The invention relates to a horizontal warm bore liquid helium cryostat for superconducting magnetic separator.
Liquid helium cryostats are used for operating superconducting magnets. The application of superconducting magnet system in magnet separators require horizontal orientation of the cryostat. In addition warm bore is required to locate process modules. This situation makes construction of the cryostat more complicated. A liquid nitrogen reservoir is used to cool the radiation shields. The performance of these cryostats are based on various factors like the weiht and volume of helium chamber, the reactive forces on the cryostat due to process conditions etc. The horizontal orientation and the presence of reactive forces requires a different approach in comparison to vertical bath cryostats often used. Due to very low boiling point ( 4.2K) and very low latent heat of liquid helium, the heat input will result in excess boil off of liquiid helium.
There are disadvantages associated with the present liquid helium cryostat for superconducting magnetic separator which uses liquid nitroen reservoir to cool the radiation shields.
The main disadvantages is that heat input to the helium environment being high the hold time is short requiringfrequent topping of liquid helium.
Another disadvantage is that there is heat input to the helium environment through the support links, current leads, neck of the Qryostat and the radiation shields thereby boiling off liquid helium requiring
frequent topping.
Therefore the main object of the present invention is to provide a high performance liquid helium cryostat which is compact and light as helium is used instead of Nitrogen and of horizontal configuration and with a warm bore for superconducting magnetic separator which limits the radiation heat and conduction heat in-flow to helium chamber and thereby reducing the boil-off of liquid helium and increases the hold time of liquid helium.
Another object of the present invention is to provide a high performance liquid helium cryostat which has cryocoolers to cool the radiation shields provided to reduce heat input to the helium chamber.
Yet another object of the present invention is to provide a high performance liquid helium cryostat with a system of intercepting conduction heat through support links and neck of the cryostat.
According to the present invention there is provided a horizontal warm bore liquid helium cryostat used for operating super conducting magnets for magnetic separators, said cryostat being provided with a warm bore to locate process modules, liquid helium being placed in a helium container, a super conducting magnet being disposed in the helium container, primary and secondary radiation shields being provided within the cryostat to minimize radiation heat input, the vertical access neck of the helium chamber interfaced to a helium recondenser having three cooling stages to intercept the conduction heat load, the said cooling stages connected to radiation heat shields and a heat exchanger finned surface which condenses evaporated liquid helium, characterized in that said radiator heat shields are cooled to different temperatures by cryocooler as well as by helium recondenser.
The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawing.
Figure i- sectionai view of horizontai warm bore cryostat showing cryocooiers interfaced to the cryostat and with horizontal links.
Figure 2 = Helium vessel suspension arrangement showing the link arrangement for support of He vessel.
Figure 3- Radiation shield suspension arrangement showing link arrangement showing link arrangement for support of radiation shield,
Figure 4- shows cryostat link details 4a- FRP link 4b- Tl alloy link 4c- Top bracket for He Vessel 4d- End bracket for Tl link
Figure 5 - Cool down characteristic of superconductor magnet by cryocoolers.
Figure 6a - cool down characteristic of primary radiation shield.
Figure 6b- cool down characteristics of secondary radiation shield
Figure 7 - shows the total evaporation rate and helium loss from the cryostat under operating conditions.
In accordance with the present nvention of a high performance liquid helium cryostat of horizontal configuration and with a warm bore which has a number of novel and useful features.
The system has cryocoolers (2) to cool the radiation shields (4,5),intercepts conduction heat through support links (9) and neck of the cryostat. This enables to limit the radiation heat and condition heat in flow to helium chamber (3) and thereby reducing the boil-off of liquid helium and increases the hold time of liquid helium (14). Helium recondensor (1) is interfaced to the neck of helium chamber (3) to intercept the heat flow through neck as well as share the heat load with shield cooler (2) in its first and second stage cooling capacity. Through the facility at its third stage, the He recondenses the evaporated helium inside the cryostat itself. This enables to reduce the effective loss of liquid helium from the cryostat so that the periodic topping of the cryostat with liquid helium can be at longer intervals. The system is interfaced with high Tc current leads (15) which further reduces heat conduction through current leads.
'The' net effect is longer interval of unattended operation of the system. A combination of link system t?,7a,7b,15) using fibre reinforced plastics and titanium alloys are used to locate the helium chamber (3) and radiation shields ( 4,5) inside the outer vessel (8) accurately. The links are designed to withstand transport acceleration forces as well as critical axial loading due to reactive forces on helium vessel due to canister movement. The optimized design of link systems takes care of extreme mechanical rigidity requirements as well as satisfies condition for good thermal performance.
Sectional view of the horizontal warm bore cryostat is as shown in Figure 1 . The cryostat has a warm bore (11) of 320 mm diameter and an outside diameter of 1200mm. The overall lenth is 1280 mm. The helium container (3) has an eccentric outer shell to reduce redundant helium inventory (Figure 1). The warm bore ( 11) is provided to facilitate canister location in the magnetic field region and carry out magnetic separation.
The helium chamber (3) houses a superconducting magnet (6) which operate in a pool of liquid helium (14). The outer chamber (8) encloses the helium chamber (3) and has a warm bore ( 11). The inner space between
the helium vessel (3) and the outer shell (8) is in vaccum
_5 and the vacuum level is better than 10 This level
of vacuum is maintained to cut out the heat input to
the system by gaseous conduction.
Since the temperature of liquid helium chamber
is at 4.2 K durin operation, the radiation from the outer
shell (8) will be enormous. As the latent heat of evaporation of liquid helium is also very low, it is essential to reduce the heat input to a very low level to hold liquid helium for a long time for convenient operation of the device.
The radiation heat input to helium chamber (3) is reduced by having radiation shields ( 4&5) cooled to intermediate temperatures. This is accomplished by two stages cryocooler (2) operating based on GM Cycle and having heat sink temperatures of 65K and 20K. This drastically reduces the net radiation heat load reaching the helium chamber, as the radiation is only from the secondary shield (5). The magnitude of radiation heat load is in m. watts/Sq.M as against; tens of watts/Sq.M in its absence. The primary shield (4) is also wrapped with 30 layers of reflective aluminium coated mylar sheets interleaved with nylon net called super insulation. This drastically reduces the heat load on the primary shield (4) from outer room temperature surfaces by an order of 80. This enables the Cooler to keep the radiation shields at these low temperatures under steady conditions during sntireoperating of the device.
The surfaces of helium chamber (3) and the radiation shields ( 4,5) are highly polished to low emissivity levels to help reflect radiation heat and limit heat absorption. The emissivity levels are further assured by sticking the polished surfaces with alumiumium self stickingtapes.
The helium chamber and the shields are located and held in position by suspension links (Figure 2 & Figure 3). The support system (9 of Figure.1 and 7a,7b,15), Figures 2 & 3) is specially designed to with- stand shock loads during transport. Since the links (9, 7a, 7b, 15) transport heat from room temperature to the helium chamber (3) by conduction mode, the following criteria is ensured. The cross section is optimum for the mechanical load conditions. The material of construction has a very low thermal conductivity. The system has links (9, 7a, 7b, 15) made of fibre reinforced epoxy links (Figure 4a,4b,4c,4d) which has comparable strength even at very low temperatures and thermal conductivity integral lower by at least 600 times in room temperature ( 300K) to 4K range. The heat input is further reduced by having heat intercepts anchored to the crycooler heat stages, limiting the net heat input to the chamber to a fraction of a watt. The intercepts are braided copper strips. Ti alloy links (9 of Figures 1, 4b) are excessively designed to counter the axial force due to process conditions which is higher than the normal axial transportation loads. The link details are as shown in Fig. 4 where top bracket (16) and end bracket (17) for the Ti link (9) are shown in detail in Fig. 4c, 4d. Figure 1 shows details of vertical support (7a) for helium chamber (3) and (7b) for radiation shield.
The helium chamber has (3) a vertical access neck of limited size to facilitate transfer of liquid
helium, connect the superconducting magnet terminal through current leads to power supply outside and also to take out measurement probe leads. The access neck also houses He recon-densor ( 1 of Figure 1 ) which has three cooling stages.The first two has 65 K and 20K stage temperatures which intercepts the conduction heat load from neck walls, current and sensor leads to helium chamber. The first two stages are also inter connected by braided strips ( 12) to radiation shields ( 4 & 5) to supplement the cooling capacity of shield cooler (2). This is to prevent warm up of the system during short interruption of any one of cryocoolers. The third stage of the cryocooler has a heat exchanger finned surface operation at 4K to enable the evaporated helium to condense and fall back inside the helium chamber itself. The cryostat is provided with flexible level sensors to monitor the level of helium inside the cryostat.
A commercial shield cooler of APD Make SH3P and having a cooling capacity of 70 watts at 65 K and 6.4 watts at 20 K and used a he recondensor of APD make - Hrs 10. has been used. This has three heat stages. First stage has a capacity of 50 watts at 65 K, second stage 2.2 watts at 20K and at third stage 0.8 4K (1.2 Ltrs./hr equivalent liquid helium evaporation). The cryocooler interface obviate the need of handling liquid cryogen on a regular basis. Figure 7 shows helium loss over a period of time. Figure 5, 6a, b shows the cool down characteristics of SC magnet, primary and secondary radiation shields respectively.
The cryostat is made from nonmagnetic stainless steel 304 L and are all weld construction with a pumping port
and a pressure relief valve. All welds are leak tested
"9 to prove a leak rate not to exceed 1.0*10 std. cc helium
per second as measured with mass spectrometer leak detector.
The system is thermally cycled to 77K twice by cooling
it with liquid nitrogen. Cumulative internal leak is
evaluated for over 24 hrs under cold conditions and with
a helium chamber pressure of nearly 10- 12 psi of helium
gas. The cryostat assembly weigh about nearly 1200Kgsand
the helium vessel with the superconducting magnet weighs
around 450 kgs. The cryostat has a capacity of around
280 ltrs of liquid helium during normal operation.
A static evaporation rate of 1.35 ltrs/hr liquid helium was achieved. With helium recondensor operating at 0.8 W capacity, near zero loss condition was achieved. The average loss of liquid helium from the cryostat under processing conditions is around 0.5 ltrs/hr (Figure 7). The thermal efficiency of the system is illustrated by the cool down characteristics of the magnet prior to helium fill ( Figure 8) and the cooldown characteristics of the primary and secondary radiation shields ( Figure 9 a&b) The characteristics indicate that the shields and links are uniformly cooled even at extreme locations (Figure 6 a&b).
The cryocoolers ensure very low heat flow to helium chamber as it cools the secondary shield to near 30 K as well as intercept the conduction heat flow throuh suspension links, access neck and leads. The invention
allows for easy assembly and exact location and alignment of the cryostat. A combination of fibre reinforced links and Ti alloy links and their specific arrangement ensures
1. Accurate location and easy assembly
2. With stand transportational forces to the extent of vertical 3g, Downwards 2g, Axial 2g
3. Ti alloy links are arranged to effectively takes care of reactive forces on the cryostat to the extent of 2.5 tons, 2.5 times more than axial forces. This is maximum possible axial load if processed at designed maximum magnetic field of the device.
The first and second cooling stages of helium recon-densor is interfaced in such a way that it works in tandem with shield cryocooler. This enables the system to function normally in the event of short interruption of any one of the cryocoolers.
The helium vessel and the magnet weighing nearly 450 Kgs could be cooled by cryocoolers alone from room temperature to near 50 K amply illustrating the thermal efficiency of the design. This ensures subsequent helium filling with minimum transfer losses ( Figure 5: Data).
The invention allows for efficient cooling of radiation shields, efficient interception of conduction heat through links and access neck by cryocoolers.
The invention allows for very low heat input to the helium chamber resulting in low evaporation rate.
The invention allows for adaptation in applications such as low loss cryostats for superconducting magnets
of various sizes and High Tc superconducting magnets operating in 4K - 35 K range.
The invention described hereinabove is in relation tP a non - limiting embodiment and as defined by the accompanying claims. 12

We Claims :-
1. A horizontal warm bore liquid helium cryostat used for operating super conducting magnets for magnetic separators, said cryostat being provided with a warm bore (11) to locate process modules, liquid helium (14) being placed in a helium container (3), a super conducting magnet (6) being disposed in the helium container (3), primary (4) and secondary (5) radiation shields being provided within the cryostat to minimize radiation heat input, the vertical access neck of the helium chamber (3) interfaced to a helium recondenser (1) having three cooling stages to intercept the conduction heat load, the said cooling stages connected to radiation heat shields (4,5) and a heat exchanger finned surface which condenses evaporated liquid helium, characterized in that said radiator heat shields (4,5) are cooled to different temperatures by cryocooler (2) as well as by helium recondenser (1).
2. A horizontal warm bore liquid helium cryostat as claimed in claim 1 wherein the said helium chamber (3) which houses a superconducting magnet (6) is enclosed by an outer chamber (8) and the space between the helium chamber (3) and the outer shell (8) is in vacuum and the said vacuum level is better than 10-5.
3. A horizontal warm bore helium cryostat as claimed in claim 1 wherein the cryocooler (2) has two stages having heat sink temperature of 65K and 20K.
4. A horizontal warm bore helium cryostat as claimed in claim 1 wherein the said primary shield (4) is wrapped with multiple layers of reflective aluminum coated myler sheets interleaved with nylon net as super insulation.
5. A horizontal warm bore helium cryostat as claimed in claim 1 wherein the support system for the said helium chamber (3) and the said shields
(4,5) are by suspension links made of Ti-ailoy (9) and fibre reinforced epoxy links (15) and vertica! supports (7a,7b) for high strength to take care of reactive forces on cryostat and iow thermal" conductivity.
6. A horizontal warm bore helium cryostat as claimed in claim 1 wherein the said cryostat is made of non magnetic stainless steel and are all welded construction.
7. A horizontal warm bore liquid helium cryostat for superconducting magnetic separator substantially as herein described and illustrated.