# Shield box proposal for Bo, as of 1-16-2007

Hans Jostlein

### Summary

In the legendary "Bo-Ducks Meeting" on Friday, January 12 a number of
decisions regarding the electronics for the Bo TPC were taken. They were
documented in Stephen's minutes:

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**Summary of items \`decided\' at the meeting on Friday 12^th^, 2007\
**\
Attendents: Bromberg, Edmunds, Finley, Gerstle, Jensen, Jaskierny,
Jostlein, Merchut, Pordes\
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Configuration - 30 central wires will be read out - decision.

*(*The remaining wires will be connected to amplifier \# 1 and amplifier
\# 32 on each chamber plane)*\
*\
 One feedthrough connector per plane = one spare connector\
\
 Stoplight plumbing - CLK, HJ\
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 NEMA box to be made at FNAL to be sent to MSU   - HJ, CLK, WJ\
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 MSU to make copper box and all decorations - DE\
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 Procure feedthroughs and flange to make dummy setup for noise tests -
WJ, CLK\
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 Isolate the reference wire lands from the frames on A and C - WJ\
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 Solder wires and pulser coaxes - WJ\
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 Clean chamber - WJ, CLK, HJ\
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 Test in Bo with dummy flange setup for noise - DE, HJ, WJ

(Target date is February 11, 2007 for the first such test)*\
*\
 Test in Bo with argon gas -- All

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**The present proposal**

builds on this, and differs only in the mechanical arrangement.

The purpose is to improve grounding and address reference wire issues
that were left hanging.

Please refer to the sketch below.

In the picture shown last Friday, there was an unshielded gap between
the stoplight feedthroughs and the entrance to the copper shield box.
There was to be some unspecified ground connection (via reference wires
or shield braid or both) across the gap.

The proposed structure provides a complete Faraday enclosure from the
stoplight flanges to the PC cards.

This is done by brazing a mounting plane (a copper sheet 0.1" thick;
Walter has the material) to the fixed stoplight flanges. The sheet has
two 6" dia cutouts for the flanges and gets attached to their periphery
at mid-point.

The copper box gets bolted to this mounting plane as shown.

We can now address one of the puzzles we did not resolve last Friday,
i.e. where do the reference wires go inside the cryostat if they are at
ground while the sense wires are at a potential of plus or minus 500
Volt. Connecting them to the chamber frame seems adventurous due to the
many opportunities for leakage currents to the sense wires. Not
connecting them to anything seems "wrong", for unclear reasons.

Remember that for the B plane there is no such quandary; both sense- and
reference-wires are near ground potential, and the reference wire can be
connected with impunity (and solder) to the chamber frame of plane B.

We can now (at least in a test) achieve this happy resolution also for
planes A and C.

This is done by connecting their reference wires to their respective
chamber frames and also to an "AC ground" which is located right next to
the feedthrough on the inside of the copper box, and designed to have
large capacity and almost no inductance to the copper box. It is DC
biased to the appropriate potential for that plane.

Outside the feedthough the reference wires are replaced by the copper
box itself which is connected at the PC card connector to the
amplifiers' reference input anyway.

Why should we use no wires at all? The return current will be attracted
to the copper box anyway because of its very small resistance and
capacitance. Wires cannot compete with that. And, we are only a few
inches away, which means that delay times and inductances are too small
to matter.

Structurally this turns out to be reasonably simple and natural. One
will need to add an internal copper "bulkhead" inside the copper box to
support the PC card connectors and to carry the good ground directly to
the connector locations. This bulkhead will look like the left side wall
of the copper box used to look.

You may also notice a slightly different way of mounting the NEMA box
off the cryostat top plate. The new arrangement was suggested by Cary
Kendziora to mitigate interferences of the NEMA box with all the SS
pipes that sprout from the top cryostat plate.

The new NEMA mounting preserves the crucial feature of being a guard
enclosure. It is connected to the cryostat top plate, but not to any
part of the stoplight or copper box. Any noise currents induced into the
NEMA box flow directly to the cryostat top plate without being given a
chance to induce voltage differences into the stoplight/ Cu box
structure.

The NEMA box mount consists of an 8" sched. 5 (or sched. 10) steel pipe,
capped with a 3/16" thick steel plate at the top. The bottom of the
steel pipe mounts to the cryostat place with a few ¼-20 screws via
welded tabs. The top plate of the steel pipe bolts to the NEMA box,
which may a copper sheet lining its bottom for better contact.

Both the NEMA box and the steel plate have large cutouts to allow them
to slip over the stoplight/ mounting plate assembly. These cutouts are
offset in the two devices to minimize the remaining hole. A patch plate
can be used to cover that hole if desired.

A minor point: The NEMA box has now traveled upward; the "decorations"
will most likely be near its top. By the way, Walter has an 18" wide x
24" high by 12" deep box at hand, but it seems that box is not wide
enough to accommodate the stoplight and the copper box with its PC
cards. The NEMA box is still 24" x 24" x 12" deep.

Please look this over and let me hear your reaction and comments.

Hans

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