1 Table of Contents Want full page pictures with most important parts labeled in the beginning. Precool Procedures (Perform before cooling down): 1. Check N2 and 4He gas tanks to be sure that there is sufficient industrial grade gas for experimental run, if not order more from Kevin (U store). 2. Order more LN2 and LHe for the cooldown day. LN2 is ordered from Valley Natural Gas (651-628-4848) with customer #M6673. LHe is ordered from William Voje (Bill) if he has any available. If Bill does not have sufficient He, it can be ordered from Praxair (1-800-772-9247) with Purchase Order #49065. Should ask Kevin Roberts to order from Praxair before doing it oneself. 3. Pump OVC below 2 10e-7 mbar (overnight) On turbo pump, top black valve needs to be unscrewed all the way, small silver valve screwed in. When venting turbo pump to change connections turn off pump and slowly open small valve to vent tubing before making new connections. Turn on the turbo pump. During overnight pumping one can turn the emis off.
2 Remove cap and unscrew the OVC valve after pumping the lines to below 1 10e-5 mbar. Note in cooldown log at what vacuum OVC was at prior to pumping. 4. Pump IVC below 2 10e-7 mbar (overnight) First flip valve and wait for vacuum to occur in T part of plumbing then open the IVC, resistance will be met by 4He bath vent tubing. Be sure to close OVC first, generally one does not want to pump both the OVC and IVC at the same time. Note in cooldown log at what vacuum IVC was at. 5. Pump sample space below 2 10e-6 mbar through sample space pumping port and probe sliding seal port (~30 minutes) using turbo pump, not roughing pump. If probe is
3 not connected can pump the sample space directly through gate valve using a Klein valve coupler. Can also pump 3He line; be sure 3He dump and cryopump are closed before doing so! 6. Pump and fill 4He bath with N2 gas. Close 4He recovery line valve (should still be closed from last run, perpendicular to line). Turn on mechanical pump.
4 Pump 4He bath (and all other pipes to remove moisture and air) through 4He bath exhaust port using big mechanical pump. V2 directed to exhaust and V1 parallel to line. Need to also have the 4He bath valve open (parallel). To use mechanical pump, open small valve slowly at first until ~ 20 mbar on vacuum gauge then large valve to ~1 mbar after small valve is opened all the way. Note how hard pump is working to decide rate of pumping. Once good vacuum is achieved, stop pumping and close the valve of 4He bath exhaust line (should form 90 o with line). Valve has to be pulled out before rotated and then pushed back to lock it in to place.
5 Connect N2 gas to 4He bath. Fill 4He bath with N2 gas to atmospheric pressure; use N2 gas from LN2 tank. 7. Pump 1K pot. Close the needle valve using log monitor program (automated). Pump 1K pumping port using the big mechanical pump. Want to attain ~10 mbar vacuum. 8. After desired vacuum is achieved need to check the 1K needle valve. Stop pumping 1K and slowly open the needle valve to send N2 gas from 4He bath. Watch how the pressure of 1K changes (note % at which pressure begins to change in cooldown log).
6 9. After checking the needle valve, pump the 1K pot to 10 mbar again and close. Be sure to not pump 1K during cool down. 10. Pump He transfer tube using turbo pump. Note in cooldown log at what vacuum transfer tube was at prior to pumping.
7 Transfer LN2: Begin by making sure that the 4He bath is not being pumped, should be at atmospheric pressure with N2 gas. 1. Fill the 4He bath with liquid N2 (3 Hr.) Replace one of the two 4He bath relief valves with a one way Tee valve for venting to nitrogen jacket for pre-cooling and connect with rubber hose. This step is done so that the LN2 that comes out of the 4He bath still cool can do double duty by precooling the LN2 jacket. Fit one filling/venting port with thick tube for venting.
8 Screw in 4He bath siphon transfer tube; do not forget the rubber stopper! Disconnect 4He level meter during filling of LN2 in case LN2 leaks during transfer but connect back for 4He filling. Connect LN2 transfer from LN2 tank to 4He bath siphon cone. 2. Depressurize tank to below 10 psi before transfer, should already be the case unless the relief vent has not been closed. Begin filling with fill rate of about 1K/min. Be mindful of filling too fast with rubber tube unfrozen as it will breathe until it is completely frozen which may result in it becoming disconnected, cracking or being destroyed. Monitor the temperature of 4He bath with AB sensors located at the top and bottom of the magnet, used for indicating the level of the collected LN2, during filling and using Sorb once it starts to register. One can stop filling after AB sensors have leveled off, signaling collection of liquid. Do not fill 4He bath completely with LN2! 3. Add exchange gas 4He into IVC Put 10cc (about the amount of the T) 4He gas into IVC through the IVC pumping port to assist cooling. Want to do this to assist cooling at the most convenient time (pumping originally is to remove anything that may not be 4He since it can not be easily pumped out when cold). This is best done by connecting 4He gas to IVC intake, filling the T, pumping out the 4He gas introduced into the T to be sure all moisture is gone, fill T again, and then introduce 4He gas in T into IVC.
9 4. Blow out liquid N2 from 4He bath to liquid N2 jacket. Remove filling tube from blow out and connect tube from one way Tee to blow out tube such that LN2 in 4He bath can be forced into the N2 jacket. PICTURE HERE Not yet taken!!!! Replace the Tee valve with relief valve. Connect 4He bath port with N2 gas, to assist in removal of LN2 from 4He bath to nitrogen jacket. Want 100-200 mbar above atmospheric pressure in 4He bath. Take the blow-out tube out and seal 4He siphon entry once LN2 removed from 4He bath. Signs that LN2 is no longer in bath: nitrogen level on ILM doesn t go up any more (if it had started registering, which it might not have) and/or pressure of 4He bath drops to atmospheric. Once all LN2 is blown out continue filling the nitrogen jacket from LN2 tank to bring the liquid N2 level up to highest level, defined as running out of LN2 in tank or LN2 spraying out the venting tube. Be sure to remove LN2 connector from tank when finished, not just the tube! Once filling of nitrogen jacket is completed replace the venting tube with thin walled tube.
10 5. Load the sample probe for pre-cooling If probe has not been connected, ground sample after checking contacts, both 2 terminal and source/drain current check. Connect appropriate wire connectors to probe head. Keep the gate valve closed and mount so that the gate valve can still be opened. Pump the probe space through pumping port and the sliding seal pumping port for ~10 minutes, alternatively could use the turbo pump to pump sample space. Once complete, close sample space pumping port valve while keeping the sliding seal pumping port open and pumped. Open the gate valve and push the sample probe down until the copper cone is 50 cm above the contact on the 1 K pot, should hear a click. 6. Add exchange gas 3He into sample space and pre-cool sample space Want to add in a small amount of 3He to help cooling, only about as much as would be available in the 3He lines. Proper procedure is to open the cryopump valve, close the cryopump valve, open the 3He port valve, close the 3He port valve. Want to attain sample temperature 100 K using LN2 only.
11 Table 1 Allen Bradley sensors in 4He bath. Location Connector color Resistance (Ω) R. T. 77 K 4.2 K 4.2 K Measured R1 10 cm above LPR Red-Yellow 158 179 961 952 R2 On LPR Red-White 156 177 927 926 R3 Top of magnet Red-Blue 160 183 937 940 Transferring liquid 4He: 1. Pump and fill 4He bath with N2 gas Pump 4He bath through 4He bath exhaust port using big mechanical pump in same manner as before. Pressure should fall steadily; if it hesitates there may still be LN2 in the bath which indicates need to vent the bath with N2 gas and go back to blow out step to remove remaining LN2 as before. If no LN2 in the bath, close the valve of 4He bath exhaust port. Connect N2 gas to 4He recovery port and fill the bath with N2 gas to atmospheric pressure. 2. Pump and fill 4He bath with 4He gas Pump 4He bath through 4He bath exhaust port using big mechanical pump in same manner as before. Close the valve of the 4He bath exhaust port. Fill the bath with 4He gas to atmospheric pressure. Pump 4He bath again and the recovery line and then fill the 4He bath back to atmospheric pressure, do not fill the 1K line. Check Sorb and 1K pot temperatures; they should be close to 77 K, higher is okay. 3. Transfer liquid 4He into 4He bath Connect recovery line to the system, should run parallel to the line.
12 Open the recovery line valve above the small mechanical pump in the pump room and connect 1K. Transfer tube needs to have direct coupler going into 4He bath and the V coupler in dewar. Transfer tube in dewar should not be all the way in the bottom if LHe obtained from Bill, need to leave some so dewar stays cold. If it is a Praxair dewar, it can be bled dry. The transfer tube can be put in the cryostat all the way into the siphon cone without pre-cooling for initial transfer. As liquid goes up to top of magnet, transfer tube should be pulled out a few inches. When pressure starts to increase and transfer starts to slow pull transfer tube out. Magnehelic gauge should read about 10 inches of water. Pressure in the storage dewar needs to be 2 to 3 psi. If T 1, which measures the temperature on the recovery line, is much lower than 266 K the transfer is going too fast. 4. Pump out exchange gas in IVC to assist cooldown Pump out the exchange gas in the IVC once Sorb/sample temperature goes to 20 K. Want to attain 2 10e-7 mbar so will be pumped overnight. 1 K needle valve can be opened a little bit to assist the cooling and close it to slow down the cooling if necessary without pumping. Do not cool the 1 K below 4.2 K while pumping out the exchange gas as it will create an unacceptable heat load later! 5. Add 3He gas into the sample space. When the temperature of the Sorb/sample drops to ~10 K, can remove 3He from cryopump by opening the 3He port valve on the insert and then opening the valve on the cryopump to remove 3He from the cryopump, then close it.
13 To remove 3He from dump, open the 3He dump, should be ~475 mbar to start with, and needs to remain open at all time! Close 3He port valve on the insert. The Sorb temperature will shoot up, after the pressure of the sample space drops to 20-30 mbar, open the 3He valve on the insert to admit more 3He, then close the valve. Repeat a few times to bring the pressure of the dump below 20-50 mbar. Final result is 3He valve on insert is closed. 6. Can stop pumping and close the IVC the next morning.
14 Recondense 3He: 1. Begin pumping 1K pot using the big mechanical pump. The pressure of 1 K pot should be a few mbar; need to constantly adjust 1 K needle valve to bring the 1 K temperature close to 1K (typically 1.5 K). 2. Make sure the probe is totally inserted and sample temperature is below 10 K. If not, lower the probe slowly until the copper cone meets the 1 K pot, wait until it cools and slowly lower it the rest of the way. Sliding seal can be closed once the probe is fully inserted. More information can be found on this in the next section. 3. Set the Sorb temperature to 32 K in incremental steps to keep from over heating the 1 K, using auto PID; should result in 3He pressures of 250 mbar, if done well, then it will decrease. 4. Set the Sorb temperature to 27 K and turn 1K needle valve off when the sample space pressure goes down to 150 mbar. Cool sample space to base temperature: 1. Let the sample temperature and the 3He pressure level off for at least 10 minutes at a Sorb value or 27 K. 2. Turn off Sorb heater and open the 1 K needle valve by 1/16 turn to assist in cooling the Sorb. Can open more to cool the Sorb later. 3. The system can then be left to cool to base temperature (~250 mk) in about 1 hour.
15 Remove the sample probe when the system is cold: 1. Check that the valve on the vacuum lock of the probe and the 3He valve on the insert are both closed. Make sure that the gate valve is fully open. Pump the probe sliding seal using the probe pump. DO NOT open the sample space valve or all 3He will be pumped away. 2. Set the Sorb such that there is ~30 mbar of exchange gas. Slowly lift the sample probe out of the cryostat, use clamp to hold it in place, (takes 10~15 min.) in several stages. Proceeding too fast if frost forms on the outside of the probe. 3. When probe is withdrawn completely and close to room temperature one can turn off Sorb heater and let it cool to reduce the vapor pressure of the 3He. 4. Once the 3He pressure equalizes at ~15 mbar and the probe is warm to the touch, close the gate valve. The sample can now be left to warm naturally to room temperature or use dry N2 gas through the small valve on the vacuum lock of the probe, found between the sample space valve and sliding seal port valve. 5. Close the probe pump and disconnect the pumping line from the probe. Remove the probe assembly by disconnecting the Klein flange. Insert the sample probe when the insert is cold: 1. Check contacts on sample about to be mounted using Fluke to make sure consistent with previous cooldowns.
16 2. Once sample put into probe, check the sample contacts again before inserting the probe using 2-terminal resistance and check that current can be sent through each contact. 3. Connect the probe assembly by connecting the Klein flange and pumping line. Pump the vacuum lock/probe space through the sample space valve and sliding seal port valve on the probe and then close the sample space valve. 4. Pump the sliding seal port while inserting probe. Open the gate valve; use the clamp to hold the probe in this position. 5. Set Sorb such that there is ~30 mbar of exchange gas to assist cooling. Lower the probe slowly until the copper cone is 30 ~ 50 cm above the contact on the 1 K pot, should hear a click. 6. Push the probe to its final position in steps. Can go fast initially but must slow down towards the end. Open the 1 K needle valve a little more if necessary to assist cooling should take at least an hour. One can tell if going to fast by sharp increase in pressure in the 4He bath and/or recovering gas monitor increasing rapidly.
17 Warming the system/removal of 3He: 1. Magnet must be de-energized (turned off) and 1K must be vented or pumped continuously. 2. To remove the 3He from the system (perform when system is between 4.2 and 80 K) open 3He valve on insert and be sure 3He dump is open. Close the 1 K needle valve and pump 1 K. Set Sorb to 50 K or possibly higher. 3. Open valve on cryopump, wait for pressure to equalize between dump and 3He insert then close 3He dump. 4. Slowly lower cryopump into liquid 4He and keep valve on cryopump open, wait 10 minutes. 5. Close the valve on 3He insert and open 3He dump. With cryopump valve open slowly remove it from the 4He. 6. When pressure on dump stabilizes close dump and lower cryopump back into 4He, wait 5 minutes. 7. Open valve on the insert, wait until it stabilizes and then close. 8. Open dump valve and lift cryopump up again. Repeat until all 3He in insert is removed, should take between 3 and 5 tries, with final dump reading of ~475 mbar. 9. Close 3He insert and dump and put cryopump into 4He again to remove 3He from lines. 10. Close the valve on the cryopump while it is still cold and remove it from the 4He. 11. Once 3He has been removed can let the system warm naturally being sure that the gases can vent, i.e. that the 4He bath is still venting to the recovery and nitrogen jacket is not pressurizing. Once the system is completely warm the recovery line can be closed.