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--- stm:first_time_cooling_procedure [2006/03/10 11:21] – (old revision restored) 127.0.0.1
+++ stm:first_time_cooling_procedure [2006/04/24 12:45] (current) – galli
@@ Line 1: / Line 1: @@
+====== First time cooling procedure ======
+Description: the procedure consists in evacuating the insulations between the tanks and the outer world and the tanks and the VTI. After flushing the needle valves and the capillaries with dry Helium, liquid Nitrogen can be poored into the Helium bath and left there to precool for 1 night. The liquid Nitrogen should be completely expelled and then liquid Helium transfer can start.
+===== Evacuating the superinsulation and the inner space =====
+  - The cryostat is warm (T1 and T2 indicate 28 Ohm = 300K).
+  - Connect the 2 tubes to the superinsulation pumping port and to the inner vacuum space (IVS) pumping port, with the valves closed. Never vent these two spaces (superinsulation and IVS) to air!
+  - Prepump the tubes with the rotary and the diffusion pump (use this as a check of the pumps).
+  - Close the back and front sides of the diffusion pump, open the bypass and slowly open the valves to the supersinsulation and the IVS. Pump these spaces with the rotary pump till the high pressure gauge indicates 10-2 mbar.
+  - When the 10-2 mbar have been reached, open the back side of the diffusion pump, close the bypass and slowly open the front side of the diffusion pump.
+  - Pump till 5 or 6 x 10-6 mbar have been reached. Normally this can take 3 to 6 days (depening on the time the cryostat has been warm).
+  - When you are done, close the valves, (disconnect the tubes), switch off the pumps.
+===== Precooling with liquid Nitrogen =====
+Before precooling, clean the capillaries of the VTI and the lambda-plate, or air freezing in the lines will cause blockages and operation of the VTI and of the lambda-plate won't be pssible.
+  - If not already done, switch the large T valve of the Helium tank from the recovery line to air (follow the flux indicator on the valve). WARNING: when doing this, although the valve goes heavy, do not put any serious force and torque!! If you do, the IVS vacuum might become worse or broken! Always switch this valve carefully!
+  - Close with a blind flange the exit to the air.
+  - Open the VTI valve to the recovery line, if not already open.
+  - Open the lambda-plate valve to the recovery line, if not already open.
+  - Open both needle valves fully. (NOTE 1: these are "heavy duty" neelde vavles. There shaft is very thick and the "needles" are large. This means that they can be open/closed with relatively larger force than common needle vavlves. Furthermore, the VTI needle valve does feel particlularly heavy and hard. Don't be worried about this. NOTE 2: when opening the needle valves, a certain amount of turns is necessary beforeyou'll hear a clear "click" sound. After this sound open the vavlve further and fully till the end. When closing, you won't here the click sound. See the manufacturer manual for details of the constructionof the needle valves.)
+  - Switch on the Helium pump (1 stage rotary pump (Leybold) in room 626, pump cabinet, top pump), with pump exaust to air.
+  - Connect a Helium storage vessel or a Helium bottle to the recovery line.
+  - Close the main He recovery line valve (on the top of the computer desk).
+  - Check that the He transfer port plug is properly closed and sealed.
+  - Close the Helium storage/He bottle valve. Pump out the He tank and the capillaries and the VTI, by opening the big red pumping valve on the pump panel. The pressure will decrease very slowly due to the large space (He tank) and narrow capillaries. Wait till the pressure is at least below 5 mbar and then close the puming red valve.
+  - Open the Helium storage valve. Let the Helium flow for some time (it has to flow through the narrow capilalries).
+  - Repeat the last 2 steps (pump and flush with He) at least 4 times. Now the lines should be cleaned and full of dry He.
+  - Close both needle valves: be sure that they are closed, but do not force them.
+  - Leave VTI and lambda-plate valves open to recovery. OPEN main Helium recovery line valve.
+  - Take a liquid Nitrogen storage dewer, full.
+  - Remove the blind flange from the He-tank exaust and temporarily close (with a ball of cotton or paper) the air exaust of the He tank.
+  - If not already done, switch now the large T valve so that the He exaust goes to air: look on flow direction indicator (operate vavlve gently, no torque!).
+  - Connect the liquid Nitrogen transfer rod (the one with white conical outlet at the bottom) to a flexible teflon tube, if not already connected. Connect the tube to the LN2 out of the storage dewer.
+  - Remove the He tank plug and insert the rod fully (NOTE: the plug sits in a sliding seal. This seal might be the one normally used for He transfer. If too small for the nitrogen transfer rod, remove the seal first. Keep the seal for the end of the procedure.)
+  - Pressurise the dewer and start filling (remove the cotton ball from exaust). Start with 400-500 mbar. When liquid will accumulate (boil off noise reduces) pump pu the pressure slowly, up to 1 bar. Filling can take 45 minutes. Filling must be stopped when the nitrogen overflows out from the exaust.
+  - Close the LN2 dewer, release the pressure. You can leave the transfer rod inside the whole night.
+===== Liquid Nitrogen Removal =====
+After one night the liquid nitrogen can be removed. It's important to remove it completely, or blockages can occur. This cryostat provides a very handy heater at the bottom of the tank, so that the last bit of liquid nitrogen can be boiled away. Do not apply a current that exceed 1 Ampere to this heater (25W), or it will break and the nitrogen removale will become tricky! The removed nitrogen can be collected in the outer nitrogen tank, and the remainig in a storage backet.
+  - Unplug the flexible teflon tube from the LN2-liquid-out of the N2 dewer and insert it in the filling hole (the only hole) of the Nitrogen tank.
+  - Take another flexible tube: connect one end to the LN2-GAS-out of the N2 dewer, and connect the other hand to the He tank air exaust. Use a "Xmas tree" connector for this purpose, if not already there.
+  - Pressurise the dewer to 300-500 mbar. Then open the gas-out. The liquid nitrogen in the He tank, pressurised by the nitrogen dry gas, will start to flow out of the tank via the transfer rod into the LN2 tank.
+  - Normally the LN2 tank will be topped up ( :-) this one to make Simon feel at home) before the Helium tank will be emptied. Take a liquid Nitrogen storage dewer or bucket and fill it with the remanent nitrogen from the tank.
+  - When there's no nitrogen coming out anymore, get the last bit out by switching on the heater: MAX: 1 Amp! (heater R=25 Ohm). Use the LTRONIX Labpac 800T2 current source, don't exceed the limit. Who breaks, fixes.
+  - Stop heating when T1~T2>8.5 Ohm. Switch off the heater. If T1 and T2 still increase, the whole nitrogen is gone and you can start the liquid Helium filling procedure.
+===== Liquid Helium transfer =====
+This goes as described in this page: [[filling_n2_and_he_tanks]]. The only difference is that you should switch the T valve again to the Helium recovery line at a certain moment in the transfer:
+  - Disconnect the nitrogen transfer rod, close the transfer hole with the sliding seal and the black plug.
+  - Disconnect the flexible tube from the nitrogen dewer, remove the dewer.
+  - At the beginning of the helium transfer, when there's still warm helium gas flowing from the liquid helium storage to the tank, leave the tank exaust to air: the remaining nitrogen gas will be expelled in the air and won't be injected in the helium recovery line.
+  - Before liquid transfer starts, switch the T valve so that the helium tank exaust is now connected to the helium recovery line. Operate valve gently, no torque!
+  - Transfer helium till 544 mm is indicated on the level meter.
+  - Switch off level meter.