2011/11/28

A leak check was conducted before opening the bottom lid. The IVC background He signal was 5.6E-7 mbar l/s. Under such a high background, no leak was detected when spraying Helium around the bottom Indium seal.

When the bottom lid was opened, most part of the Indium wire came off really easily. It was pretty much like too much vacuum grease was applied to the wire. This could be the cause of the leak detected at 77K.

A few dents were found on the sealing surfaces of the flanges after they were cleaned. There must be some hard chips falling between the sealing surfaces when the lid was put on. The chips could open a gap between the flanges and cause the leak. And the gap could give us the wrong impression that too much grease was applied to the wire.

I don't know where the chips were from. There are a few possibilities. First, they could be in the Indium wire. But I don't think this is very likely. Second, some chips migrated from the dirty scissors when cutting the indium wire. In recent runs including the last, the two ends of the wire were left outside the flanges. So this should not have caused the dents. Third, they might be in the screw holes before the lid was closed. And they were pushed out when inserting screws into the holes. I think this is possible. Fourth, they were from somewhere else and fell/jumped into the gap between the two flanges when the lid was closed. I don't know if this could happen. To avoid such a problem, the wire cutting tool and the screw holes of the bottom lid should be cleaned as carefully as the flanges.

There are a few other scratches on the bottom lid. They don't trigger single-point failures but possibly make the task of making a good Indium seal harder. A few pictures were taken to document them. The top flange is scratch free except the 'big' dents looking by eye.

Fig 1. I don't remember seeing these dents last time. They also show up on the top flange. Fortunately, they are outside the sealing part of the surface.

Fig 2. There are tiny scratches between the two screw holes. Indium should be able to fill the scratches when pressed.

Fig 3. Little dents on the outer edge of the flange close to the black spot. Other scratches are also on the outer edge.

Fig 4. A scratch at the inner edge pointed by the black spot.

Fig 5. Same scratch as in Fig 4.

Fig 6. A scratch close to the inner edge pointed by the black spot.

Fig 7. Same scratch as in Fig 6.

Fig 8. A tiny groove very close to the inner edge. Hard to see because of light.