1. Diodes qualification: Results and plans for LHC and spares MSC Technical Meeting on 2013-12-12 F. Savary, on behalf of the diode working group, with special contributions from M. Bajko, M. Bednarek, Z. Charifoulline, K. Dahlerup-Petersen, G. D’Angelo, G. Dib, C. Giloux, L. Grand Clement, R. Moron Ballester, V. Roger, A. Verweij, G. Willering

2. Outlook Reminder Why worry about the protection diode? Description of the diode stacks Quadrupole diode stacks Dipole diode stacks COMSOL analyses + Measurements in SM18 Measurements in the LHC machine Plan for spares – New production and tests Diodes qualification: results and plans for LHC and spares2

3. Why worry about the protection diode? Diodes qualification: results and plans for LHC and spares3 Quadrupole diode lead resistance during a quench @ 5 kA in the LHC machine (first half of 2011): there is a sudden jump of resistance, peculiar and permanent, which could not be explained with the expected behavior of the diode stack Step in  t<50 ms D16R5 a Courtesy A. Verweij.

4. Quad. Diode: key parts, and contacts Diodes qualification: results and plans for LHC and spares4 Diode bus bar Nickel plated everywhere SSS bus bar Silver plated over 40 mm Connection plate Nickel plated everywhere R BB-CP < 2 µΩ R HS-BB < 2 µΩ R Diode-HS < 5 µΩ

5. Dip. Diode: key parts, and contacts Diodes qualification: results and plans for LHC and spares5 Lower diode bus bar R Half-moon < 2 µΩ R HS-BB < 2 µΩ Diode box, He content:  5 liter R Diode-HS < 5 µΩ Voltage taps on the diode ‘press pack’ Upper Heat Sink Lower Heat Sink Bolted contacts with 4 M6 + helicoil, 4 spring washers, 10 Nm Diode compressed by a stack of spring washers to 40 kN All copper parts Ni-plated, 2-3 µm

6. Diode to heat sink behavior R Diode-HS : has typical behavior, confirmed by production data, tests in Bloc 4, and more recent tests @ 4.2 K in SM18 Diodes qualification: results and plans for LHC and spares6 Diodes from S34 All resistances go up at 5 or 10 kA Most of the resistances go down at 13 kA Except for all 16 quad diode resistance (4 stacks measured at the same time). R max up to 43 µΩ at 5 and 10 kA MQB0112 Courtesy V. Roger, G. Willering

7. Quad. connection plates Connection plate Made of Cu ETP Ni-plated Two screws per contact – M5 Three spring washers – CuBe Screws made of St. Steel A4-70 (R p0.2 = 700 N/mm 2 ) Thread lock (Loctite) Bus bars Magnet side: Ag-plated Diode side: Ni-plated Tightening torque 8 Nm (break @ 12 Nm) Diodes qualification: results and plans for LHC and spares7 Original design

8. Quad. connection plates – Behavior Diodes qualification: results and plans for LHC and spares8 Original design Courtesy V. Roger et al.

9. Quad. connection plates Connection plate Made of Cu ETP Ni-plated Two screws per contact – M5 Three spring washers – CuBe Screws made of St. Steel A4-70 (R p0.2 = 700 N/mm 2 ) Thread lock (Loctite) Bus bars Magnet side: Ag-plated Diode side: Ni-plated Tightening torque 8 Nm (break @ 12 Nm) EDMS: 1281434 Connection plate Made of Cu OFE (higher RRR) Ag-plated Two studs per contact M5, screwed in a back plate (made of Inconel 718) Made of Inconel 718, of much higher yield limit (1100 N/mm 2 ), preload increased with same M5 Tightening torque 12 Nm (break @ 20 Nm) Add washer plate to distribute better contact pressure Two spring washers – Inconel 718 Thread lock (Loctite orapi 303) Bus bars Magnet side: Ag-plated Diode side: Ni-plated Diodes qualification: results and plans for LHC and spares9 New design vs. Original design

10. Quad. connection plates – Behavior Diodes qualification: results and plans for LHC and spares10 Original design New design - Stable Due to higher resistance of Cu at RT Courtesy V. Roger et al. Thermal cycle

11. In addition … Diodes qualification: results and plans for LHC and spares11 Shims To (re)center the diode stack... needed for the installation of the back plates New parts (cross) For electrical insulation of each pair of bus bars one w.r.t. each other, especially at the bottom Courtesy L. Grand Clement

12. Procurement challenges Material (un)availability for the fasteners made of Inconel 718, even though not exotic! Contractor Youparts-DE, …and Fdb-Mechanik-CH Issues during manufacture due to the hardness of the material The contractor is not the manufacturer, and the manufacturer has subcontracted to its daughter company in SL, … + lack of QC Mitigation actions were launched: other contracts were placed with two other firms Bold King (UK), delivery expected next week, 540 threaded plates (back plate) Coudray (F), for 1000 studs and 500 threaded plates. This firm may become a fall back solution Diodes qualification: results and plans for LHC and spares12 Courtesy R. Moron Ballester

13. Consolidation status Diodes qualification: results and plans for LHC and spares13 Consolidated assembly Original design 180 done out of 391, i.e. 46% SMACC START Courtesy L. Grand Clement

14. NCR’s regarding insulation sleeves Diodes qualification: results and plans for LHC and spares14 Missing cutouts (QBQI.30R7) Wrong angular position Insulation after consolidation Wrong position - glued Courtesy L. Grand Clement

15. Other NCR’s Diodes qualification: results and plans for LHC and spares15 QBQI.30R7: bare copper in lieu of silver coated bb (magnet side) Nickel coated bb in lieu of silver coated bb (magnet side) Contact resistance HS to BB > 2 µΩ QBQI.21R8 (A: 20.77 µΩ / D: 4.75 µΩ) QBQI.21L8 (A: 3.67 µΩ) Resolution: extraction of the diode, and contact redone Courtesy L. Grand Clement Contact resistance diode to HS > 15 µΩ in QBQI.31R6 (A: 19 µΩ) The diode stack was replaced

16. Dipole diode ½-moon – Behavior @ RT From coordination of LHC cryodipoles (http://lhc- dipcoor.web.cern.ch/lhc-dipcoor/new_reports/dioderepairs.html)http://lhc- dipcoor.web.cern.ch/lhc-dipcoor/new_reports/dioderepairs.html Following repair @ CERN, after delivery from Cold Mass Assembler, before installation 255 magnets from the 3 CMAs Diodes qualification: results and plans for LHC and spares16

17. Dipole diode ½-moon – Behavior @ RT CMA series 2000, data to be checked, also for the other 2 CMA’s series 1000 and 3000 Diodes qualification: results and plans for LHC and spares17

18. Dip. diode ½-moon – Behavior @ cold Diodes qualification: results and plans for LHC and spares18 Measured for the first time in SM18 in Feb. 2012 Are stable at cold / high current R < 2 µΩ required R < 1 µΩ measured Courtesy V. Roger et al.

19. In addition … COMSOL simulations were carried out to determine hot spot temperatures in the MB and MQ diode leads Measurements were done @ cold in SM18 Measurements are being carried out, in the framework of SMACC, in the LHC machine, to determine the actual resistance of the diode leads in the dipole circuits … @ RT Done in S67 On-going in S56 The remaining 6 sectors will be done early next year Diodes qualification: results and plans for LHC and spares19

20. Diodes qualification: results and plans for LHC and spares20 Rc [  ] T hot [K] 9350 11400 13450 Current12 kA Decay time constant104 s RRR (all copper parts) 80 Cooling to heliumNone (adiabatic) R HM =2,5,10,15 , no cooling T MAX over all volume T MAX of contact area COMSOL simulations – ½ moon Courtesy Z. Charifoulline, A. Verweij

21. Diodes qualification: results and plans for LHC and spares21 Rc [  ] T hot [K] 20220 50300 100525 Current12 kA Decay time constant104 s RRR (all copper parts) 80 Cooling to heliumNone (adiabatic) R HS-BB =2,20,30,50,70,100 , no cooling T MAX over all volume T MAX over Bus Bar COMSOL simulations – BB to HS Courtesy Z. Charifoulline, A. Verweij

22. Cold tests in SM18 – Feb. 2012 Diodes qualification: results and plans for LHC and spares22 Courtesy V. Roger et al.

23. Measurements in the LHC machine Measurement procedure, see EDMS 1334252, without opening the diode container The measured magnet is powered via the I-taps of the neighboring magnets of the same line. The test current is limited to 10 A. Measurements through the voltage taps (D20 and P10 connectors) of the measured magnet are then carried out, in both conducting (10 A), and reverse (1.5 A) modes Copper contributions are subtracted Diodes qualification: results and plans for LHC and spares23 MBB dipole Courtesy M. Bednarek MBB dipole MBA dipole

24. Results from S67 ‘as recorded’, … Question: where is the high contact resistance? Diodes qualification: results and plans for LHC and spares24 Courtesy M. Bednarek

25. … Compared to S23, and Frascati Beware that the measuring conditions, and the quantities measured are different Notwithstanding the above, the ‘signature’ is comparable S23 is special ‘after CSCM’ Diodes qualification: results and plans for LHC and spares25 Courtesy M. Bednarek, G. Willering,

26. Another view on these data … Diodes qualification: results and plans for LHC and spares26 Courtesy M. Bednarek, V. Roger Let’s look at the production data of the 30 ‘worst’ diode stacks!

27. Another view on these data … Generally, the higher the resistance at cold, the higher the R at RT MDB0273 was excellent ‘as produced’ Either it has degraded, or another contact is responsible for the higher R Diodes qualification: results and plans for LHC and spares27 Entire production, Frascati, R DIODE-HS, @ cold, 10 th 13 kA decay S67 R DIODE-HS + R HS-BB + R HM, RT, Low current MDB0273 Anode side Courtesy V. Roger

28. Diodes qualification: results and plans for LHC and spares28

29. Why can we feel confident? If any of the resistances approaches 14 µΩ, it is still ok at 12 kA, 104 s, adiabatic conditions, RRR = 80, worst-case conditions (ref. to COMSOL simulations) For the moment, the highest resistance over 3 contacts found in S67 is 14 µΩ, i.e. ok All the half-moon resistances were below 2 µΩ at the installation of the diode stacks in the magnets. Measurements data are available. The ½-moon assembly is robust and stable (4 x M6 on thick flange) There will be other opportunities with diode leads showing higher R (other than MDB0273 in S67) for which we may decide to open the diode container, and to carry out local measurements … hopefully showing that the higher R is located at the diode to heat sink contact Diodes qualification: results and plans for LHC and spares29

30. Other considerations The CSCM that will be done in 3 sectors before the end of LS1 will hopefully confirm safe operation for these sectors. The other sectors may be done before LS2 The two diode leads in S23 that showed resistance higher than 12 µΩ were localized in the same diode stack and on diode to heat sink contacts. This diode stack has been removed and will be retested in SM18 (R DIODE-HS = 22 and 25 µΩ) … we will learn from this test S23 is special, as it will show higher resistances due to CSCM already done in Feb. 2013. We know that all diode to HS resistances are changing during high current runs, and expect that the high resistances are localized there Diodes qualification: results and plans for LHC and spares30

31. Spare diode stacks Contract with Prodtek AB (SE), now finished, after a series of difficulties (due to other commitments of the contractor) For sets of parts, heat sink, bus bars, flanges, helicoils, tie rods, … for 30 dipole stacks, and 10 quadrupole stacks Diodes (‘press packs’) are from DYNEX (UK); 270 units were bought, and there are about 180 left, all Ni-coated Assembly of the diode stacks @ CERN – MPE A new specification will be edited for another IT for 60 sets of parts, to be defined how many dipole stacks and how many quadrupole stacks, and up to MSC to decide whether finished/tested stacks are needed, or whether sets of parts can be stored till the moment they are needed Diodes qualification: results and plans for LHC and spares31 Courtesy K. Dahlerup-Petersen

32. New production – What’s ‘new’? On the 40 sets of parts delivered: Silver coating on all contact surfaces (4 µm thick), except that of the diode (press pack) that remains Ni-coated No EB welding nearby the half-moon, only one remaining Varnish applied on the G10 insulation parts to avoid absortion of humidity … and electrical failure What will be changed for the new contract (the additional 60 sets): Avoid machined insulation parts (G10), consider instead molded parts (not porous) Insulation U-shapes shall also be redesigned to avoid cutting fibers in the angles during machining (otherwise cracking occurs) Diodes qualification: results and plans for LHC and spares32 Courtesy K. Dahlerup-Petersen

33. Test results – 4 dip. d. stacks - New SES37 / 2013-11-05 Diodes qualification: results and plans for LHC and spares33 Courtesy V. Roger et al.

34. Test in SM18 – MDA0001 Diode to heat sink: Max 5 µΩ @ 10 th 13 kA decay Diodes qualification: results and plans for LHC and spares34 Look at the scale! Heat sink to bus bar: Max 2 µΩ throughout the test Courtesy V. Roger et al.

35. Test results – 4 quad. d. stacks - New SES38 / 2013-11-21 Diodes qualification: results and plans for LHC and spares35 Courtesy V. Roger et al.

36. Test in SM18 – MQB2006 Diode to heat sink: Max 5 µΩ @ 10 th 13 kA decay Diodes qualification: results and plans for LHC and spares36 Heat sink to bus bar: Max 2 µΩ throughout the test Look at the scale! Courtesy V. Roger et al.

37. Status on warm / cold testing WhenStack IDNote Total # ‘spare’ Sept. 2013MDB 2005, MDB 2006, MDB 2007, et MDB 2008 Certified4 MDB Sept. 2013MQA 0423Certified, and installed in a magnet- Oct. 2013MDA 2001, MDA 2002, and MDA 2003 Certified, and MDA 2001 installed in 3097 at BBI.A20R2 2 MDA Nov. 2013MQB 2006, MQB 2007, and MQB 2008 Certified3 MQB To come2 MQA ready for cold testing To come4 MQB ready for cold testing To come8 MDA ready for cold testing, and 4 ready for warm testing To come4 MDB ready for cold testing, and 4 ready for warm testing Diodes qualification: results and plans for LHC and spares37

38. Conclusions Consolidation of the unstable bolted assembly of the connection plates in the quadrupoles progressing well (close to 50% done) Difficulties with the procurement of the fasteners made of Inconel now overcome A few other minor NCR’s were discovered, and resolved Further investigations on-going for the dipole diode leads COMSOL simulations were done to determine operation limits (also for the quads) Measurements @ cold, and @ warm, both in SM18, and in LHC The dipole diode leads are safe. The new test results coming soon, should confirm this New production finished, assembly work finished with MPE, and testing on-going at both MPE and MSC New IT will be launched in early 2014 for 60 sets of parts Diodes qualification: results and plans for LHC and spares38

39. Diodes qualification: results and plans for LHC and spares39 Courtesy G. Willering