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The diode lead resistance ‘issue’

The diode lead resistance ‘issue’. Contents: Diode geometry Measurements performed in the past Measurements in S56 on 6 dipole diodes Measurements in S56 on 4 quad diodes Origin of the large resistances / discussion / conclusion. A. Verweij, TE-MPE, CSCM workshop 7/10/2011.

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The diode lead resistance ‘issue’

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  1. The diode lead resistance ‘issue’ Contents: • Diode geometry • Measurements performed in the past • Measurements in S56 on 6 dipole diodes • Measurements in S56 on 4 quad diodes • Origin of the large resistances / discussion / conclusion A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  2. The goal of the test Investigate in situ in the machine if the heat developed in the magnet & diode & diode leads will propagate towards the 13 kA joint, eventually causing it to quench. I Dipole magnet (1.8 MJ @ 6 kA) Dipole magnet (1.8 MJ @ 6 kA) 21 cm Joint I ‘heat sink’ contacts ‘half moon’ contacts Diode Diode Similar for a quadrupole A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  3. While performing these tests, unexpectedly large voltages were measured on the diode leads I Dipole magnet (1.8 MJ @ 6 kA) Joint I Udiode Ulead,A Diode Ulead,C A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  4. The dipole diode Rc,moon Lower diode busbar Diode box, Helium contents : 5 liter Upper heat sink Rc,hs Voltage taps on the diode Rc,diode Lower heat sink A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  5. Dipole to diode connection Voltage taps Main busbars Upper diode busbar (partially flexible) ‘Half moon’ contact towards diode A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  6. The quadrupole diode Diode 2 Diode 1 Rc,hs Ansys model from S. Izquierdo Rc,CL Rc,CU A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  7. The diode lead resistance is the sum of: A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  8. The bolted contacts in the diode leads have been discussed many many times in the EEWG in the years 2003-2006 (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.” Large values of Rc,moon were reported in the early production, both for ‘cold’ and ‘warm’ measurements. This lead to a slightly modified design of the half moon, and to more stringent procedure for the cleaning and assembly of the diodes, with increased torques. This resulted in a clear reduction of the contact resistances to a few mW. Testing at warm in SMA18, and tests at cold in SM18 were added to the magnet reception test program. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  9. About 250 diodes have been repaired at CERN. The plots below show the resistances Rc,moon and Rc,hs before the repair. After the repair all resistances were below 5 mW. cathode anode cathode anode A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  10. What was measured in the past? Dipole diodes: A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  11. Measurements on 6 dipole diodes in S56 - Diode voltages at 6 kA - Diode lead voltages/resistances at 2 kA - Diode lead voltages/resistances at 6 kA A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  12. Measured magnets All numbers in kA In total 28 heater induced quenches. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  13. Diode voltages for 6 kA quenches Conclusion: Forward voltage (and hence the heating) over the 6 diodes is very uniform. (s<10 mV) Diode cooling down Diode blocks Magnet not yet fully s.c., all current in magnet Magnet s.c., all current in magnet, U=L*dI/dt A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  14. Diode lead voltages for 2kA quenches Note the large difference between voltages of B15R5-run2 and run1. }At 1.5 kA A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  15. From voltage to resistance • The current in the diode lead is not known, but should be equal to the current in the power converter (IPC), except for: • t<t1, because the current is still diverting from the quenching magnet into the diode. • (t110 s for 2 kA, t13 s for 6 kA) • t>t2, because the magnet starts recovering, and the current is slowly transferring back from the diode into the magnet. • (t230-40 s for 2 kA, t2120 s for 6 kA) • So the effective resistance of the diode lead is obtained by dividing the voltage by IPC, valid for t1<t<t2 Validity range t1 t2 A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  16. Diode lead ‘resistances’ for 2 kA quenches }At 1.5 kA • Conclusion: • - Resistances constant in the 10-40 s range. • Resistances up to a factor 2 larger than • measured at cold in SM18. 1.8-3.6 mW: measured at cold reception in SM18 A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  17. Diode lead voltages for 6 kA quenches • Conclusion: • - Large spread among the 12 leads. • ‘Steps’ occurring in first 15 s. • Significant difference between the • voltages of B15R5-run1, run2, and run3. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  18. Diode lead ‘resistances’ for 6 kA quenches These curves cannot be explained by ‘normal’ Joule heating in the resistive busbars and in the contacts. 1.8-3.6 mW: measured at cold reception in SM18 A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  19. Diode lead ‘resistances’ for B15R5 Anode The results are not reproducible!!! - The two 2 kA curves differ a factor 3-4. - The three 6 kA curves differ a factor 2. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  20. Summary dipole diodes C=Cathode, A=Anode R1: resistance measured during cold reception in SM18 R2: maximum resistance measured during the 2 kA quench(es) R3: maximum resistance measured during the 6 kA quench(es) Rmax: maximum resistance measured during all quenches Umax: maximum voltage measured during all quenches A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  21. Measurements on 4 quad diodes in S56 • Main differences w.r.t. dipole diodes: • Circuit time constant is 9.2 s instead of 50 s. • Resistance of a quadrupole aperture is about 6x smaller than a dipole (for T>10 K). • Opening of the diode after a quench takes longer. • Current transfer from a quenched magnet into the diode is therefore slower. • The upper diode busbar is much longer (about 1.6 m instead of 0.4 m). • 3 bolted connections (2xM5, 2xM5, 3xM5) instead of 2 (4xM6, 4xM6). • The quad diode busbars have 2x smaller cross-section A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  22. Measured magnets All numbers in kA In total 9 heater induced quenches in the 4 apertures in parallel. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  23. Summary quad diodes (preliminary, analysis on-going) All resistances in mW “Difference in resistance between 1st and 2nd/3rd quench at 3000 A A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  24. D16R5 anode: 2 consecutive quenches at 5 kA Step in Dt<50 ms Probably a movement in one of the bolted connections during the first tests, resulting in an permanent increase in the contact resistance. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  25. What could be the origin of • these excessive resistances? • Lorentz force causing a reduction of the force on the contacts, possibly resulting in (micro)movement of the contact. • Thermal gradients, especially between the heat sink and the lower diode busbar. • Local heating at the microscopic contact points. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  26. 2 rules of thumb: 1. Interface temperature: Tc=(Tbulk2+V2/4L)0.5 The microscopic contact spots can reach very high temperatures at high currents and may then soften/deform or even melt, altering the contact resistance from quench to quench. Tbulk=10 K Ni-Ni contacts with 4 M6 bolts 2. Interface contact resistance: Rc=(r2pH/4F)0.5 r=resistivity H=Vickers hardness F=applied force Rc can change strongly with temperature because r=f(T) and H=f(T). T can increase strongly with current because P = I2Rc. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  27. Simulations Electro-thermal simulations are ongoing with QP3 and Comsol. However, there are many unknown parameters, microscopic effects, and irreproducible results. At present we are unable to simulate what is going on in the diode lead and, more important, what would happen at 12 kA…. Comsol output for the final temperature of the dipole diode after a 6 kA quench with Rc,moon=40 mW (adiabatic conditions) 90 K 180 K 95 K D. Molnar A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  28. A working group is looking into detail into the functioning of the diode and the leads. Analysis and tests are on-going/planned to find out what is happening • Cold tests in SM18 on several diodes (planned for Nov) • As similar as possible to the machine but with additional instrumentation. • Currents up to 12 kA. Tests at 1.9, 4.3, and maybe 20 K. • Tests on a few diodes at 300 and 80 K (ongoing) • Small current (10-1000 A). • Applying a force on the diode busbarsimulating the Lorentz force in the machine. • Electro-thermal computations using QP3 and Comsol (ongoing) • Mechanical computations using Ansys (ongoing) • Mechanical measurements (ongoing) • Torque preload characteristics of the bolts, behavior of the washers, … • 4th series of quench tests in the machine (to be discussed if useful) • Tests in SM18 on a dipole + diode (not before 2012, but probably not possible due to stability issues in the cryogenic feed box) A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  29. Conclusion 1/2 Voltages over 12 dipole diode leads and 8 quad diode leads have been measured at currents between 0.76 and 6 kA. Resistances are up to 15 times larger than measured during cold reception tests in SM18, and seem to strongly increase with the current. The observed spread in the resistance of these leads is very large (factor 20), indicating that much larger resistances are likely to be present in some of the other 4000 diode leads of the machine. There seem to be at least two different phenomena at the origin of the excessive increase in resistance. The results are irreproducible, and correct simulation is presently not possible due to the large number of unknowns. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

  30. Conclusion 2/2 Large, and rather erratic high resistances in the diode leads are very worrying, because safe operation at 12 kA cannot be guaranteed. But on the positive side, we have not experienced any problem with the high current training quenches during the 2008 hardware commissioning. Many tests and simulations are on-going to understand the origin of the excess resistance, and investigate the diode lead behaviour at higher currents. The CSCM test is a relatively fast method to map all the diode leads of one (or more) sectors at several current levels. Note that this test would very likely permanently increase the resistance of many bolted connections. Proper setting of the thresholds is important and not at all trivial. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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