The Large Hadron Collider The 19 th Sep 2008 incident

The Large Hadron Collider

The 19th Sep 2008 incident

[R. Alemany][CERN AB/OP]

[Engineer In Charge of LHC]NIKHEF Seminar (12.12.2008)

Incident of September 19th 2008• During a few days period without beam• Making the last step of hardware commissioning of

dipole circuit in sector 34 to 9.3 kA 5.5 TeV• At 8.7 kA, a resistive zone developed in the dipole bus

bar splice between Q24 R3 and the neighboring dipole• Electrical arc developed which punctured the helium

enclosure• Helium released into the insulating vacuum• Rapid pressure rise inside the LHC magnets

• Large pressure wave travelled along the accelerator both ways

• Self actuating relief valves opened but could not handle all

• Large forces exerted on the vacuum barriers located every 2 cells

• These forces displaced several quadrupoles and dipoles

• Connections to the cryogenic line affected in some places

• Beam vacuum also affected: soot and superinsulation material

R.Bailey, DESY, December 2008 3

LHC cryodipole (1232 of them)7TeV• 8.33T• 11850A• 7MJ

R.Bailey, DESY, December 2008 4

All have to be interconnected (quads too)

5

Interconnections

Bus bar interconnection

Hypothesis

• Temperature increase due to an excessive resistance• Superconductor quenches and becomes resistive at high

current (temperature increase due to the resistance).• Up to a certain current, the Copper can take it (cooled by

the He II).• Beyond a certain current, ‘run-away’ of the temperature,

splice opens, electrical arc …

Splice insulation Length He II @ 1.9K, 1Bar

currentcurrent

Heat exchange with He IIBus Bar’s Insulation

R.Bailey, DESY, December 2008 8

! Considerable collateral damage over few hundred metres Contamination by soot of beam pipes Damage to superinsulation blankets Large release of helium into the tunnel (6 of 15 tonnes)

Consequences

Insulating vacuum barrier every 2 cells in the arc Some moved

R.Bailey, DESY, December 2008 9

!

Repair

• Present strategy assumes treating all magnets Q19 to Q31

• May have to treat slightly further outside this zone (to Q33)

• Nearly all the components are at CERN• Critical components are beam screens and SSS

bottom trays• Estimate for magnets (preliminary) November 08

to March 09• Then have to finish interconnection, cool down,

power test• 53 magnets to be removed (39 dipoles + 14

quadrupoles): they will be thoroughly inspected and either clean or repaired or substituted by a spare magnet.

Outside sector 34• All data from hardware commissioning

carefully scrutinized• Anomalous cryogenic behaviour found in

sector 12 at 7kA• Higher than nominal heat load in

cryogenic sector 15 R1• Controlled tests made late October at

different currents• Calorimetric measurements

• Measure temperature increase at XkA mK/h

• Derive rate of energy deposition J/s = W

• Fit Energy deposition vs current• Deduce equivalent resistance nΩ

y = 9.2E-08x2.0E+00

0

5

10

15

0 5000 10000

Addi

tiona

l loc

al

diss

ipati

on [W

]

Dipole current [A]

Nominaldissipation

R.Bailey, DESY, December 2008 11

Calorimetric results so far (November)

-10

-5

0

5

10

15

20

25

30

35

40

07R1

11R1

15R1

19R1

23R1

27R1

31R1

29L2

25L2

21L2

17L2

13L2

09L2

07R6

11R6

15R6

19R6

23R6

27R6

31R6

29L7

25L7

21L7

17L7

13L7

09L7

07R7

11R7

15R7

19R7

23R7

27R7

31R7

29L8

25L8

21L8

17L8

13L8

09L8

Spec

ific r

esisti

ve h

eatin

g [m

W/m

]

3000 A 5000 A 7000 A

S1-2 S6-7 S7-8

25 nW

50 nW

75 nW

100 nW

• 3 sectors, 4 suspicious cases

Electrical results so far (November)

• Electrical measurements• Dedicated electronics needed for inter-magnet splices• QPS system used for internal magnet splices• S12 15R1

• All inter-magnet splices measured to be similar, around 0.3nΩ

• Magnet B16.R1 measured to have 100nΩ !!!• S12 19R1

• Nothing found; traced to a feature of cryogenic system• S12 31R1

• Nothing found; calorimetric fit in any case is very poor• S67 31R6

• Magnet B32.R6 measured to have 45nΩ !!!• S78

• Nothing found

Other measures• From the analysis of the incident, the following modifications and

consolidations are under consideration:

• Upgrade of the quench protection system for protection against symmetric quenches (was already in the pipeline before Sector 34 incident)

• Upgrade of the quench protection system for precision measurements and protection of all interconnects

• Modifications of commissioning procedure to include calorimetric information and systematic electrical measurements

• Addition of pressure release valves on EVERY dipole cryostat• note that this probably requires warming up

Strategy for implementing this is not yet finalised

Timescales for restart will be determined by

• Efficiency of logistics of magnets removal / installation

• Efficiency of magnet repair• Efficiency of beam pipe repair / cleaning• Efficiency of interconnection activities• Strategy adopted to ensure no repeat is

possible• Time to cool down• Time to re-commission power circuits