Critical beam losses during Commissioning

& Initial Operation

Guillaume Robert-Demolaize

(CERN and Univ. Joseph Fourier, Grenoble)

with R. Assmann, S. Redaelli, C. Bracco & T. Weiler;

thanks to B. Dehning, B, Holzer & L. Ponce

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OUTLINE

Introduction

Loss distribution from betatron cleaning

Minimum workable BLM system for collimation studies

Conclusion – Future studies

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Introduction

Purpose of the LHC Collimation system: provide cleaning efficiency and protection, using collimators and absorbers

=> ~ 40 elements per ring~ 40 elements per ring are being implemented in the machine (Phase 1 of the system)

About 3700 Beam Loss Monitors (BLMs) can be counted around the two rings of the machine

=> do we need all BLM information to understand the cleaning => do we need all BLM information to understand the cleaning performance and losses from the “leaking halo” ?performance and losses from the “leaking halo” ?

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Base principles of the LHC collimation system

Collimators intercept beam halos (first, secondary, …) with some leakage which gets lost around the ring: the cleaning inefficiency of the system is then defined as:

The leakage lost over a given length of the machine (10 cm in our studies) is then counted as local cleaning inefficiency (unit = m-1).

Goal of this presentation is to show that it is sufficient to usesufficient to use only a only a limited number of BLMs for commissioning the collimation systemlimited number of BLMs for commissioning the collimation system.

systemtheby )cleaned"(" absorbed protons of #

systemcleaning the escaping protons of #

~

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Critical BLMs for collimation

There are two types of critical BLMs for the collimation system:

-- BLMs at the collimators: needed from early on for the set-up of the collimators (experiments in SPS performed successfully in Fall 2004 for the first time),

-- BLMs at loss locations of “leakage halo”: the halo exiting IR3/IR7 is lost in characteristic locations and not spread everywhere around the ring (implying all BLMs should be used)

=> critical loss locations characterize the efficiency of our system: can => critical loss locations characterize the efficiency of our system: can we identify those critical locations (= BLMs) ??we identify those critical locations (= BLMs) ??

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How to address this question

Performing full simulations with ALL movable LHC Collimation System equipments: 41 collimators/absorbers per ring for Phase 1.

Only betatron cleaning is considered in the following for on-momentum beam halo

Check leakage halo losses in cold elements of the machine

Notes: * results presented for Beam 1 only (Beam 2 tracking in preparation) * heavy computing effort in resources and time (CPU limited) * local energy deposition: FLUKA takes our data as input * losses at collimators: induced showers can propagate downstream

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CPU usage

2 students - 2 fellows

Tracking on 64+ CPUs

← limit of Collimation allocated CPUs

← granted by share with experiments

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OUTLINE

Introduction

Loss distribution from betatron cleaning

Minimum workable BLM system for collimation studies

Conclusion – Future studies

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Parameters for obtaining loss maps

Data done for the two types of tracked halo (horizontal and vertical) and the two optics defined as reference cases:

-- injection optics: 450 GeV, * = 17 m at all IPs,

-- collision optics: 7 TeV, * = 0.55 m at IP1 & IP5 (else 10 m).

Intermediate * values can be studied if necessary (in case of big losses in experimental insertions).

Assumed quench limit values: 10-3 m-1 (injection) 2 x 10-5 m-1 (collision)

Results presented in the following slides focus on the horizontal halo only.

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Collimators settings – Injection (1/2)

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Collimators settings – Injection (2/2)

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Collimators settings – Top energy (1/2)

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Collimators settings – Top energy (2/2)

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Error scenarios

In the following we consider free orbit oscillations, always at the worst phase (found in simulation scan), following 2 scenarios:

Static case:

-- collimators are always re-centered around the perturbed orbit

-- the error amplitude can reach the estimated aperture tolerances of 4 mm (injection optics) / 3 mm (collision optics)

Dynamic case:

-- collimators are still centered on the nominal closed orbit

-- peak amplitude of error is ~ 1.5

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Loss map – 450 GeVIdeal case

=> Ideal case: below the quench limit (factor 5); not true during start-up though

▬► halo

↕ x 5

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Sample perturbed orbit

▬► halo

↑│││ ± 4 mm││↓

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Loss map – 450 GeV Perturbed orbit – worst phase, 4 mm amplitude

=> Loss of a factor 2 in efficiency at worst locations !!!

▬► halo

↕ x 2.5

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Going downstream from IR7

▼▼ ▼ ▼ ▼ ▼ ▼

=> Same loss locations !!! Modulation of the peaks: a way to measure orbit ???

▼

▼

▼

▼▼

▼

▼▼

high dispersion

↓high dispersion + high ↓ ↓ ↓ ↓

Ideal case

4 mm orbit

▼ = critical BLM

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Effect of optic (dispersion)

=> Losses due to first high dispersion location !!!

Characteristic loss locations can be understood from halo properties and optics.

↑ peak loss location

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Why off-momentum losses for on-momentum primary halo ?

Collimators in IR7 intercept off-axis particles => induced proton-collimator material interaction follows several processes.

Single-diffracting scatteringSingle-diffracting scattering: generates off-momentum halo=> always lost at one of the first high dispersion points: critical locations for limiting losses are therefore well defined (as seen in the IR7 +Arc 7-8 case)

Sets fundamental limitation of the LHC betatron cleaning insertion: single-diffracting scattering can never be avoided !!!

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IR8 + Arc 8-1

▼

▼

▼

▼

Ideal case

4 mm orbit

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Effect of optic (beta)

=> Losses due to high betatron location !!!

← peak loss location

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IR1

Ideal case

4 mm orbit

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IR2

Losses here are due to scattering from TDI

TDI.4L2

TCLIA.4R2

Ideal case

4 mm orbit

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IR3

▼

▼

Remember: only betatron cleaning

Ideal case

4 mm orbit

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IR4

Ideal case

4 mm orbit

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IR5

Ideal case

4 mm orbit

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IR6

▼

Losses at the TCDQ equipment: → problem of local showers downstream of it under study

Ideal case

4 mm orbit▼

=> We made one turn after IR7: 13 critical BLMs identified13 critical BLMs identified at injection (in addition to the ones foreseen at the locations of collimators).

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Going further in error amplitude

│ │ ← specified orbit │ │ │ │ │ │ │ │ │ │ │ │ │ │

+ 80 %

+ 130 %

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7 Tev Study

IR2, IR5 & IR8: nominal crossing schemes IR1: where orbit perturbation is applied

halo▬►

Static orbit: ± 4 mm in the arcs, ± 3 mm in the insertions (many thanks to W. Herr !!!) .

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Going downstream from IR7

▼▼

▼

▼▼

▼▼ ▼▼

▼

▼

▼Ideal case

With orbit error

↓ high dispersion

high dispersion + high ↓ ↓ ↓ ↓

=> From below quench limit to about twice above; additional BLMs show up, but most of them are at the same locations than injection case.

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IR8 + Arc 8-1

← TCTs: generate a new quartiary halo => critical BLMs to be located here as well

▼

▼ ▼ ▼

▼▼

Ideal case

With orbit error

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IR1

Ideal case

With orbit error

← TCTs

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IR2

← TCTs

Ideal case

With orbit error

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IR3

Remember: only betatron study so far

Ideal case

With orbit error

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IR4

Ideal case

With orbit error

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IR5

Ideal case

With orbit error

← TCTs

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IR6

Losses at the TCDQ equipment: → problem of local showers downstream of it under study

Ideal case

With orbit error

=> After one complete turn: 18 critical locations (in addition to collimator ones and at the triplets)

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Dynamic scenario - Process

Dynamic studies: collimators are not re-centered around the perturbed orbit.

Purpose of this scenario: check the sensitivity of the system to fast orbit changes

=> how does the system behave if a secondary collimator gets closer to become a primary (back to a single-stage system) ? What is the effect on the cleaning efficiency ?

In the following, only the collision optics case is presented (results for injection optics still being analyzed).

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Effect on the cleaning system - LatticeTCP.C6L7 TCSG.B4L7 TCSG.6R7

↓ zero orbit change

↑ critical secondary

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Loss Map for a 0.95 offset (only IR7 elements)

=> Loss of a factor 4 in local cleaning efficiency in IR7 !!!

same critical locations !!!↓

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OUTLINE

Introduction

Loss distribution from betatron cleaning

Minimum workable BLM system for collimation studies

Conclusion – Future studies

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Requirements for commissioning

For commissioning of the LHC and its collimation system, one needs to be sure to operate in safe conditions

=> with the results presented here, we can already point out critical locations !!

The determined positions and peak values of losses can then be used to define a minimum workable LHC BLM system for collimation studies.

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Summary table for injection

black: nominal & perturbed case

red: only in nominal case

+ collimator locations

+ critical locations for IR3

=> 13 critical locations in total

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Summary table for collision

black: nominal & perturbed case

red: only in nominal case

blue: only in perturbed case

+ collimator locations

+ triplets

+ critical locations for IR3

=> 18 critical locations in total, 6 of which being identical as in identical as in the injection casethe injection case !!

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Critical loss locations

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Longitudinal distribution of beam losses – detailed studies for BLM positioning

Dipole: all along the magnet

Quadrupole: up to the middle of the magnet

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Remarks

Early scenario checked (as seen in R. Assmann’s previous talk) as well: identical loss locations

Cases studies here refer to closed orbit perturbation spread all along the lattice: do not take into account possible local bumps in orbit !!!=> expect certainly some few additional high loss locationsexpect certainly some few additional high loss locations.

injection optics: many regions, not that criticalinjection optics: many regions, not that critical

collision optics: few regions, more criticalcollision optics: few regions, more critical

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OUTLINE

Introduction

Loss distribution from betatron cleaning

Minimum workable BLM system for collimation studies

Conclusion – Future studies

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Conclusion

The tools we developed allow us to study where the most critical regions of the machine are expected:

-- for both mode of operation of the LHC (injection & collision), with still other optics possible,

-- for any given scenario of beam losses, to check how flexible the system can be depending on the mode of operations.

In close collaboration with the BLM team, detection and detection and monitoring of these critical regions shall be achieved to allow monitoring of these critical regions shall be achieved to allow efficient commissioning of the LHC Collimation Systemefficient commissioning of the LHC Collimation System.

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Conclusion

SC ring losses: 25 per ring + 8 triplet locations

Collimator losses: 41 collimator locations at Phase 1

Out of these 74 locations, only 3 of them are not yet foreseen as BLM locations: MB9, MB11 and MB13 downstream of IP7

=> would it be sufficient to rely on the information delivered by => would it be sufficient to rely on the information delivered by the BLM located at the closest quadrupole magnets ?the BLM located at the closest quadrupole magnets ?

Results will be used to prepare commissioning tools: number of channels displayed, etc…

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Future works

Static case at collision optics: in this talk, we set IR1 as the disturbed insertion => estimation of losses for other IRs ??

Dynamic studies: check of the influence on efficiency at injection still ongoing; accident cases can de derived from this scenario (e.g. a secondary IR7 collimator becoming a primary)

Other error models are foreseen to be studied, mainly beta-beating tolerances, inclusion of the map of non-linearities of the LHC magnets, more complete imperfection models.

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BACKUP SLIDES

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Critical BLMs for collimation

S. Redaelli,Chamonix 2005

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Parameters for obtaining loss maps

Quench limit values used in the following are derived from the values of the loss rates at the quench limit as given in the LHC Project Report 44:

(assuming simplified quench limits)

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Cases studied so far

Various models are available; to compare with the Perfect Machine case, we started studying the effect of Closed Orbit variation, depending on:

-- the phase of the error with respect to the IR7 insertion,

-- the amplitude of this error,

-- the speed of this error: is the perturbation fast enough so that collimators become off-centered from the new closed orbit ?

=> 2 scenarios: static study and dynamic study

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Beam Loss Tracking

A package of state-of-the-art 6D tracking tools has been set up in 2005, which includes:

-- scattering routines applied to all of the 43 equipments foreseen for Phase 1 of the Collimation System,

-- LHC aperture model with a 10 cm resolution level,

-- full 6D treatment of error models (closed orbit deviation, beta-beating, magnet non-linearities)

=> this talk will focus on the critical beam loss locations due to various closed orbit error scenarios

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Static case – Injection Optics

Aim: scan all possible phases between [ -π ; + π ] and find the worst one, i.e. the one phase for which the highest local loss peak comes the closest to the design quench limit.

Once this phase is found, do a scan over the amplitude of the closed orbit deviation (peak value of the error always taken in the arc as reference).

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Peak losses - local

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First Step – Scan in Phase

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List of “golden” BLMs - Injection

12 critical positions listed:

Q11 @ right of IR3

DFBA behind Q5 @ right of IR6

Q11, MB13, Q13, Q23, Q27, Q31 @ right of IR7

Q33, Q29, Q25 @ left of IR8

Q2 before D1 @ right of IR8

Q6 @ right of IR8

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Loss map – 7 TevIdeal case

=> Ideal case: just below the quench limit downstream of IR7 !!!!

▬► halo

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Worst phase – Static case, Collision optics

The static scenario for a closed orbit perturbation at collision optics is different than the previous study, the maximum tolerance in orbit distortion being:

-- ± 4 mm in the arcs

-- ± 3 mm in the insertion regions

In the collision scheme we consider, IR1 & IR5 are squeezed: in the following we will consider a maximum perturbation in the arcs and IR1 and an orbit corrected to the nominal schemes in all other IRs.

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Loss map – 7 TeV Perturbed orbit – worst phase for IR1 scenario

=> Now a factor 2 over quench limit at worst locations !!!

▬► halo

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Collision Optics – 7 TeV Perturbed orbit – phase with high IR3 losses

=> Getting closer to quench limit in IR3: Q6 (left) !!!

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Effect of optic parameters

=> Losses due to first high dispersion location !!!

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Effect of optic parameters

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List of “golden” BLMs – 7 TeV

17 critical positions listed:

Q6 @ left of IR3

Q8, MB9, Q9, MB11, BS.11, Q11, Q13, Q19, Q21, Q27 @ right of IR7

Q33, Q25, Q17 @ left of IR8

Q16, Q30 @ right of IR8

Q22, Q14 @ left of IR1

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Dynamic scenario – Considered cases

TCP.C6L7.B1 TCSG

↔

DX = 0.95 s

+6 s+7 s-6 s -7 s

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Dynamic scenario – Considered cases

TCP.C6L7.B1 TCSG

↔

DX = 1.1 s

+6 s +7 s-6 s -7 s

=> In that case, the TCSG becomes a primary collimatorTCSG becomes a primary collimator !!!

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Second step – Phase selection

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Effect on the cleaning system – Cleaning Inefficiency for a 0.95 s offset

=> At 10 s, we loose a factor 2 in cleaning efficiencyfactor 2 in cleaning efficiency !!!

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Critical loss locations

Injection optics:

-- the critical losses are distributed over the end of IR7, the Arc 7-8 and IR8,

-- the IR2 region should also be monitored: losses there are due to interaction of secondary halo particles with the TDI collimator, protecting the machine from Beam 1 injection errors,

-- the IR3 region (dedicated to momentum cleaning) should also be monitored: studies presented here only consider on-momentum particles, and loss spikes can already be noticed.

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Critical loss locations

Collision optics:

-- static scenario: * critical losses are at the very beginning of the dispersion suppressor downstream of IR7,

* some other noticeable spikes are seen in arcs 7-8 and 8-1, but none in IR8,

* for some particular situation, we also noticed high losses at the Q6 of IR3: this location should as well be monitored

-- dynamic scenarios: this case shows how much the system relies on a good control of the orbit => for a 0.95 s offset at the worst location in IR7, the cleaning efficiency drops by a factor 2 (significant at collision optics).