LHC diodes: Status report (for information) A. Verweij, TE-MPE, Splice review, 29/11/2011 Arjan Verweij, TE-MPE With input from many people involved in

LHC diodes: Status report (for information)

A. Verweij, TE-MPE, Splice review, 29/11/2011

Arjan Verweij, TE-MPE

With input from many people

involved in testing and analysis

Reliability of power connectionsM. Braunovic, J Zhejiang Univ Sci A 2007 8(3):343-356

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=100 s (RB) or t=30 s (RQ). Any high resistive singularities (e.g. contact resistances) can cause local overheating. Rupture of a diode lead can result in local dissipation of up to 1 GJ !!! The LHC contains 1232 dipole diode stacks and 392 quadrupole diode stacks.


The dipole diode stack

Rbus-bus(aka ‘half moon’)

RHS-bus

Diode box, Helium contents : 5 liter

Lower diode busbar

Rdiode-HSLower heat sink

Upper heat sink

Voltage taps on the diode


The quadrupole diode stack

RHS-bus

Diode 1

Diode 2

Rbus-bus

Ansys model from S. Izquierdo

Rdiode-HS

Lower diode busbar

Upper diode busbar


Dipole Quad

Upper diode busbar About 410 mmR4K=0.23 mW (RRR=100)

About 1.6 mR4K=1.8 (mW RRR=100)

Rbus-bus Contact surface=1600 mm2

Dipole side: Ag coated (5-10 mm)Diode side: Ni coated4xM6, 10 Nm, 4-5 CuBe spring washers

Contact surface=2x560 mm2

Dipole side: Ag coated (5-10 mm)Diode side: Ni coated2x2xM5, 8 Nm, 3 CuBe spring washers

Lower diode busbar Maximum 480 mmR4K=0.3 (mW RRR=100)

135 to 370 mmR4K=0.15-0.4 (mW RRR=100)

RHS-bus Contact surface=950-1070 mm2

Both sides are Ni coated4xM6, 10 Nm, 4-5 CuBe spring washers

Contact surface=440-640 mm2 Both sides are Ni coated3xM5, 8 Nm, 3 CuBe spring washers

Heat sink R4K<<0.1 (mW RRR=100)

Rdiode-HS Contact surface=5000 mm2

The diode lead resistance is the sum of:

The electro-thermal behavior of the copper leads and heat sinks is well understood, and they are sufficiently over-dimensioned.


Contacts are often a source of problem because deterioration of the contact resistance can occur due to oxidation, movement, local heating, etc.

The bolted contacts in the diode leads have been discussed many 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.

About 250 diodes have been repaired at CERN, since they had Rbus-bus and RHS-bus up to a few 100 mW. After the repair all resistances were below 5 mW, according to measurements at warm (up to 10 A) and at cold (up to about 1000 A).

During 4 technical stops in 2011 the resistances of the leads of 6 dipole diodes and 6 quadrupole diodes in S56 were measured after heater induced quenches in the magnets.


http://lhcp.web.cern.ch/lhcp/tcc/powering/eewg/eewg.htm


Constant resistance of the diode leads

Redistribution of current from diode into magnet

Results for the dipole diode lead ‘resistances’ for 2 kA quenches

Results for the dipole diode lead ‘resistances’ for 6 kA quenches

1.8-3.6 mW: measured at cold reception in SM18

The results showed non-reproducible resistances much larger than 5 mW. The graphs could not be explained by ‘normal’ Joule heating in the resistive busbars including constant contact resistances.


A. Verweij, TE-MPE, 15/11/2011

All resistances in mW

I D14-c D14-a F14-c F14-a D16-c D16-a F16-c F16-a D18-c D18-a F18-c F18-a

HM1 HM2 HM1 HM2 HM3 HM4 HM3 HM4

2No diode opening

2.7 2.8 3.7 3.9

2 2.5 2.6 3.6 3.8

3 5.2 6.3 6.1 5.6 3.3 3.6 4.5 4.7

5 7.8 8.4 9.3 7.5 3.8 43.3 5.9 6.9

5 9.1 8.6 9.3 8.2 3.8 38.6 6.3 7.4

6 11.4 9.4 11.9 28.3 4.0 45.1 9.6 13.0

6 12.2 9.8 11.8 25.0 4.0 45.6 10.3 14.1

3 9.4 7.6 9.2 18.6 3.4 26.4 9.3 11.1

3 9.3 7.5 9.1 18.7 3.4 25.9 9.3 11.2

3 3.9 32.0 11.1 13.1 3.7 3.7 5.4 5.4

3 after 10 s 4.1 28.2 11.6 13.8 3.9 5.0 5.7 7.1

4 4.3 40.1 11.4 14.0 4.3 5.8 6.1 7.1

4 after 10 s 4.7 25.0 12.4 14.5 6.3 10.0 9.5 17.1

3 4.1 28.6 11.6 13.3 6.1 8.2 8.5 11.6

3 after 10 s 4.3 25.4 12.1 13.9 6.4 8.6 8.9 12.1

5 4.0 44.5 13.4 15.4 8.3 9.1 10.1 13.8

5 after 10 s 11.5 23.8 20.8 25.8 20.6 14.8 20.8 24.0

3 7.1 21.1 19.4 25.9 15.3 9.2 12.2 12.6

3 after 10 s 7.1 20.0 20.0 26.4 15.4 9.3 12.5 12.9

Similar results were seen on the quad diodes


D16R5 anode: 2 consecutive quenches at 5 kA

Probably a movement in one of the bolted connections during the first tests, resulting in a permanent increase in the contact resistance.

Step in Dt<50 ms



The diode lead resistances measured in the machine are much larger than expected,

are non-reproducible, and show degradation at 3 kA. The resistance strongly depends on the ‘quench’ current and on the current and/or Idt during previous tests.

Different behaviors are observed so different physical/mechanical phenomena may play a role.

The origin of the large resistance is not known, and extrapolation 12 kA is not possible.

In order to better understand the observed behavior, an informal working group was created in Aug 2011. Main activities are:

Mechanical measurements and calculations (Ansys). Electro-thermal simulations (Comsol). Data collecting from the series production (ENEA) and reception

tests at SM18 and Block 4. Resistance measurements at 80/300 K under loading (up to 6 kA). Diode measurements at cold in SM18 (up to 13 kA). Microscopic analysis of contact surfaces.

The progress on these activities was presented during a small workshop

on 15 Nov with already a lot of very interesting data/results.

MembersFrédéric SavaryLuca BotturaAndrzej SiemkoKnud Dahlerup-PetersenMarta BajkoGaelle DibChristian GilouxHerve PrinSusana Izquierdo BermudesPhilippe PerretGerard WilleringElvis FornasiereMateusz Jakub BednarekGiorgio D'AngeloMichael GuinchardLudovic Grand-ClementArjan Verweij


Block-4 measurements show high resistances during the first runs, saturating to low values during the 13 kA runs.

0

5

10

15

20

25

30

35

40

1300 A 5 kA 10 kA 13 kArun1

13 kArun2

13 kArun 3

13 kArun 4

13 kArun 5

13 kArun 6

13 kArun 7

13 kArun 8

13 kArun 9

13 kArun 10

Max

imum

resi

stan

ce (µ

Ω)

Measurement run

Heatsink-diode resistance Bloc 4 measurements

MDA0106-Rha D1MDA0106-Rhc D1MDA0437-Rha D2MDA0437-Rhc D2MDA0682-Rha D1MDA0682-Rhc D1MDA0761-Rha D1MDA0761-Rhc D1MDA0788-Rha D2MDA0788-Rhc D2MDA0790-Rha D1MDA0790-Rhc D1MDA0792-Rha D2MDA0792-Rhc D2MDA0805-Rha D3MDA0805-Rhc D3MDA1067-Rha D3MDA1067-Rhc D3MDA1085-Rha D2MDA1085-Rhc D2MDA1109-Rha D4MDA1109-Rhc D4MDA1117-Rha D3MDA1117-Rhc D3MDA1240-Rha D3MDA1240-Rhc D3MDA1267-Rha D4MDA1267-Rhc D4MDACERN3-Rha D4MDACERN3-Rhc D4MDB0842-Rha D4MDB0842-Rhc D4

Data recovery and analysis: G.Willering, TE-MSC

But how would the curve look like if the 1st quench would be at 13 kA??


During the Frascati tests different behaviors were observed, see the example.

The diode to heat-sink resistance might decrease or increase with the number of runs.

Usually the resistance reduces during the decay of the current!

The behavior in the machine was unexpected, but analysis of data from the past shows that a similar behavior was already present during reception tests.

There is a strong indication that most of the resistance is at the “diode - heat sink” contact.

The heat sink has a lot of margin, and a large Rdiode-HS is acceptable as long as there is sufficient thermal contact between diode and heat sink.

Opening of the circuit in case of overheating of the diode or the contact between diode and heat sink is unlikely due to the presence of the 40 kN force.


Conclusion

The bolted contacts should also be carefully looked at, even though opening of a properly bolted contact (with washers) is not very likely.

Restarting the diode tests at CERN (in SM18) with more instrumentation will surely give more insight in the behavior of the contacts.

During the dipole training campaign in S56 in 2008 no problems have been observed in the functioning of the diodes.

Documents

LHC diodes: Status report (for information) A. Verweij, TE-MPE, Splice review, 29/11/2011 Arjan Verweij, TE-MPE With input from many people involved in