1. Rüdiger Schmidt and Karl Hubert Mess
The challenges of protecting the super-conducting magnets and powering system
Rüdiger Schmidt and Karl Hubert Mess
Academic Training May 2008

2. During tests the energy of 2 MJ from the SPS beam was directed into a metal target (LHC 360 MJ)
V.Kain

3. During tests the energy of 7 MJ in one of 154 magnets was released into one spot in the coil (interturn short)
P.Pugnat

4. Overview Energy in beams and magnets ….and their controlled discharge
Beam and powering interlock systems
Quenches in superconducting magnets
LHC powering architecture
Powering system: magnet types and electrical circuit types
Protection for the electrical circuits
Procedure for commissioning of (the protection systems) an electrical circuit and recent experience
Interlocks systems during beam operation

5. Energy stored in magnets and beam
E dipole = 0.5  L dipole  I 2dipole
Energy stored in one dipole operating at 7 TeV with A is 7.4 MJoule
For 154 dipoles in one sector: ~1.2 GJoule
For all 1232 dipoles in the LHC: ~9 GJoule
E beam = N p  N bunches  Energy
Energy stored in one beam with 2808 bunches, each 1.15  1011 protons at 7 TeV is 360 MJoule

6. LHC cycle and stored beam energy
7000
6000
energy ramp 25 MJ  360 MJ circulating beam
2808 bunches
5000
Energy [GeV/c]
coast
360 MJ circulating beam
4000
3000
beam dump
discharge of energy into beam dump block
injection phase
3 MJ  25 MJ
beam transfer
circulating beam
2000
1000
12 batches from the SPS (every 20 sec)
one batch 216 / 288 bunches
3 MJ per batch
-4000
-2000
2000
4000
time from start of injection (s)

7. LHC cycle and stored magnet energy (main dipoles)
12000
10000
current ramp 36 MJ  9 GJ for 1232 dipole
magnets
8000
coast
9 GJ
Current [kA]
6000
4000
in case of quench
discharge of energy into resistors
injection current
760 A
0.53 T
2000 1
-4000
-2000
2000
4000
time from start of injection (s)

8. Livingston type plot: Energy stored magnets and beam
based on graph from R.Assmann

9. What does this mean?
The energy stored in the LHC magnets corresponds approximately to 8 such trains running at 280 km/h
Sufficient to heat up and melt more than 10 tons of Copper!!
90 kg of TNT
The energy of an 200 m long fast train at 155 km/hour corresponds to the energy stored in one LHC beam
8 litres of gasoline
15 kg of chocolate
It’s the time for the energy release (instantaneous power) that matters !!

10. Machine Protection during all phases of operation
During commissioning of the powering system and operation: protection of magnets, busbars connecting magnets and High Temperature Superconductor current leads is mandatory
The magnet protection and powering interlock systems become operational during this time, long before starting beam operation
In case of failure, the energy of the superconducting magnets must be discharged into resistors
During beam commissioning and operation: protection from the injection process, during the energy ramps and at 7 TeV is mandatory
The only component that can stand a loss of the full beam is the beam dump block - all other components would be damaged
The LHC beams must ALWAYS be extracted into the beam dump blocks, at the end of a fill and in case of failure
In general, a failure in an electrical circuit leads to beam extraction

11. Beam dumping system in IR6
Septum magnet deflecting the extracted beam
H-V kicker for painting the beam
Q5L
Beam Dump Block
Q4L
Fast kicker magnet
about 700 m
Q4R
about 500 m
Q5R
Beam 2
AB - Beam Transfer Group

12. LHC Machine Protection Overview
Beam
Dumping System
Beam Interlock System
Injection Interlock

13. LHC Machine Protection Powering System >50% of all interlocks
Beam
Dumping System
Beam Interlock System
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor
Magnets
Power Converters
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK

14. LHC Machine Protection Access System
Beam
Dumping System
Beam Interlock System
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor
Access
System
Magnets
Power Converters
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK
Doors
EIS

15. LHC Machine Protection Vacuum System
Beam
Dumping System
Beam Interlock System
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor
Access
System
Vacuum
System
Magnets
Power Converters
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK
Doors
EIS
Vacuum
valves
Access
Safety
Blocks
RF Stoppers

16. LHC Machine Protection Beam Loss Monitors
Dumping System
Beam Interlock System
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor
Beam loss monitors
BLM
Access
System
Vacuum
System
Magnets
Power Converters
Monitors
aperture limits
(some 100)
Monitors in arcs
(several 1000)
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK
Doors
EIS
Vacuum
valves
Access
Safety
Blocks
RF Stoppers

17. LHC Machine Protection Collimation System ~99% of all interlocks
Collimator
Positions
Environmental
parameters
Collimation
System
Beam
Dumping System
Beam Interlock System
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor
Beam loss monitors
BLM
Access
System
Vacuum
System
Magnets
Power Converters
Monitors
aperture limits
(some 100)
Monitors in arcs
(several 1000)
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK
Doors
EIS
Vacuum
valves
Access
Safety
Blocks
RF Stoppers

18. LHC Machine Interlocks
Devices
Movable Detectors
Beam Loss
Monitors
BCM
Experimental Magnets
Collimator
Positions
Environmental
parameters
Safe Beam Parameter Distribution
LHC
Devices
LHC
Devices
Safe
LHC
Parameter
Software
Interlocks
Sequencer
Operator Buttons
CCC
LHC
Experiments
Transverse Feedback
Beam
Aperture
Kickers
Collimation
System
SpecialBLMs
Beam
Dumping System
Beam Interlock System
Safe
Beam
Flag
Injection Interlock
Powering
Interlocks
sc magnets
Powering
Interlocks
nc magnets
Magnet Current Monitor RF
System
Beam loss monitors
BLM
Beam
LifetimeFBCM
Access
System
Vacuum
System
Screens / Mirrors
BTV
Timing System
(Post Mortem Trigger)
Magnets
Power Converters
Monitors
aperture limits
(some 100)
Monitors in arcs
(several 1000)
MPS
(several 1000)
Power
Converters
~800
AUG
UPS
Cryo OK
Doors
EIS
Vacuum
valves
Access
Safety
Blocks
RF Stoppers

19. Operational margin of a superconducting magnet
Applied Magnetic Field [T] Bc
Bc critical field
8.3 T
quench with fast local loss of ~5 · 106 protons
QUENCH
Tc critical
temperature
quench with fast local loss of ~5 · 109 protons
0.54 T
1.9 K
9 K
Temperature [K]

20. Quench - transition from superconducting state to normalconducting state
Quenches are initiated by an energy in the order of mJ
Movement of the superconductor by several µm (friction and heat dissipation)
Failure in cooling
Beam losses
To limit the temperature increase after a quench
The quench has to be detected
The magnet current has to be switched off within << 1 sec
For main magnets: the energy stored in the quenching magnet is distributed inside the magnet by force-quenching the magnet coils using quench heaters
For magnets powered in series: the quenching magnet is isolated from the other magnets using a power diode

21. Superconducting wire and cable
Filament diameter 6 m
Wire diameter 1 mm
Typical value for operation at 8 T and 1.9 K: 800 A
width 15 mm
Rutherford cable current ~12000 A

22. Power into superconducting cable after a quench
Specific heat of copper at 300 C:

23. LHC Powering in 8 Sectors5
Powering Sector:
154 dipole magnets
about 50 quadrupoles
total length of 2.9 km 4 6
DC Power feed
LHC
Octant 3
DC Power 7
27 km Circumference
Powering Subsectors:
long arc cryostats
triplet cryostats
cryostats in matching section 2 8
Sector 1
P.Proudlock

24. Magnet in one arc cell of 110 m length
6 main dipole magnets (12 kA)
2 arc quadrupole magnets (12 kA)
Lattice sextupole magnets in arcs (600 A)
Multipole and other correctors in arcs
Powered in series
752 arc orbit corrector magnets powered individually (60 A)
Correctors to adjust beam parameters (trim quadrupoles, orbit correctors, etc., 80 – 600 A)
Powered individually
SSS
sextupole
corrector
(MCS)
quadrupole
MQF
orbit corrector
quadrupole
MQD
orbit corrector
quadrupole
MQF
orbit corrector
main
dipole MB
main
dipole MB
main
dipole MB
main
dipole MB
main
dipole MB
main
dipole MB
special corrector (MQS)
lattice sextupole (MS)
decapole
octupole
corrector
(MCDO)
special corrector (MO)
lattice sextupole
(MS)
special corrector (MO)
lattice sextupole
(MS)
F0D0 cell 110 m

25. Types of electrical circuits
There are more than 50 different types of magnets
Magnets can be powered in series or individually
As an example, powering and protection of an individual orbit corrector magnet (60 A, 9 kJ) and a circuit including 154 main dipole magnets (12 kA, 1.2 GJ) is very different
risks, magnet protection, interlocks, commissioning procedures, etc.
1618 electrical circuits grouped into nine “Electrical Circuit Types” (eight for circuits with superconducting magnets, one for circuits with normal conducting magnets)
The attribution of an electrical circuit to a type depends on the energy stored in the magnets of the electrical circuit, and the way of protecting magnets, busbars and current leads
Commissioning procedures are essentially identical for electrical circuit types

26. Sector 7-8 and magnet cryostats
IR7
Cleaning
Matching
section
Arc cryostat (3 km)
Matching
section
Inner
Triplet
IR8
LHCb
DFBMH
DFBAN
DFBAO
DFBMC
DFBMA
DFBX
IR Quadrupoles
Correctors
154 Arc dipole magnets and correctors
Short straight sections with quadrupoles and correctors
IR Quadrupoles
Correctors
Insertion dipoles
IR Quadrupoles
Correctors
Each cryostat has its own set of PC. Only the beam, the access, the vaccuum, and the cryogenics couple
Each cryostat one PPC (arc needs two, because fed from both sides)
PPC close to the PC
Power Converters (60A) for 94 orbit corrector magnets

27. Sector 7-8 and power converters
IR7
Cleaning
Matching
section
Arc cryostat (3 km)
Matching
section
Inner
Triplet
IR8
LHCb
DFBMH
DFBAN
DFBAO
DFBMC
DFBMA
DFBX
Energy extraction
Each cryostat has its own set of PC. Only the beam, the access, the vaccuum, and the cryogenics couple
Each cryostat one PPC (arc needs two, because fed from both sides)
PPC close to the PC
UJ76
RR77
UA83
Power Converters
for 34 electrical circuits
and other equipment
Power Converters
for 72 electrical circuits
and other equipment

28. Conditions for powering
Cryogenics: correct conditions
1.9K, 4.5K, other conditions
Safety systems ready (AUG – arret urgence general, UPS – uninterruptible power supplies, …)
Power converter ready
Magnet protection system ready
Power converters
Energy extraction
Operator / Controls: must give permission to start powering
Powering Interlock Controller (PIC)
Beam Interlocks
Quench in a magnet inside the electrical circuit
Warming up of the magnet due to quench in an adjacent magnet
Warming up of the magnet due to failure in the cryogenic system
AUG or UPS fault
Power converter failure

29. Main dipoles in arc cryostat
Time for the energy ramp is about min (Energy from the grid)
Time for regular discharge is about the same (Energy back to the grid)
DFB
DFB
Magnet 2
Magnet 4
Magnet 152
Magnet 154
Magnet 1
Magnet 3
Magnet 5
Magnet 153
Energy Extraction: switch closed
Energy Extraction: switch closed
Power Converter

30. Main dipoles: quench of a magnet
Quench in one magnet: Resistance and voltage drop across quenched zone
Quench is detected: Voltage across magnet exceeds 100 mV for >10 ms
DFB
DFB
Magnet 2
Magnet 4
Magnet 152
Magnet 154
Magnet 1
Magnet 3
Magnet 5
Magnet 153
Energy Extraction: switch closed
Energy Extraction: switch closed
Quench
Detector
Power Converter

31. Main dipoles: magnet protection
Quench heaters warm up the entire magnet coil: energy stored in magnet dissipated inside the magnet (time constant of 200 ms)
Diode in parallel becomes conducting: current of other magnets through diode
Resistance is switched into the circuit: energy of 153 magnets is dissipated into the resistance (time constant of 100 s for main dipole magnets)
DFB
DFB
Magnet 2
Magnet 4
Magnet 152
Magnet 154
Magnet 1
Magnet 3
Magnet 5
Magnet 153
Energy Extraction: switch open
Energy Extraction: switch open
Quench
Detector
Quench
Heater PS
Power Converter

32. Magnet and busbar quench detection
To detect a quench: U = R  I (about 1 V need to be detected)
But one needs to substract from U1:
a) U2 = Rwarm  I
b) During energy ramp: U = dI/dt  L  154 = 77  (U3 + U4)
DFB
DFB
Magnet 2
Magnet 4
Magnet 152
Magnet 154
Magnet 1
Magnet 3
Magnet 5
Magnet 153 U2 U3 U4 U1
Power Converter
World FIP - deterministic Fieldbus

33. Simpler way to discharge the energy?
assume one magnet quenches
assume the magnets in the string have to be discharged in, say, 200 ms
the inductance is about 15 H, the current about 12 kA
with U = l  dI/dt
Discharge with about 1 MV: not possible

34. Challenges for quench protection
Detection of quenches for all main dipole and quadrupole magnets (1600 magnets powered in 24 electrical circuits)
Voltage across the dipole magnet chain up to 180 V during ramping - 1 V needs to be detected in presence of noise etc.
During discharge the quench detectors are connected to equipment at high voltage (1000 V)
Detection of quenches in about 800 other circuits
Global quench detection for circuits operating at 600 A
Inductance of magnets change as a function of current
Detection of quenches across all HTS current leads (2000) with very low voltage threshold ~ 3 mV for 1 sec across HTS part
Systems must be very safe in order not to damage equipment (any quench must be detected)
Systems must be very reliable in order not to disturb operation (the system should not trigger in case of noise etc.)

35. Energy extraction switch house 13 kA Energy extraction resistors MB
Energy extraction switch 13 kA
Diode for 13 kA

36. Objective of powering tests
Why the lengthy commissioning?
Most components in the electical circuit have been tested before (e.g. all magnets)
Short circuit tests of the power converters have been done
Validate the entire electrical circuits for the first time
Busbars ok? superconducting connections ok? Magnets still ok (after storage)? All other systems ok?
Protection systems work? Switch-off for different failure cases ok?
Discharge of 1 GJoule ok?
All interlock systems ok?
Test protection in case of quench, of water cooling problems, of a failure in the UPS system, of an AUG activation
Test protection in case of a problem in the cryogenic system

37. One of ~1700 interconnections: busbars and tubes

38. Steps in the commissioning procedure
Verification of the correct functioning of the interlock systems at low current
in case of quench: fire quench heaters and fast switch off power converter
in case of power converter failure: extraction of energy stored in the circuit
in case of UPS failure: switch off power converter
in case of CRYO failure: slow abort of power converter
Verification of magnet protection by firing quench heaters at low current
Verification of magnet protection by firing quench heaters at high current
For circuits with energy extraction systems
Verification of the energy extraction functionality by switching the resistor into the circuit at different current levels

39. Commissioning procedure for quadrupoles (example)
Tests of interlock system at very low current
Tests at intermediate current
Tests at nominal current

41. RD4: Fast Power Abort from 200A (circuit quench via magnet protection system)
time [s] 41

42. Fast Power Abort from 350A (Fast Abort request via powering interlock system)
time [s] 42

43. Fast Power Abort from 5500A (PNO.C2)
time [s] 43

44. Quench from 5500A
time [s] 44

45. Quench - detail
time [s] 45

46. Quench detection and beam dump trigger
detector
threshold
reached
Quench
trigger
Quench
Diode opens Current
bypasses
magnet
Quench
Heaters
fire
15 – 130 ms ms 10 ms
time
1.5 ms
7 - 9 ms
Energy extraction
Current decay
starts
time
3-4 ms
0.2 ms
< 0.4 ms
Powering Interlock
triggered
Beam
Interlock
triggered
Beam
extracted

47. Response to Quenches Quench detected (+ 0 ms)
Current decay starts (+ 9 ms)
3.4 ms
73 ms
Interlocktriggered (+ 5 ms)
Beam would be dumped (+ 5.6 ms)
0.01% of beam would lost (+91 ms)
11/11/2018 47

48. Conclusions
Protection has high priority, a failure can lead to substantial equipment damage
Several systems are involved in protection: magnet protection system, powering interlock system, power converters, cryogenics, UPS, AUG …
The commissioning of the machine protection systems is mandatory for safe operation
The complexity of the LHC powering system is unprecedented and requires the application of strict procedures for commissioning
The procedures have been automised, and therefore commissioning could become very efficient
In case of a failure (quench, failure of a power converter, failure of a supply system such as water and electricity) the beam will be dumped. During hardware commissioning more than 50% of all interlock channels are being tested.
Commissioning experience: worked in general very well, but the lengthy work is fully justified (a number of non-conformities were detected)

49. Acknowledgement
Many colleagues have been involved in the work that is presented here, as CERN staff, as project associates and as industrial support.
It is not possible to list all the names, but I very much appreciate the enthusiastic work of all them.
Teams responsible for protection systems:
Magnet Protection, Powering Interlocks, Beam Interlocks, Beam Dumping System, Beam Loss Monitors, Collimation System
Teams responsible for systems involved in powering:
Power Converters, Vacuum, Cryogenics, (cold) Electrical Engineering
Teams responsible for service systems:
Water cooling, Ventilation, Access System, AC and DC distribution
Other teams:
Controls and Networking, Operation, Magnet “Owners”
Collaborators from abroad (US, Japan), Project Engineers