Update on recent work in MPE & MSC on the large diode lead resistances

1. Update on recent work in MPE & MSC on the large diode lead resistances Arjan Verweij, TE-MPE Contents: Diode geometry Observations in the tunnel R&D outside the tunnel Conclusion A. Verweij, MPE-TM meeting, 15/3/2012

2. The diode (incl. heat sinks and diode leads) is the bypass in case a magnet quenches. • Similarly to the bus, the diode has to carry 12 kA, t=100s (RB) or t=30 s (RQ). • Any high resistive singularities (e.g. contact resistances) can cause local overheating. • Opening of the diode lead can result in local dissipation of up to 1 GJ !!! A. Verweij, MPE-TM meeting, 15/3/2012

3. The dipole diode Rbus-bus (aka ‘half moon’) Lower diode busbar Diode box, Helium contents : 5 liter Upper heat sink RHS-bus Voltage taps on the diode Rdiode-HS Lower heat sink A. Verweij, MPE-TM meeting, 15/3/2012

4. The quadrupole diode Lower diode busbar Upper diode busbar Diode 2 Diode 1 RHS-bus Ansys model from S. Izquierdo Rdiode-HS Rbus-bus A. Verweij, MPE-TM meeting, 15/3/2012

5. The diode lead resistance is the sum of: The electro-thermal behavior of the copper leads and heat sinksis well understood, and they are sufficiently over-dimensioned. Rdiode-lead=Rdiode-HS + RHS + RHS-bus + Rlower bus+ Rbus-bus + Rupper bus Only local heat sources in the contacts may cause problems. A. Verweij, MPE-TM meeting, 15/3/2012

6. The past At the start of the production, very large ‘half moon’ resistances were frequently observed, up to values of a few 100 mW. The bolted contacts in the diode leads have been discussed many times in the EEWG in the years 2003-2006, especially the “half moon” contact. (see: http://lhcp.web.cern.ch/lhcp/tcc/powering/eewg/eewg.htm). The minutes of 18/9/2003 state: “…the baseline design leaves the possibility for potential dangers.” An agreement was set during reception to target Rdiode-lead<15 mW, in order to make sure that Thalf-moons<340 K and Twafer<300 K. Rbus-bus was closely monitored, and production values show and average of 2.5 mW. Rdiode-HS was measured for all diodes during reception with 10 powering tests at 13 kA, t=120 s. Final values were typically a few mW. However, the fully assembled diode leads have never been tested in situ in the machine. A. Verweij, MPE-TM meeting, 15/3/2012

7. First surprise in May 2011 when we measured diode lead resistances in several dipoles of the LHC that were much larger than expected. These curves cannot be explained by ‘normal’ Joule heating in the resistive busbars and in contacts of a few mW. A. Verweij, MPE-TM meeting, 15/3/2012

8. As a consequence more tests were performed in TS’s in July, Aug and Nov on 6 dipole diodes and 6 quadrupole diodes. A. Verweij, MPE-TM meeting, 15/3/2012

9. Members: MSC Marta Bajko Luca Bottura Gaelle Dib Elvis Fornasiere Christian Giloux Ludovic Grand-Clement Michael Guinchard Susana Izquierdo Bermudes Philippe Perret Herve Prin Frédéric Savary - chair Gerard Willering MPE Mateusz Bednarek Knud Dahlerup-Petersen Giorgio D'Angelo Daniel Molnar Andrzej Siemko Arjan Verweij The Working Group on Diodes in the LHC Main Magnetswas set up in Aug. 2011. The first meeting of this informal WG was to agree between the groups MPE and MSC on a work plan for off-line (outside the LHC machine) testing of the diode leads. Since the start we had about 10 progress meetings and a workshop (on 15 Nov 2011). cern.ch/diodes A. Verweij, MPE-TM meeting, 15/3/2012

10. Main topics: Computations (see next talk by Daniel). Mechanical tests. Electrical measurements under loading. Data history from series production and testing at ENEA. Data history from tests in SM18. New diode tests in SM18. A. Verweij, MPE-TM meeting, 15/3/2012

11. 2. Mechanical tests: focus on bolts, washers, helicoil, loctyte, surface quality, … From S. Izquierdo A. Verweij, MPE-TM meeting, 15/3/2012

12. 2. Mechanical tests: Conclusion • There are several design “issues” concerning the 3 different contacts. • However, there is no clear systematic problem that could cause large contact resistance. • A large contact resistance can only be explained if the assembly procedure is not followed precisely, and if the Quality Control is not done properly. A. Verweij, MPE-TM meeting, 15/3/2012

13. 3. Electrical tests under loading: focus on RHS-bus M. Bednarek, G. D’Angelo, K. Dahlerup-Petersen A. Verweij, MPE-TM meeting, 15/3/2012

14. 3. Electrical tests under loading: focus on RHS-bus 50 N 100 N M. Bednarek, G. D’Angelo, K. Dahlerup-Petersen A. Verweij, MPE-TM meeting, 15/3/2012

15. 3. Electrical tests under loading: Conclusion • The effect of a repelling force is visible, but is not the key parameter that could explain the effects seen in the tunnel. • No clear difference between 300 K and 80 K testing. • High resistances can be reproduced by not following the assembly procedure when redoing the connections. • A dirty contact can be vulnerable to the forces and deteriorate in consequence, but such contacts should have been observed during reception tests. M. Bednarek, G. D’Angelo, K. Dahlerup-Petersen A. Verweij, MPE-TM meeting, 15/3/2012

16. 4. Data history from series production and testing at ENEA Focus on Rdiode-HS testing at 4 K, I=13 kA, τ=120 s. Frascati test. CERN test. Training effect clearlyvisible in CERN measurements. G. Willering

17. 4. Data history from series production and testing at ENEA

18. 4. Data history from series production and testing at ENEA: Conclusion • ENEA reported the Rdiode-HS as measured in the final test, at the end of the current decay. These values were usually very small, although values in the first few runs and/or values at the start of the decay were often >10 mW. • Rdiode-HS seems to deteriorate during time. • Consecutive testing improves the contact. • Nobody really knows what is happening, but anyhow, dangerously high contact resistances, or a‘run-away’ is never observed. A. Verweij, MPE-TM meeting, 15/3/2012

19. 5. Data history from tests in SM18: Focus on Rbus-bus(‘half-moons’) Data from M. Bajko, F. Bertinelli

20. 5. Data history from tests in SM18: Conclusion • Only the resistances of about 60% of the magnets/diodes are reported in the database. • The average diode lead resistance was about 2.5 mW, but several diodes with much larger values (up to 10mW) have been accepted. • Warm measurements are performed at small DC current (<10 A) and cold measurements at an AC pulse of about 1 kA. This is not representative for the behavior at high currents. A. Verweij, MPE-TM meeting, 15/3/2012

21. 6. New diode tests to be done in SM18 • Several dipole and quadrupole diodes have now been tested in SM-18, some of them with additional instrumentation. • More tests to be done, possibly with different surface condition of the contacts. • Also a test of multiple Rbus-bus contacts of the quadrupole diode lead is in the pipeline. A. Verweij, MPE-TM meeting, 15/3/2012

22. 6. Rdiode-HS Different non-understood behaviors observed Data from Chr. Giloux A. Verweij, MPE-TM meeting, 15/3/2012

23. 6. RHS-bus Different non-understood behaviors observed Data from Chr. Giloux A. Verweij, MPE-TM meeting, 15/3/2012

24. Conclusion: • Large diode lead resistances, as observed in the machine, were already observed in the past (at ENEA and in SM18), but ‘kind of forgotten’. • Testing of contact resistances at small currents (<several kA) is not representative fro the behavior at nominal current. • A high resistance at the diode to heat sink contact is not very critical because: • the heat sink is sufficiently over-dimensioned • it does not run-away during the current decay • it does not increase considerably at higher currents • However, the soundness of all the bolted contacts is not proven because they have never been tested in-situ in the machine at high current. • Even though opening of a bolted contact is not very likely, a test in the machine at high current (CSCM) should be envisaged (if possible). A. Verweij, MPE-TM meeting, 15/3/2012