JP5969967B2 - Cryostat pressure control device - Google Patents

Description

  The present invention relates to a pressure control device that is provided in a cryostat that cools a superconducting magnet or the like and controls the pressure of a gas-phase refrigerant in a refrigerant tank provided in the cryostat.

  In a cryostat that cools a superconducting magnet or the like, the refrigerant evaporated by heat entering from the outside into the refrigerant tank containing the refrigerant is recondensed by a refrigerator. However, because the refrigeration capacity of the refrigerator is superior to the amount of heat entering from the outside by design, the gas-phase refrigerant is excessively condensed, and as a result, the pressure of the gas-phase refrigerant in the refrigerant tank is negative. become. When the pressure of the gas-phase refrigerant in the refrigerant tank becomes negative, air enters from a weak seal and causes freezing.

  Therefore, in Patent Document 1, when the pressure of the gas-phase refrigerant in the refrigerant tank becomes negative, the refrigerator is heated by a heater to promote the evaporation of the liquid-phase refrigerant, A superconducting magnet that releases a negative pressure state is disclosed. Further, Patent Document 2 includes a pressure control device that controls the amount of heat of the heater so that the pressure of the gas-phase refrigerant in the refrigerant tank does not become negative, and pressure control in the refrigerant tank by the pressure control device does not work. A superconducting electromagnet is disclosed that prevents the inside of the refrigerant tank from becoming negative pressure in a state where only the refrigerator is working by stopping the operation of the refrigerator when it is in a state.

JP-A-6-283329 JP 2009-267273 A

  By the way, in the pressure control by the heater, the pressure and current including a straight line having a predetermined slope through the point where the pressure value of the gas-phase refrigerant is the reference pressure value and the current value applied to the heater is the reference current value. By substituting the measured value of the pressure sensor for measuring the pressure of the gas-phase refrigerant into the relational expression, the current value applied to the heater is obtained. Here, the reference pressure value and the reference current value are provided for convenience in order to set a relational expression between pressure and current. The heater generates an amount of heat corresponding to the applied current value. The amount of heat generated by the heater balances the refrigerating capacity of the refrigerator and the amount of heat entering the refrigerant tank from the outside, so that the pressure of the gas-phase refrigerant in the refrigerant tank is stabilized.

  The current value applied to the heater is stabilized at a value when the refrigerating capacity of the refrigerator and the amount of heat entering the refrigerant tank from the outside balance with the amount of heat generated by the heater. Therefore, the pressure of the gas-phase refrigerant in the refrigerant tank is stabilized at a value obtained from the current value at the balance and the relational expression between the pressure and the current.

  However, the freezing capacity of the refrigerator changes with time, and also changes due to the difference in frequency between East Japan and West Japan. Further, the amount of heat entering the refrigerant tank from the outside also changes due to environmental changes. If the freezing capacity of the refrigerator or the amount of heat that enters the refrigerant tank from the outside changes, the amount of heat that balances the freezing capacity of the refrigerator and the amount of heat that enters the refrigerant tank from the outside changes, so the current value at the time of balance is Change. As a result, the value at which the pressure of the gas-phase refrigerant in the refrigerant tank is stabilized changes. When the pressure value at the time of stable pressure changes, the uniformity of the magnetic field by the superconducting magnet changes, which affects measurement results such as high resolution NMR.

  An object of the present invention is to provide a cryostat pressure control device capable of suppressing the influence on measurement results such as high-resolution NMR caused by the change in the uniformity of a magnetic field.

  A cryostat pressure control device according to the present invention is provided in a cryostat for recondensing a refrigerant evaporated in a refrigerant tank accommodated in a state in which a superconducting magnet is immersed in a liquid refrigerant by a refrigerator. In the pressure control device that controls the pressure of the refrigerant, a pressure sensor that measures the pressure of the gas-phase refrigerant, a heater that heats the liquid refrigerant by generating an amount of heat according to an applied current value, and the pressure A heat quantity control device that controls the amount of heat generated by the heater by applying a current having a current value corresponding to a measured value of the sensor to the heater, and the heat quantity control device includes a pressure of a gas-phase refrigerant. A value of a reference pressure value Ps, and a current value applied to the heater passes through a point of the reference current value, and a relational expression between pressure and current including a straight line having a predetermined slope, and a measured value of the pressure sensor A current value calculation unit for obtaining a current value to be applied to the heater, a pressure correction coefficient n, an initial reference pressure value Ps0, and a correction amount (Ps0−Pp) / n obtained from the pressure sensor measurement value Pp. A correction unit that corrects the reference pressure value Ps by adding to the reference pressure value Ps and sets the corrected reference pressure value Ps as a new reference pressure value Ps at predetermined intervals; It is characterized by having.

  According to the above configuration, the pressure control is started with the target value of the pressure of the gas-phase refrigerant in the refrigerant tank as the initial reference pressure value Ps0, and the correction amount (Ps0−Pp) / n is used as a reference every predetermined interval. By adding to the pressure value Ps, the reference pressure value Ps is corrected, and the corrected reference pressure value Ps is set as a new reference pressure value Ps. Specifically, first, (1) the pressure value of the gas-phase refrigerant is an initial reference pressure value Ps0, the current value applied to the heater passes through the reference current value point, and includes a pressure including a straight line having a predetermined slope. A current value to be applied to the heater is obtained using a relational expression with the current, and the amount of heat generated by the heater is controlled by applying the current having the current value to the heater. At the same time, the initial reference pressure value Ps0 is corrected to a new reference pressure value Ps. Next, (2) the relationship between the pressure and the current including a straight line having a predetermined slope with the pressure value of the gas-phase refrigerant being a new reference pressure value Ps and the current value applied to the heater passing through the reference current value The amount of heat generated by the heater is controlled by obtaining a current value to be applied to the heater using the equation and applying a current of this current value to the heater. At the same time, the current reference pressure value Ps is corrected to a new reference pressure value Ps. By repeating the above (2), the reference pressure value Ps is cumulatively corrected from the initial reference pressure value Ps0. Thereby, since the reference pressure value Ps changes, the relational expression between the pressure and the current changes.

  Here, the current value applied to the heater is stabilized at a value when the refrigerating capacity of the refrigerator and the amount of heat entering the refrigerant tank from the outside balance with the amount of heat generated by the heater. Therefore, the pressure value when the pressure of the gas-phase refrigerant in the refrigerant tank is stabilized can be obtained from the current value at the balance and the relational expression between the pressure and the current. However, since the relational expression between the pressure and the current changes due to the cumulative correction of the reference pressure value Ps, the pressure value when the pressure is stable changes. When the pressure value at the time of pressure stabilization changes to the initial reference pressure value Ps0, the measured value of the pressure sensor, which is the pressure value at the time of pressure stabilization, becomes the initial reference pressure value Ps0, so the correction amount becomes zero. As a result, the reference pressure value Ps settles to the value when the correction amount becomes zero. Therefore, the relational expression between pressure and current settles to include a straight line passing through a point where the pressure value of the gas-phase refrigerant is the reference pressure value Ps at this time and the current value applied to the heater is the reference current value. As a result, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is the target value.

  Then, no matter what the current value at the time of balancing becomes, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is a value at which the correction amount becomes zero. Therefore, the pressure of the gas-phase refrigerant in the refrigerant tank is stabilized at the initial reference pressure value Ps0 that is the target value even if the current value at the time of balance changes. In this way, by setting the target value of the pressure of the gas-phase refrigerant and stabilizing the pressure of the gas-phase refrigerant in the refrigerant tank at the target value, the uniformity of the magnetic field becomes constant. Thereby, the influence on measurement results, such as high-resolution NMR, resulting from a change in the uniformity of the magnetic field can be suppressed.

  In the cryostat pressure control device according to the present invention, the heat control device may further include a storage unit that stores the reference pressure value Ps. According to the above configuration, the reference pressure value Ps stored in the storage unit is stored even if an unexpected situation occurs in which the reference pressure value Ps disappears from the current value calculation unit by storing the reference pressure value Ps in the storage unit. Using Ps, the cumulative correction of the reference pressure value Ps can be resumed from the middle. Thus, time loss due to unforeseen circumstances can be reduced.

  In the cryostat pressure control apparatus according to the present invention, the correction unit may provide an upper limit value for the correction amount. According to the above configuration, the correction unit provides an upper limit value for the correction amount so that the correction amount does not become too large. As a result, the current value obtained from the pressure sensor measurement value and the relational expression between pressure and current will not be too large or too small, and pressure overshoot will be suppressed. Therefore, stable pressure control with small pressure fluctuation can be performed.

  In the cryostat pressure control apparatus according to the present invention, the correction unit may change the pressure correction coefficient n stepwise. According to said structure, a correction | amendment part increases or reduces the effect by correction | amendment by changing the pressure correction coefficient n in steps. The greater the pressure correction coefficient n, the smaller the effect of the correction and the smaller the pressure fluctuation, but the greater the pressure deviation (difference from the average value). Conversely, the smaller the pressure correction coefficient n, the greater the effect of correction and the greater the pressure fluctuation, but the smaller the pressure deviation. Therefore, by changing the pressure correction coefficient n stepwise from large to small, it is possible to stabilize the pressure with a target value with high accuracy after stabilizing the pressure to some extent while suppressing pressure fluctuation. Further, by changing the pressure correction coefficient n stepwise from small to large, it is possible to perform stable pressure control while suppressing pressure fluctuations after quickly stabilizing the pressure to a certain level.

  In the cryostat pressure control apparatus according to the present invention, the correction unit may stop the cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. According to said structure, a correction | amendment part stops the accumulation correction | amendment of the reference pressure value Ps, when predetermined conditions are satisfied. When the time required for the pressure to stabilize to a certain extent or sufficiently elapses, by canceling the cumulative correction of the reference pressure value Ps and performing pressure control independent of the correction, fine pressure fluctuations caused by the correction can be obtained. It can be prevented from occurring. Further, when there is an instruction from the operator, the cumulative correction of the reference pressure value Ps is stopped, so that the cumulative correction can be stopped at a timing desired by the operator.

  In the cryostat pressure control apparatus according to the present invention, the correction unit may start cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. According to the above configuration, the correction unit starts cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. Accumulation correction can be started at a desired timing by starting the accumulation correction of the reference pressure value Ps when an instruction from the operator is given or when a predetermined time has elapsed. Further, when there is an instruction from the operator after canceling the cumulative correction, by restarting the cumulative correction of the reference pressure value Ps, even when the pressure value at the time of the pressure stabilization after the stop is deviated from the target value. The pressure of the gas-phase refrigerant in the refrigerant tank can be stabilized at the target value.

  In the cryostat pressure control device according to the present invention, the heat quantity control device further includes a current upper limit value setting unit that sets an upper limit value of a current value applied to the heater, and the current upper limit value setting unit A moving average may be calculated using a predetermined number of current values applied to the heater, and a value obtained by adding the predetermined value to the moving average may be set as an upper limit value of the current value applied to the heater. According to the above configuration, the current upper limit value setting unit calculates a moving average using a predetermined number of current values applied to the heater most recently, and applies a value obtained by adding the predetermined value to the moving average to the heater. Set as the upper limit of the current value. By providing an upper limit value for the current value applied to the heater, the current value obtained from the pressure sensor measurement value and the relational expression between pressure and current will not be too large, and pressure overshoot will be suppressed. Since the error between the value and the target value becomes small, stable pressure control with small pressure fluctuation can be performed.

  According to the cryostat pressure control apparatus of the present invention, the reference pressure value Ps is cumulatively corrected, so that even if the current value at the time of balance changes, the pressure of the gas-phase refrigerant in the refrigerant tank is an initial value that is the target value. The reference pressure value Ps0 is stabilized. Thus, by setting the target value of the pressure and stabilizing the pressure of the gas-phase refrigerant in the refrigerant tank at the target value, the uniformity of the magnetic field becomes constant. Thereby, the influence on measurement results, such as high-resolution NMR, resulting from a change in the uniformity of the magnetic field can be suppressed.

It is a side view which shows the internal structure of a cryostat. It is a circuit diagram of a pressure control device. It is an example of the relational expression of a pressure and an electric current. It is the figure which simulated the time change of the pressure of helium gas. It is the figure which simulated the time change of heater current. It is an example of the relational expression of a pressure and an electric current. It is the figure which simulated the time change of the pressure of helium gas. It is the figure which simulated the time change of heater current. It is a circuit diagram of a pressure control device.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

(Composition of cryostat)
A cryostat pressure control device (pressure control device) 100 according to an embodiment of the present invention is provided in a cryostat 50. As shown in FIG. 1, the cryostat 50 includes a helium tank (refrigerant tank) 2 that stores liquid helium, which is a liquid refrigerant, and a refrigerator 5 provided above the helium tank 2. The cryostat 50 of the present embodiment is used for an NMR apparatus, but is not limited to this, and may be used for an MRI apparatus, for example. Further, the refrigerant is not limited to helium.

  The helium tank 2 is provided with a gas discharge port (not shown). This gas discharge port is a path through which helium gas evaporates when the capacity of the refrigerator 5 is lost, and is provided at the upper end of a cylindrical member 13 described later. A check valve for preventing air from entering the helium tank 2 from the outside is attached to the tip of the gas discharge port. This check valve has a performance capable of sufficiently safely processing a large amount of helium gas generated when a superconducting magnet 1 described later is quenched. Therefore, even if the helium gas in the helium tank 2 is cooled by the refrigerator 5 and liquefied, the total amount of helium in the helium tank 2 does not change. In order to prevent air from entering the helium tank 2, the pressure in the helium tank 2 is controlled to a positive pressure slightly higher than the atmospheric pressure. Examples of the material of the helium tank 2 include aluminum and stainless steel.

  A superconducting magnet 1 is accommodated in the helium tank 2 while being immersed in liquid helium. The superconducting magnet 1 is formed by spirally winding a superconducting wire around a winding frame (not shown). The superconducting wire may be a metal-based superconducting wire or an oxide-based superconducting wire. A cylindrical space S (bore) extending in the vertical direction is provided at the center of the helium tank 2. A sample is placed in the cylindrical space S, and various analyzes and experiments are performed. In the helium tank 2, a gas space 10 filled with helium gas is provided above the liquid helium surface in which the superconducting magnet 1 is immersed.

  The helium tank 2 is surrounded by a radiation shield 3. The radiation shield 3 is a shield container that is cooled by the cold heat of the helium gas so as not to let the cold heat escape. Further, the radiation shield 3 is forcibly cooled by a first cooling stage 6 described later of the refrigerator 5. Examples of the material of the radiation shield 3 include aluminum and copper.

  Further, the helium tank 2 and the radiation shield 3 are accommodated in a vacuum vessel 4. The vacuum container 4 is a container whose inside is maintained at a high vacuum and suppresses heat intrusion into the superconducting magnet 1 and the helium tank 2. A neck member 12 having a cylindrical member 13 inside is attached to the upper portion of the vacuum vessel 4. The cylindrical member 13 is used as a passage for inserting a current lead (not shown) or a replenishment passage for liquid helium into the helium tank 2. The vacuum vessel 4 is supported on the floor by a plurality of stands 9. Examples of the material of the vacuum vessel 4 include aluminum and stainless steel.

  The refrigerator 5 is for reliquefaction (recondensation) of liquid helium evaporated in the helium tank 2, and a pulse tube refrigerator is used in this embodiment. A first cooling stage 6 (1nd stage) is provided in the middle part of the refrigerator 5 in the vertical direction, and a second cooling stage 7 (2nd stage) is provided at the lower end of the refrigerator 5. Each of the first cooling stage 6 and the second cooling stage 7 has a flange shape, and is cooled by the refrigerator 5 to, for example, about 40K and about 4K, respectively. The material of the first cooling stage 6 and the second cooling stage 7 is mainly copper or a copper alloy. The refrigerator 5 is not limited to the pulse tube refrigerator, and may be a GM refrigerator, a Stirling refrigerator, or the like.

  The lower part including the second cooling stage 7 in the refrigerator 5 is accommodated in the cylindrical member 15. A tubular member 16 is further disposed outside the tubular member 15. The internal space of the tubular member 15 is a recondensing chamber 8, and the recondensing chamber 8 and the helium tank 2 are communicated with each other by a tubular communicating member 14 having a smaller diameter than the tubular member 15.

(Configuration of pressure control device)
The pressure control device 100 is provided in the cryostat 50 and controls the pressure of the helium gas in the helium tank 2. The pressure control device 100 includes a pressure sensor 21 attached to the branch pipe 17 connected to the cylindrical member 16 and a heater 22 attached to the superconducting magnet 1.

  The pressure sensor 21 measures the pressure of helium gas in the recondensing chamber 8. In the present embodiment, the pressure sensor 21 is a strain gauge. The pressure sensor 21 may be provided at the gas discharge port at the upper end of the cylindrical member 13 or in the helium tank 2 so as to measure the pressure of the helium gas in the gas phase space 10. Also good. Further, the pressure sensor 21 is not limited to a strain gauge. Moreover, the pressure sensor 21 is not limited to the structure which measures a pressure continuously, The structure which measures a pressure intermittently with a certain period may be sufficient. For example, the pressure sensor 21 may be configured to measure the pressure at intervals of 1 minute in accordance with the microcomputer 26 described later correcting the reference pressure value every minute.

  The heater 22 heats liquid helium by generating an amount of heat corresponding to the applied current value. The heated liquid helium evaporates into helium gas. Thereby, the pressure of the helium gas in the helium tank 2 increases. The heater 22 may be attached to the refrigerator 5 or the like so as to indirectly heat liquid helium.

  Here, in the cryostat 50, the liquid helium evaporated by the heat that has entered the helium tank 2 from the outside is recondensed by the refrigerator 5. Here, if the refrigerating capacity of the refrigerator 5 is inferior to the amount of heat entering from the outside, the evaporation of liquid helium cannot be stopped. Therefore, the refrigeration capacity of the refrigerator 5 is superior to the amount of heat entering from the outside by design. However, since the refrigerating capacity of the refrigerator 5 is superior to the amount of heat entering from the outside, the helium gas is excessively condensed, and as a result, the pressure of the helium gas in the helium tank 2 becomes negative. When the pressure of the helium gas in the helium tank 2 becomes a negative pressure, air enters from a weak sealing property and causes freezing.

  Therefore, the liquid helium is heated by the heater 22 to promote the evaporation of the liquid helium, and the pressure of the helium gas in the helium tank 2 is controlled to be a positive pressure slightly higher than the atmospheric pressure. That is, the amount of heat generated by the heater 22 balances the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside.

  As shown in FIG. 2, which is a circuit diagram, the pressure control device 100 includes a heat quantity control device 23. This heat quantity control device 23 is provided on the circuit board and provided outside the cryostat 50, and the heater 22 is generated by applying a current having a current value corresponding to the measured value of the pressure sensor 21 to the heater 22. The amount of heat to be controlled is controlled.

  The heat quantity control device 23 includes a converter 24, an A / D converter 25, a microcomputer 26, a setting device 27, a D / A converter 28, and an amplifier 29. The converter 24 converts the change in electrical resistance measured by the pressure sensor 21 that is a strain gauge into a voltage value. The A / D converter 25 converts an analog voltage value into a digital value. The microcomputer (current value calculation unit) 26 obtains a current value to be applied to the heater 22 by substituting the measured value of the pressure sensor 21 into a relational expression between pressure and current. The setter 27 sets a value used to set a relational expression between pressure and current used in the microcomputer 26. The D / A converter 28 converts a digital current value into an analog value. The amplifier 29 amplifies the current value. The heater current having the amplified current value is applied to the heater 22.

  Here, an example of a relational expression between pressure and current is shown in FIG. This relational expression passes through a point A where the pressure value of helium gas is the reference pressure value (4 kPa) and the current value applied to the heater 22 is the reference current value (30 mA), and the straight line C having an inclination of 12 [mA / kPa]. Is included. Here, the reference pressure value and the reference current value are provided for convenience in order to set a relational expression between pressure and current. The heating effect of the heater 22 is the same whether the current value is positive or negative. Therefore, the current value is raised by the reference current value so that the current value becomes the reference current value when the pressure of the helium gas in the helium tank 2 is the reference pressure value, and the current value when the pressure is 6.5 kPa or more. Is set to 0, and pressure control is always performed with a positive current value (including 0). Further, by setting an upper limit value for the heater current so that the current value is constant at 90 mA when the pressure is −1 kPa or less, damage to the heater 22 and wiring burnout are prevented.

  The setting device 27 (see FIG. 2) includes a reference pressure value (4 kPa) that is a pressure value at point A, a reference current value (30 mA) that is a current value at point A, and a slope of a straight line C (12 [mA / kPa). ]). Thereby, the straight line C is set. The heat quantity control device 23 controls the amount of heat generated by the heater 22 by applying a current having a current value obtained by the microcomputer 26 to the heater 22. Thereby, the pressure of the helium gas in the helium tank 2 is controlled. Such pressure control stabilizes the pressure of the helium gas with time.

  A diagram simulating the time change of the pressure of helium gas is shown in FIG. After about 30 hours from the initial value of 2 kPa, the pressure of the helium gas in the helium tank 2 is stable. Moreover, the figure which simulated the time change of heater current is shown in FIG. After about 30 hours, the heater current is asymptotic to 20 mA and stabilized. At this time, the amount of heat generated when 20 mA is applied to the heater 22 balances the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside. 4 and 5, in order to balance the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside, the current value applied to the heater 22 does not change at 20 mA (is constant). Modeled.

  Theoretically, the amount of heat generated by the heater 22 at the time of balance can be obtained from the difference between the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside, but in the actual cryostat 50, the heater The amount of heat generated by 22 cannot be directly detected.

  As can be seen from FIG. 5, the current value (20 mA) at which the heater current is stabilized is not the reference current value (30 mA) shown in FIG. In FIG. 3, the pressure value at the point B where the straight line D having a current value of 20 mA and the straight line C intersect with each other is 4.83 kPa. This value is not the reference pressure value (4 kPa). Thus, the reference pressure value is originally provided for convenience in order to set a relational expression between the pressure and the current together with the reference current value, and is a value at which the pressure of the helium gas in the helium tank 2 is stabilized. is not.

  Here, the refrigeration capacity of the refrigerator 5 changes with time, and also changes depending on the frequency difference between eastern Japan and western Japan. Further, the amount of heat that enters the helium tank 2 from the outside also changes due to environmental changes. When the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside change, the amount of heat for balancing the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside changes. The current value changes. Thereby, the value at which the pressure of the helium gas in the helium tank 2 is stabilized changes. When the pressure value at the time of stable pressure changes, the uniformity of the magnetic field by the superconducting magnet 1 changes, which affects measurement results such as high resolution NMR.

  Therefore, the microcomputer (correction unit) 26 shown in FIG. 2 corrects the reference pressure value at predetermined intervals. That is, the microcomputer 26 corrects the reference pressure value Ps by adding the correction amount (Ps0−Pp) / n to the reference pressure value Ps, and sets the corrected reference pressure value Ps as a new reference pressure value Ps. This is performed at predetermined intervals. Here, n is a pressure correction coefficient, Ps0 is an initial reference pressure value, and Pp is a measured value of the pressure sensor 21. In the present embodiment, the microcomputer 26 corrects the reference pressure value Ps every minute. Therefore, the correction amount is calculated every minute.

  In the present embodiment, the reference pressure value Ps is corrected every minute, but this interval is not limited to one minute. However, in the conventional control without correction, as shown in FIG. 4, the pressure is stabilized over approximately one day, so that one minute can be said to be fine enough as a correction cycle.

  An example of the relational expression between pressure and current is shown in FIG. In the pressure control with correction, the pressure control is started with the target value as the initial reference pressure value Ps0 (4 kPa). Then, by adding the correction amount (Ps0−Pp) / n to the reference pressure value Ps every minute, the reference pressure value Ps is corrected, and the corrected reference pressure value Ps is set as a new reference pressure value Ps. To do.

  Specifically, first, (1) the pressure value of helium gas is an initial reference pressure value Ps0 (4 kPa), the current value applied to the heater 22 passes through the point A of the reference current value (30 mA), and a predetermined slope ( 12 [mA / kPa]) using the relational expression between the pressure and the current including the straight line C having a current of 12 [mA / kPa]), a current value to be applied to the heater 22 is obtained, and the heater current of this current value is applied to the heater 22 to 22 controls the amount of heat generated. At the same time, the initial reference pressure value Ps0 is corrected to a new reference pressure value Ps.

  Next, (2) the pressure value of helium gas is a new reference pressure value Ps, the current value applied to the heater 22 passes through the point of the reference current value (30 mA), and a predetermined slope (12 [mA / kPa]) is obtained. A current value to be applied to the heater 22 is obtained using a relational expression between pressure and current including a straight line, and the amount of heat generated by the heater 22 is controlled by applying the heater current of this current value to the heater 22. At the same time, the current reference pressure value Ps is corrected to a new reference pressure value Ps.

  By repeating the above (2), the reference pressure value Ps is cumulatively corrected from the initial reference pressure value Ps0. Thereby, since the reference pressure value Ps changes, the current value applied to the heater 22 is the reference current value (30 mA) when the pressure value of the helium gas is the reference pressure value Ps and the current value applied to the heater 22 is the reference current value. It moves from point A along a certain straight line E. Accordingly, a straight line having a predetermined slope (12 [mA / kPa]) passing through the point of the reference current value when the pressure value of the helium gas is the reference pressure value Ps and the current value applied to the heater 22 is along the straight line E. It moves from the straight line C.

  Here, the current value applied to the heater 22 is stabilized at 20 mA, which is a value when the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside are balanced by the amount of heat generated by the heater 22. Therefore, the pressure value when the pressure of the helium gas in the helium tank 2 is stabilized is obtained from the current value (20 mA) at the time of balance and the relational expression between the pressure and the current. On the straight line C, the pressure value at the point B stabilizes the pressure of the helium gas in the helium tank 2. However, since the relational expression between the pressure and the current changes due to the cumulative correction of the reference pressure value Ps, the pressure value when the pressure is stable moves from the point B along the straight line D where the current value is 20 mA. Go.

  When the pressure value when the pressure is stable changes to the initial reference pressure value Ps0, that is, when the point B changes to the point G, the measured value of the pressure sensor 21 which is the pressure value when the pressure is stable is the initial reference pressure value. Since Ps0, the correction amount becomes zero. Thereby, the reference pressure value Ps settles to a value when the correction amount becomes zero, that is, a pressure value at the point F. Accordingly, a straight line having a predetermined slope (12 [mA / kPa]) passing through the point where the pressure value of the helium gas is the reference pressure value Ps and the current value applied to the heater 22 is the reference current value is a straight line passing through the point F. Settle to H. As a result, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is the target value. Thereafter, the amount of heat generated by the heater 22 is controlled using a relational expression between the pressure and the current including the straight line H.

  Here, when the amount of heat for balancing the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside changes, the current value at the time of balance changes. However, even if the current value at the time of balance changes, the pressure value at the time of stable pressure settles down to the initial reference pressure value Ps0 that is the target value.

  For example, when the current value at the time of balancing changes to 10 mA, the correction amount becomes zero when the straight line C moves to the straight line I. As a result, the straight line through which the pressure value of the helium gas is the reference pressure value Ps and the current value applied to the heater 22 passes through the point of the reference current value is settled to the straight line I, and the pressure value when the pressure is stable is the initial value that is the target value. The reference pressure value Ps0 is settled.

  Thus, no matter what the current value at the time of balancing becomes, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is a value at which the correction amount becomes zero. Therefore, the pressure of the helium gas in the helium tank 2 is stabilized at the initial reference pressure value Ps0 that is the target value even if the current value at the time of balance changes. Thus, by setting the target value of the pressure of the helium gas so that the pressure of the helium gas in the helium tank 2 is stabilized at the target value, the uniformity of the magnetic field becomes constant. Thereby, the influence on measurement results, such as high-resolution NMR, resulting from a change in the uniformity of the magnetic field can be suppressed.

  FIG. 7 shows a diagram simulating the time change of the pressure of the helium gas when the pressure is controlled while accumulating the reference pressure value Ps. FIG. 7A shows a pressure correction coefficient n of 60, FIG. 7B shows a pressure correction coefficient n of 120, and FIG. 7C shows a pressure correction coefficient n of 600. Further, FIG. 8 shows a diagram simulating the temporal change of the heater current when the pressure control is performed while accumulating the reference pressure value Ps. 8A shows a pressure correction coefficient n of 60, FIG. 8B shows a pressure correction coefficient n of 120, and FIG. 8C shows a pressure correction coefficient n of 600. 7 and 8, the current value applied to the heater 22 is not changed (constant) at 20 mA in order to balance the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside. Modeled.

  In FIG. 7, when the pressure correction coefficient n is 60, the pressure overshoot is the largest and the fluctuation cycle is the smallest. As the pressure correction coefficient n increases, the pressure overshoot decreases and the fluctuation period increases. Here, the refrigeration capacity of the refrigerator 5 varies depending on the size of the refrigerator 5 and the like. Further, the amount of heat that enters the helium tank 2 from the outside varies depending on the size of the cryostat 50 and the like. Therefore, the pressure correction coefficient n needs to be changed according to the size of the cryostat 50 and the size of the refrigerator 5. However, in the general cryostat 50 and the refrigerator 5, as shown in FIG. 7, it takes about 30 hours for the pressure of the helium gas to be stabilized. It is about three times 600 minutes (n = 600). Therefore, it can be seen that in the general cryostat 50 and the refrigerator 5, the pressure correction coefficient n is preferably about 600.

  As shown in FIG. 2, the microcomputer 26 includes a memory (storage unit) 26a. The memory 26a stores a reference pressure value Ps. The memory 26a is preferably a non-volatile memory that can be electrically rewritten and does not lose its memory even in a non-energized state. Note that the memory 26a may be configured to maintain the storage with power from a battery or the like.

  As shown in FIG. 7, it takes about one day for the pressure of the helium gas to become stable and suitable for measurement such as high resolution NMR. In the meantime, if the microcomputer 26 is accidentally turned off or a power failure occurs, the accumulated reference pressure value Ps disappears, and the accumulated correction is restarted from the beginning, resulting in a maximum time loss of about one day. Become.

  Therefore, the cumulatively corrected reference pressure value Ps is stored in the memory 26a at intervals of 10 minutes or 1 hour. As a result, even if an unexpected situation occurs in which the cumulatively corrected reference pressure value Ps disappears from the microcomputer 26, the cumulative correction of the reference pressure value Ps is performed halfway using the reference pressure value Ps stored in the memory 26a. You can resume. Thus, time loss due to unforeseen circumstances can be reduced. Note that the memory 26a is not limited to the configuration for storing the cumulatively corrected reference pressure value Ps, and may be configured to store the cumulative correction amount from the initial reference pressure value Ps0.

  The microcomputer 26 (see FIG. 2) provides an upper limit value for the correction amount. The correction amount per time is (Ps0−Pp) / n. However, in the initial stage of pressure control, the difference between the reference pressure value Ps and the measured value Pp of the pressure sensor 21 is large, and thus the correction amount increases. . Also, when the pressure correction coefficient n is set too small, the correction amount increases. When the correction amount is large, the current value obtained from the measured value of the pressure sensor 21 and the relational expression between the pressure and the current becomes abnormally large or becomes zero. Therefore, by providing an upper limit value for the correction amount, the correction amount is prevented from becoming too large. As a result, the current value obtained from the measured value of the pressure sensor 21 and the relational expression between the pressure and the current does not become too large or too small, the pressure overshoot is suppressed, and the pressure value and the target Since the error from the value becomes small, stable pressure control with small pressure fluctuation can be performed. Note that the microcomputer 26 may provide an upper limit value for the current value applied to the heater 22 instead of providing an upper limit value for the correction amount. In this case, the current value applied to the heater 22 may become zero, but since the current value applied to the heater 22 does not become abnormally large, the same effect can be obtained.

  Further, the microcomputer 26 changes the pressure correction coefficient n step by step. As shown in FIG. 7, the greater the pressure correction coefficient n, the smaller the effect of the correction, and the smaller the pressure fluctuation, but the greater the pressure deviation (difference from the average value). Conversely, the smaller the pressure correction coefficient n, the greater the effect of correction and the greater the pressure fluctuation, but the smaller the pressure deviation. Further, if the correction effect is reduced by increasing the pressure correction coefficient n, the pressure of the helium gas in the helium tank 2 greatly fluctuates due to a change in the state of the refrigerator 5 or the cryostat 50 or a change in the environmental pressure. However, since the correction amount is small, the resistance force and response force against these disturbances are reduced, and the pressure value when the pressure is stable deviates from the target value.

  Therefore, if you want to stabilize the pressure to a certain level while suppressing the pressure fluctuation, but then want to stabilize the pressure accurately with the target value, increase the pressure correction coefficient n at the initial stage of pressure control to reduce the pressure fluctuation. The pressure correction coefficient n is decreased to reduce the pressure deviation after the pressure has been stabilized to a certain level. In this way, by changing the pressure correction coefficient n stepwise from large to small, it is possible to stabilize the pressure with a target value with high accuracy after stabilizing the pressure to some extent while suppressing pressure fluctuation.

  In addition, when it is desired to stabilize the pressure to a certain degree quickly, but thereafter to suppress the pressure fluctuation, the pressure correction coefficient n is reduced in the initial stage of pressure control to reduce the pressure deviation, and the pressure is maintained to some extent. After stabilization, the pressure correction coefficient n is increased to reduce the pressure fluctuation. As described above, by changing the pressure correction coefficient n stepwise from small to large, it is possible to perform stable pressure control while suppressing pressure fluctuation after quickly stabilizing the pressure to a certain level.

  Further, the microcomputer 26 stops the cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. As shown in FIG. 7, when about one day has passed, the pressure of the helium gas in the helium tank 2 is stabilized. Therefore, the microcomputer 26 stops the cumulative correction of the reference pressure value Ps when a predetermined time (for example, two days) which is a time required for the pressure to stabilize to a certain extent or sufficiently elapses, and thereafter the correction is performed. The conventional pressure control that does not depend is performed. Here, after the accumulation correction is stopped, the refrigerating capacity of the refrigerator 5 and the amount of heat entering the helium tank 2 from the outside or the ambient atmospheric pressure fluctuates, so that the pressure value at the time of pressure stabilization becomes the target. May deviate from the value. In conventional pressure control, control is not performed so that the pressure value at stable pressure becomes the target value, so even if the pressure value at stable pressure deviates from the target value, it cannot be corrected to the target value. Therefore, it is possible to prevent the pressure variation from occurring.

  Note that a stop button that can be operated by the operator is provided, and the microcomputer 26 may stop the cumulative correction when the operator operates the stop button. In this case, the cumulative correction can be stopped at a timing desired by the operator.

  Further, the microcomputer 26 starts cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. The pressure value at the time of stable pressure deviates from the target value due to fluctuations in the refrigerating capacity of the refrigerator 5 or the amount of heat entering the helium tank 2 from the outside or fluctuations in the atmospheric pressure after stopping the cumulative correction. There is. Therefore, a start button that can be operated by the operator is provided, and when this start button is operated by the operator's judgment, the microcomputer 26 resumes the cumulative correction. Thereby, the pressure of helium gas can be stabilized at the target value.

  Note that the microcomputer 26 may not start the correction when the pressure control is started, and may start the correction when the start button is operated or when a predetermined time has elapsed. In this case, cumulative correction can be started at a desired timing.

  The microcomputer (current upper limit setting unit) 26 sets an upper limit value of the current value applied to the heater 22. This upper limit value is a value obtained by adding a predetermined value to the moving average calculated using a predetermined number of current values applied to the heater 22 most recently. In the present embodiment, the upper limit value is a value obtained by adding 5 mA to the moving average calculated using the 30 current values applied to the heater 22 most recently. The predetermined number is not limited to 30, and the predetermined value is not limited to 5 mA.

  For example, if the current value of the heater current calculated at the start of pressure control is 10 mA, the upper limit value of the heater current is 15 mA. In the examples of FIGS. 5 and 8, the pressure is controlled in the direction of decreasing the heater current. On the contrary, when the pressure is controlled in the direction of increasing the heater current, the current value in the initial stage of the pressure control. Overshoots in an increasing direction. In the process, the current value reaches a peak at 15 mA. Further, 30 minutes after the start of the pressure control, the moving average is calculated using the 30 current values applied to the heater 22 most recently. When the moving average is 13 mA, the upper limit value of the heater current is 18 mA. Thereafter, the moving average is updated at 1-minute intervals, and the upper limit value of the heater current gradually increases. Thus, by providing the upper limit value for the heater current, the current value obtained from the measured value of the pressure sensor 21 and the relational expression between the pressure and the current does not become too large, and the pressure overshoot is suppressed. Since the error between the pressure value and the target value becomes small, stable pressure control can be performed.

(Modification)
In addition, as shown in FIG. 9 which is a circuit diagram, the calorie | heat amount control apparatus 23 which the pressure control apparatus 100 has may be comprised by the analog circuit. That is, the heat quantity control device 23 includes a current value calculation circuit 31, a correction circuit 32, a memory 33, and a current upper limit value setting circuit 34 in addition to the converter 24 and the setting device 27 described above. . The current value calculation circuit (current value calculation unit) 31 is a circuit that determines the relationship between pressure and current, using the measurement value of the pressure sensor 21 as an input and the current value applied to the heater 22 as an output. The correction circuit (correction unit) 32 corrects the reference pressure value Ps. The memory (storage unit) 33 is a circuit such as a capacitor that stores the reference pressure value Ps. The current upper limit value setting circuit (current upper limit value setting unit) 34 sets an upper limit value of the current value applied to the heater 22. The heater current having the current value calculated by the current value calculation circuit 31 is applied to the heater 22.

  The pressure sensor 21 continuously measures the pressure of helium gas. The correction circuit 32 continuously corrects the reference pressure value Ps using the measurement value of the pressure sensor 21. Here, the pressure correction coefficient n is a gain (magnification) in the circuit configuration. The cumulative correction of the reference pressure value Ps is performed by an integration circuit. If conditions such as the pressure correction coefficient n are not frequently adjusted, the pressure control device 100 can be manufactured in large quantities at low cost by configuring the heat control device 23 with an analog circuit.

(effect)
As described above, according to the pressure control apparatus 100 according to the present embodiment, the pressure control is started with the target value of the pressure of the helium gas in the helium tank 2 as the initial reference pressure value Ps0, and every minute, By adding the correction amount (Ps0−Pp) / n to the reference pressure value Ps, the reference pressure value Ps is corrected, and the corrected reference pressure value Ps is set as a new reference pressure value Ps. Since the relational expression between the pressure and the current changes due to the cumulative correction of the reference pressure value Ps, the pressure value when the pressure is stable changes. When the pressure value at the time of pressure stabilization changes to the initial reference pressure value Ps0, the measured value of the pressure sensor 21, which is the pressure value at the time of pressure stabilization, becomes the initial reference pressure value Ps0, so the correction amount becomes zero. . As a result, the reference pressure value Ps settles to the value when the correction amount becomes zero. Therefore, the relational expression between the pressure and the current settles to include a straight line passing through a point where the pressure value is the reference pressure value Ps at this time and the current value is the reference current value. As a result, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is the target value. Then, no matter what the current value at the time of balancing becomes, the pressure value at the time of stable pressure settles to the initial reference pressure value Ps0 that is a value at which the correction amount becomes zero. Therefore, the pressure of the helium gas in the helium tank 2 is stabilized at the initial reference pressure value Ps0 that is the target value even if the current value at the time of balance changes. Thus, by setting the target value of the pressure of the helium gas so that the pressure of the helium gas in the helium tank 2 is stabilized at the target value, the uniformity of the magnetic field becomes constant. Thereby, the influence on measurement results, such as high-resolution NMR, resulting from a change in the uniformity of the magnetic field can be suppressed.

  Further, by storing the reference pressure value Ps in the memory (storage unit) 26a, even if an unexpected situation occurs in which the reference pressure value Ps disappears from the microcomputer (current value calculation unit) 26, the memory 26a stores it. The accumulated correction of the reference pressure value Ps can be resumed from the middle using the reference pressure value Ps. Thus, time loss due to unforeseen circumstances can be reduced.

  The microcomputer (correction unit) 26 sets an upper limit value for the correction amount so that the correction amount does not become too large. As a result, the current value obtained from the measured value of the pressure sensor 21 and the relational expression between the pressure and the current does not become too large or too small, the pressure overshoot is suppressed, and the pressure value and the target Since the error from the value becomes small, stable pressure control with small pressure fluctuation can be performed. Note that the same effect can be obtained by providing an upper limit value for the current value applied to the heater 22 instead of providing an upper limit value for the correction amount.

  Further, the microcomputer (correction unit) 26 increases or decreases the effect of the correction by changing the pressure correction coefficient n in a stepwise manner. The greater the pressure correction coefficient n, the smaller the effect of the correction and the smaller the pressure fluctuation, but the greater the pressure deviation (difference from the average value). Conversely, the smaller the pressure correction coefficient n, the greater the effect of correction and the greater the pressure fluctuation, but the smaller the pressure deviation. Therefore, by changing the pressure correction coefficient n stepwise from large to small, it is possible to stabilize the pressure with a target value with high accuracy after stabilizing the pressure to some extent while suppressing pressure fluctuation. Further, by changing the pressure correction coefficient n stepwise from small to large, it is possible to perform stable pressure control while suppressing pressure fluctuations after quickly stabilizing the pressure to a certain level.

  Further, the microcomputer (correction unit) 26 stops the cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. When the time required for the pressure to stabilize to a certain extent or sufficiently elapses, by canceling the cumulative correction of the reference pressure value Ps and performing pressure control independent of the correction, fine pressure fluctuations caused by the correction can be obtained. It can be prevented from occurring. Further, when there is an instruction from the operator, the cumulative correction of the reference pressure value Ps is stopped, so that the cumulative correction can be stopped at a timing desired by the operator.

  Further, the microcomputer (correction unit) 26 starts cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. Accumulation correction can be started at a desired timing by starting the accumulation correction of the reference pressure value Ps when an instruction from the operator is given or when a predetermined time has elapsed. Further, when there is an instruction from the operator after canceling the cumulative correction, by restarting the cumulative correction of the reference pressure value Ps, even when the pressure value at the time of the pressure stabilization after the stop is deviated from the target value. The pressure of the helium gas in the helium tank 2 can be stabilized at the target value.

  Further, the microcomputer (current upper limit setting unit) 26 calculates a moving average using a predetermined number of current values applied to the heater 22 most recently, and applies a value obtained by adding the predetermined value to the moving average to the heater 22. Set as the upper limit of the current value. By providing an upper limit value for the current value to be applied to the heater 22, the current value obtained from the measured value of the pressure sensor 21 and the relational expression between the pressure and the current is not excessively increased, and the pressure overshoot can be suppressed. Since the error between the pressure value and the target value becomes small, stable pressure control with small pressure fluctuation can be performed.

(Modification of this embodiment)
The embodiment of the present invention has been described above, but only specific examples are illustrated, and the present invention is not particularly limited, and the specific configuration and the like can be appropriately changed in design. Further, the actions and effects described in the embodiments of the invention only list the most preferable actions and effects resulting from the present invention, and the actions and effects according to the present invention are described in the embodiments of the present invention. It is not limited to what was done.

1 Superconducting magnet 2 Helium tank (refrigerant tank)
DESCRIPTION OF SYMBOLS 3 Radiation shield 4 Vacuum container 5 Refrigerator 6 1st cooling stage 7 2nd cooling stage 8 Recondensing chamber 10 Gas phase space 12 Neck member 13 Cylindrical member 14 Communication member 15 Cylindrical member 16 Cylindrical member 17 Branch pipe 21 Pressure sensor 22 heater 23 heat quantity control device 24 converter 25 A / D converter 26 microcomputer (current value calculation unit, correction unit, current upper limit value setting unit)
26a Memory (storage unit)
27 Setting Device 28 D / A Converter 29 Amplifier 31 Current Value Calculation Circuit (Current Value Calculation Unit)
32 Correction circuit (correction unit)
33 Memory (storage unit)
34 Current upper limit setting circuit (current upper limit setting section)
50 Cryostat 100 Pressure control device

Claims (7)

In a pressure control device that is provided in a cryostat for recondensing a refrigerant evaporated in a refrigerant tank containing a superconducting magnet immersed in a liquid refrigerant with a refrigerator, and controls a pressure of a gas-phase refrigerant in the refrigerant tank ,
A pressure sensor for measuring the pressure of the gas phase refrigerant;
A heater that heats the liquid refrigerant by generating an amount of heat corresponding to the applied current value;
A calorific value control device that controls the amount of heat generated by the heater by applying a current having a current value corresponding to a measurement value of the pressure sensor to the heater;
Have
The calorific value control device
The pressure value of the gas phase refrigerant is the reference pressure value Ps, the current value applied to the heater passes through the point of the reference current value, and a relational expression between the pressure and the current including a straight line having a predetermined slope, and the pressure sensor A current value calculation unit for obtaining a current value to be applied to the heater using the measured value;
By adding a correction amount (Ps0−Pp) / n obtained from the pressure correction coefficient n, the initial reference pressure value Ps0, and the measured value Pp of the pressure sensor to the reference pressure value Ps, the reference pressure value Ps is obtained. A correction unit that corrects and sets the corrected reference pressure value Ps as a new reference pressure value Ps at predetermined intervals;
A pressure control device for a cryostat, comprising:   The cryostat pressure control device according to claim 1, wherein the heat quantity control device further includes a storage unit that stores the reference pressure value Ps.   The cryostat pressure control device according to claim 1, wherein the correction unit has an upper limit value for the correction amount.   The cryostat pressure control apparatus according to claim 1, wherein the correction unit changes the pressure correction coefficient n stepwise.   5. The cryostat pressure control apparatus according to claim 1, wherein the correction unit stops the cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. 6.   6. The cryostat pressure control apparatus according to claim 1, wherein the correction unit starts cumulative correction of the reference pressure value Ps when a predetermined condition is satisfied. 7. The calorific value control device further includes a current upper limit value setting unit for setting an upper limit value of a current value applied to the heater,
The current upper limit setting unit calculates a moving average using a predetermined number of current values applied to the heater most recently, and a value obtained by adding the predetermined value to the moving average is an upper limit of the current value applied to the heater. It sets as a value, The pressure control apparatus of the cryostat of any one of Claims 1-6 characterized by the above-mentioned.

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