Accidental beam losses and protection in the LHC. R.Schmidt and J.Wenninger f or the Working Group on Machine Protection HB 2004 Bensheim. LHC parameters and associated risks Overview accidental beam losses Aperture and accidental beam losses Protection and redundancy Conclusions. - PowerPoint PPT Presentation
HB 2004 1 Accidental beam losses and Accidental beam losses and protection in the LHC protection in the LHC R.Schmidt and J.Wenninger f or the Working Group or the Working Group on Machine Protection on Machine Protection HB 2004 Bensheim HB 2004 Bensheim LHC parameters and associated LHC parameters and associated risks risks Overview accidental beam losses Overview accidental beam losses Aperture and accidental beam Aperture and accidental beam losses losses Protection and redundancy Protection and redundancy Conclusions Conclusions
Transcript
HB 2004 1
Accidental beam losses and protection in Accidental beam losses and protection in the LHC the LHC
R.Schmidt and J.Wenninger
for the Working Group on or the Working Group on Machine ProtectionMachine Protection
HB 2004 BensheimHB 2004 Bensheim
LHC parameters and associated risksLHC parameters and associated risks
Aperture and accidental beam lossesAperture and accidental beam losses
Protection and redundancyProtection and redundancy
ConclusionsConclusions
HB 2004 2
LHC tunnel
Beam dump tunnel
HB 2004 3
Some numbers for 7 TeVSome numbers for 7 TeVMomentum at collision 7 TeV/cBeam intensity 2808 1.1 1011 protons per beamLuminosity 1034 cm-2s-1
Dipole field at 7 TeV 8.33 TeslaTypical beam size 200-300 µm
• Energy stored in the magnet system: 10 GJoule
• Energy stored in one (of 8) dipole circuit: 1.1 GJoule
• Energy stored in one beam: 350 MJoule
• Average beam power to compare with
high power accelerators, both beams: some 10 kWatt
• Instantaneous beam power for one beam: 3.9 TWatt….during 89 µs
….corresponds to the power of 1700 nuclear power plants
• Energy to heat and melt one kg of copper: 700 kJ
HB 2004 4
Bunch intensities, quench and damage levelBunch intensities, quench and damage level
• Intensity one “pilot” bunch 5109
• Nominal bunch intensity 1.11011
• Batch from SPS (216/288 bunches at 450 GeV) 31013
• Nominal beam intensity with 2808 bunches 31014
• Damage level for fast losses at 450 GeV 1-21012
• Damage level for fast losses at 7 TeV 1-21010
• Quench level for fast losses at 450 GeV 2-3109
• Quench level for fast losses at 7 TeV 1-2106
Damage and quench assessment approximative, supported by experience in SPS and calculations
Further calculations and material tests at SPS in two weeks planned
HB 2004 5
Livingston type plot: Livingston type plot: Energy stored in the beamEnergy stored in the beam
0.01
0.10
1.00
10.00
100.00
1000.00
1 10 100 1000 10000Momentum [GeV/c]
En
erg
y st
ore
d in
th
e b
eam
[M
J]
LHC topenergy
LHC injection(12 SPS batches)
ISR
SNSLEP2
SPS fixed target
HERA
TEVATRON
SPSppbar
SPS batch to LHC
Factor~200
RHIC proton
HB 2004 6
Failure scenarios and accidental beam losses Failure scenarios and accidental beam losses
A large number of different mechanisms can cause accidental particle losses: Classification of accidental beam losses according to time constant for the loss
• Ultra fast beam losses (single turn or less) • to be avoided, beam dump block is the only element that can safely
absorb the 7 TeV LHC beam
passive protection with collimators and beam absorber
• Very fast beam losses (some turns to some milliseconds)• Fast beam losses (5 ms – several seconds)• Slow beam losses (several seconds – 0.2 hours)
active protection, by detecting failure and extracting the beams into beam dump block
HB 2004 7
Single turn accidental beam lossesSingle turn accidental beam losses
Failure mechanisms
• Failure of beam dump kicker (prefiring, asynchronous beam dump)• Failure of kickers for tune measurements and aperture exploration• During transfer and injection
• failure of injection kicker • wrong trajectory or mismatch of beam energy• obstruction of beam passage
• Recent studies on protection during transfer and injection of the beams from SPS at 450 GeV to the LHC (see H.Burkhardt)
Strategy for protection
• Avoid such failures (systems with high reliability)• Block beam transfer from SPS to LHC if parameters are not correct (i.e. magnet
current)• Beam trajectory after such failure is reasonably well defined • Passive protection: rely on collimators and beam absorbers
HB 2004 8
Consequence of a failure scenario: Full 7 TeV LHC beam deflected into copper target
Target length [cm]
vaporisation
melting
Copper target
2 m
Energy density [GeV/cm3] on target axis
2808 bunches7 TeV 350 MJoule
collaboration with N.Tahir (GSI) et al.
HB 2004 9
Density change in target after impact of 100 bunches
• Energy deposition calculations using FLUKA• Numerical simulations of the hydrodynamic and thermodynamic response of the target with two-dimensional hydrodynamic computer codeFrom this calculations one can estimate the longitudinal range of full beam in copper
between 10m and 40m
Target radial coordinate [cm]
radial
copper solid state
collaboration with N.Tahir (GSI) et al.
100 bunches – target density reduced to 10%
Copper target
HB 2004 10
LHC Layout
eight arcs (sectors with a length of about 2300 m)
Collimators for cleaning the beam halo • close to the beam between 5-10 • must be accurately adjusted (within a fraction of one )• position depends on optics and possibly on energy
Collimators for protection of equipment against single turn beam losses• shadow equipment downstream • must be adjusted (better than one σ)• position depends on LHC operational mode (injection, energy ramp, …) and on optics
Collimators for protection and cleaning of the low-beta insertions, mainly in IR1 and IR5 • close to the beam about 10 • must be accurately adjusted (within about one )• mainly required during squeeze and for squeezed beams
Injection
Transfer Line Transfer Line
Injection
HB 2004 15
Lifetime of the beam - for Lifetime of the beam - for nominal intensity at 7 TeVnominal intensity at 7 TeV
Beam lifetime
Beam power into equipment (1 beam)
Comments
100 h 1 kW Healthy operation, beam cleaning should capture > 99% of the protons
10 h 10 kW Operation acceptable, beam cleaning should capture 99.9% of the protons
(approximate beam losses = cryogenic cooling power at 1.9 K)
0.2 h 500 kW Operation only possibly for 10 s, beam cleaning must be VERY efficient
1 min 6 MW Equipment or operation failure - operation not possible - beam must be dumped
• Quench of superconducting magnets• Discharge of superconducting magnets switching a
resistance into the circuit (after quench, or by accident) • Failure of magnet powering
For some magnets very fast beam loss (several turns): D1 normal conducting magnet
• Electric short in the coil of a normal conducting magnet
HB 2004 18
Multiple turn failures: Other failuresMultiple turn failures: Other failures
• Aperture limitation in beam pipe (circulating beam)• Vacuum valve moves into beam
• Collimator moves into beam
• Other element moves into beam
• Loss of beam vacuum
• Failure in the RF system• Debunching of beam and number of protons in the abort gap… could
lead to single turn failure when beam is dumped
• Operational failures
• Combined failures, for example after Mains Disturbances (thunderstorm, …)
HB 2004 19
Beam losses and apertureBeam losses and aperture
The aperture of the LHC at 450 GeV is limited (about 7.5σ, assuming
closed orbit excursions=4 mm, beta-beating, ….)
Critical operation at 7 TeV with squeezed optics: -function up to 4850 m in insertions IR1 and IR5 • very strong low- quadrupole magnets with orbit offset • normal conducting dipole magnets • superconducting dipole magnets
• In general, particle losses first at collimators
• Fast orbit changes are the most critical failures• collimators at about 6-9 from the beam• 1% of the beam would damage the collimators for fast beam loss
HB 2004 20
Critical apertures around the LHC (Critical apertures around the LHC ( illustration drawing)illustration drawing)
in units of beam size in units of beam size at 450 TeV at 450 TeV
IR1 IR2 IR3 IR4 IR5 IR6 IR7 IR8
collimators (betatroncleaning)
collimators(momentumcleaning)
aperture in cleaning insertions about 6-9
6-9
arc aperturedown to about 7.5
aperture in cleaning insertions about 6-9
HB 2004 21
Critical apertures around the LHC (Critical apertures around the LHC ( illustration drawing)illustration drawing)
in units of beam size in units of beam size 7 TeV and 7 TeV and * = 0.55 m in IR1 and IR5* = 0.55 m in IR1 and IR5
Most likely failures for fast losses: quenchesMost likely failures for fast losses: quenchesFailures leading to the fastest multiturn losses: D1 magnet Failures leading to the fastest multiturn losses: D1 magnet
Quench of:- MQX - D2- MB
Powering Failure of D1 normal conducting
D1 normalconducting
very fast loss
Squeezed optics with max beta of 4.8 km
All 4 quadrupole magnets (inner triplet MQX) quench , approximately Gaussian current decay with time constant 0.2 s
Powering failure for D1, exponential current decay, time constant 2.5 s
Quench of one MB, approximately Gaussian current decay with time constant 0.2 s
D2 quenchfast loss
time [seconds]
orbit [mm]
MB quench
fast loss
MQX: 2 quads quench
fast loss
V.Kain Diploma thesis 2001 / O.Brüning
HB 2004 23
Particles that touch collimator after failure of normal conducting D1 magnets
After about 13 turns 3·109 protons touch collimator, about 6 turns later 1011 protons touch collimator
V.Kain
“Dump beam” level
1011 protons at collimator
HERA experience confirmes worries: very fast beam losses
HB 2004 24
Protection and redundancy: what triggers a beam Protection and redundancy: what triggers a beam dump? dump?
1. Hardware diagnostics
2. Quench signal from Quench Protection System
3. Beam loss monitors at the collimators and other aperture limitations
PLUS• Does not require collimators to have correct settings• If early enough, can dump the beam before particle losses
MINUS• For many type of failures the beam dump comes too late • Complexity of hardware: not all failures are detected• Too many channels: too many “False Beam Dumps” • Risk of including failures that would not lead to particle losses
HB 2004 27
Quench detection Quench detection
1. Magnet starts to quench
2. Resistive Voltage across magnet > 0.1 V
+10 ms: quench detection
• fire quench heater
• requests energy extraction
• requests a beam dumpThe quench heaters become effective
Magnet current starts to debypass magnet by diode
+5 ms: current starts to decay exponentially
+300 s: the beam dump kicker extracts beam
+ 3 ms: the interlock system transmits the request to the beam dump
HB 2004 28
Quench detection Quench detection
PLUS• Does not require collimators to have correct settings• If early enough, beam gone before losses• Dumps beam for failures of the quench protection system• Does not reduce the availability of LHC: Quench protection is
always required. After a quench the beam must be dumped
MINUS• Only covers beam losses due to magnet quenches• Might be too late (…being further analysed, efficiency depends on
quench process, magnet field, beam loss pattern, etc…)• Large complexity (several 1000 channels) – good post mortem
analysis required
HB 2004 29
Beam loss monitoring at aperture limitations Beam loss monitoring at aperture limitations
In general, collimators are limiting the aperture• Always true for beam blow up• Mostly true for orbit changes
Beam loss monitors at aperture restrictions continuously measuring beam losses
• Losses are detected within less than a turn• After detection it takes 2-3 turns to extract all particles into beam
dump block
HB 2004 30
Beam loss monitoring at aperture limitations Beam loss monitoring at aperture limitations
PLUS• Should capture (nearly) all types of accidental beam losses• Dumps the beam if there are really particle losses• Very fast (< 100 s)• Limited complexity (some 100 channels)• Expected to be very reliable
MINUS• Does require collimators to have correct settings and defining the
aperture• Does not catch beam losses in the arcs (for example, closed orbit
bumps)• Random spikes might trigger beam dump• Setting of thresholds not obvious - if too low, False Beam Dumps –
if too high - risk of damage
HB 2004 31
Beam loss monitoring around the acceleratorBeam loss monitoring around the accelerator
Beam loss monitors continuously measuring beam losses• Together with the BLMs at aperture limitations, covers most
of the LHC (all arcs)• Losses can be detected within less than a turn• After detection it takes 2-3 turns to extract all particles into
beam dump block
HB 2004 32
Beam loss monitoring around the acceleratorBeam loss monitoring around the accelerator
PLUS• Dumps the beam if there are really particle losses• For failures leading to orbit changes and emittance growth• Detection can be made very fast (< 100 s)• Does not require collimators to have correct settings • Catches failures that appear only in the arcs (for example, closed
bump)
MINUS• Large complexity (some 1000 channels)• Could increase number of False Beam Dumps• Setting thresholds: delicate balance between avoiding magnet
quenches and avoiding False Beam Dumps
HB 2004 33
Magnet current decay monitoring for critical magnets Magnet current decay monitoring for critical magnets
Very fast detection of power converter / magnet failures • Monitors change of magnet current (Hall probes, voltage, …)• Prototype “quick and dirty” gave promising results (M.Zerlauth)• Similar technique recently successfully implemented at HERA
(M.Werner)• Should be possible to detect powering failures in less than one
millisecond
Interlock signal creation using Hall-Probe
-1
0
1
2
3
4
5
6
5 10 15 20 25
Time [ms]
Vo
ltag
e [
V]
Reference Ramp Converter Hall probe readout Interlock Signal
HB 2004 34
Magnet current decay monitoring for critical magnets Magnet current decay monitoring for critical magnets
PLUS• Independent method to monitor failures in the powering system:
power converter fault / thunderstorm / short circuit in magnet / other problems
• Does not require collimators to have correct settings • Can be made fast (< 1ms)• Mainly for normal conducting magnets
MINUS• Needs to be demonstrated if practical (EMC, …) – wait for HERA
experience• Setting of thresholds required – could be delicate• Should be limited to a few electrical circuits with normal conducting
magnets – otherwise too complex
HB 2004 35
Beam position change monitors Beam position change monitors
If the orbit start to moves very fast, dump the beam
• Fast orbit changes can be observed anywhere around the LHC
• Observation for each beam, each plane, two monitors with 90 degrees phase advance: in total 8 BPMs• …system with limited complexity
• BPMs at location of high beta function, using the same monitors that are already required for machine protection (to ensure x < 4 mm in the insertion IR6 for the beam dumping system)
HB 2004 36
Beam position change monitors: thresholdsBeam position change monitors: thresholds
450 GeV: fastest orbit movement during normal operation by an orbit corrector magnetSuperconducting orbit correctors : 2 mm/s … 15 mm/s
Normal conducting orbit correctors: 0.6 mm/s … 1.7 mm/s
450 GeV: if the change of the orbit exceeds, say, some 10 mm/s corresponding to 0.01 mm/msec, there is something wrongDetection of very fast orbit drifts:
(IF dx/dt > 0.1 mm/ms) OR
(IF dx/dt > 1 mm/100ms) THEN beam dump
7 TeV: if the change of the orbit exceeds, say, some 1 mm/s corresponding to 0.001 mm/msec, there is something wrong(IF dx/dt > 0.05 mm/ms) OR
(IF dx/dt > 0.3 mm/100ms) THEN beam dump
numbers prelim
inary
HB 2004 37
Beam position change monitors Beam position change monitors
PLUS• Independent method to measure fast orbit drifts due to failures• Does not require collimators to have correct settings • Can be made fast (< 1ms)• Beam dump before particles are lost• Limited complexity
MINUS• Is it practical ? False Beam Dumps ?• Setting of thresholds delicate• What to do during injection, during kicks for Q-measurements, …
to be studied• Only for beam orbit changes, not for emittance growth• Studies needed
HB 2004 38
Fast beam current decay monitoring Fast beam current decay monitoring
Very fast beam current monitor, could detect losses within short time
• Measuring proton losses with, say, N / t = 1010 protons • Interlock condition
• if (N / t > Nthreshold(E) · 1010) THEN BEAM DUMP
t could be as short as one turn
• Nthreshold decreases with energy to be always efficient
• For start of LHC operation, when intensity is limited, resolution should be no problem
• Challenge: must be fast and accurate• First discussions with experts - looks promising
HB 2004 39
Fast beam current decay monitoring Fast beam current decay monitoring
PLUS• Independent method to measure beam losses• Does not require collimators to have correct settings • Fast for reduced accuracy (< 1ms)• Slow for high accuracy (> 10ms)• Limited complexity – one instrument
MINUS• Needs to be demonstrated if practical • Setting of thresholds required• Not sufficient for all LHC operation modes and for shortest
accidental beam losses time constants • Could be ok for 450 GeV, not for 7 TeV • Studies needed
HB 2004 40
Protection and redundancy at 450 GeVProtection and redundancy at 450 GeV
System Needs collimators
In time? Complete protection
Add. develop.
effort
Complexity
Y=1, N=3 Y=3, N=1 Y=3, N=1 Y=1, N=3 Y=1, N=3
Hardware diagnostics
3 1 1 3 2
Quench signal 3 3 1 3 3
Beam loss monitors collimators and aperture limitations
1 2 2 3 2
Beam loss monitors in the arcs
3 2 2 3 1
Magnet current change monitors
3 2 1 2 2
Beam position change monitors
3 3 2 2 3
Fast beam current decay (“lifetime”) monitors
2 3 3 2 3
HB 2004 41
Protection and redundancy at 7 TeVProtection and redundancy at 7 TeV
System Needs collimators
In time? Complete protection
Add. develop.
effort
Complexity
Y=1, N=3 Y=3, N=1 Y=3, N=1 Y=1, N=3 Y=1, N=3
Hardware diagnostics
3 1 1 3 2
Quench signal 3 2 2 3 3
Beam loss monitors collimators and aperture limitations
1 3 3 3 2
Beam loss monitors in the arcs
3 2 1 3 1
Magnet current change monitors
3 2 1 2 2
Beam position change monitors
3 3 2 2 3
Fast beam current decay (“lifetime”) monitors
2 2 2 1 3
HB 2004 42
ConclusionsConclusions
• Protection for LHC starts before extraction from SPS• Protection is required during the entire cycle
Large redundancy in protection
Availability of the machine due to the complex protection is an important issue
• Large energy: stringent protection required - too few interlocks could lead to severe damage of the LHC
• Unprecedented complexity: too conservative interlocking of the machine protection systems could prevent efficient LHC exploitation
• Initial operation with part of the protection systems• Commissioning of other protection systems during initial operation
HB 2004 43
Some questions to the workshop……..Some questions to the workshop……..
Fast beam current decay monitoring
Fast beam position change monitoring
• What can be achieved?
• Who has experience?
• Where else might such systems be required?
HB 2004 44
AcknowledgementsAcknowledgements
The presentation is based on the work that was performed in many groups in several CERN Departments, as well as collaborators from other labs (Fermilab, GSI, Protvino, Triumf, ….)
Contributions of many colleagues are acknowledged, in particular for the discussions in the Machine Protection WG, Collimation WG and Injection WG
Recent question in MPWG: Can we dump the beam in time after a quench of a dipole magnet?
Beam is to be dumped before the current in the dipoles starts to decay
The sequence of following actions has to be determined beam loss causes the magnet to quench the voltage builds up and exceeds the threshold of the quench detector the quench detector detects the voltage after some time the quench detector fires quench heaters or triggers the energy extraction at the same time, the PIC is informed
PIC sends a dump request to the BIC the heaters become efficient the BIC sends a dump request to the beam dumping system the voltage exceeds the diode voltage, and the current starts to bypass the magnet the switch opens, and the current in the string of magnets decays
The Powering Interlock Controller is the only system sending (direct) beam dump request after powering failures
‘Secondary’ protection with collimators, BLM (possibly BPM / beam lifetime)
Def
ined
seq
uen
ce
Wh
o’s
qu
ick
est?
HB 2004 46
Hardware configuration (main dipole circuit) and signal transmission
Quenching (dipole) magnet in the arc
Beam DumpRequest
Beam Dumping System
Beam Interlock Controller
Beam DumpRequest
‘Quench Loop’ stretching over the arcs
HB 2004 47
The PIC process times
For main circuitsHardware Matrix (CPLD)Process time < 0.2ms
PIC Controller (PLC)Process time < 5ms
Quench Protection
System
Power Converters
Quench Signals, Discharge Requests
Power Permit, Powering Failure,Discharge Requests
Beam Dump Requests
Interlock Controller is based on a PLC controlled process, monitoring and controlling up to 45 electrical circuits (>200 signals)
For time (beam) critical circuits -> configurable hardwired matrix in parallel
HB 2004 48
The Timescales
T0 Quench signal
T2 PIC reads signal
T1 Quench Loop Controller Receives signal
T3 PIC issues beam dumpT4
Beam dump is received by the BIC
T5 Completion of beam dump
0.2 … 5ms < 100μs < 270μs
< 15 μs < 100 μs 5..7 ms
T1 EE system readsQuench signalT2
Switch Opening
T3 Last branch open, arc is extinct –Current in dipole magnets decays
Current in dipole circuitDiode becomes conductive > 80ms
< 15 μs< 100 μs
Decay starts somewhat before the complete arc extinction due to resistive arc // resistor
Mechanic opening & arc extinction
PIC process
HB 2004 49
Conclusions
Due to the delay in the switch opening of the 13kA energy extraction system, the beam is dumped before the current in the magnet starts decaying also true in case of self triggering of EE
For all other sc magnets the time constraints are less critical
For time critical signals (mainly main dipole and quadrupole circuits due to large effects on the circulating beams), a hardwired matrix within the PIC can be configured in parallel to the PLC process
