Installing the TPC Cathode Plane

During the period 25 July — 3 August 2000, we unpacked the TPC from its shipping crate, installed the cathode plane, and applied high voltage. Out of caution, this work was done in at Industrial Building 4, where we had use of a clean room for opening the TPC. We then removed the cathode plane, re-crated the TPC, moved it to our assigned space in Lab 7, and uncrated it again. This note documents the process for installing and removing the cathode plane. Separate notes describe crating and handling the TPC and the high voltage tests.

There are four steps to installing the cathode plane:

The cathode plane was shipped wrapped in paper or foam, tied to the top of the TPC inside the TPC shipping crate. Inside a clean work area (we used the clean room at Industrial Building 4), we unwrapped the plane.

The following picture, taken after the plane was installed, shows the screw holes around the perimeter, the slots and vent holes, and the pair of "feedthroughs" (purpose not known to me).

We discovered that roughly half of one long edge had become badly scored. There were also numerous other small marks and scratches scattered about. We polished out the smaller scratches and worked on the larger ones until the edges were smooth. We used the finest sand paper we had at hand, 320 grit, followed by Scotch Brite, followed by KimWipes, all wetted with alcohol, periodically removing the gray sludge with KimWipes and alcohol.

When we finished dressing all the scratches, we wiped down the entire plane with KimWipes and alcohol until the wipes came away with just the aluminum oxide gray, and not any dirt. We also cleaned the screw holes and vent slots with Q-tips and alcohol.

The TPC lid is a 1/4" aluminum plate with a high voltage ladder and isolation plane glued to the underside.

The high voltage plane has two spring clips that contact the cathode plane, which sets the potential.

The inner surface of the lid contacts an O-ring at the top of the TPC, which makes a gas tight seal.

To open the TPC, first, we removed all the screws from the lid. Then we used our fingers and round edge tools, such as nut drivers, to lever it up without translating. We checked that the O-ring was completely detached from the lid and resting in its groove on the TPC.

Because of the resistor chain and the spring clips, a few inches of clearance is required when removing the lid. Because of its weight and size, the lid is awkward to handle. We had six people, one at each corner, and two on the ground to receive it. This job might be accomplished with only four, but two would have to hold it while the other two changed positions.

We did not clean the inner surface of the lid where the O-ring mates, though we probably should have.

With the lid removed, it was straightforward to install the cathode plane. We carried it over the field cage, then lowered it onto the flange. By lowering it in a rotated position, we could set it down gently without trapping our fingers, then slide it into final position so the screws lined up. Note that the copper feed-through plates are on top.

The plane is fixed to the field cage flange by 24 1/4-20 x 1/2" silver-platted socket head cap screws, cleaned for vacuum use. Because of a late delivery from the supplier, we did not have these screws on hand when we installed the plane, so we went through some rigmarole to clean the screws from the lid, which were the same size. I now have a package of new clean screws for the next time we install the plane.

Installing the lid is just the reverse of removing it. Because of the O-ring, it should be lowered into the correct position, not rotated like the cathode. To get all the screws to line up, we found it necessary to leave them very loose until all screws were started.