Description and Operation of the System

General Description

This system comprises a special horizontal field 5T split pair magnet mounted in the tail section of a large capacity, liquid nitrogen shielded, vacuum insulated cryostat. The magnet has a cold split of 100 mm x 40 mm and a cold bore diameter of approximately 120 mm, accessed through aluminium windows. Alternatively, room temperature tubes may be inserted to allow the magnetic field to be plotted. The vertical split access allows a large cooling power helium-4 insert to be fitted from above.

The cryogenic efficiency of the cryostat is very high due to the small helium neck diameters and the way in which the exhausting helium gas is used to cool them. The helium can heat load is minimized by the use of a nitrogen cooled shield which will minimize both the conducted and radiated heat to the minimum levels possible.

The current leads are all fixed for high reliability and safe operation.

The magnet is 5 T with a homogeneity of better than 1 in 10^-4 over a length of 20 mm and a diameter of 80 mm. See the (insert link to Test Results) for more detailed information. Access is provided through aluminium windows which may be demounted if required for access to the magnet cold bore.

The weight of the system is approximately 1,000 kg. Appropriate lifting gear must be used to move the cryostat.

This system, when energized to full field has a considerable stray field, extending over many meters, and the system stored energy is approximately 0.5 MJ. It is therefore VITAL that the safety section is read by ALL personnel coming near the system.

PARTICULAR CARE MUST BE TAKEN TO ENSURE THAT THE SYSTEM IS WELL ANCHORED TO THE FLOOR, AND ANY STEEL OBJECTS IN THE VICINITY ARE SIMILARLY WELL BOLTED TO THE FLOOR.

Cryostat Description

The cryostat is of a vacuum insulated, all metal construction with intermediate temperature radiation shielding. The outside surfaces of the helium and nitrogen vessels are wrapped with single or multi-layer super-insulation to reduce emissivity. The outer vacuum case (OVC) of the dewar will be fitted with an evacuation valve incorporating a pressure relief safety feature that will operate in the event of a cryogen leak to the vacuum space. In addition there is a drop-off plate at the base or side of the dewar.

The siphon entry port has an associated cone located within the cryostat. A tube runs from the cone to the bottom of the cryostat and ensures that all liquid nitrogen can be removed from the helium reservoir after pre-cooling the magnet and that filling with helium is from the bottom.

All cryomagnet service ports should be sealed with the plugs provided when not in use. In all cases, the boil-off of cryogens is minimized by taking great care in the design to prevent heat entering from the following main sources:

Gaseous conduction. An evacuation /  pressure relief valve allows the insulating vacuum space to be evacuated to less than 10^-4 torr.

Metallic conduction.Great care is always taken to use materials of low thermal conductivity combined with mechanical strength to support the cryogens in their vacuum. The supports (usually tubes) are of minimum cross sectional area and maximum effective length within overall size constraints. Neck tubes are thermally anchored with a copper thermal link to the top of the nitrogen vessel and good use is made of the enthalpy of the exhausting gas to minimize incoming conducted heat.

Radiation. The radiation load is reduced to reasonable values by the introduction of intermediate temperature radiation shields. These are usually cooled by a reservoir of liquid nitrogen surrounding the helium bath. The enthalpy of the exhausting helium gas is sometimes used to cool a radiation shield inside the nitrogen shield. The emissivity of cold surfaces can also be reduced. This is achieved using many interleaved layers of aluminium and insulation known as super-insulation.

Ohmic heating. The principal sources of ohmic heating are the current leads and the superconducting switch. In some systems the current leads are made demountable to minimize the cryogen boil-off with a persistent magnet, the remainder of systems feature carefully designed current leads which do not impose a significant heat load. All systems now feature low-loss switches.

Evacuating the Cryostat OVC

In order to maintain the thermal isolation of the liquid helium it is necessary that a high vacuum be maintained within the cryostat outer vacuum case.

IMPORTANT In many cases the thin wall construction of the helium reservoir will not support an external pressure differential of one atmosphere. The helium reservoir must therefore NEVER be evacuated unless the OVC is first evacuated. The recommended pumping equipment consists of an oil diffusion pump of 50 mm (2 inch) diameter or, even better, a turbomolecular pump fitted with a liquid nitrogen cold trap. This pump should be backed by a rotary pump of not less than 12-15 m³/hr pumping speed, fitted with a gas ballast facility. All connecting lines should have an internal diameter of not less than 50 mm and be as short as possible. Tubes must NOT have been used previously to carry or pump helium.

       a. Connect the valve on the cryostat top flange to the pumping equipment. Using the rotary pump, evacuate the cryostat slowly (approximately half hour) to prevent any possible collapse of internal shielding, until the pressure is less than 0.05 mbar.

       b. Switch over to the diffusion pump and evacuate the cryostat to less than 5 x 10^-4 mbar. Continue pumping at least overnight to ensure the removal of residual gases trapped in the super-insulation.

Inspecting the vacuum:-

If the cryostat is already evacuated and it is desired to inspect the pressure only, the pumping tube should be evacuated and the diffusion pump operating before the OVC valve is opened. If the pressure is greater than 10^-3 mbar with the system warm, the cryostat should be evacuated overnight with the diffusion pump to less than 5 x 10^-4 mbar. It is recommended that the cryostat is always pumped overnight before use.

Flushing the vacuum space:-

If the vacuum space has been accidentally contaminated with helium gas or moisture evacuation can be improved by flushing the space. NOTE: Never vent cryostats with helium gas as this will 'stick' in the super-insulation.

1. Using a rotary pump, evacuate cryostats to less than 1 mbar.
2. Admit an atmosphere of DRY nitrogen gas, preferably through a 1 mm orifice, and pump out to less than 1 mbar.
3. Repeat (2) several times, then pump to less than 0.05 mbar.
4. Switch over to the diffusion pump as in (b) above.

Precooling the Magnet

Before filling the cryostat with liquid helium, the magnet and system must be cooled to a temperature below 100 K, this will save a considerable amount of liquid helium which is much more expensive than liquid nitrogen. To perform the precool, fill the liquid helium container with liquid nitrogen, completely above the magnet. Use a length of 9.6 mm diameter stainless steel tubing inserted into the transfer tube entry port (this is the 'blow-out' tube supplied with the system). Ensure that the tube is located in the cone fitting below the siphon entry port inside the cryostat, the liquid nitrogen storage dewar should be conveniently positioned and connected to the blow-out tube with flexible plastic tubing (once the transfer has started this should not be moved as it is very brittle and will break easily). Allow the liquid nitrogen to remain for one or two hours and then fill it completely again.

The liquid nitrogen should then be removed. Insert the stainless steel tube into the transfer entry fitting and ensure that it is firmly fitted into the cone on the top of the magnet. Blow out all the liquid nitrogen by pressurizing the liquid helium container with helium gas to not more than 0.25 atmospheres overpressure, the blown out liquid nitrogen may then be usefully fed into the nitrogen can, see Filling the Liquid Nitrogen Container (the next section). Do not stop this prematurely as removing the remaining nitrogen could cause problems. Use the heaters on the helium can to remove the last few liters of liquid nitrogen that will be left in the helium can. Monitor the background in the OVC with a leak detector connected to the OVC pumping line to check for low temperature leaks from the main bath to the OVC. It is important that all the liquid nitrogen is removed. Failure to do this properly will make filling with the liquid helium difficult, and may impair the performance of the magnet. When all the nitrogen has been removed, release the pressure in the liquid helium bath and evacuate the liquid helium container using a rotary pump and then fill it with helium gas. If during pump down, a pause is seen in the range of 70-100 mbar, and the pumping line becomes very cold, then liquid nitrogen is still present. Stop pumping immediately and flush out the helium bath with helium gas fed down the blow out tube (which should be located in the cone fitting). Failure to do this will result in solidification of the nitrogen. Repeat this procedure at least two times in order to thoroughly purge the magnet of nitrogen. As an indication that all the liquid nitrogen has been removed, check that it is possible to evacuate the liquid helium container to a pressure less than 10 mbar.

Allen-Bradley and Rhodium-Iron Sensor Characteristics

Please not that this table is a guide for cooldown monitoring purposes only, and is not a substitute for a full calibration.

Approximate % error due to magnetic fields:

Filling the Liquid Nitrogen Container

In the interests of economy it is advisable to precool the magnet before filling the nitrogen can. The procedure for this is described previously and will ensure that the cryogens are most efficiently used.

Connect one of the three filler / vent tubes of the liquid nitrogen container to a storage vessel using flexible polythene pipe. Transfer the liquid nitrogen by pressurizing the storage vessel to approximately 0.25 atmospheres above atmospheric pressure. Violent boiling will occur initially until the radiation shield has cooled down. When liquid nitrogen sprays out of the filler tubes release the pressure on the storage vessel to stop the transfer.

The storage vessel can be pressurized using a valve on the outlet. By using an electronically controlled valve, the liquid nitrogen container can be filled and the level maintained using a Liquid Nitrogen Level Controller. Inspect the liquid nitrogen at intervals appropriate to the overall system hold time.

All Oxford Instruments cryostats are fitted with overpressure relief valves which are not customer removable. The problems caused by ice formation in the filling tubes can be reduced by slipping 0.25 m lengths of plastic tubing over them. These tubes also prevent any overflow of liquid nitrogen from cooling the top flange and its 'O' ring. This can be important if an autofilling system fails to stop the nitrogen transfer when the tank is full.

Transfer Tube and Storage Dewar Adapter for Liquid Helium

The transfer tube optionally provided with the system is of a stainless steel construction. It takes the form of a tube surrounded by a second tube with a vacuum of better than 10^-4 mbar maintained between them. The assembly of the two tubes usually takes the form of a large 'n' shape.

Occasionally re-pumping of the tube will be necessary in service, particularly during the first few months while the materials in the tube are still outgassing.

The ST9 Siphon Evacuation Fitting

The transfer siphons supplied by Oxford Instruments are supplied pre-evacuated, however re-evacuation may become necessary after a period of operation. To evacuate an Oxford Instruments standard siphon, an ST9 fitting is needed to operate the vacuum valve.

1. Remove the yellow nylon dust cap from the transfer tube valve. Connect the ST9 fitting to the high vacuum pumping system.
2. Place the ST9 fitting over the transfer tube valve. Evacuate the pumping lines and check the system for leaks.
3. Using the red anodized aluminium knob, which is connected to the hexagonal key internally, open the transfer tube valve. Pump out the siphon to 10^-4 mbar or better.
4. Close the transfer tube valve using the red knob, isolate the pump and remove the ST9.
5. Replace the dust cap.
6. Try to avoid getting dirt in the ST9 fitting.

Note: The cryostat overpressure relief valve must be in position and not restricted. If the cryostat is connected to a recovery system any flow meter should be capable of high flow rates and should not introduce a restriction (it may be sensible to fit a bypass flap valve to accommodate the high flows during a possible quench, ensuring that all the helium is recovered).

Initial Filling with Liquid Helium

1. Check that the transfer tube has the correct leg lengths and diameters to be compatible with the cryostat and storage dewar. Connect the cryostat and storage dewar to the helium recovery system or put a one-way valve on the cryostat exhaust port (if the system is large and a one-way valve is found too restrictive, it may be replaced by a 2 m length of convoluted tubing). Position the liquid helium storage vessel so that the transfer tube can be inserted easily and is close to the cryostat to be filled.
2. Remove the plug from the cryostat transfer tube entry port and also from the top of the storage vessel. Insert the transfer tube legs into the cryostat and, slowly, into the storage dewar, allowing it to cool gradually. Ensure that the end of the transfer tube in the cryostat is fitted into the cone on top of the magnet. In this way, cold gas and then liquid is introduced at the bottom of the magnet which is then cooled by the enthalpy of the gas as well as by the latent heat of evaporation.
3. Start transferring the liquid helium by pressurizing the storage vessel. (This is generally done by gently squeezing a rubber bladder). The transfer rate should be such that the vent pipe is frozen for not more than 2 m of its length. The initial transfer rate should be equivalent to about 10 liters of liquid per hour. This rate can be increased as the magnet cools and the boil-off reduces. Typically the cool-down from 77 K to 4.2 K will take between 10 and 60 liters depending on the system size and the care taken in the transfer.

By monitoring the Allen-Bradley sensors, when the magnet temperature falls below 10 K, the transfer rate can be further increased in order to fill the liquid helium container. This should occur when a further 10 to 50 liters of liquid have been transferred, depending on the size of the magnet and dewar.

4. When the liquid helium reservoir has been filled, stop the transfer by releasing the pressure in the storage vessel. Remove the transfer tube and replace the plug. Inspect the liquid helium level at appropriate intervals.

Refilling with Liquid Helium

The cryostat should be refilled before the level reaches the 10% mark (if a helium level meter is in use). In refilling, care should be taken not to evaporate the liquid in the cryostat with the hot gas which initially comes through the transfer tube. (N.B. Failure to take care can cause the magnet to quench).

With Oxford Instruments siphons, a 'phase separator' is supplied. This is a small brass cylinder approximately 25 mm long x 10 mm diameter with an internal screw thread at one end and two angled cuts in the curved surface, it does not have a hole right through. The phase separator may be screwed to the end of the siphon leg which enters the cryostat, the liquid / gas passing through the transfer line is then separated as it is thrown upwards by the angled slots and the liquid simply falls back under gravity to collect when the refilling is intermittent (e.g. with autofilling systems, or transfer lines left permanently in the cryostat) as liquid in the transfer line may have been vaporized and this is then not passed through the colder liquid in the cryostat, which would cause it to boil off. The phase separator should not be used for initial cooling of the system.

The correct procedure is as follows:

1. Insert one leg of the transfer tube into the storage vessel, but leave the other one outside of the cryostat. The cryostat siphon entry fittings (the O-ring, washer, and the knurled ring) should be undone and slid onto the transfer leg to go into the cryostat, reseal the cryostat entry with the bung provided, the siphon may now be precooled without warm gas entering the cryostat. Pressurize the transport dewar in the normal way, as if transferring helium. After about a minute liquid will issue from the transfer tube, indicated by a blue tongue of vapor. (Prior to this a white vapor plume will have been seen for about 20 seconds).
2. Quickly release the pressure in the transport dewar and insert the open end of the transfer tube into the cryostat.
3. Lower the transfer tube until it reaches the bottom of the necktube. DO NOT push the tube into the cone on top of the magnet, or on the magnet support structure. Transfer liquid helium in the usual way.

If the helium level has fallen below 5% and the magnet is still energized there are two courses of action available:

      i. If the level is below 0% or if the user is not certain that a careful transfer can be done DE-ENERGIZED THE MAGNET, refill and then re-energize the magnet.

     ii. Refill the dewar, but be careful as the siphon is introduced and as the transfer starts.

The cyrogen boil-off test results are given in the results section.

Cooldown Fault Diagnosis

Superconducting Magnet

The magnet consists of a number of concentric solenoid sections together with compensating coils including shimming coils (when required to achieve the specified level of homogeneity). Each section is wound from multifilamentary superconducting wire formed from Niobium Titanium (NbTi) filaments surrounded by a stabilizing matrix of copper. High field magnets i.e. those with maximum fields of greater than 11 Tesla will be fitted with inner coil sections of Niobium Tin (Nb₃Sn). All sections are constructed to the MAGNABOND system, an integration of proprietary techniques, developed by Oxford Instruments, to give a structure which is both physically and cryogenically stable under the considerable Lorentz forces generated during operation. All the constituent sections of the magnet are connected to allow series energization except when independently excited shims are fitted.

The Superconducting Switch

A superconducting switch is used to establish 'persistent mode operation', this is the temporary connection of a superconducting short circuit across the magnet leads when the magnet has the desired current flowing within. In this way the magnet may be set (persistently) at a given field, and the current in the supply leads reduced to zero. This will save a considerable amount liquid helium due to the ohmic heating in the current leads.

The switch consists of a length of superconducting wire non-inductively wound with an electrical heater. The superconducting switch, as supplied, has this length of superconductor wired in parallel with the entire magnet. The superconducting wire is made resistive by raising its temperature using the heater. The switch is then in its open state and current, due to a voltage across the magnet terminals, will flow in the superconducting magnet windings in preference to the resistive switch element. The switch is in its closed state when the heater is turned off and the switch element becomes superconductive again. The process of establishing persistent mode operation of the magnet consists of energizing the magnet to give the required field with the switch in the open state, closing the switch and then reducing the current flowing through the magnet current leads to zero, leaving the magnet in its previously energized state. The current flowing in the magnet windings remains constant as the magnet lead current is reduced, the current flowing in the closed switch then being the difference between the magnet and lead currents.

Magnets are specifically constructed for fast sweep applications may not be fitted with a switch, the advantages of this are a reduction of the boil-off whilst sweeping as switch heater current is not required, and, secondly, all the power supply current is forced through the magnet and is not shunted by the switch. This shunt current would otherwise lead to non-linearity between the power supply current and the field, which may be undesirable for some applications.

Magnet Quench Protection