4 Operating the System

4.1 Evacuating the Cryostat

In order to maintain the thermal isolation of the liquid helium, it is necessary that a high vacuum be maintained in the cryostat outer vacuum case (OVC). The recommended pumping equipment consists of an oil diffusion pump of 50mm diameter, fitted with a liquid nitrogen cold trap. The diffusion pump should be backed by a rotary pump of not less than 25 L/min pumping speed, fitted with a gas ballast facility. All connecting lines should have an internal diameter of not less than 1 5mm and be as short as possible. If plastic or rubber link tubes are used, these must NOT have been used previously to carry or pump helium when evacuating from atmospheric pressure.

1. Connect the valve on the cryostat top flange to the pumping equipment. Using the rotary pump, evacuate the cryostat until the pressure is less than 1 torr, then admit an atmosphere of DRY nitrogen gas and pump out again. Repeat several times, then finally pump to less that 0.005 torr.
2. Switch over to the diffusion pump and evacuate the cryostat to less than 5 x 10-5 torr. Continue pumping at least overnight, to ensure the removal of residual gases trapped in the superinsulation.

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 pressure is greater than 10-4 torr with the system warm, the cryostat should be evacuated overnight with the diffusion pump to less than 5 x 10-4 torr It is recommended that the cryostat is always pumped overnight before use.

4.2 Filling the System

4.2.1 Filling the liquid nitrogen container

Fill helium gas into the variable temperature insert sample space and heat exchanger and the lambda point refrigerator. Close off the needle valve of the VTI and lambda fridge to avoid contamination. A helium filled rubber bladder can be fitted to each space to give an idea of the pressure that exists within. In addition, the main liquid helium reservoir should also be evacuated and Evacuate and flush with dry flushed with dry helium gas to remove any residual moisture.

Connect one of the filler/vent tubes of the liquid nitrogen container to a storage vessel using flexible plastic pipe. Transfer the liquid nitrogen by pressurizing the storage vessel to approximately 0.25 atm. 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 high-pressure gas cylinder fitted with a reducing valve. By using an electrically operated valve between the gas cylinder and the storage vessel, the liquid nitrogen container can be filled and the level maintained using a Liquid Nitrogen Level Controller.

Inspect the liquid nitrogen level daily.

The problems caused by ice formation in the filling tubes can be prevented 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 'O' rings. This can be important if an auto filling system fails to stop the nitrogen transfer when the tank is full. At least one of the liquid nitrogen fill/vent tubes should be connected to a one-way valve (e.g. a Bunsen valve), as a precaution against ice blockage.

Important:

• The nitrogen vent ports should be periodically checked for blockages and cleared when necessary. Ice blockage can occur on the unprotected vents due to air being sucked in through the vents when the LN2 is sub cooled by a relatively high flow of cold helium gas during helium transfer or a quench.

4.2.2 Precooling the Magnet

Before filling the cryostat with liquid helium, the magnet must be cooled to a temperature below 100K. This is done using liquid nitrogen. However the lambda point refrigerator and variable temperature insert must be pumped and flushed with helium gas before any liquid nitrogen is transferred into the helium reservoir. The lambda point refrigerator must be filled with helium gas at 760 torr and the needle valve closed during precooling. The respective exhaust line valve should also be shut. To transfer liquid nitrogen use a length of close fitting stainless steel tubing inserted into the transfer tube entry port.

Control the flow of LN2 to the helium tank, to slowly precool and thereby protect the magnet, by connecting a restriction to the cryostat exhaust line (e.g. the one way valve provided or 12 ins. of 0.25 ins. bore tube) and by transferring the LN2 at an over pressure of approximately 260 torr

Allow the liquid nitrogen to remain around the magnet for approximately 2 hours. To remove the liquid nitrogen from the liquid helium reservoir, insert the stainless steel tube into the reservoir's transfer entry fitting and ensure that it is firmly fitted in the cone mounted on the magnet support plate. Blow out the liquid nitrogen through the tube by over-pressurizing the liquid helium reservoir to not more than 200 torr (4 p.s.i.g.). Ensure that all the liquid nitrogen is removed. Failure to remove all the liquid nitrogen will make transferring liquid helium into the reservoir difficult and may impair the performance of the magnet.

When the liquid nitrogen has been removed, seal off any open access ports on the cryostat and evacuate the liquid helium reservoir using a rotary pump. If, during the evacuation process, a pause is seen in the pressure range 70 - 100 torr, then liquid nitrogen is still present and immediate action should be taken to repeat the blow out procedure, or alternatively, warm helium gas should be blown down the liquid nitrogen blow out tube to the bottom of the liquid helium reservoir, to evaporate the remaining liquid nitrogen. The helium reservoir should then be let back up to atmospheric pressure, with dry helium gas. Repeat the pumping and flushing procedure at least four 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 reservoir to a pressure of less than 10 torr Finally, evacuate the liquid helium reservoir to a pressure of less than 1.0 torr, for approximately 15 minutes, before refilling the reservoir with helium gas.

Pump and flush the lambda point refrigerator with helium gas and check the operation of the needle valves after the liquid nitrogen has been removed.

4.2.3 Initial filling with liquid helium4

Connect the cryostat to the helium recovery system or put a one-way valve on the cryostat exhaust. Position the liquid helium storage vessel so that the transfer tube can be inserted easily into the port on the cryostat. Ensure that the transfer tube is not blocked by blowing helium gas through it.

Remove the plug from the transfer tube entry port and also from the top of the storage vessel. Insert the transfer tube slowly, allowing it to cool gradually. Ensure that the end of the transfer tube is fitted into the cone at the bottom of the services neck. In this way, 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.

Start transferring the liquid helium by pressurizing the storage vessel. (This is generally done by gently squeezing a rubber bladder full of warm gas attached to a port on the storage dewar top fitting). 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 ten litters of liquid per hour. This rate can be increased as the magnet cools.

When the magnet resistance drops to zero, the transfer tube should be lifted slightly so that it is no longer in the cone. The transfer rate can then be further increased in order to fill the liquid helium container.

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 daily.

4.2.4 Refilling with liquid helium

The cryostat should be refilled before the level reaches the 10% mark as indicated by the helium level meter. 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). The correct refilling 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. 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 services neck. DO NOT push the transfer tube into the socket on top of the magnet. Transfer liquid helium in the usual way. If the helium level has fallen below 5% and the magnet is still energised there are two courses of action open:

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

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

Important:

• Beware of the stray magnetic field when bringing magnetic objects close to the system with the magnet energised.

4.3 Operating the Magnet

The following instructions assume that an OXFORD INSTRUMENTS PS120-10 magnet power supply is being used. 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 PS120-10 allows operation of the magnet either manually or under control of a computer (using the RS232 link, or IEEE 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
• Resistances of the heaters for each switch
• Isolation of the heaters for each switch

1. Before connecting the PS120-10 to the electricity supply, connect the magnet current leads and the persistent mode switch heater leads to the terminals inside the rear cover of the power supply.
2. Connect the leads to the relevant cryostat magnet terminals (depending on the mode of operation required) and to the switch heater ten pin seal. Check for electrical isolation from the cryostat. Switch on the magnet power supply.
3. The power supply will initialize by displaying, "PASS", then 0.00
4. Select the power supply output to be displayed in amps. Set the current to which the 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 Section 5 of this manual for advised energisation limits. The SET RATE can be changed while the magnet is being energized so the SET POINT can be the value desired ultimately.
5. The magnet is equipped with two persistent mode switches in series, so press the HEATER ON button on the SWITCH HEATER panel. The button should be held down for about five seconds until the indicator light remains on when the button is released.
6. The magnet energisation 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 over to the voltage needed to overcome the magnet impedance on the analogue meter.
7. When the set point has been reached, the switch heater can be turned off by pressing the HEATER ON button. After waiting about ten 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 software.
8. 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 vale. Turn the switch heater current 'on' by pressing the HEATER ON button for five seconds until the indicator light remains on when the button is released.

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. If it is desired to change the value of the magnetic field, sweep the current leads to the present current or field of the magnet, open the switch by turning on the heater. Press the SET POINT button and RAISE and LOWER to change the Set Point to the new desired value. Make changes to the Set Rate of sweep in a similar manner. Press the SET POINT button on the SWEEP CONTROL panel and the magnet will either energize or de-energize to the new field. If the voltage needed to drive the magnet at a given rate is such that the maximum voltage of the power supply will be exceeded, the power supply will deliver its full voltage and the RATE LIMITING light will illuminate, on the SWEEP CONTROL panel. The sweep will continue, but at a slower rate than intended. In the event of a magnet quench, the power supply will trip to zero amps and the QUENCH light will illuminate.

4.4 Magnet Operation using the Lambda Point Refrigerator

Important:

• When operating the Lambda point refrigerator, boil off from the main helium reservoir will decrease and may reach negligible proportions. It is important therefore that precautions are taken to ensure that there is no chance of the cryostat 'sucking back', thereby introducing contaminants into the helium. It is recommended that the system is linked as in the figure overleaf, with the Lambda fridge pump exhaust pump connected to the main helium exhaust. The oil mist filter prevents possible contaminants entering the cryostat. Also ensure that there is sufficient liquid helium in main reservoir to achieve and maintain low temperature operation of the magnet.

A pump of minimum 1000 I/min. (60 m3/hr) displacement should be connected to the pump exhaust port, as shown overleap. The pressure gauges should be connected to the exhaust port on the cryostat, not on the pump inlet. With the needle valve closed, evacuate the pumping lines. Crack open the needle valve sufficient to maintain the pressure at approximately 50 mbar. Monitor the temperature using the four carbon sensors R1, R2, R3, R4 and a digital resistance meter (use on as high a range scale as possible to minimize self heating of the resistor). R2, R3 and R4 will show a reducing temperature whilst R1 should maintain approximately 4K.

Once at 2.2K, as measured by R2, the flow can be reduced to give approximately 30 mbar pressure. The aim is to provide just sufficient cooling power to maintain the gradient between sensors R1 and R2. If the phase boundary begins to move upwards, the flow should be reduced and vice-versa if sensor R2 shows signs of warming. When adjusting the flow, take into consideration the long time constant of the system and also that greater cooling power will be required if the magnet is being energized or de-energized, to counteract the power dissipation in the current leads and superconducting switches. The lambda fridge can be used with the magnet energized.

A typical lambda point refrigerator performance characteristic can be found in the test section of this manual.

4.5 Operating the Variable Temperature Insert

Changing the samples

1. Warm insert to approximately 300K.
2. Place gas filled bladder on sample space exchange gas port and open the valve to ensure an over pressure of helium in the sample space.
3. Remove sample holder.
4. Immediately seal sample space with baffle set provided.
5. To replace sample holder, follow the same procedure, noting that it may be necessary to turn the sample holder as it enter the sample space

Operating above 4.2K

Above 4.2K the insert can be operated in two modes, namely gravity feed or pumped feed. The former is obviously easier, since no pump is required but provides limited flow and therefore limited cooling power.

a) Gravity Feed

It is recommended that the insert exhaust and pot are initially evacuated with the needle valve closed, and vented to an atmosphere of helium gas, to minimize the chances of a blockage when the needle valve is first opened.

1. Connect the insert exhaust port to the helium vent / recovery position.
2. Open the needle valve to give the required flow and cooling power.
3. Once the required temperature has been attained, restrict the flow until the temperature settles at a value just below that required, or continues to drift slowly downwards.
4. Set the required temperature on the temperature controller and use the heater to bring the insert to the required value.

b) Pumped Feed

1. Connect the insert exhaust to a suitable pump (approximately 450 1/min).
2. Initially evacuate with the needle valve closed.
3. Open the needle valve to give the required cooling power.
4. Operation and control are as for (a) above.

Operation below 4.2K

Temperatures below 4.2K are achieved by pumping against the needle valve, thereby reducing the vapor pressure and therefore the temperature of the helium. Operation can be either continuous or single shot, the latter providing the ultimate base temperature.

Subsequent instructions assume that the insert has been cooled to 4.2K.

a) Continuous operation

Operation is basically as for operation above 4.2K, except that the needle valve is merely cracked open with the pump running. Temperature control is achieved by controlling the pumping pressure via the manostat, the operation of which is described in its own manual. This method will provide a base temperature of 2 - 2.5K, depending on the heat input from the sample support rod.

b) Single shot operation

This mode involves filling the insert pumping pot with helium, closing the needle valve, and then pumping on the helium to reduce its pressure and therefore temperature. It allows the ultimate base temperature to be reached, but obviously the size of the pot restricts the operating time before the helium is exhausted.

With the insert at 4.2K, turn off the pump and revert to gravity feed operation (see Section 2 above). Fully open the needle valve and allow the pot to fill completely. Left unattended the helium would fill the pot to the same level as the helium in the main bath. However, it is advisable to restrict the flow before this level is reached, as it would cause the helium to run up the insert exhaust line, and therefore increase the helium boil off. During the filling the helium boil off from the pot should be monitored, and the filling curtailed when a sharp increase in flow indicates that the helium is entering the exhaust line.

Once the pot is full, disconnect the exhaust from the vent/recovery line and connect to the pump through the manostat. Firmly close off the needle valve and begin pumping, controlling the pressure through the manostat if required. When the base temperature has been attained, the operating time is limited by the capacity of the pot. It is possible to slowly bleed helium into the pot whilst maintaining pumping to prolong this time, although this will tend to restrict the base temperature.

It the pot should run out, simply crack open the pot with the pump running, and refill the pot. Subsequent refills will prolong the hold time as a greater thermal mass is cooled.

4.6 System Close down

Temporary Close down

If the system is to be left unattended for any length of time (e.g. over a weekend period) it is preferable, but not absolutely necessary to de-energize the magnet. However, the following precautions MUST be taken to ensure the safety of the system.

1. Ensure that the cryostat contains sufficient helium to last for the required period.
2. Ensure that there is no chance of the exhaust ports becoming blocked or iced up. The helium exhaust should be connected to a helium recovery system or vented through a one-way valve.

In order to minimize helium consumption the current leads should be deenergised to zero current. The VTI sample space and heat exchanger should be vented to an atmosphere of helium gas - do not allow spaces to become pressurized.

Warming the system

Before warming the system, it is imperative that there is no trapped volume of gas or liquid within the cryostat. In particular the lambda point refrigerator and the variable temperature insert sample space and heat exchanger should be linked into the main bath exhaust and the VTI needle valve left open. (Alternatively, the needle valves can be closed off and the spaces pumped out continually during the warming procedure).

Having adopted the above precautions, and with the magnet de-energized, the system can simply be allowed to run out of liquid helium and left to warm up. If a rapid warm up is desired, either transfer the helium out of the cryostat into a transport dewar or insert the blowing out tube into the transfer entry port and gently pass DRY helium gas through it. This will boil off the remaining liquid.