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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.
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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
Protection resistors, and diodes if appropriate, are provided for all magnet sections, restricting the development of potentially high voltages in the event of a magnet quench (rapid conversion from the superconducting to the normal resistive state). The resistors also dissipate some of the energy stored in the magnet during a quench thereby reducing the energy dissipation within the magnet windings. The resistors are mounted on baffles attached to the magnet support structure or on plates above the magnet itself and hard wired or coupled to the magnet via an electrical connector. The connector will also incorporate the wiring for the superconducting switch heater, making it impossible to run the magnet without the protection circuit attached.
If barrier diodes are used in the protection circuit then, under limited voltage conditions, e.g. energization or de-energization of field and when the field is static, all the current passes through the magnet and ensures proportionality between energization current and magnetic field. As no current is flowing through the protection circuit the heat load from the protection resistors and hence system boil off are reduced.
Under quench conditions, the barrier voltage is exceeded and the protection circuit shunts a proportion of the current away from the magnet windings.
Equivalent Circuit of a Superconducting Magnet
A Superconducting magnet can be considered to be a pure inductor, however connections from the power supply to the magnet will of course be resistive and a small voltage will be dropped along the length of the leads (typically <0.7 V for 5 m leads at 120 A) and along the leads inside the cryostat (typically 0.15 V at 120 A). This voltage will be roughly proportional to the current in the leads. It is a good idea to check these voltages and the cryostat boil-off, with the switch heater off (i.e. magnet not energized) and full current in the leads, after commissioning and any subsequent disassembly.
The switch will shunt a small amount of the magnet current whilst where is a voltage across the magnet i.e. when the field is changing. This current is minimized by running the magnet up slowly and using a high resistance (100 Ω) switch, a worst case may be energization at 10 V (the maximum output voltage from a PS120-10) through 100 Ω i.e. 0.1 A. For experiments requiring an extremely linear field versus current ratio, such as VSM measurements, a switch may not be fitted.
The protection circuitry is generally fitted with special diodes and will not pass current until a certain voltage is exceeded. This is generally 4 V, however some special magnets and those designed for fast ramping may have a "protection voltage" of 10 V (or more if a special power supply is used, such as the PS120-20). Some small magnets and magnets designed for very infrequent running up and down may have no diodes. In this case the protection circuit will dissipate power whenever a voltage is present across the magnet terminals.
To run the magnet up to field a voltage must be applied to the magnet leads to overcome the inductance of the coil. The magnitude of this voltage will govern the speed at which the magnet will run-up, this is defined by the equation:
Where:
L is the magnet inductance given in the specification section.
For magnetic circuits with no iron (i.e. they are linear) the magnetic field at any point is proportional to the magnet current i.e.:
k is the current to magnetic field ratio given in the specifications section.
Operating the Magnet
Check that the quench valve is in position on the outlet from the main bath, this is VITAL as a magnet quench liberates hundreds or thousands of liters of helium and any restriction on the recovery line of exhaust port could cause an explosion. After cooling down the system and collecting liquid helium in the helium can the magnet is ready for energization.
A magnet power supply is needed to energize the magnet. Typically an Oxford Instruments PS120-10, PS120-3, or PS120-10HS power supply would be used, however any power supply with the necessary current rating to achieve the full field of the magnet, and a voltage suitable to allow field sweeping at the desired rates may be used. The following instructions are general. Read the relevant Power Supply Unit (PSU) handbook for specific information. The magnet field strength is determined by the current available (the Tesla/Amp ratio is given in the specifications), while the voltage determines the rate of change of field (the inductance is also given in the specifications).
The magnet can be operated manually or under control of a computer. Three modes of magnet energization exist namely:
Current control mode with voltage trip. This allows the solenoid to be swept to a set current or zero current at a constant rate of change of current, which with resistor-diode protection allows a constant rate of energization. If the maximum output voltage of the power supply is not capable of energizing the magnet at the set rate due to the sum of the inductive back EMF and the lead drop, the power supply will trip and go into the 'Hold' state i.e. energization halts and can be restarted by the user at a lower sweep rate. All recent power supplies manufactured by Oxford Instruments work in this mode.
Current control mode with voltage limit. This is a similar mode to voltage trip constant current sweep rate but in the event of the voltage limit being reached, if the rate of energization demands more voltage at the power supply than it is allowable, then the power supply will limit at that voltage but will continue to sweep to the set current.
Constant voltage. This allows the solenoid to be swept to a set current or zero at a rate dictated by a constant voltage at the power supply terminals, the voltage drop in the current leads and the inductance of the solenoid. This will not give a constant rate energization with increment of time and is therefore not any interest for VSM experiments. It is mentioned here for historical reasons only and is now rarely used.
IMPORTANT Before initial use, and if the system has not been used for sometime the following measurements should be made, and compared with the quoted values.
- Magnet continuity.
- Magnet / cryostat, switch heater / cryostat, and magnet / switch heater isolation.
- Switch heater resistance.
Suggested sweep rates are described in the specification section of this manual.
Running the magnet with a PS120-10, PS120-3, or PS120-10 HS Power supply
The following Instructions assume that an OXFORD INSTRUMENTS PSxxx-yy type magnet power supply is being used. (xxx defines the maximum current, yy defines the maximum output voltage).
The instructions that follow are sufficient to cover the basics of running a magnet. For more detailed instructions and description, consult the power supply instruction manual.
The PSxxx-yy allows operation of the magnet either manually or under control of a computer (using the RS232 link, of IEEE488 interface if the optional converter is fitted).
IMPORTANT: Before initial use, and if the system has not been used for some time the following measurements should be made, and compared with the quoted values.
- Magnet resistance
- Magnet to cryostat isolation
- Switch heater resistance
- Switch heater to cryostat isolation
- Magnet to switch heater isolation
- Before connecting the PSxxx-yy to the electricity supply, check the rating plate on the rear of the unit corresponds with the supply voltage being used. Now connect the magnet current leads and the persistent mode switch heater lead to the terminals inside the rear cover of the power supply.
- Connect the leads to the cryostat magnet terminals and the appropriate ten pin seal. Check for electrical isolation from the cryostat.
- Switch on the magnet power supply. The power supply will indicate successful initialization by displaying the firmware version e.g. 'PS2.04' then 0.00.
- Select the mode of display required, this can be in Amps or Tesla by pressing the button labelled CURRENT/FIELD (the ratio of these is set in the software for a given magnet). Set the current or magnetic field to which magnet is to be energized by pressing the RAISE and LOWER buttons on the ADJUST panel while depressing the SET POINT button on the DISPLAY panel. Set the rate of change in a similar way by pressing RAISE and LOWER while depressing the SET RATE button. Please consult the results section of this manual for advised energization limits.
- If the magnet is equipped with a persistent mode switch, press the HEATER ON button on the SWITCH HEATER panel. The button should be pressed, the indicator light will come on. Wait 30 seconds to allow the switch to heat up before proceeding.
- The magnet energization can now be started by pressing the SET POINT button on the SWEEP CONTROL panel. The current or field will be seen to increase on the digital display and the output voltage will have been seen to kick to the voltage needed to overcome the magnet impedance on the analogue meter (if fitted, i.e. not the PS120-3).
- When the set point has been reached, the switch heater can be turned off by pressing the HEATER ON button again. After waiting about 30 seconds for the switch to become superconducting, press the ZERO button on the SWEEP CONTROL panel. The current in the magnet leads will decrease to zero leaving the magnet, still energized, in persistent mode. The rate at which the leads alone can be swept is faster than the magnet and leads, this is automatically taken into account in the power supply firmware.
- The magnet can be taken out of persistent mode by using the following procedure:
Pressing the SET POINT button on the SWEEP CONTROL panel (the switch heater is left 'off'). The current leads will be swept at a fast rate to the Set Point value. Turn the switch heater current 'on' by pressing the HEATER ON button. Wait 30 seconds for the switch to warm up. Press the ZERO button on the SWEEP CONTROL panel and the magnet will start to de-energize. The Set Rate can be increased during the sweep without stopping.