1. Collimators: Operations - Baseline Assumptions
As indicated the collimators have to protect the machine and experiments while we’re spraying beam around at all stages of operations
Types of losses
Beams
Operational cycle & role of collimators
Lifetime limits
Collimator efficiency
Operational constraints on beam parameters
Annual losses
LHC commissioning - phased approach
30/06/2004
LHC collimator review

2. Types of loss Abnormal (Fast & Ultra fast loss) Short lifetimes Stable
Equipment malfunction etc.
Short lifetimes
Operator error
Beam instabilities
Parameter control challenges (persistent currents etc.)
Stable
Transverse
Beam gas
Nonlinearities
Long range beam-beam
Electron cloud
IBS
Collisions
Longitudinal
Touschek RF
Other: e.g. electron-capture by pair production
30/06/2004
LHC collimator review

3. Short Lifetime/Stable conditions
Required Beam Intensity
Operational cycle
Permitted beam loss
Acceptable Lifetimes
Collimator efficiency
Operational tolerances
Beam Instrumentation
Coming later…
Abnormal losses
Collimator design
Transfer Line collimation
Other protection devices
Machine Protection
30/06/2004
LHC collimator review

4. Here to Protect 1. Damage: 2. Quenches
Dangers clear and well enumerated.
2. Quenches
For example, local transient loss of 4 × 107 protons at 7 TeV
Nominal beam energy →
One British aircraft carrier at 11 knots
One girl in a Porsche at 1600 mph
30/06/2004
LHC collimator review

5. Beams Beam No. bunches Protons/bunch Total Intensity Emittance Pilot 1
5 – 10 x 109
1 – 3.75 µm
Intermediate 12
1.15 x 1011
1.4 x 1012
3.75 µm
Nominal
2808
3.23 x 1014
Ultimate
1.67 x 1011
4.7 x 1014
Ions
592
7 x 107
4.1 x 1010
1.5 µm
Totem
43/156
3 x 1010
1.3/4.4 x 1012
1.0 µm
30/06/2004
LHC collimator review

6. Nominal cycle
30/06/2004
LHC collimator review

7. Injection – 450 GeV Big beams, lower dynamic aperture
Pilot & Intermediate beam to check & adjust beam parameters, position collimators etc.
12 SPS batches per ring, 1 batch up to 288 bunches
Big beams, lower dynamic aperture
Protection of cold aperture in arcs
Collimators to protect during:
Injection process (injection oscillations etc.)
Accidents: kicker misfires, timing errors
Inevitable lifetime dips
30/06/2004
LHC collimator review

8. Ramp & Squeeze Start ramp - out of bucket flash:
~5% total beam primarily onto the momentum collimators
Start ramp - snapback:
Tune, chromaticity, momentum, orbit, -beating. Lifetime.
Ramp:
Collimators stay (more-or-less) where they are. Beam emittance shrinks. Still protecting arc cold aperture.
Scraping at end of ramp?
Squeeze:
Aperture limit now becomes inner triplet [IR1 & 5]. Collimators need to move in before/during the squeeze to protect the insertion quadrupoles.
Tune, chromaticity, orbit, -beating. Lifetime.
30/06/2004
LHC collimator review

9. Beam lifetimes 7 TeV - Physics
The contributions for collisions have to be doubled up to get an estimate for an intensity lifetime of around 17.8 hours. NB figures preliminary
Plus: Lifetime dips, background optimisation, abort gap
30/06/2004
LHC collimator review

10. Emittance growth rates
Plus random power supply noise, ground motion, RF noise, electron cloud, nonlinearities . Small contribution to beam lifetime at 7 TeV especially given the presence of synchrotron radiation damping
30/06/2004
LHC collimator review

11. Minimum beam lifetimes
Mode
T [s]
 [h]
Rloss [p/s]
Ploss [kW]
Injection
continuous
1.0
0.8 x 1011 6 10
0.1
8.6 x 1011 63
Ramp
≈ 1
0.006
1.6 x 1013
1200
Top energy 97
0.2
4.3 x 1011
487
30/06/2004
LHC collimator review

12. Allowable Intensity in the LHC
Allowed
intensity
Quench threshold
(7.6 ×106 7 TeV)
Cleaning inefficiency =
Number of escaping p (>10s)
Number of impacting p (6s)
Beam lifetime
(e.g. 0.2 h
minimum)
Dilution
Length
(50 m)
The nominal intensity of 3 × 1014 protons per beam requires a collimation inefficiency of 2 × 10-5 m-1. Injection has less strict requirements.
30/06/2004
LHC collimator review

13. Operations Limitations on the allowed minimal collimator gap:
The beam core must not be scraped by collimation, usually requiring collimator settings above 4-5 .
The collimator gap must be wide enough to avoid excessive impedance from the collimators and to maintain beam stability.
The two-stage functionality of the collimation system must be maintained during the whole operational cycle, e.g. the primary collimators must always remain primary and the secondary must always remain secondary collimators. Usually a relative offset of 1 nominal sigma is required, corresponding to about 200 µm at 7 TeV. Operational and mechanical tolerances are specified for this offset.
30/06/2004
LHC collimator review

14. Operations - implications
The settings n1, n2 and n3 of primary, secondary and tertiary collimators must be carefully adjusted in order to minimize the leakage rates of the cleaning insertions → tight demands on beam optics and stability. To go to significant intensity therefore:
Design aperture must be established
Max. -beating ≈ 20%
Max. orbit deviation ≈ 4 mm.
Transient changes in orbit and -beating under control (tune & orbit feedback, etc.)
Max. transient -beating ≈ 8%
Max. orbit shift ≈ 0.6 
Nominal beam loss rates established
Min. beam lifetime > 0.2 hours. Dump beam otherwise
30/06/2004
LHC collimator review

15. Annual Doses
Take: assumed operational efficiency, number of days of operation, turn around → number of fills
For a fill, estimate:
Injection oscillation losses, lifetime at 450 GeV, scale to 7 TeV
Start ramp: out of bucket flash, snapback
Lifetime in ramp
Squeeze: lifetime, lifetime dips
Physics: lifetimes (plus lifetime evolution) - halo versus luminosity etc.
Dump
Plus some lost fills
30/06/2004
LHC collimator review

16. IR3 IR7 First Year - 1.3 x 1016 Nominal 8.0 x 1015 3.5 x 1016 Ultimate
Annual loss estimates
IR3
IR7
First Year -
1.3 x 1016
Nominal
8.0 x 1015
3.5 x 1016
Ultimate
1.1 x 1016
7.3 x 1016
30/06/2004
LHC collimator review

17. Initial commissioning of phase 1
Phased commissioning
Initial commissioning:
Ending with Pilot physics: 43 on 43 with x 1010 (if we’re lucky)
Year one[+] operation:
Lower beam intensity/luminosity:
Event pileup
Electron cloud
Phase 1 collimator impedance etc.
Equipment restrictions
Relaxed squeeze, lower intensities, 75 ns. bunch spacing
Initial commissioning of phase 1
Use this period to stage commissioning of collimator systems & to optimise cleaning efficiency
30/06/2004
LHC collimator review

18. Phased commissioning Parameter
Tolerances for 50% increase in cleaning inefficiency
Nominal Injection
(6/7 )
Nominal Collisions
Collisions (Relaxed *) (7/10.5 )
Beam size at colls.
≈ 1.2 mm
≈ 0.2 mm
Orbit change
0.6  ≈ 0.7 mm
0.6  ≈ 0.12 mm
2.0  ≈ 0.4 mm
Transient -beat 8%
80%
Collinearity beam-jaw
50 µrad
75 µrad
30/06/2004
LHC collimator review
R. Assmann, J.B. Jeanneret, E. Metral,

19. Conclusions Difficult beams, potential for quenches/damage high
Operational cycle will include challenges
effective collimation essential at all stages
Reasonable limits on lifetimes assumed
Tight limits on collimator settings
Tight limits on operational beam parameters to ensure required collimator efficiency
Annual dose estimates for IR3 & IR7
Phased commissioning foreseen
Acknowledgements…
30/06/2004
LHC collimator review