Insulation Test at IVW

Insulation Test at IVW

Feb the 3rd, 2006

Insulation test at IVW

IVW/Wuppertal has presented the new stave assembly, hereafter called Dummy_3, meant to qualify the lab for producing ~30 staves with new CC and partially new Omega..

This stave has been assembled with the 25um KAPTON foil underneath of the Omega, ~100um filaments at ~60mm pitch and 60um KAPTON tape underneath of the end of the pipe.

The stave shown a perfect insulation during several measurements done in the past days but yesterday after some marginal manipulation of the pipe (basically pushing and pulling) shown a variable resistance around ~50MOhm.

Stave has been cooled down to -25deg and brought back to room temp in few minutes. Electrical impedance became of the order of KOhm.

To understand what is the reason of the degradation of the insulation the following test has been setup:

  1. A ~100um monofilament has been wrapped around a squared aluminum bar previously polished and cleaned. Since it is difficult to see in the picture they have been marked in red :
  1. Bar was posed on a steel bent. The insulation even pressing hardly was good

  1. Bar is then posed on top of a TMTs and the isolation was good.

  1. At the end the pipe was gently moved longitudinally on top pf the TMTs. The resistance went down to few hundreds Ohms
  1. Looking at the surface of the bar in contact with the TMTs one can clearly see the carbon dust that is collected on the monofilaments or close to it. The wires turn with the motion of the bar, get polluted of carbon and produced the short.
  1. Aiming to proof that the insulation failure was caused by the carbon dust, the same bar has been equipped with <1mm wide KAPTON strip placed at a distance of ~20mm pitch. Bar was passed on the TMT and the short appeared.

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Even in this case is as been noticed carbon dust close to the KAPTON strip that acted like dust collector until a conductor is formed between the aluminum and the CC. Picture shows it rather clearly:

  1. At the end a section of an old stave pipe has been wrapped with an 100um monofilament. Parylene has been removed by sand paper. No contact on the steel bent, no contact on the TMTs, contact appeared after sliding the pipe onto the TMT. Picture is not clear but the presence of carbon dust on and around strings is evident on the real part.

CONCLUSIONS

  • The hypothesis that the electrical shorts observed in some of the old , new and repaired staves are due to the carbon dust from the CC that directly or indirectly (polluting the strings or the grease) put in contact areas of the pipes (with defective parylene) to the CC is supported by the observation.
  • Grease is not necessary for the phenomena to happen.
  • The worsening of the phenomena with thermal cycling is well coherent with this interpretation. The supposed motion of the pipe during the thermal variation should produce an effect similar to the sliding of the pipe onto the CC surface.
  • It seams reasonable to believe that to get rid of the problem a full layer of insulator (KAPTON, epoxy, etc) needs to be interposed between the pipe and the CC. This will obviously deteriorate the thermal properties. We can conclude that the monofilament, and the grease are not reliable to avoid shorts.
  • Although the anodization of the pipe seems a rpiori an appealing solution because it could replace the parylene protecting the pipe with an hard surface, it has to be mentioned that a test has been done at IVW in which an “unfilled” anodized aluminum bar was made sliding on the CC obtaining rapidly a short.

U. Schmitt, C. Glitza, D. giugni

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