefly. 
     &lt;&lt;When Pressure Variation is in Sine Curve Form&gt;&gt; 
     When the pressure variation is in a sine curve form, the amount V1 of the gas required to cause the inside of the sealed space R 2  to effectively function as the gas damper is
 
 V 1 ={q×K ×( Pm÷P 1) 1/n }÷{1−( Pm÷P 2) 1/n }  [1]
 
     Here, q represents a discharge rate [l] per a single reciprocation and K represents a constant according to a pump type and is 0.25 in a case of a single double-action reciprocating pump as in the embodiment. Pm represents discharge average pressure [MPa] and P 1  representing sealed gas pressure is (0.6 to 0.8)×Pm [MPa] when a temperature does not change. For example, P 1 =0.7×Pm [PMa]. n represents a polytropic index and is 1.41 when the gas is nitrogen gas. 
     Furthermore, P 2  represents target maximum pipe internal pressure and
 
 P 2={1+(pulsation rate÷100)}× Pm [Mpa]
 
     The “pipe” corresponds to the supply passage K 1  and the return passage K 2  in the embodiment. The “pulsation rate” refers to a value obtained by dividing a pressure difference between the target maximum pipe internal pressure and the discharge average pressure by the discharge average pressure. In other words, the “pulsation rate”={(P 2 −Pm)÷Pm}×100. 
     &lt;&lt;When Pressure Variation is in Square Wave Form&gt;&gt; 
     When the pressure variation is a square wave form, the amount V2 of the gas required to cause the inside of the sealed space R 2  to effectively function as the gas damper is
 
 V 2 =Va ×( Pa÷P 1)
 
     Here, Pa represents pressure (normal operation pressure) in the pipe (the supply passage K 1  and the return passage K 2 ) when shock pressure is not applied. P 1  is (0.8 to 0.9)×Pa [MPa]. For example, P 1 =0.9×Pa [MPa]. 
     Va representing a gas amount when the pressure is Pa is
 
 Va={W×v   2 ×( n− 1)}÷{200× Pa ×(( Pb/Pa ) (n-1)/n −1}
 
     Here, W represents a fluid mass in the pipes (the supply passage K 1  and the return passage K 2 ) and W=(π/4)×d 2 ×L×ρ×10 −6  [kg]. d represents a diameter (inner diameter) [mm] of the pipes and L represents a length [m] of the pipes, and ρ represents a fluid density [kg/m 3 ]. v represents a flow velocity and v=21.23×Q/d 2  [m/s]. Here, the flow velocity v is an average flow velocity in the supply passage K 1  and the return passage K 2 . Q represents a flow rate [l/min]. n represents a polytropic index and is 1.41 when the gas is nitrogen gas. Furthermore, Pb represents permissible shock pressure and is the maximum permissible shock pressure. The permissible shock pressure Pb is normally set to 110% of the normal operation pressure Pa. In other words, Pb=1.1×Pa [MPa]. 
     Comparison Between Prior Art and Embodiments 
     With reference to  FIGS. 6(   a ) to  6 ( d ), comparison results between pressure variation (pulsations) in the prior art and the above-described respective embodiments will be described. In  FIGS. 6(   a ) to  6 ( d ), the variation in the pressure (vertical axis) with respect to elapsed time (horizontal axis) is shown in graphs. 
       FIG. 6(   a ) shows cases in which the pressure variation is in the sine curve form in the prior art (when the pump function is performed only by the first pump chamber), wherein the left drawing shows a case in which the damper is not provided and the right drawing shows a case in which the damper is provided. 
       FIG. 6(   b ) shows cases in which the pressure variation is in the sine curve form in the embodiment (when the pump function is performed by the first pump chamber and the second pump chamber), wherein the left drawing shows a case in which the damper is not provided (Embodiments 1 to 3) and the right drawing shows a case in which the damper is provided (Embodiment 4). Here, as described above, if the amount of gas is set to an amount satisfying the above-described expression of V1, it is possible to suppress the difference between Pmax and Pmin to 30% or lower (pulsation rate of 30% or lower) as compared with the case in which the damper is not provided. 
       FIG. 6(   c ) shows cases in which the pressure variation is in the square wave form in the prior art (when the pump function is performed only by the first pump chamber), wherein the left drawing shows a case in which the damper is not provided and the right drawing shows a case in which the damper is provided. 
       FIG. 6(   d ) shows cases in which the pressure variation is in the square wave form in the embodiment (when the pump function is performed by the first pump chamber and the second pump chamber), wherein the left drawing shows a case in which the damper is not provided (Embodiments 1 to 3) and the right drawing shows a case in which the damper is provided (Embodiment 4). Here, as described above, if the amount of gas is set to an amount satisfying the above-described expression of V2, it is possible to suppress the difference between Pmax and Pmin to 30% or lower (pulsation rate of 30% or lower) as compared with the case in which the damper is not provided. Although the graphs are simplified in the basic application (Japanese Patent Application No. 2011-56426), to put it more concretely, the pressure rises to reach Pmax for an instant and then drops as shown in  FIG. 6(   d ), if the damper is provided. 
     If the linear actuator drives the shaft  150  with a crank shaft or the like not at a constant velocity, the pressure variation is in a waveform like the sine curve. If the shaft  150  is driven at a constant velocity, the pressure variation is in the square wave form. 
     As is clear from the graphs in  FIGS. 6(   a ) to  6 ( d ), if the pump function is performed by the first pump chamber and the second pump chamber, the pressure variation (pulsations) can be suppressed. It is possible to effectively suppress the pressure variation especially in the case of the square wave. As in Embodiment 4, by providing the damper in the system, it is possible to effectively suppress the pressure variation in cooperation with suppression of the pressure variation itself. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100  liquid supply system 
               110  first vessel 
               120  second vessel 
               121  first intake port 
               122  first delivery port 
               123  second intake port 
               124  second delivery port 
               121   a ,  122   a ,  123   a ,  124   a  one-way valve 
               130  bellows 
               140  linear actuator 
               150  shaft 
               151  relief hole 
               160  buffer structure 
               161  pipe portion 
               162 ,  163  small bellows 
               164  safety valve 
               171 ,  172  heater 
               181 ,  181   a  shaft member 
               182  bearing 
               182   a ,  182   b  bearing member 
               200  cooling device 
               300  cooled device 
               310  vessel 
               320  superconducting coil 
             K 1  supply passage 
             K 2  return passage 
             K 3  branch passage 
             L liquid 
             P 1  first pump chamber 
             P 2  second pump chamber 
             R 1 , R 2  sealed space