lhc machine protection: an introduction

LHC Machine Protection:LHC Machine Protection:an introduction an introduction

JJöörg Wenninger rg Wenninger

OP training OP training

March March 20062006Acknowledgments to my colleagues of the MPWGfor input and material.

2

Machine protection at the LHCMachine protection at the LHC

• Machine protection activities of the LHC are coordinated by the LHC Machine Protection Working Group (MPWG), co-chaired by R. Schmidt & J. Wenninger.http://lhc-mpwg.web.cern.ch/lhc-mpwg/

• Since 2004 the MPWG is also coordinating machine protection at the SPS (ring & transfer lines).

http://lhc-mpwg.web.cern.ch/lhc-mpwg/

http://lhc-mpwg.web.cern.ch/lhc-mpwg/

OutlineOutline

• Energy stored in the LHC magnets and beams• Charging the energy• LHC dipole magnets – quench protection• Beam induced damage – what is a safe beam?• Beam dumping system• Collimation system• Strategy for Protection of the LHC machine

OutlineOutline

• Energy stored in the LHC magnets and beams• Charging the energy• LHC dipole magnets – quench protection• Beam induced damage – what is a safe beam?• Beam dumping system• Collimation system• Beam interlock system

Energy stored in a dipole magnetEnergy stored in a dipole magnet

Most energy is stored in the magnetic field of the dipoles

Dipole magnetfield map for one

aperture

B = 8.33 Tesla I = 11800 A L = 0.108 H

Energy stored in LHC magnetsEnergy stored in LHC magnets

Approximation: energy is proportional to volume inside magnet aperture and to the square of the magnet field

about 5 MJ per magnet

Accurate calculation with the magnet inductance:

E dipole = 0.5 L dipole I 2dipole

Energy stored in one dipole is 7.6 MJoule

For all 1232 dipoles in the LHC: 9.4 GJ

0

22

2

dipoledipoledipolestored

RLBE

Energy stored in the beamsEnergy stored in the beams

Stored beam energy: Proton Energy Number of Bunches Number of protons per

bunch

Proton Energy: 7 TeV

In order to achieve very high luminosity:

Number of bunches per beam: 2808

Number of protons per bunch: 1.05 ×1011

Stored energy per beam: 362 MJoule

25 ns

3×1014 protons / beam

8

Stored energy comparisonStored energy comparison

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00

1 10 100 1000 10000Momentum [GeV/c]

En

erg

y s

tore

d in

th

e b

ea

m [

MJ

]

LHC topenergy

LHC injection(12 SPS batches)

ISR

SNSLEP2

SPS fixed target HERA

TEVATRON

SPSppbar

SPS batch to LHC

Factor~200

RHIC proton

LHC energy in magnets

9

The energy stored in the magnets The energy stored in the magnets corresponds to ..corresponds to .. an A380 flying at 700 km/han A380 flying at 700 km/h

a US aircraft carrier at battle-speed of a US aircraft carrier at battle-speed of 55 km/h55 km/h

10

The stored energy also corresponds to …The stored energy also corresponds to …

10 GJoule corresponds to…

• the energy of 1900 kg TNT• the energy of 400 kg Chocolate

• the energy required to heat and melt 12 tons of copper

• the energy produced by a nuclear power plant during 10 seconds

An important point to determine if there is an equipment damage issue:

How fast can this energy be released?

OutlineOutline


0

2000

4000

6000

8000

10000

12000

-4000 -2000 0 2000 4000

time from start of injection (s)

dip

ole

cu

rre

nt (A

)

energy

ramp

preparation and access

beam dump

injection phase

coast

coast

LHC cycle: charging the magnetic energyLHC cycle: charging the magnetic energy

L.Bottura

450 GeV

7 TeV

start of the

ramp

13

Sector

1

5

DC Power feed

3

Oct

ant

DC Power

2

4 6

8

7LHC27 km Circumference

Powering Sector:

154 dipole magnets &about 50 quadrupolestotal length of 2.9 km

LHC Powering in 8 Sectors

Powering Subsectors:

• long arc cryostats• triplet cryostats• cryostats in matching section

Ramping the current in a string of dipole Ramping the current in a string of dipole magnetmagnet

Magnet 1 Magnet 2

Power Converter

Magnet 154Magnet i

• LHC powered in eight sectors, each with 154 dipole magnets• Time for the energy ramp is about 20-30 min (Energy from the grid)• Time for discharge is about the same (Energy back to the grid)• Note : if you switch off the main dipoles PC, the current decays with a time

constant of ~ 6 hours.

0

2000

4000

6000

8000

10000

12000

-4000 -2000 0 2000 4000

time from start of injection (s)

dip

ole

cu

rre

nt (A

)

injection phase12 batches from the SPS (every 20 sec)

one batch 216 / 288 bunches

LHC cycle – charging the beam LHC cycle – charging the beam energyenergy

L.Bottura

450 GeV

7 TeV

OutlineOutline


17

QuenchQuench

A Quench is the phase transition of a super-conducting to a normal conducting state.

Quenches are initiated by an energy in the order of mJ• Movement of the superconductor by several m (friction and

heat dissipation)• Beam losses• Failure in cooling

To limit the temperature increase after a quench• The quench has to be detected• The energy is distributed in the magnet by force-quenching the

coils using quench heaters• The magnet current has to be switched off within << 1 second

Operational margin of a superconducting Operational margin of a superconducting magnetmagnet

Temperature [K]

App

lied

field

[T]

Superconductingstate

Normal state

Bc

Tc

9 K

Applied Field [T] Bc critical field

1.9 K

quench with fast loss of

~5×109 protons

quench with fast loss of

~5×106 protons

~ 0.00001% total no. protons/beam

8.3 T

0.54 T

QUENCH

Tc critical temperature

Temperature [K]

Power into Power into superconductingsuperconductingcable after a quenchcable after a quench

Cross section : Asc 10 mm2

Current : Isc 10000 A

Length of superconductor : Lsc 1 m

Copper resistance at 300 C: cu 1.76 10 6 ohm cm

Psc cu Isc2

Lsc

Asc Psc 1.76 105 watt

Specific temperature of copper at 300 C : cvcu 3.244joule

K cm3

Temperature increase of copper TPsc

Asc Lsc cvcu

Temperature increase within one second: T 5.425 103K

s

Quench - Quench - transition from superconducting state to transition from superconducting state to normalconducting statenormalconducting state - Emergency discharge of energy - Emergency discharge of energy

Magnet 1 Magnet 2

Power Converter

Magnet 154

Magnet i

To limit the temperature increase after a quench• The quench has to be detected : use voltage increase over coil• The energy is distributed in the magnet by force-quenching using quench heaters• The current in the quenched magnet decays is < 200 ms• The current of all other magnets flows through the bypass diode (triggered by the

voltage increase over the magnet) that can stand the current for 100-200 s.• The current of all other magnets is dischared inot the dump resistors

Discharge resistor

Energy extraction system in LHC Energy extraction system in LHC tunneltunnel

Resistors absorbing the energy

Switches - for switching the resistors into series with the magnets

Challenges for quench protectionChallenges for quench protection

• Detection of quench for all main magnets (1600 magnets in 24 electrical circuits)

• Detection of quench across all HTS current leads (2000) with very low voltage threshold ~ 1 mV across HTS part

• Detection of quench in about 800 other circuits• Firing heater power supplies, about 6000 units

• Failure in protection system• detection when there is no quench: downtime of some hours• no detection when there is a quench: damage of magnet,

downtime 30 days

• Systems must be very reliable

Powering InterlockPowering Interlock

• PLC-based Powering Interlock Controllers (PIC) are used to manage the interlock signal between the power converters and the quench protection system.

• The PIC also interfaces to the Beam Interlock System and will request a beam dump if the electrical circuit that fails is considered to be critical for beam operation.

OutlineOutline


A proton injected into the LHC will end its life…A proton injected into the LHC will end its life…

• In a collision with an opposing beam proton• The goal of the LHC !• The experiments are designed to withstand very high particle

fluxes and high doses of radiation.• On the LHC beam dump

• At the end of a fill, be it scheduled or not. • On a collimator or on a protection device/absorber

• The collimators must absorb protons that wander off to large amplitudes to avoid quenches.

• Protons that escape the collimation system or are pushed to large amplitudes by a ‘failure’ (operation or equipment).

• On the machine aperture• Protons that escape the collimation system…

Beam loss into materialBeam loss into material

• Proton losses lead to particle cascades in materials• The energy deposition leads to a temperature increase• The temperature increase may lead to damage : melting, vaporisation, pressure waves…

Magnets could quench…..• beam lost - re-establish condition will take hours

The material could be damaged…..• melting• losing performance (mechanical strength)

Repair could take several weeks or years !

From SPS we (OP) know by experience that ~ 1013 protons at 450 GeV (1 MJ) we can damage equipment !

27

Beam induced damage : SPS Beam induced damage : SPS experimentexperiment

25 cm

Controlled experiment:• Special target installed in the TT40 transfer line• Impact of 450 GeV LHC beam (beam size σx/y = 1.1mm/0.6mm)

Beam

28

Results….Results….

A B D C

Shot Intensity / p+

A 1.2×1012

B 2.4×1012

C 4.8×1012

D 7.2×1012

TT40 damage test presented by

V. Kain at Chamonix 2005:

• Melting point of Copper is reached for an impact of 2.5×1012 p.

• Stainless steel is not damaged, even with 7×1012 p.

• Results agree with simulation

Based on those results the MPWG has adopted for the LHC a limit for safe beams with nominal emittance @ 450 GeV of:

1012 protons ~ 0.3% of the total intensity

Scaling the results yields a limit @ 7 TeV of:

1010 protons ~ 0.003% of the total intensity

Full LHC beam deflected into copper target

Target length [cm]

vaporisation

melting

N.Tahir (GSI) et al.

Copper target

2 m

Energy density [GeV/cm3] on target axis

2808 bunches

The beam will drill a hole along the target axis …

30

Beam absorber challengesBeam absorber challenges

• The stored energy in the LHC beam is so huge that designing absorbers for the beams that are not destroyed by an impact is a real challenge !

• Almost all protection elements are made of Graphite or other forms of Carbon: very robust low density absorber!

• The beam dump block is the ONLY element of the LHC that can safely absorb all the beam – will be discussed in a moment.

• All other absorbers in the LHC (collimators and protection devices) can only stand partial losses – typically up to a full injected beam, i.e. equivalent to the energy stored in the SPS at 450 GeV.

OutlineOutline


LHC Layout

IR3, IR6 and IR7 are devoted to protection and collimation ! IR6: Beam

dumping systemIR4: Radio frequency

acceleration

IR5:CMSexperiment

IR1: ATLASexperiment

IR8: LHC-BexperimentIR2: ALICE

experiment

InjectionInjection

IR3: Momentum Collimation (normal

conducting magnets)

IR7: Collimation (normal conducting magnets)

Beam dump blocks

Schematic layout of beam dump system in Schematic layout of beam dump system in IR6IR6

Q5R

Q4R

Q4L

Q5L

Beam 2

Beam 1

Beam Dump Block

Septum magnet deflecting the extracted beam H-V kicker

for painting the beam

about 700 m

about 500 m

15 kicker magnets

Dumping the LHC beamDumping the LHC beam

about 8 m

concrete shielding

beam absorber (graphite)

about 35 cm

35

Requirements for a clean dumpRequirements for a clean dump

•Strength of kicker and septum magnets must match the beam energy:

•Very safe beam measurement based on the current of the magnets !

•Dump kickers must be synchronized to the « Particle free gap »:

•Accurate and reliable synchronization.

•Abort gap must be free of particles: gap cleaning with damper.

particle free abort gapof 3 s

Kicker magnets constant angle

Beam dump block

Time

Kicker strength

Illustration of kicker risetime

Large graphite absorbers in the beam dump area protect downstream elements (including dump septa themselves) against badly ‘kicked’ particles.

OutlineOutline


37

Beam collimation (cleaning)Beam collimation (cleaning)The very high stored energy, combined with a very low thresholds for quench requires a

complex two-stage cleaning system:• Large amplitude protons are scattered by the primary collimator (closest to the beam).• The scattered particles impact on the secondary collimators that should absorb them.• The efficiency of the collimation must be larger than 99.9% to be able to run under reasonable

conditions, i.e. with lifetimes that can drop down to less than 1 hours from time to time… This requires settings tolerance of < 0.1 mm.

60 collimators/beam!

38

Beam +/- 3 sigma

56.0 mm

1 mm

+/- 8 sigma = 4.0 mm

Example: Setting of collimators at 7 TeV - with luminosity opticsExample: Setting of collimators at 7 TeV - with luminosity optics Very tight settings Very tight settings orbit feedback !! orbit feedback !!

Ralphs Assmanns EURO

Collimators at Collimators at 7 TeV, 7 TeV, squeezedsqueezedopticsoptics

39

Prototype collimators Prototype collimators

Robustness maximized with Robustness maximized with C-C jawsC-C jaws

and water cooling!and water cooling!

40

Robustness test at SPSRobustness test at SPS

C-C jaw

C jaw

TED Dump

No sign of jaw damage!No sign of jaw damage!(but some deformation (but some deformation was observed on the was observed on the

supporting structures)supporting structures)

Test condition:

each jaw hit 5 times!

• 450 GeV SPS LHC beam• 3×1013 protons• 2 MJ• 1 mm2 beam area• equivalent to:

Full Tevatron beam

½ kg TNT

OutlineOutline


Beam loss over multiple turnsdue to many types of failures

Passive protection • Avoid such failures (high reliability

systems)• Rely on collimators and beam

absorbers

Active Protection• Failure detection (from beam monitors

and / or equipment monitoring)• Fire Beam Dump

Beam loss over a single turn during injection, beam dump or any other fast ‘kick’.

In case ofIn case of any failureany failure oror unacceptable beam lifetimeunacceptable beam lifetime, , thethe beam beam must must bebe dumped dumped immediately, immediately, safely into thesafely into the beam dump block beam dump block

‘Unscheduled’ beam loss due to failures

Two main classes for failures (with more subtle sub-classes):

43

Beam interlock systemBeam interlock system

BIS Dump kickerBeam ‘Permit’

User permitsignals

Actors and signal exchange for the beam interlock system:Actors and signal exchange for the beam interlock system:

• ‘‘User systemsUser systems’ : systems that survey equipment or beam parameters and that ’ : systems that survey equipment or beam parameters and that are able to detect failures and send a HW signal to the beam interlock system.are able to detect failures and send a HW signal to the beam interlock system.

• Each user system provides a HW status signal, the Each user system provides a HW status signal, the user permituser permit signal. signal.

• The beam interlock system combines the user permits and produces the beam The beam interlock system combines the user permits and produces the beam permit.permit.

• The The beam permitbeam permit is a HW signal that is provided to the dump kicker (also is a HW signal that is provided to the dump kicker (also injection or extraction kickers) : injection or extraction kickers) : absence of beam permit absence of beam permit dump triggered ! dump triggered !

Hardware links and systems

Core of the Machine Core of the Machine Protection SystemProtection System

Beam Interlock System

Fire kicker magnets

Beam Dumping System

User system detects failure

Beam dump request to Beam Interlock

System

Beam dump request to Beam Dumping System

dump beam

Protection for powering operation• Quench Protection System (4000 channels)• Power Interlocking Controller (36 crates for

800 electrical circuits)

Protection for beam operation• Beam Loss Monitors System (3500 channels)• Special beam instrumentation (few channels) • Beam Interlock System (16 crates for 150 user

connections)• Beam Dumping System (2 complex systems)

45

Schematic of the beam interlock systemSchematic of the beam interlock system

BEAMINTERLOCK

CONTROLLERMODULE

(BIC)

BEAM1_PERMIT

BEAM_PERMIT STATUS SIGNALS

BEAM2_PERMIT

to User Systems

SPS Extraction System for beam 1

SPS Extraction System for beam 2

for beam 2

Beam Dumping System

USER_PERMIT SIGNALS

UN

MA

SK

AB

LE

INP

UT

SM

AS

KA

BLE

IN

PU

TS

MaskSettings

Safe Beam Flag

PM event Trigger TimingSystem

LHC protectionsystems

User System #2

User System #9

User System #16

User System #1

User System #8

User System #10

for beam 1

Beam Dumping System

for beam 1LHC Injection System

for beam 2

LHC Injection System

ArchitectureArchitecture of the BEAM INTERLOCK SYSTEM of the BEAM INTERLOCK SYSTEM

Beam-1 / Beam-2 are Independent!

- fast reaction time (~ s)- safe- limited no. of inputs- Some inputs maskable for safe beam intensity

Up to 20 Users per BIC system:

6 x Beam-18 x Both-Beam

6 x Beam-2

Connected to injection IR2/IR8:-In case of an interlock (=NO beam permit),

the beam is dumped & injection is inhibited.

- It is not possible to inhibit injection

ALONE.

47

BIS reaction timesBIS reaction times

UserSystemprocess

a failure has been detected… beam dump

request

Beam Dumping System waiting for beam gap

89μs max

Signalssend

to LBDS

t2 t3

Beam Interlocksystemprocess

~70μs max.

t1

> 10μs

USER_PERMIT signal changesfrom TRUE to FALSE

Kicker fired

t4

all bunches have been extracted

~ 89μs

Achievable response time ranges between 100 s and 270 s

(between the detection of a dump request and the completion of a beam dump)

SummarySummary

• The LHC is one of the most complex instruments that has ever been The LHC is one of the most complex instruments that has ever been conceived.conceived.

• The LHC is the first accelerator where the machine protection systems The LHC is the first accelerator where the machine protection systems are vital.are vital.

• LHC commissioning progress will be strongly influenced by the LHC commissioning progress will be strongly influenced by the understanding of the components of the protection systems.understanding of the components of the protection systems.

• The LHC performance will be strongly affected by the protection The LHC performance will be strongly affected by the protection systems: systems:

• due to the large number of interlock channels the reliability of the systems must be very high/ Reliability studies have been performed (and there are more to come).

• The very tight tolerance on machine parameters and collimation will make LHC operation totally different from SPS or LEP:

Play once and the beam is gone !

lhc machine protection: an introduction

Documents