Diode simulations and measurements

1. Diode simulations and measurements Daniel Molnar

2. Dipole diode in magnet cryostat

3. Dipole diode Rc,moon Lower diode busbar Diode box, Helium contents : 5 liters Voltage tap Upper heat sink Rc,hs Rc,diode Lower heat sink From A. Verweij, TE-MPE, 26 Aug 2011

4. Quadropole diode “Half-moons” Leads with 3(or 2) bolts Diodes(identical to dipole)

5. Dipole diode connections to the main BUSbars Voltage tap From A. Verweij, TE-MPE, 26 Aug 2011 Main busbars Upper diode busbar (partially flexible) ‘Half moon’ contact towards diode

6. Dipole and QuadropoleDiode measuring schematics HM3 HM4 UM2 Dipole diode layout(from Arjan’s slides) Quadropole layout(from Wednesday measurement doc. )

7. Tests in May and July at the dipoles Most plausible explanation is that the contact resistances are bigger then design values and they’re even temperature dependent(and current dependent)

8. Quench propagation tests in quadropole circuits(31.08-1.09) • Test @2 ,3,5,5,6,6,3,3 kA • Some of the leads showed similar as observed in the dipole ones, one of exceptionally high(“the wall”) • The real resistances may differ from these in the table, because not all the current is present in the diode(part in the magnet for smaller currents) • Better approximation: we saw a more or less constant lead resistance –surprisingly the pair of the worst-so we can use it to determine the “real” current (most interesting case is shown next slide) • Also the opening voltage is proportional to the Temperature of the diode wafer: good to determine the temperature of the wafer

9. Results of the quad quench propagation tests “real”:30 uOhm From Arjan’s doc. “real”:45uOhm

10. Different current level quenches for the worst observed

11. Modeling the diodes • The models takes the Joule heating as the base physical description, but later other physics were added • Reasonable simplifications i.e.: insignificant, tiny details are excluded, such as the actual shape of the heat sink • Different contact resistance values have been investigated, to check how we can come up with the results shown above, and to check the maximum temperature • But : a contact is a micro effect, we model macro,so we also need the micro model (according to literature!) of a contact • The same stand for the Quadropole diodes

12. Modeling in Comsol • Model of the diode itself is just a resistor, which is varying with the current, since the voltage is assumed as a constant(1V) • Adiabatic thermal boundary conditions, with initial temperature of 10 K(in some cases cooling to He as well ) • Electrically is a current constraint problem, with the exponential decaying current; most cases for 6kA and 50 sec of Tau • First the modeled Temperatures have been compared to previous measurements(in SM18 or other place not in the machine!) • First assumption 0 contact resistance everywhere: impossible but ideal world must be nice in other aspects of life too • With no contact resistances we should measure a few micro-Ohms

13. Comsol 4.1 tool and discretization

14. Hot spot in diode-0 contacts

15. Comparisons to older tests,Temperature of the heat sink Also with an other code, Arjan’s QP3 was compared

16. Contact resistances • In the lead to heat sink or in the half-moons • Different values have been investigated, to see the maximum temperature • Both electrical and thermal contact resistances have been assumed, the latter is still an open question; The thermal conductivity of the contact is obtained from Wiedemann-Franz law (proportional to electrical conductivity) • Voltages compared to latest measurements, the conclusion is that the observed behavior is the result of more complex phenomena, although contact resistance

17. The contacts and the diode 2 Half-moons Lead to Heat sink upper(1120mm^2) Diode to heat sink(possible) Lead to Heat sink lower(947mm^2)

18. Thermal barrier example(12kA,100uOhm)

19. Electrical contact resistance example, voltage

20. Results for contact resistances

21. Hot spot in the half-moons(high contact resistance)

22. Half moon contacts,6kA 50 sec of Tau

23. Temperature of the half-moon

24. Temperature of the half-moon

25. 100 uOhm contacts at the leads to Heat sink, good thermal contact

26. 100 uOhm contacts at the leads to Heat sink, good thermal contact

27. Lead to heat sink summary,6kA 50 sec of Tau

28. The resistance of a lead for 100uOhm and bad thermal contact The kink is still missing

29. Consequences of high resistances • Are they really dangerous? : Yes, even now, but more over when 9kA with time constant of 68 sec, or for 12kA 100 sec • 9 kA, Tau 68 sec, Tmax=1200K 12kA,Tau 100 sec, Tmax=2500 K (!) • Even in this model the hot spot is a very local property, so the bus could remain under 100 K for 9 kA

30. Other effects

31. Force calculation • Two parallel wires carrying current opposite direction, there’s Lorentz force between them, if the wires are lines and the current is distributed evenly over the length easy analytical solution(as approximation of reality and check the results of simul.): • In Comsol the actual force calculation, but with a bit simplified geometry • It is true, that because of the actual bended shape of the diode leads, the torque resulting from the force could be different (and the stress as well) I is the current, L is the length of the wires, r is the original separation btw them

32. Results of force calculations Vertical part Bus to heat sink Bus to main bus

33. Micro contact model • In reality there’s a 2 micron thick Ni coating layer on the Cu elements(probably anti corrision) , and in the half-moon one part is coated by Ag • One can find nice formulas for the voltage difference and the temperature of the bulk and the actual hot spots, also for the value of the contact resistance The value of contact resistance, H is hardness in Pa(MPa), F is contact force Temperature drop along a contact, U is the voltage difference, L is the Lorenz number

34. Micro contact overview

35. Temperature drop for microcont.

36. Quad diode modeling • They’re quite similar, except a few things: the diode leads are straight (smaller bolts, and bus), no half-moon less contact surface and the heat sinks are smaller as well (less energy dissipated) • For the contacts we can say the same on micro level • Similar measured data, except the very steep, step in voltage • For the later a possible guess is now being investigated: the moving of the contacts, and after the bolts carrying the current

37. The contact movement Stainless steel bolts Contact btw surfaces varies from 0 to even a few um Cu of Lead (or one Halfmoon) Ni(orAg coating for one Halfmoon) Cu of Heatsink (or one Halfmoon)

38. Some possible explanations • Probably we observe these voltages(resistances) as a result of multiple effects, superimposed on each other, An example: as a result of the repelling force the actual contact force is less, so it causes higher contact resistance, which could lead to high temperatures, and after all it results in lower resistances through the melting of the coating • These effects are hard to simulate, and more input data is needed, the best would be an actual metallurgic investigation of the surfaces (for diodes in the machine quite impossible)

39. overview • It is hard to give a straight answer what’s going on inside these resistances, although some plausible explanations we already have • It is clear that it has to do sth with the contact surfaces • The most scary thing is, that it’s very hard to estimate the actual risk, what these diodes mean for the machine for long time period of operation (self healing or not?) • Are the quads similar?: Yes they showed similar behavior, except for the D16-a (is it exceptional, or we should expect from other thousands)

40. Backup slides

41. Bolts in quad diode Steel bolts

42. torque etc. From Arjan Dipole diode Quadropole diode

43. Temperature values for half-moons and heat sink to lead

44. Forces

45. Melting temperatures and voltages From Arjan

46. Physical description

47. Material properties, some examples Nb-Ti electrical resistivity Copper thermal conductivity Copper electrical resistivity

48. Comsol 4.1