Disassembling the cryostat (optistat)

Check list before disassembling:


Remove the vacuum hose, the transfer tube, and the electrical connections such that the cryostat is left as shown above.

Unscrew the four black plastic headed screws and then the four M4 screws.

With the screws removed the cryostat looks like this.

The inner parts can now be lifted up by re-inserting the black headed screws into two of the threaded holes in the gray ring.

The cryostat inner parts are now free to be lifted up.

The inner parts can conveniently be placed in a tri-fork.

Remove the radiation shield by removing the four brass screws. When removing the last one, take care that the shield does not fall down on the table.

With the radiation shield removed, the crystal mount is exposed. Be gentle with these parts, i.e. do not place finger prints on the crystal, do not tilt the cryostat since the crystal is only fit loosely, take care that the RF-coils are not bumped into such that they fall out of the groove.

The tension on the crystal is governed by the brass screw shown above. Adjusting this screw is a science in it self. Going to low temperatures the copper and the brass screw contract more than the crystal. This enhances the thermal contact between the copper and the crystal. It also increases the tension on the crystal. One may increase thermal contact by applying Apiezon N grease on the crystal rim, but this also affects the tension. Getting good performance is a matter of trial and error. If you want to mess around with this, ask Brian for advice. If he does not work in Lund anymore, give him a call and save yourself numerous hours of work!

The other side of the crystal mount. The RF-leads carrying the 10MHz and 17MHz fields to the coils are clearly seen. Desolder these leads if you wish to dissasemble the cryostat further (there are no pictures of this at present but is is straightforard).

Reassembling the cryostat:

Reassembling the cryostat is essentially just the reverse of the above. There are a few things to note, though:

  1. The windows on the radiation shield are quite difficult to clean since there is Apiezon N grease between the glass and the metallic parts of the shield. Practically, two sets of windows may be cleaner than the other two. Make sure, that the cleanest set is placed in the direction where the laser light passes the crystal.
  2. Before re-inserting the inner parts of the cryostat into the outer shield, make sure that the gaskets are clean and greased.
  3. After the electrical connections have been re-established, always check the resonance conditions of the RF-eraser circuit. To do this consult the manual pages on adjusting the RF-eraser circuit.