CoolDown

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The cryostat is equipped with two vacuum ports, one for evacuation and the other for gauges, leak testing and/or a secondary pump if required.

The Edwards Model EXT70 turbo pump (rated at 45 litres/second, and fitted with a KF40 flange) must only be operated in orientations where it is hanging below (or at) the horizontal plane. It should never be run above the horizontal plane.

1.1 Reduce the vacuum vessel pressure to <0.2mBar using a roughing pump (this should take about 8 hours), then use the turbo pump to get the pressure down to <1E-3 mBar.

1.2 Leak test the cryostat, remembering to check the window seal (this may mean removing the window cover).

1.3 If the instrument is planned to be kept cold for several months, it should be pumped for a minimum of three days, with five days recommended. The cryostat may also be back-filled with grade 3 nitrogen on the second day of pumping to help clean it out, though this process can be ignored if it has not been opened up.

1.4 The pre-cool procedure should only be started once the measured outgassing rate is well below 1E-4 torr litres per second (cryostat volume = 1.5 cubic metres). In other words, if the turbo pump is valved off, the pressure should rise at less than 0.2E-3 mBar per hour. For information: at this outgassing rate, ice will be formed at a rate of 15 milligrammes per day.

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Wear suitable clothing (gloves, face-mask etc.).   The nitrogen pre-cool produces a lot of nitrogen gas, especially in the first couple of hours. Be sure that the area is well ventilated and use an oxygen level monitor with an audible alarm.   Do not under any circumstances remove the fill pipes from the cryostat while Nitrogen cans are full.   This would cause liquid Nitrogen to spurt out under pressure causing serious injury.

Be careful not to put any excessive strain where the pipe goes into the instrument as this might cause a vacuum leak into the main cryostat. The pipes need to be supported and you should monitor the vacuum pressure closely during dewar changeovers.

   This is currently a ~26 hour procedure when done without interrupts. The temperature v time plot for the September 1999 cooldown  is shown in Figure 2.1.

PIC?

Figure 2.1. Pre-cool curves from log file 990908_0849.log. The horizontal scale is time [hours], and the vertical scale is temperature [K].

   The volume of liquid nitrogen required is approximately 600 litres. Figure 8.2.2 shows the rate at which the nitrogen is consumed and the corresponding temperature reached by the grating drum.

Figure 2.2. Typical rate of consumption of liquid nitrogen during the cooldown procedure.

AIM To leave the cryostat in a safe state during or after the nitrogen pre-cool procedure by emptying the pre-cool chambers of liquid nitrogen and evacuating them of all gas.

3.1   Changing the Dewars

A sharp temperature rise on radiation shield LN2 can indicate an empty dewar.

Two people are needed for this operation equipped with protective clothing (gloves, face-mask).   Close the N2 supply valve on the storage dewar and switch off the auto-fill system.   One person should support the fill pipe while the other removes the solenoid from the dewar.   Take the empty dewar away and replace with a full one, reconnect the solenoid, open valve, switch on auto-fill system and check the flow of N2.

3.2 Close the N2 supply valve on the storage dewar.

3.3 Remove the fill pipe from the storage dewar and aim it at a container suitable for liquid nitrogen. Remove the drain pipe from the auto-fill sump and quickly insert a bung or plug in its end. The nitrogen should be blown out of the instrument through the fill pipe. If necessary use a low pressure supply of nitrogen gas to pressurise the tank using the drain pipe.

3.4 Remove the drain and fill pipes, then defrost and dry out the bare pipe flanges where they exit the cryostat. Fit a safety pop-off valve on every flange (to avoid an over-pressure explosion caused by any liquid that might be left in the tank or one of the fill pipes).

3.5 Fit a vacuum valve and evacuate the nitrogen tank to a pressure of <2E(-3)mBar. Close the vacuum valve and remove the pump. The pipework should then appear as shown in Figure 3.1. (NEED THIS PHOTO)

3.6  To Restart the pre-cool, backfill the tanks with dry nitrogen gas until the pop-off valves pop-off.

Figure 4.1 Cooling with the CTI1020, October 1998. The main optics (red) are seen to reach their final temperature (32K) after some 200 hours. The target for the main optical bench is 25K in 140 hours.

  The following table records the dates when the main Michelle optical bench was cooled to <100K.