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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:
- 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).
- Quickly release the pressure in the transport dewar and insert the open end of the transfer tube into the cryostat.
- 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
| Helium level meter has erratic display whilst refilling. | Rotate helium level probe to prevent splashes of liquid helium entering the small breather holes in the probe. |
| Helium level probe has continual erratic display. | If demountable, remove probe and warm up to remove ice. If probe is not demountable warm system and pump out the helium can for 24 hours. |
| During helium filling magnet temperature does not drop and helium will not collect. | There is liquid nitrogen from the pre-cooling (which may have now frozen) in the helium can. Allow system to warm slightly and then repump the helium can. Check base pressure is less than 10 mbar. Poor OVC vacuum - does the outside or the cryostat feel cold or has ice formed on the outsider? - Re-pump and / or check for leaks. |
| Allen-Bradley resistances do not appear to correlate with calibration given (particularly at low temperatures). | Are you looking at the right calibration (100 Ω or 270 Ω)? If sensor is 100 Ω a high impedance meter is required otherwise resistance will appear to be lower than it really is. |
Liquid helium transfer tube (a) has ice spots on the exterior (b) has ice all over exterior | (a) Internal capillary is touching outer tube, continue to use if feasible, replace or return to factory for repair. (b) Loss of internal vacuum. |
| Lack of vacuum in outer vacuum container of cryostat. | Leak on pumping system, isolate cryostat and check pumping system base pressure. Leak on dewar, use mass spectrometer to identify source of leak, check all 'o' rings for cleanliness (e.g. a human hair). Excessive moisture in the OVC - pump and flush with dry nitrogen gas several times then re-pump thoroughly - preferably 24 hours. |
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.