GB2463869A - An Arrangement for Cooling Fixed Current Leads of a Cryogenic Apparatus - Google Patents

Abstract

The arrangement is for cooling fixed current leads 30 of a cryogenically cooled apparatus 1 housed within a cryogen vessel 20, and the arrangement has a separate cryogen reservoir 10 which, in use, is partially filled with liquid cryogen. The cryogen reservoir is linked with the cryogen vessel, and the link includes an opening 22 located in a venturi-like constriction 12. The link provides part of a gas escape path 22, 23, 24, which extends between the cryogen reservoir and a turret 13. A first heating element 14 is located inside the cryogen reservoir, and in use the heating element is located in contact with liquid cryogen. During ramping the gas flow through the venturi creates a low pressure region near the opening to the cryogen vessel, and causes a reduced pressure in the cryogen vessel leading to a reduction in cryogen vessel temperature. The arrangement may also include a second heating element located within the cryogen reservoir, but located above the level of liquid. The second heating element can be used for heating cryogen gas, and neither heater introduces heat into the cryogen vessel. The cryogen can be helium, or other cryogens such as nitrogen, hydrogen, neon, etc.

Description

AN ARRANGEMENT FOR COOLING A CRYOGEN VESSEL

Superconducting magnets typically comprise coils of superconductive wire housed within a cryogen vessel, itself within a cryostat, and cooled to cryogenic temperatures such that, in use, the coils of superconductive wire exhibit superconductivity. Typically, cooling is achieved by partial immersion of the wire coils in a liquid cryogen such as liquid helium.

A superconducting magnet is typically energised by connecting a low voltage, high current power supply across suitable input terminals. The input terminals are typically linked by a superconducting switch, which is held in a resistive state during application of energising current to the magnet windings. The energisation process generates large amounts of heat in non-superconducting current leads. The temperature of those portions of current leads which enter the cryostat must be controlled to prevent excessive temperature rises. Typically, fixed current leads in a superconducting magnet system are manufactured from materials that are chosen for their relatively low thermal conductivity, such as stainless steel, to reduce the thermal load to the low temperature magnet environment during operation of the magnet.

Known arrangements may employ fixed current leads, which are permanently attached to the magnet, or removable current leads which are withdrawn from contact with the magnet when they are not being used to introduce or withdraw current. The present invention relates to fixed current leads.

A typical solution to prevent excessive temperature rises in fixed current leads during magnet energisation is to ensure there is a sufficient outflow of gaseous cryogen past the current leads, removing heat. This gas flow may result from thermal influx to the cryogen vessel from the surrounding environment.

The resistive heating due to the introduction of current into the magnet through resistive current leads acts to increase the temperature and pressure inside the cryogen vessel and eventually causes a pressure-limiting valve to open to allow cooling gas egress past the current leads.

During the energisation ("ramping") process the pressure inside the cryogen vessel rises above the equilibrium pressure. This leads to an increased cryogen vessel temperature during magnet energisation which reduces the current-carrying capacity of the superconducting wire. If the pressure rise and therefore temperature rise is too high, the reduction in current carrying capacity may prevent the magnet from being energised to

a desired magnetic field strength.

The invention provides methods and apparatus for cooling a superconducting magnet, such that the fixed current leads are cooled whilst the pressure and temperature inside the cryogen vessel is reduced or maintained at a low value. This provides at least some of the following advantages: maintains high current carrying capacity of the superconductive coils; enables reduced cryogen vessel temperature during magnet energisation; and so reduces a risk of quenching during magnet energisation.

Accordingly the present invention provides methods and apparatus as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will become apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings wherein: Fig. 1 shows an example arrangement for cryogen vessel cooling during magnet energisation, according to an embodiment of the present invention; Fig. 2 shows results of simulations performed to show the effects of differing temperatures of effluent gas, and differing diameters of effluent gas flow path, in reducing the temperature inside a cryogen vessel; Figs. 3 and 4 show alternative embodiments of the arrangement for cryogen vessel cooling; Fig. 5 shows an arrangement for cryogen vessel cooling, according to the present invention, placed within a cryogen vessel; and Fig. 6 shows an arrangement for cryogen vessel cooling, according to the present invention, placed on a side of a cryogen vessel.

The present invention will be descrthed with particular reference to helium as the cryogen. However, as will be apparent to those skilled in the art, other cryogens such as nitrogen, hydrogen, argon, neon could be used, largely as determined by the characteristics of the wire of the magnet coil(s), without departing the scope of the present invention.

Fig. 1 rather schematically illustrates an example of the present invention.

In a certain embodiment, the invention consists of a superconducting MRI magnet 1 within a helium vessel 20, provided with a separate helium reservoir 10. An access turret 13 is provided to the cryogen vessel 20 containing the magnet 1, with one or more fixed current lead 30 running through the turret into the cryogen vessel.

As shown, a helium reservoir 10 is provided, comprising a lower part, partially filled with liquid helium, and an upper part which is provided with a valve 23 allowing egress of helium gas. A helium recondensing refrigerator 17 may be provided. Between the upper part and the lower part is a venturi-like constriction 12. At the constriction 12 is a direct connection 24 to the helium vessel 20. This is preferably provided by a tube 18, akin to a Pitot tube but with a downstream-facing opening 22.

The helium reservoir 10 may be located within the cryogen vessel 20 or, preferably, may be located outside the cryogen vessel. Particular proposed embodiments will be discussed in more detail below with reference to Figs. and 6.

In normal steady-state operation, the helium reservoir 10 and the helium vessel 20 are isobaric and are sealed from the outside world by a differential pressure valve 23. If the pressure inside the helium reservoir and helium vessel 20 exceeds a predetermined value, the differential pressure valve 23 will open and helium gas 16 will escape from the system, removing heat from the fixed current leads 30 as it does so.

During magnet energisation, a heating element 14 inside the helium reservoir 10 is run at a constant or near constant rate to generate large quantities of helium gas 16 from the helium liquid inside the reservoir 10.

The boiled-off helium gas 16 increases the pressure within the helium reservoir 10, and opens the valve 23. The boiled-off helium gas 16 passes through the venturi-like constriction 12 and then passes the fixed current leads, providing a cooling effect. The increase of helium gas velocity through the venturi-like constriction 12 reduces the pressure at the opening 22 to the helium vessel 20 because of the Bernoulli principle. The low pressure region in the venturi-like constriction 12 causes a pressure drop inside the helium vessel 20 and so reduces the magnet temperature, avoiding reduction in the current-carrying capacity of the superconducting wire of the magnet 1.

The reduced temperature inside the helium vessel 20 will avoid, or reduce, the reduction in current-carrying capacity of the superconducting magnet coils and so will lead to reduced risk of quenching during magnet energisation. A secondary heater 26 may be positioned inside the gas volume of the helium reservoir 10. The gas heater 26 allows the temperature of the helium gas 16 to be increased to allow more efficient flow of the gas 16 through the venturi 12 to create a larger pressure drop and so abigger temperature drop in the helium vessel 20.

At the end of the magnet energisation process the heater(s) 14, 26 is/are turned off and the gas flow 16 inside the helium reservoir 10 reduces until the differential pressure valve 23 closes, at which point the helium reservoir 10 and the helium vessel 20 will again be isobaric.

Simulations have been performed to determine the pressure drop expected across the venturi-like constriction 12, with certain values of diameters Dl and D2. These dimensions, the gas flow rate and the gas temperature are all input variables in this simulation.

In general, the pressure drop scales as a function of the gas density, itself a function of temperature, the square of the volumetric flow rate and inversely with d4 (where d is the diameter of the flow path in the constriction, assumed to have a circular cross-section). As is typical with a venturi-type arrangement, the pressure drop increases as the ratio of D1ID2 increases.

From the calculated pressure drop, the resultant reduction in gas temperature within the helium vessel 20 can be deduced from the saturation properties of helium.

An example calculation to determine the pressure drop across the venturi-like constriction 12 maybe summarised as follows: Assume: 1) Dl > D2, for example Dl = 0.05 m and D2 = 0.0029 m 2) the helium gas passing through the venturi-like constriction is at a temperature of 30 K 3) the volume of liquid helium lost during ramp = 20 litres 4) ramping of current into the magnet, from zero to full operating current takes 30 mins 5) the volumetric flow rate of effluent helium gas during ramp = 1.11x105m3/s (through opening 22) 6) the density of liquid helium at 4.2 K = 125.01 kg/m3 7) the density of helium gas at 30 K= 1.6006 kg/m3 (giving a ratio of helium gas to liquid density = 78.10) These figures are used to provide a calculated volumetric flow through the venturi-like constriction, during current ramping = 86.78x1ft5 m3/s (through the constriction 12).

The pressure drop across the venturi-like constriction is calculated as: P1 -P2 = 13.814 kPa (2.00 psi).

In the described example, P1, the pressure in the turret 13 causing the pressure relief valve 23 to open at a differential pressure of 15.8 psia (109 kPa), which equates to a helium gas temperature of 4.30 K. The pressure P2 at the outlet 22 of the tube 18 within the venturi-like constriction is calculated to bereduced by 2 psia (13.8 kPa), which equates to a helium bath temperature of 4.16 K. The pressure drop across a venturi constriction may be calculated by the formula: Q = A1*A2 * sqrt[2(P1-P2)/p(A12-A22)] Where: Q is the mass flow rate through the venturi constriction; Al and A2 are the cross-sectional areas of the tubes (elsewhere represented by their diameters Dl and D2); and p is the density of the fluid flowing through the venturi constriction (which will itself vary with the pressure).

By exposing the interior of the helium vessel 20 to the reduced pressure within the venturi-like constriction, the temperature within the helium vessel may correspondingly be reduced to 4.16K. In this way, the current carrying capacity of the superconducting windings forming the superconducting magnet is increased, and the likelihood of quench during ramping is reduced.

The heat introduced by heater(s) 14, 26 arises in the helium reservoir 10, and is carried away by the effluent helium gas 16. The heater(s) 14, 26 accordingly do not introduce any heat into the helium vessel 20.

Under the assumption that the gas flow rate through the venturi-like constriction 12 should approximate that experienced during energisation of a known magnet system, further calculations have been performed to deduce the likely pressure drop across the venturi-like constriction for a range of helium gas temperatures. From these values, and assuming a start pressure of 16.0 psia (110.3 kPa), which is determined from helium saturation properties to be equivalent to a helium temperature of 4.3K, the temperature expected in the helium vessel 20 can be deduced.

Results are shown below tabulated, and are shown graphically in figure 2.

Using a diameter of the helium reservoir Dl = 50 mm; and a helium gas pressure P1 within the turret 13 = 16 psia (110.3 kPa), the following possible temperature reduction effects within the helium vessel 20 are predicted, with a selection of temperatures of helium gas flowing through the venturi-like constriction and a selection of diameters (D2) of the venturi-like constriction: T=30K T=20K T=1OK D2 [mm] dT [mK] dT [mK] dT [mK] 2.0 715 441 200 2.5 257 165 78 3.0 118 77 37 3.5 62 41 20 4.0 36 24 11 4.5 22 15 7 5.0 15 10 5 Clearly, the most effective cooling of the helium vessel 20 is achieved with a small D2, and a higher temperature of the effluent gas 16. As helium has a particularly large thermal expansion coefficient, the volume flow rate of 30K helium will be significantly larger than the volume flow rate of 10K helium, leading to an increased reduction in pressure within the helium vessel, and so to a more reduced temperature within the helium vessel.

Fig. 2 graphically illustrates these results, showing the temperature drop from 4.3K achieved within the helium vessel 20 when helium gas at 10K, 20K and 30K flows through the venturi-like constriction of varying diameters D2. From Fig. 2, and the tabulated results above, it can be seen that the gas temperature drop within the helium vessel 20 is a strong function of gas temperature in the venturi-like constriction 12.

A temperature drop of 250 mK, leading to a significant improvement in the current carrying capacity of the superconducting coils, can be achieved by passing 30K helium gas through a venturi-like constriction of diameter 2.5 mm, assuming typical values for volumetric flow rate.

Fig. 3 shows an alternative embodiment of the present invention, in which the opening 22' of the connection 24 to the helium vessel is not pointed downstream in the effluent gas flow path, but is pointed perpendicular to the path of effluent gas 16. The venturi-like constriction 12 will cause effluent gas to flow at an increased velocity past the opening, causing reduced helium gas pressure at the opening, in turn causing reduced temperature within the helium vessel.

Fig. 4 shows another alternative embodiment of the present invention. As with the embodiment of Fig. 3, the opening 22' of the helium vessel is not -10-pointed downstream of the effluent gas, but is pointed perpendicular to the path of effluent gas 16. Furthermore, the venturi-like constriction is absent. Even the resulting flow in this arrangement, at a volume flow rate higher than the flow of gas in tube 24 from the helium vessel, will cause some reduction in helium gas pressure at the opening 22', in turn causing reduced temperature within the helium vessel.

As has been mentioned above, the arrangement represented in Fig. 1 is schematic only. Figs. 5 and 6 more closely represent possible physical arrangements according to the present invention. As shown in Fig. 5, the helium reservoir may be located within the cryogen vessel, within the turret. An opening 24' between the helium reservoir and the helium vessel need only be a very short tube, which essentially serves to direct the opening 22" in a required direction at a required position within the effluent gas flow path 16.

Fig. 6 shows a preferred embodiment in which a turret 18 is formed to the side of the cryostat, and the helium reservoir 10 is formed on the outside of the helium vessel, still below the access turret 13.

While the present application has been descrthed with reference to a limited number of particular embodiments, various modifications and variations of the described embodiments will be apparent to those skilled in the art and fall within the scope of the present invention as defined by the appended claims. For example, while the different parts of the venturi-like constriction have been defined by their diameters Dl, D2, those parts of the venturi-like constriction need not have circular cross-sections. With -11 -non-circular cross sections, similar results may be obtained by considering the respective cross-sectional areas Al, A2.

Claims (3)

1. -12 -CLAIMS1. An arrangement for cooling fixed current leads of a cryogenically cooled apparatus housed within a cryogen vessel (20), the arrangement comprising:-a separate cryogen reservoir (10) which, in use, is partially filled with liquid cryogen, and is linked with the cryogen vessel; -a heating element (14) located inside the cryogen reservoir, located, in use, in contact with liquid cryogen within the cryogen reservoir; -a gas escape path extending between the cryogen reservoir and a turret (13), said gas escape path including a section (12) of reduced cross-section, defining a venturi-like constriction, the link between the cryogen vessel (20) and the cryogen reservoir (10) being provided by an opening (22), located in the venturi-like constriction.
2. 2. An arrangement according to claim 1, further comprising a gas heating element located within the cryogen reservoir, in use above the level of liquid cryogen in the cryogen reservoir, for heating cryogen gas.
3. 3. An arrangement according to claim 1 or claim 2 wherein the opening (22) is in the form of atube having an opening directed in ta downstream direction f the gas escape path.Amendments to the claims have been filed as follows 1. An arrangement for cooling fixed current leads of a cryogenically cooled apparatus housed within a cryogen vessel (20), the arrangement comprising: -a separate cryogen reservoir (10) which, in use, is partially filled with liquid cryogen; -a heating element (14) located inside the cryogen reservoir, located, in use, in contact with liquid cryogen within the cryogen reservoir; -a gas escape path extending between the cryogen reservoir and a differential pressure valve (23), said gas escape path including a section (12) of reduced cross-section, defining a venturi-like constriction, 0) the cryogen vessel (20) and the gas escape path being linked by a direct connection (24) which terminates in an opening (22) located in the CO venturi-like constriction (1 2).2. An arrangement according to claim 1, further comprising a gas heating element located within the cryogen reservoir, in use above the level of liquid cryogen in the cryogen reservoir, for heating cryogen gas.3. An arrangement according to claim 1 or claim 2 wherein the opening (22) is in the form of a tube having an opening directed in the downstream direction of the gas escape path.