Third LHC Splice Review Geneva – 12 to 14 November 2012 Diodes Status F. Savary

Outline Dipole and quadrupole diodes – Description The early signs of a potential flaw Study of the different contacts Diode to heat sink contact Heat sink to bus bar contact Bus to bus contact – dipoles / quadrupoles Outcome of the study Consolidated design for the quad. diode stacks Construction of the diode stacks at CERN Conclusions F. Savary - Consolidation of the superconducting splices and circuits3

Dipole diode stack F. Savary - Consolidation of the superconducting splices and circuits4 Lower diode bus bar R Bus-bus (‘half moon’) R HS-bus Diode box, He content:  5 liter R Diode-HS Voltage taps on the diode Upper Heat Sink Lower Heat Sink Bolted contacts with 4 M6 + helicoil, 4 spring washers, 10 Nm Diode compressed by a stack of spring washers to 40 kN All copper parts Ni-plated, 2-3 µm

Quadrupole diode stack F. Savary - Consolidation of the superconducting splices and circuits5 R HS-bus Diode 1 R Bus-bus Ansys model from S. Izquierdo R Diode-HS Lower diode bus bar Diode 2 Primarily, bolted contacts with 2-3 M5 + helicoil, 1 washer, 4 Nm Later, 2-3 M5 + helicoil, 3 spring washers, 8 Nm Diode compressed by a stack of spring washers to 40 kN All copper parts Ni-plated, 2-3 µm, except the magnet bb, which are silver plated over 40 mm

Specification Temperature [K] Parameter Before Test After Test Visual inspection U reverse [V], I r max =1 mA≥ 550 a -≥ 300 a ≥ 550 c ≥ 300 d ≥ 550 c ≥ c U forward max [V]< 10 U forward [V]≤ 1.0 a ≤ 1.6 b ≤ 1.0 e ≤ 1.6 e Insulation test [kV], 3 min5.0 ± 0.1 g h T wafer (calculated) [K]< 300 T heat sink (measured) [K]< 230 R diode to heat sink [µΩ] ˂ 5 f R heat sink to bus bar [µΩ] ˂ 2 F. Savary - Consolidation of the superconducting splices and circuits6 12 kA ± 50 A, 293 ± 2 K 12 kA ± 50 A, 77 ± 2 K 293 ± 5 K 77 ± 2 K 12 kA ± 50 A f. After 10 runs at 13 kA g. I max = 5 µA, RH < 50% h. I max = 5 µA Diode wafer Diode stack After fabricationCold tests in SM18

The early signs of a potential flaw Technical Stop May 2011: Diode lead voltages (dipole 2 kA as 6 kA much larger than expected Worrying because Origin not clear (R bus to bus/half-moon ; R bus to HS ; R HS to Diode ) Large variations amongst the 6 diode leads measured Measured resistance higher than specified values in production (5 µΩ) The contacts may deteriorate with time F. Savary - Consolidation of the superconducting splices and circuits7 Courtesy A. Verweij Technical Stop July 2011 A maximum value of 48 µΩ was measured on 3 kA! (this magnet had one natural quench at 11.2 kA in the machine) All measurements indicate that the resistance of many bolted connections is irregular and unpredictable Measurements in the LHC R in µΩ

Dipole diode 6 kA quenches F. Savary - Consolidation of the superconducting splices and circuits8 Non-reproducible resistances, much larger than 5 µΩ. The graphs could not be explained by normal Joule heating in the resistive bus bars including constant contact resistances Measurements in the LHC Courtesy A. Verweij

Quadrupole diode 5 kA quench F. Savary - Consolidation of the superconducting splices and circuits9 The sudden jump of resistance is peculiar and permanent, and could not (yet) be explained with the behavior identified later in the diode stack Measurements in the LHC Courtesy A. Verweij

Look at the series production data (from the vendor) At the additional data from tests in Bloc 4 (CERN) Carry out, again, tests at 4.2 K CERN)

Diode to heat sink contact (history) First 13 kA compared to last run at 13 kA F. Savary - Consolidation of the superconducting splices and circuits11 Frascati (4187 contacts) First run Last runDifference Bloc-4 Spare diodes (122 contacts) First run Last runDifference First run Last runDifference Bloc-4 S34 diodes (60 contacts) No big difference between run 1 and 10 Some increase and some decrease Generally, some decrease in resistance Large portion with R > 8 µΩ during run 1. R decrease in most cases Courtesy G. Willering

Diode to heat sink contact (SM18) F. Savary - Consolidation of the superconducting splices and circuits12 First 13 kA MQB0112 Not stable During the tests carried out at CERN in Bloc 4, and more recently in SM18, the diode to heat sink contact, has shown typical behavior

Diode to heat sink contact (SM18) F. Savary - Consolidation of the superconducting splices and circuits13 First 13 kA Standard Test 4.2 K Spec. [µΩ] # of contacts measured Min. measured [µΩ] Max. measured [µΩ] Average [µΩ] rms [µΩ] First 13 kA Last 13 kA< 5 µΩ # of contacts measured Contact resistance [µΩ] Bad assemblies 7.5 µΩ 14 µΩ 4.5 µΩ 30 µΩ The diode is protected against excessive temperature by the heat sink (T DIODE < 300 K and T HEAT SINK < 230 K) In the machine, the diode sees a “first run” Heat sink Diode wafer

Heat sink to bus contact F. Savary - Consolidation of the superconducting splices and 4.2 K, standard tests Test 4.2 K Spec. [µΩ] # of contacts measured Min. measured [µΩ] Max. measured [µΩ] Average [µΩ] rms [µΩ] Standard< 2 µΩ Special< 2 µΩ Contact resistance [µΩ] # of contacts measured R HS-bus is stable at cold/high current, for both dipoles and quadrupoles stacks (also for 24 stacks measured in Bloc 4 after incident in S34) Like the diode to heat sink contact, is OK thanks to the HS

Bus to bus contact – dipole F. Savary - Consolidation of the superconducting splices and circuits15 R Bus-bus – ½ moons contacts for dipoles Measured for the first high current, in SM18 in February 2012 (tested at only a few hundreds amps during the magnet reception cold in SM18) Are stable at cold/high current R < 2 µΩ required R < 1 µΩ measured

Bus to bus contact – quad. F. Savary - Consolidation of the superconducting splices and circuits16 R < 2 µΩ required R > 2 µΩ measured R Bus-bus – connection plate contacts for quads Measured for the first high current, in SM18 in July 2012 (tested at only a few hundreds amps during the magnet reception tests)

On the mechanical side ParameterValueRemark No. of screws2Per contact TypeM5 x 20 QualityA2-70AISI 304 σ 0.2 [MPa], screw MPa for 90% usage σ 0.2 [MPa], Cu OFE240 HelicoilM5 – 5 x 7.5In bb – Cu OFE Torque [Nm]8 ~ 40% more than recommended (VDI2230) Pre-load [kN]6.3 -7, measured -Yield limit exceeded Max current density [A/mm 2 ] kA, average -~ 3 times more than in dipole ½ moons Contact P [MPa]22.6-Average Lubricant/LockLoctite ® (blue, medium thread-locker) F. Savary - Consolidation of the superconducting splices and circuits17 Friction coeff. Torque [Nm] Pre-load [N] According to VDI2230 Courtesy S. Izquierdo Bermudez

Outcome of the tests The recent measurements in 4.2 K, indicate that: The diode to heat sink contacts, even if not stable, are safe thanks to the heat sink, which was designed for 13 kA, tau = 120 s / 50 s for the dipole and quadrupole circuits, respectively. In addition, all the stacks have seen at least 8 runs at 13 kA, and survived The heat sink to bus, and ½ moon contacts (dipoles), are stable with small resistances, hence safe The resistance of the connection plate contacts (quads) depends on the current and current history. We cannot guarantee that all (400 x 8) contacts in the machine will safely carry one or more current decays of 12 kA, tau=30 s (even if reduced). None of these contacts was measured at high current during the production. The bolted assembly lacks rigidity and robustness, hence is not safe F. Savary - Consolidation of the superconducting splices and circuits18

Decision (endorsed by TE & LMC) F. Savary - Consolidation of the superconducting splices and circuits19 Re-do all the bus bar to connection plate contacts of the almost 400 quad. diode stacks installed in the machine R BB Diode/Plate R BB SSS/Plate Stud

2 test runs, 4.2K, with thermal cycle Heavily instrumented quadrupole stacks F. Savary - Consolidation of the superconducting splices and circuits20 M5 studs – Torque 8 N/m Helicoils in the bus bars removed Silver plated connection plates Std 12 mm between the studs axis M6 studs – Torque 11 N/m Helicoils in the bus bars removed Silver plated connection plates Threaded holes drilled to larger diameter 15 mm between the studs axis MQB0529 MQB0528

Instrumentation 64 V-taps (for 32 contact resistances) 48 T-sensors (4 wires each) 8 PT100 (square) on the connection plates 32 «CERNOX» on the bus bars 8 PT100 (pipe) on the heat sink mid-height) F. Savary - Consolidation of the superconducting splices and circuits21 V-taps T-sensors

Test set up in SM18 F. Savary - Consolidation of the superconducting splices and circuits22

Test results – first run MQB0528 – M6 F. Savary - Consolidation of the superconducting splices and circuits23 Resistances are constant during the tests at 13 kA No degradation throughout the test run Resistances are very warm, and still very cold without degradation At the end of this run, additional tests at 13 kA were done with different tau: 60 s, 65 s, and 75 s. The resistances increase due to the copper resistivity High resistances due to the copper resistivity at warm I [kA] Tau [s] T of the connection plate [K]

Test results – after TC – MQB0528 F. Savary - Consolidation of the superconducting splices and circuits24 Run 1 No degradation The resistance remains constant throughout the test, for runs 1 and 2, at 13 kA Increase of the resistance due to the copper resistivity High resistances due to the copper resistivity at warm Run 2 M6

Test results – first run MQB0529 – M5 F. Savary - Consolidation of the superconducting splices and circuits25 The resistances are constant during the tests at 13kA No degradation throughout the test run Resistances are low at RT, and remains low at cold, without degradation High resistances due to the copper resistivity at warm

Test results – after TC – MQB0529 F. Savary - Consolidation of the superconducting splices and circuits26 These resistances are constant during the tests at 13kA Test run 1 No degradation, even after thermal cycle Increase of the resistance due the resistivity of the copper High resistances due to the copper resistivity at warm Test run 2 M5

Further developments M5 retained, to avoid unnecessary and risky work during the consolidation Use studs for stronger tightening, and counter-plates to apply more uniform contact pressure Possibly, use studs, counter-plates, and washers, made of Inconel ® alloy 718 to increase preload, i.e. increase contact pressure (reasonably), and ensure, with margin, the overall rigidity/stability of the joint A2-70: σ 0.2 = 450 MPa Inconel ® alloy 718: σ 0.2 = 1000 to 1250 MPa depending on the delivery conditions Enquiries ongoing Insulation cover likely needed at the bottom The consolidation procedure will be verified, and repeated, on representative mock-ups in B-180  we have already checked that it can be done without removing the stack from its container F. Savary - Consolidation of the superconducting splices and circuits27

Assembly of the stacks Long learning process to assemble correctly diode stacks at CERN Performance of the diode stack depends a lot on many details (cleanliness, surface conditions, storage conditions, tightening torques, quality of the materials, …) Recent diode stacks assembled in B287: Quadrupole diode stacks MQB0456, MQB0506 Dipole diodes stacks MDB0397, MDB0642, MDB01162, and MDB1323 Tested at RT and 77 K prior to transport to SM18 for testing at 4.2 K F. Savary - Consolidation of the superconducting splices and circuits28

Quad: MQB0506, Resistance F. Savary - Consolidation of the superconducting splices and circuits29

Quad.: MQB0456, 0506, U reverse F. Savary - Consolidation of the superconducting splices and circuits30

MDB0397, 0642, 1323, and 1162 Resistance F. Savary - Consolidation of the superconducting splices and circuits31

MDB0397, 0642, 1323, and 1162 U reverse F. Savary - Consolidation of the superconducting splices and circuits32

4.2 K in SM18 F. Savary - Consolidation of the superconducting splices and circuits33

4.2 K in SM18 F. Savary - Consolidation of the superconducting splices and circuits34

4.2 K in SM18 F. Savary - Consolidation of the superconducting splices and circuits35

MDB0397, 0642, 1323, and 1162 T [K] Location Run # 300 B B SM18 Before R1 4.2 SM18 Before R1 4.2 SM18 After R1 4.2 SM18 After R2 4.2 SM18 After R3 4.2 SM18 After R4 4.2 SM18 After R5 4.2 SM18 After R6 4.2 SM18 After R7 4.2 SM18 After R8 4.2 SM18 After R9 4.2 SM18 After R10 MDB MDB MDB MDB F. Savary - Consolidation of the superconducting splices and circuits36 U reverse

For LS1 15 dipole diode stacks needed 3 quad. diode stacks needed The assembly of the diode stacks in B287, and their cold testing in SM18 on the critical path (need to be done by the end of 2012) F. Savary - Consolidation of the superconducting splices and circuits37 Diode IDStatusNote D i p o l e MDB0333 * MDB0397 MDB0430 * MDB0608 * MDB0636 * MDB0641 * MDB0642 MDB1162 MDB1194 * Q u a d. MQB0456 MQB0506 ** MQB0528-*** MQB0529-*** *Bus bar coating, and hence contacts also, were redone, therefore these stacks may need to be 4.2 K **The bad bus bar to heat sink contact was redone, stack may need to be 4.2 K *** Have seen special test sequence

Conclusions The 3 contacts present in the diode stacks were studied One appears to be weak, not stable: bus bar to connection plate contact in the quadrupole diode stacks It will be redone during LS1 A solution was developed to reinforce the weak contact 4.2 K, including thermal cycle, have shown that it satisfies the rigidity/stability requirements The assembly procedure of the stacks is (re)understood and applied to produce the stacks missing for LS1 The production and testing of these items is on the critical path F. Savary - Consolidation of the superconducting splices and circuits38