Top Loading Procedure
Introduction:
The dilution refrigerator is designed to allow samples to be loaded into the cryostat without warming. A special top loading siphon (two parts) is used to pre-cool the sample and sample-holder to liquid helium temperature and then insert the holder into the tail piece attached to the mixing chamber.
Care must be taken not to let air into the system, to pre-cool the sample-holder to liquid helium temperature and not to damage the baffles during loading and unloading the sample.
Loading the sample into the DR (two persons are required)
1. The two parts of the sample holder should be cleaned and screwed together finger tight (anti-clockwise). Then the sample holder should be screwed hand tight (anti-clockwise, left hand thread) into the lower end of the top loading siphon. Finally, use a wrench and the DR torque wrench and apply a maximum torque of 25 in-lbs between the siphon and holder.
Use Teflon tape to mark a distance of 154 cm from the top of the siphon (this marks how far to insert the siphon into the IVC before pre-cooling, step 2 below). Also, note the sample face orientation with respect to the crosspiece at the top of the siphon (this will help you find the correct orientation for seating the sample, step 7 below).
2. Insert the siphon into the vacuum lock and pump it out. Make sure the two little green valves (on top of the DR) are open to pump both halves. When the pressure is reduced to 10-4 mbar, close the lower valve then open the gate valve and lower the siphon until the Teflon tape reaches the top of the vacuum lock system. Distance X in Fig. 1 should be about 190 cm, in this position the sample holder will be about 12 cm above the 4.2 K baffle (take care not to go too far and damage the top baffle). During this procedure the sliding seal should be pumped on (keep upper green valve open) to prevent air which passes through the O-ring from entering the IVC.
3. Insert the pre-cooling siphon into the filling port at the top of the DR.
Ask some one to close valve 32 and make sure the pressure in the main bath does not go above 5 PSI (above atmosphere). Slowly insert the pre-cooling siphon into the main bath and connect the two siphons together. Check that they are not blocked by observing the gas coming out the siphon exit. Attach pumping line to little gas pump and turn it on.
4. Prepare the DR by turning off the mixing-chamber heater, still heater and stopping circulation by making sure both the QSB valve and valve 12 are closed.
WAIT 20 MINUTES the sample holder should be at about 4.2 K.
5. Stop the little gas pump and release main bath pressure (open valve 32).
Remove the pumping line from the siphon a jet of liquid will be seen.
Remove pre-cooling siphon and put the cork on main bath port on top of the DR.
6. Turn the radiation baffles handle to "OPEN" (1/4 turn) and remove the Teflon tape before lowering the top loading siphon through the mixing chamber to the tail piece.
7. The face of the sample holder should be oriented correctly to fit into the seat and stop muons in the sample. If this is not the case then when the distance (X in Fig. 1) is 75.5 cm the holder will hit the sample seat.
Turn the sample rod right and left with slight downward pressure until the flats are aligned and the holder goes through to the seat then the distance X should be 72.0 cm.
8. Rotate the siphon clockwise to load the sample.
The clockwise motion first unlocks the coupling thread between the two parts of the sample holder Fig. 2 (the operator will feel the break). Continued clockwise rotation will locate and lock the upper part of the sample holder into the tail piece. The locking of this piece insures that thermal contact between the sample holder and the tail is obtained. Continued clockwise rotation (the operator will feel the break between the sample holder and the siphon) the siphon must be rotated further until it is free to be withdrawn. The siphon is then pulled out completely, the baffles and gate valve are then closed.
9. When the sample holder cools to the mixing chamber temperature the pressure on G4 begins to fall then helium-3 from the dumps can be allowed back into circulation by slowly opening valve 12. When the pressure in P2 drops to about 1mbar transfer circulation back to the booster pump and then turn on the still and mixing chamber heater.
Unloading the DR sample}:
The sample is unloaded by essentially following the same procedure described above with the following exceptions:
1. The wait time for pre-cooling the siphon can be reduced to 15 minutes.
2. First touch on removal the distance X = 71 cm.
3. To unload the sample the siphon must be rotated counter-clockwise
4. A break is felt after the insertion into the pressure plug is complete. Second break is felt when the pressure plug is free.
SKIP TO NEXT STEP --- DON’T DO THIS
After the second break lift the siphon about 1 cm to engage the flat surface and continue to turn counter-clockwise until it is tight, this ensures that the transition screw is engaged and that there will not be much play in the holder when it is removed.
5. Raise the siphon all the way up and make sure you feel the holder hit the top of the vacuum lock before closing the baffles and lock.
Loading Samples
1) Miss orient by 90˚.
2) X = 190 cm pre cool to 4.2 K wait 20 min.
3) Open baffles and insert until hit, X = 75.5 cm.
4) Re orient 90˚ to line up flats, finish insertion X = 72.0 cm.
5) Rotate clockwise to break coupling thread, continue for about 8 turns to release.
6) Rotate
clockwise
7) Continue to rotate clockwise to break the coupling between the siphon and insertion tool, rotate about 10 more turns to disengage the top loading siphon.
8) Remove siphon to vacuum lock, close gate valve and baffles.
Unloading Samples
1) With siphon above baffles pre cool to 4.2 K wait 15 min.
2) Open baffles insert siphon to mate insertion thread X = 71 cm.
3) Rotate counter clockwise about 10 turns to engage insertion thread and break the coupling between the pressure thread and mounting thread.
4) Continue to rotate
counter clockwise
5) SKIP TO STEP 6 --- DON’T DO THIS
Raise about 1 cm to engage coupling thread. Slowly rotate counter clockwise until it is tight in mount. Do not over tighten as you will damage the sample holder and it will get stuck inside the DR.
6) Remove to vacuum lock.
7) Close gate valve.