Herman ten Kate and Alexey Dudarev

1. MCS 600A sextupole circuits (update on study performed in May 2010) Herman ten Kate and Alexey Dudarev Content: References General observations Electrical circuit Insulation issues Forces Conclusion (update of status mentioned in this color) LHC Task Force Splices, Review-1, 19 Oct 2010

2. References Based on interviews of Alexey Dudarev and myself with: • Mikko Karppinen • Paolo Fessia • Marta Bajko • Nuria Catalan Lasheras • Antonio Perin • Reiner Denz Charge: Find issues that may impact • safety of the correction coil circuits (not to lose them) • main Q and D circuits (arc to Q or D bus) • cause severe damage to He circuits, insulation vacuum (arc to ground)

3. General observations • There are about 106k low current splices in the various corrector coils circuits • Obviously we can’t check them all from a-to-z • We studied what is accessible: open magnets in B180 and some documentation • And we tried by interviewing the responsible engineers to find critical areas that may be improved • This focus in this report is mainly on the sextupole correctors MCS, but many observations hold for all circuits as well • 12 Recommendations formulated and status updated

4. MCS: how it looks

5. Electrical circuit and documentation • MCS coil with internal protection resistor, 154 in series • Connected to HTS leads, dump resistor, cascade of 3 switches and power convertor • Internal protection resistor can take up all energy during fast dump • 550A, 0.8mH, 19 kJ, R// = 0.08Ω • Integration documentationis incomplete: • electrical scheme showing all details, including routing, all welds, bus sections, voltage taps does not exist, hindering fast analysis and will slow down intervention repairs • Cure 1: make integration drawings of all circuits (!) • Update: acknowledged, work started

6. Electrical insulation: test voltage • In factory: 1000V, but in SM18 mostly 600V, in tunnel 600V • But is this enough? What is the maximum differential voltage between the corrector coil circuits/bus mutually and to the Q bus? • Following A.Verweij: - 600A circuits show 0.7Ω at ±600A, thus some ±420V - Main Q bus shows Ƭ=9.2s at 6kA, thus 190V max • Meaning that ΔV_max = ~610V between MCS and bus and 840 mutually! • There is no or negative margin in the test voltage and thus the risk of electrical breakdown is real, in particular by aging with time • Normal safety factors are 2-3 at warm and 1.1-1.5 at cold, here <1 • Cure 2: Do check all circuits again, make table of maximum circuit voltages; agree on a sensible higher test voltage for new assemblies, however, not too high in order not to risk damage; say 900 or 1000V • Update: acknowledged, all V’s are verified, new table in progress

7. Quench protection, V-taps, leak current • Safety of the 600A bus lines in the case of a bus quench under various cooling conditions not fully analyzed, not proven, nor documented (though there are some indications that it is safe) Cure 3: Analysis report requested for all spoolscircuits considering the case of single or combined quenches anywhere in the bus Update: acknowledged, analysis to start • Today there isno diagnostics on the corrector magnets, identification of quenched unit not possible, have to go into the tunnel Cure 4: Consider extra voltage taps, wire the taps up to a diagnostic unit Update: to be discussed, not yet decided • There is no ground insulation current monitoring (only interlock at >50mA in power convertor), no early warnings, no degradation monitoring Cure 5: Consider to install leakage current monitoring at μA level Update: to be discussed, no decision yet

8. Electrical insulation of bus to ground • The 15m long bus has Kapton wraps and is encased by G10, however, at its terminals, the last 15cm, it has a weak glass-braid insulation and no G10 casing to ground (easy to do), potentially a weak spot, also because conductors move/deflect in field • Insulation is requested on all bare metal surfaces within the end cap & interconnect • Cure 6: insert an insulation plate (or spray coating) between bus and steel, and if possible on entire interior of the end caps • Update: under discussion, no decision yet

9. Insulation between 600A and Q-bus • Worries: in the case of repair, 1 joint is replaced by 2 and then not covered by the casing • New insulation and new extended casing needed to support the wires (already being done in H180 on the magnets being repaired) • Breakdown voltage between 600A circuit, bus and ground not known • Cure 7: Qualify the present casing by electrical breakdown tests of this arrangement, engineer a new full-coverage solution for repaired areas • Update: under discussion, see presentation Fessia, new insulation casing under development, extra insulation layer present, new solution to be qualified, check what to do with old splice casing

10. Metallic spider between bus and ground • Another worry is the metallic spider with short insulation gaps to 600A circuits, to Q-circuit and to ground • Again, breakdown voltage of present lay-out not known, never measured on many samples Cure 8: Do break down test between spider, all 600A circuits and Q bus Replace them by non-conductive spiders or put insulation tube/sheet between metallic spider and the cryostat tube Update: to be discussed, no decision yet but repair highly recommended !; see presentation Fessia, small series not tested and some spiders show ~3kV, too low value and worst values to be expected in large series

11. Missing clamps on 600A joints • The welded 600A joints have no clamps • Risk of opening of circuits by aging even when initial weld looks fine • Like in the case of 13kA splices one should never relay on single welding but always add a clamp, easy to do, see the pressure clamps on the soldered joints in the DFB’s Cure 9: Consider to put clamps on all new joints (like in the DFB’s); not reasonable to do this for all welds; too many and to much work; Consider best –effort inspection during interconnects opening to reduce the risk Update: to be discussed, no decision yet for new splices, inspection recommended

12. Missing protection resistors on V-taps • In LHC voltage taps do not have a protection resistor, usually 5-10kΩ(incomprehensible !) • Such resistors are required to limit the short-circuit current to ground or other circuits in the case of damaged insulation • If not present the high circuit voltage is in the long cables and on the connectors and everywhere to ground • The voltage tap cables are very long in LHC, • the risk of long range shorts between circuits • through voltage tap cables, cards and racks is considered substantial Cure 10: Put in all new assemblies protection resistors Update: under discussion, no decision yet

13. Bending of 600A bus wires by Lorentz force • The 600A leads show big open loops exposed to stray field of the 6 and 13kA bus • Quick estimate shows ~ 0.1 kg/dm between 600A wires ~ 0.2-0.3 kg/dm between 600A and 6/13kA • The loops will bend in field • The wires are not sufficiently supported • Risk of aging effect with time and finally shorts Cure 11: improve support in all new assemblies • Update: under discussion, • no decision

14. Circuits not yet fully charged Note! • Correction coils and main magnets were not fully charged in SM18 at the same time, nor in the tunnel • Thus the 600A circuits have not yet seen the full mechanical and electrical load of ±600A in combination with 6 and 13 kA in the bus bars and coils • We have to be careful and may see surprises in terms of training and shorts when charging in full • This argument holds for current and voltage Cure 12: carefully check readiness for full load and be prepared for surprises, perform additional test before repair shut-down Update: to be discussed, no decision yet, test program before shut-down planned

15. Cabling and joints in the DFBs • So far no surprises found in the DFBs • Layout and technical realization well documented • Insulation looks solid using right materials, Nomex in stead off glass braids and spacer spiral wraps to keep distance to ground • Joints look fine • Clamps on all soldered joints present Remark: Techniques, materials choices and methods are different here...... Why not uniform solutions throughout the entire LHC, communication and collaboration!

16. Conclusion so far....... • 600A circuits, in particular MCS were screened for weak spots, other circuits to follow • Documentation incomplete, in particular electrical installation/integration schemes • Circuit analysis for faults missing • Flaws on electrical insulation found • Test voltage of 600V is too low to guarantee insulation • Quality of insulation not known, breakdown tests to do • Not protected for circuit joints that open, clamps missing • Protection resistors in voltage taps missing • Many 600A wires not well supported, cyclic deflection/bending under varying loads, risk of weakening/degrading the insulation • Correction coils circuits are not yet fully charged! Update: most recommendations accepted, others still to discuss and decision making is pending, MCO circuit screened as well, next talk