Consolidation of the dipole diode insulation Technical aspects M. Bednarek, G. D’Angelo, M. Duret, L. Grand-Clement, S. Le Naour, F. Lackner, H. Prin, T. Sahner, F. Savary, C. Scheuerlein, J.-P. Tock

Metal chip on top of half-moon splice seen by radiography Radiographs of diode C12L4 half moon connection (a) before and (b) after the short-to-ground was eliminated. Courtesy A. Piguiet, S. Le Naour and J.-P. Tock. MSC-TM meeting, 4Apr2017

Insulation plates LHCMB_E0077 and E0078 By design the electrical insulation of the upper half moon splice surface is done with the insulating plates LHCMB__E0077 and E0078. However, the procedure LHC-DQDB-AP-0001 rev 0.2 (EDMS No. 851517) explicitly requests NOT to install LHCMB_E0077 and E0078. We estimate that LHCMB_E0077 and E0078 are present on about 15% of the half moon splices (see presentation of Sandrine). It is not clear to us why the procedure LHC- DQDB-AP-0001 requests not to mount these insulation plates, and we don’t see any risk associated to their presence. The stainless steel screws that fix the plates E0077/78 are electrically insulated with Araldite, which would make it difficult to remove them. Diode busbar with half-moon splice and insulation plate LHCMB_E0078. MSC-TM meeting, 4Apr2017

Missing insulation plates on top of the half-moon splice as observed during LS1 The half moon splices represent a bottle neck for the He flow. The insulation tubing LHCMB_E0070 extends about 1 cm above the upper splice surface. Metal chips transported by He flow from the magnet towards the diode container are likely to be trapped on top of the half moon splice. Pieces longer than about 1 cm can bridge the space between splice and container. MB3097 diode bus bar half moon splice insulation (a) before and (b) after repair during LS1. Courtesy L. Grand-Clement. MSC-TM meeting, 4Apr2017

Diode bus bar half-moon splice electrical insulation The design of the present half-moon splice insulation pieces is satisfactory. Taking into account the criticality and the shorts to ground already encountered, a more robust insulating system could be achieved for example by adding insulation to the existing one. The present insulation pieces MB_E0070/E0077/E0078 can stay in place provided that they are properly installed. The impact of a reinforced insulation on cryogenics must be verified. Half-moon splice insulation tubing LHCMB_E0070. Catia 3D model of diode busbar connection inside the diode container. Courtesy T. Sahner. MSC-TM meeting, 4Apr2017

Insulation consolidation scenario 1 Open upper flange of T pipe. Remove all metallic debris and other contamination. Add insulation plates if missing. Additional insulation could be added between busbar and diode box. A grid that prevents large metal chips reaching the half-moon splice could be added. Consolidation should be compatible with existing insulation pieces. Schematic of a grid to prevent movement of large metal chips. Courtesy T. Sahner. Diode mock-up. Courtesy M. Duret. MSC-TM meeting, 4Apr2017

Insulation consolidation scenario 2 In addition to the works of scenario 1 also open the lower flange of the diode box. This would allow to: Remove debris from the diode box. Add insulation to the presently blank diode busbars below the half moon splices. Measure the individual half moon splice resistances at warm on the about 85% splices where the insulation plates are not yet mounted. Blank diode busbars. Schematic of additional busbar insulation. MSC-TM meeting, 4Apr2017

Insulation consolidation scenario 3 In addition to the works of scenario 2, disconnect the half-moon splices and extract the diodes. Main advantage: The individual diodes could be tested and test results could be compared with those obtained before LHC installation. Main disadvantages: We would lose the previous qualification of the diode busbar leads and half moon splices (at warm all lead resistances are <15.5 µΩ, and CSCM test at high current). Opening and reconnecting the bolted splices might degrade their contact resistance. We don’t think that additional insulation between the diode container and the diodes is required because they are both at ground potential, although we do not understand the possible signs of an arc that seem to have been observed once. MSC-TM meeting, 4Apr2017

Possible timeline April 2017: First concepts for the LHC dipole diodes insulation reinforcement July 2017: Functional specification for the LHC dipole diodes insulation reinforcement September 2017: Insulation concept and first insulation pieces for testing December 2017: Validation of the concept and test of a dipole magnet with reinforced dipole diode insulation in SM18 (e.g. MB2638 and/or MB2950) February 2018: Review March 2018: Finalised new insulation design and technical specification April 2018: Mock up tests, start training, finalise procedures December 2018: Ready for dipole diodes insulation reinforcement MSC-TM meeting, 4Apr2017

Conclusion We think that it is very likely that the reason for the 8 shorts to ground that occurred at the diode half-moon splices are metal chips that are transported by He flow onto the top of half-moon splices with missing insulation pieces. We estimate that only about 15% of all half-moon splices have the insulation pieces LHCMB_E0077 and E0078 mounted. The design of the present insulation pieces is satisfactory. The installed pieces can stay in place provided that they are properly installed. Nevertheless, taking into account the criticality and the shorts already encountered, a more robust insulating system is recommended, for example by adding insulation to the existing one. 3 main diode insulation consolidation scenarios are proposed: Scenario 1- open the top flange, clean and remove all debris that is accessible, add the insulation pieces where they are missing, add additional insulation and/or a grid that would prevent large chips to reach the half-moon splice. Scenario 2 - open both flanges. This would allow to insulate the presently blank diode busbars below the half-moon splices, to measure most the half-moon splice resistances at warm, and to remove debris from the diode box. Scenario 3 - disconnect the half-moon splices and systematically remove the diodes. This would allow to re- test all diodes. But: opening and reconnecting the bolted half-moon splices might degrade the splices an their coating, and we would lose the previous circuit qualification. We don’t think that additional insulation between the diode container and the diodes is required because they are both at ground potential, although we do not understand the possible signs of an arc that seem to have been observed once. We could be ready to start a systematic consolidation of all LHC dipole diode splice insulations by December 2018. MSC-TM meeting, 4Apr2017

Back-up slides

LHC dipole mock up.