ASG-INFN CERN meeting about NC on YT112 QH electrical short issue

The objectives of meeting were to introduce the findings during the electrical inspection in final magnet configuration after rod tightening and discuss the next actions steps to carry out.

1. Findings:

The electrical test with Ohmmeter of V1 AP bottom A coil QH confirmed that the short is on the YT112+ wire. The inspection visual findings was presented showing the YT112 + end wire side with respect to untouched opposite wire. The YT 112 + side does not show damaged QH insulation wire but some unusual view of transverse tape of Kapton. This additional transverse adhesive tape seen was clarified by ASG and is used for maintaining the GI layers in position during collaring assembly.

The question on previous QH NC due to tracking distance spark was raised and it was confirmed that it was related to the other NCS side of quench heater, hence not to CS.  

1. QH wire connection layout
1. ASG  presented the layout of the welded wire connection to tape, with the end shoe groove position. The start of insulated wire is further protected with thermoshrinkage sleeves.
2. The exit part of wires in the end shoe groove is not glued into the bottom of groove before D2 apertures collaring

1. Discussion:
1. The insertion of an insulated Kapton sleeve proved that the short is not located in the outside part of the QH wire but in the upstream part of G10 end shoe of towards the connection. In this region the only grounded piece is nominally the protection SS sheet but is separated and electrically isolated from QH by 4 layers of Kapton Ground insulation. At moment, there is no clear root cause explanation of possible punching from quench heater active parts of ground insulation layers to Ground.
2. It was reminded that the actual design of D2 includes a redundancy of 3 QHs on the two apertures out of the nominal 8 operating QHs.
3. It was asked how much time would require decollaring to investigate and repair this QH. It would actually take as much time ( > 5 months)  as during fabrication of MBRDs as it involved removal of front end plates, pieces, unyoking, removal of aluminium sleeves and uncollaring of individual aperture.

1. Actions:

1.       Although next magnet are in intermediate preloaded state , they will be tested at truck reception step at intermediate values ( tbd) to check any potential issue which could occur after transport

2.       MBRD01 tie rods will be unloaded, not fully but at intermediate load for instance around 10 % of full tightening load using similar hydraulic way as the initial tightening. This will confirm or not that unloading as no influence on the direct short insulation level observed. The rods will be then retighten according to previous nominal applied load.

a-      Planning to be confirmed with 180, including INFN attendance

3.       As a final corrective action step, depending on the evolution of short resistance situation, it will be decided to isolate the QH channel.

PS: ASG slides are attached.