1. Functional specification for the consolidated LHC dipole diode insulation system and consolidation strategy
C. Scheuerlein
on behalf of the LHC dipole diode insulation consolidation working group: M. Bednarek, G. D'Angelo, M. Duret, L. Grand-Clement, F. Lackner, F. Meuter, S. Le Naour, H. Prin, T. Sahner, F. Savary, C. Scheuerlein, J.-P. Tock, R. Van Weelderen, G. Willering)
First Internal LHC Dipole Diode Insulation Consolidation Review, 10 October 2017

2. Outline The present insulation system
Insulation system consolidation: assumptions, strategy and associated risks
Functional requirements of the consolidated insulation system
Compatibility requirements with existing equipment
Geometrical
Assembly and disassembly
Electrical
Mechanical
Hydraulic
Radiation
Quality control
Conclusion

3. Assumptions for the insulation system consolidation
The design of the present half-moon splice insulation pieces is satisfactory, and the present insulation pieces can stay in place, provided that they are properly installed.
The installation of the missing insulation plates is the key for the successful consolidation of the dipole diode insulation system.
Taking into account the criticality and the shorts to ground already encountered, a more robust insulating system shall be achieved.
Once they are mounted the insulation plates never need to be disassembled.
Only in rare cases the insulation tubes need to be disassembled. The only reason for future dismantling of the insulation tubes is the need to open the half-moon splice, for instance in case of magnet replacement or diode exchange.

4. Consolidation strategy
Open both flanges of T-piece and of diode container
Half-moon splices will not be disconnected
Measure half-moon splice resistances at warm before consolidation
Clean and remove debris from the diode box and cold mass extremity where accessible
Present insulation plates will stay in place if installed
Verify maximum gap size between insulation pieces and busbars; if necessary close gaps with an epoxy adhesive
Add missing insulation pieces onto half-moon splices
Insulate the presently blank diode busbars below the half- moon splices using a single easily mountable insulation piece
Measure half-moon splice resistances at warm after consolidation

5. Insulation plates LHCMB_E0077 and E0078
By design the electrical insulation of the upper half moon splice surface is done with a pair of insulating plates (LHCMB_E0077 and E0078).
Assembly of the insulation plates can be conveniently done before welding the diode container to the cold mass, but is more tricky afterwards (see presentation of F. Meuter).
It is estimated that insulation plates are only present on about 15% of the half moon splices.
The reason for this is probably that connecting screws have extended in some cases out of the upper half moons, which would make it impossible to tighten the splice properly when the insulation plates are in place.
The stainless steel screws that fix the plates E0077/78 are electrically insulated with epoxy adhesive, which would make it difficult to remove them.
Diode busbar with half-moon splice and insulation plate LHCMB_E0078.

6. Main risks when mounting the missing insulation plates
Risk: It is not possible to mount the insulation plates with standard geometry, for instance because the busbars are not perfectly aligned.
Mitigation: Have available different sets of insulation plates with different geometry. Add preformed Polyimide foil that covers the half moon splice (see presentations of T. Sahner and F. Meuter). If needed fill larger gaps between busbars and insulation plates with an epoxy adhesive.
Risk: Screws extend through the upper half moon splice
Mitigation: New insulation plates have blind holes that can accommodate screws extending up to 5 mm.
Risk: We need to disconnect a larger number of half-moon splices because of non conformities, e.g. to repair damaged insulation tubes.
Mitigation: Foresee enough time margin and have an effective half moon splice quality control available.

7. Main risks when insulation plates are already installed
Risk: Present insulation is damaged or not well mounted.
Mitigation: Prepare a repair solution that allows to disassemble the existing insulation plates. This may imply mechanical stress on the half-moon splice, grinding, and heating (radiation protection, possibility to degrade half-moon splices).
Risk: Gaps remain between the insulation pieces that are so large that metal chips can fall into them and make a short.
Prepare efficient and solid repair solutions, for instance using epoxy adhesive to close larger gaps.
Risk: When using epoxy adhesive this may not be compatible with the existing insulation system and the half-moon splices.
Mitigation: Select suitable epoxy and perform careful test campaigns prior to the use of epoxy in the LHC.
Risk: Part of the bypass circuit (e.g. of replaced magnets) will not be validated by a CSCM test.
Mitigation: Further develop the half moon-splice quality control at room temperature (RT), establish the relation between RT and 1.9 K half moon splice resistance for splices without defect and with different defect types.

8. Main risks when mounting the insulation insert
Risk: The insert may be damaged during LHC operation and the connecting metallic screws become exposed.
Mitigation:
Minimise cross sectional area exposed to He pressure differences (see presentation T. Sahner)
Foresee enough mechanical stress and withstand voltage margin
Perform careful FE calculations to optimise the insert design and materials (see presentation F. Lackner).
Verify FE results by experiments (see presentation F. Meuter).

9. Functional requirements
See LHC-DQD-ES-0003
To be completed …

10. Conclusion
The installation of the presently missing insulation plates is key for the successful diode insulation consolidation.
Mounting of the insulation plates in the present situation is much more difficult than it was during first magnet installation. The insulation plate geometry needs to be adapted.
A careful quality control of the already installed insulation plates is needed, and large gaps between insulation pieces and busbars need to be closed.
The functional requirements of the consolidated insulation system have been defined and are summarised in the draft functional specification LHC- DQD-ES-0003.
Design optimisation of the new insulation pieces is ongoing, taking into account FE calculation and mechanical test results.
The final design solutions need to be qualified in a series of mechanical and electrical tests, as well as by mounting tests in dedicated mock ups.
It may be required in certain cases to disconnect the half-moon splices.