The diode lead resistance ‘issue’ A. Verweij, TE-MPE, CSCM workshop 7/10/2011 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

Dipole magnet (1.8 6 kA) Diode Joint 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. The goal of the test I I Dipole magnet (1.8 6 kA) Diode ‘half moon’ contacts ‘heat sink’ contacts 21 cm Similar for a quadrupole A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

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

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

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

DiodeQuad Upper diode busbarAbout 410 mm, RRR spec =100 R 4K =0.23  R 40K =0.6  R 100K =5  About 1.6 m, RRR spec =100 R 4K =1.8  R 40K =4.4  R 100K =40  Contact resistances between upper and lower diode busbars (R c,moon or R c,CU and R c,CL ) Contact surface=1600 mm 2 Dipole side: Ag coated (5-10  m) Diode side: Ni coated 4xM6, 10 Nm, 4-5 CuBe spring washers Contact surface=560 mm 2 Dipole side: Ag coated (5-10  m) Diode side: Ni coated 2xM5, 8 Nm, 3 CuBe spring washers Lower diode busbarMaximum 480 mm, RRR spec =100 R 4K =0.3  R 40K =0.7  R 100K =6  135 to 370 mm, RRR spec =100 R 4K =  R 40K =0.4-1  R 100K =3.3-9  Contact resistance between lower diode busbar and heat sink (R c,hs ) Contact surface= mm 2 Both sides are Ni coated 4xM6, 10 Nm, 4-5 CuBe spring washers Contact surface= mm 2 (?) Both sides are Ni coated 3xM5, 8 Nm, 3 CuBe spring washers Heat sink RRR spec =100, R 4K <<0.1  R 100K <0.1  Contact resistance between heat sink and diode (R c,diode ) Contact surface=5000 mm 2 Apparent resistance from inductive voltage <1  assuming a few mV at 6 kA decay Apparent resistance from thermal voltage <1  assuming a few mV for  T=200 K The diode lead resistance is the sum of: A. Verweij, TE-MPE, CSCM workshop 7/10/2011

The bolted contacts in the diode leads have been discussed many many times in the EEWG in the years (see: The minutes of 18/9/2003 state: “…the baseline design leaves the possibility for potential dangers.” Large values of R c,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 . 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

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

What was measured in the past? Dipole diodes: Dipole diodesQuad diodes Diode acceptance tests in FRASCATI at 4.3 K 8-10 endurance tests were carried out with 13 kA U diode, T hs and R c,diode were measured on all diodes R c,diode was decreasing with number of tests, reaching <1  No contact resistances were measured All reports are available Diode tests at CERN at 4.3 KSimilar testing as in FRASCATI, but only on about 76 diodes Warm measurements of the resistance of the diode leads (with I=5-20 A) 691 diodes were measured at CERN aver=2.5 ,  =1.1 , max=11  At Accel and CERN Spec: R c,hs <5 , R c,CU and R c,CL <2  1.9 K measurements in SM18 of the resistance of the diode leads during a  1 s long transient following a heater provoked quench at 3 kA 677 diodes measured (aver=3.3 ,  =1 , max=9.8  ) None A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

Measured magnets DipoleTraining during reception tests in SM18 * Training quenches during HWC in 2008 Tests during TS in May 2011 Tests during TS in July 2011 A15R , (5.2, 4.6) 2, 6 B15R (6.3, 7.4, 6.8)2, 62, 5, 0.76, 4, 6, 6, 3 C15R , 12.7 (2.4) 10.9, , 6 A16R , , 0.76, 4, 6, 3, 5 B16R , 12.4(7.4, 2)2, 5, 0.76, 6, 3, 4 C16R , 11.8, 12.4, 12.8 (0.6, 5.2)2, 6, 4 All numbers in kA In total 28 heater induced quenches. A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

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

From voltage to resistance The current in the diode lead is not known, but should be equal to the current in the power converter (I PC ), except for: - t<t 1, because the current is still diverting from the quenching magnet into the diode. (t 1  10 s for 2 kA, t 1  3 s for 6 kA) - t>t 2, because the magnet starts recovering, and the current is slowly transferring back from the diode into the magnet. (t 2  s for 2 kA, t 2  120 s for 6 kA) So the effective resistance of the diode lead is obtained by dividing the voltage by I PC, valid for t 1 <t<t 2 Validity range t1t1 t2t2 A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

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

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

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

Summary dipole diodes A15R5B15R5C15R5A16R5B16R5C16R5 CACACAACCACA R 1 (  ) R 2 (  ) R 3 (  ) R max (  ) U max (mV) C=Cathode, A=Anode R 1 : resistance measured during cold reception in SM18 R 2 : maximum resistance measured during the 2 kA quench(es) R 3 : maximum resistance measured during the 6 kA quench(es) R max : maximum resistance measured during all quenches U max : maximum voltage measured during all quenches A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

Measured magnets SM18LHC 2008Aug 2011 D14R5 (SSS251) Defocusing A Focusing D16R5 (SSS150) Defocusing - Focusing 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

All resistances in  CurrentD14-cD14-aF14-cF14-aD16-cD16-aF16-cF16-a D-HM1D-HM2F-HM1F-HM2D-HM3D-HM4F-HM3F-HM No diode opening Summary quad diodes (preliminary, analysis on-going) “Difference in resistance between 1 st and 2 nd /3 rd quench at 3000 A A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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

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

2. Interface contact resistance: R c =(  2  H/4F) 0.5  =resistivity H=Vickers hardness F=applied force R c can change strongly with temperature because  =f(T) and H=f(T). T can increase strongly with current because P = I 2 R c. 1. Interface temperature: T c =(T bulk 2 +V 2 /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. Ni-Ni contacts with 4 M6 bolts 2 rules of thumb: T bulk =10 K A. Verweij, TE-MPE, CSCM workshop 7/10/2011

Comsol output for the final temperature of the dipole diode after a 6 kA quench with R c,moon =40  (adiabatic conditions) 95 K 90 K 180 K 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…. Simulations D. Molnar A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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 ( A).  Applying a force on the diode busbar simulating 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, … 4 th 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

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. Conclusion 1/2 A. Verweij, TE-MPE, CSCM workshop 7/10/2011

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. Conclusion 2/2 A. Verweij, TE-MPE, CSCM workshop 7/10/2011