05.03.2003chamonix 03 / presentation 5.5 / j. wenninger1 orbit control for machine operation and...

05.03.2003 Chamonix 03 / Presentation 5.5 / J. Wenninger

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Orbit control for machine operation Orbit control for machine operation and protection and protection

• Orbit control requirements• Feedback performance & limitations• Feedback architecture• Summary & Outlook

J. Wenninger AB/OP

Main persons involved in orbit FB (past & present) :

L. Jensen, R. Jones AB/BDIJ. Andersson, S. Chtcherbakov, K. Kostro, T. Wijnands AB/COM. Lamont, R. Steinhagen, J. Wenninger AB/OPQ. King AB/PO


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Stabilization requirements IStabilization requirements I

Collimation (see also R. Assmann): Cleaning section : < 0.3 m TCDQ absorber in IR6 : < 0.5 m @ 7 TEV

On day 1 + some @ injection and during the ramp. For larger * in physics.

Collimation inefficiency versus position error

…for nominal performance in physics and for * = 0.5 m !!

Stabilization to 200 m is sufficient :


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Stabilization requirements IIStabilization requirements II

Beam dumping system (see also B. Goddard) : CO stabilized to 1 mm (peak) @ kicker & septa in IR6 – H plane only !

in the shadow of the collimation requirements / TCDQ.

Injection : CO stabilized to 0.2 mm rms at the TDI.

Machine protection : Stabilize CO around the WHOLE ring to ensure that the aperture limits

are always in the collimation section. Very important for the triplets.

Machine performance & operation : Minimize beam excursions with respect to reference CO to help control

feed-downs from multipoles (injection & snapback). Stabilize the orbit during the squeeze. Minimize beam movement at the IRs in physics. Make life (much) easier for operation !


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Observed orbit drifts : ~ 200-500 m rms over a few hours ~ 20-50 m rms over ~ minute(s)

LEP/LHC tunnel is a quiet place. Ground motion spectrum ~ f-3

Ground motion @ LEPGround motion @ LEP

orbit rms ground movement

Uncorrelated motion : 35 Waves (E. Keil):

f < 5 Hz 1f > 5 Hz 1 < < 100

CO movements at f > 0.1 Hz arein or below the few m range !

1 m

1 nm

OPAL cavern

IP4

(S. Redaelli)Ocean waves

@ = 100 m

@ 100-150 m


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Vertical low- quadrupoles @ LEP moved vertically ~ 100 m during the machine cycle :

• Orbit drifts of 2-5 mm rms dominant effect on LEP orbit

• Not entirely reproducible• Related to temperature • Lot’s of problems in the ramp due to the absence of areal-time feedback.

Magnet girders @ LEPMagnet girders @ LEP

We must watch out for :• Triplet movements• Vibrations (cryo…)

= kick due to low- movement @ one IP1 rad 40 m rms @ =100 m


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Orbit movements during Snapback and decayOrbit movements during Snapback and decay

Random b1 errors (~ 0.75 units) 1 mm rms in the horizontal plane (with a large spread).

Random a1 errors (~ 2.6 units) 3-4 mm rms in the vertical plane.

Feed-down from b2 errors 0.2 mm rms in both planes !


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Ramp, squeeze, collisionsRamp, squeeze, collisions

Ramp : “Experience” shows that drifts of few mm rms have to be expected. Magnetic centre of the warm quads expected to move by ~ 100 m.

(should be Ok !)

Squeeze : Large drifts – up to 20 mm rms (IR1 & IR5 * : 18 m 0.5 m) Effects are very sensitive to the input conditions :

orbit offset, -fct and strength change in IR quads.

Collisions :Ground motion … (Parasitic) beam-beam kicks.

@ LEP the inability to control the orbit in real-time during ramp & squeeze probably cost us ~ 5% overall efficiency !

and was responsible for > 30% of the lost ramps.


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Orbit drifts & requirements in shortOrbit drifts & requirements in short

Most drifts occur / build up on time scales of few seconds to minutes. need a good feedback gain at and above ~ 0.1 Hz.

The squeeze could be the most delicate phase for the orbit FB.

The most critical requirement apply during collisions where slow ground motion is hopefully the main ‘enemy’…

During the initial operation, requirements are not as stringent – 200 m rms tolerance is probably OK.

Most perturbations produce ~ REPRODUCIBLE drifts (except ground motion) 80% (?) or more of those effects can be anticipated and feed-forward. reduces load on FB.


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Power converters & magnetsPower converters & magnets

Cold orbit correctors : Circuit time constants 10 to 200 s (arc correctors ~ 200 s). For small signals the PC is limited to frequencies of ~ 1 Hz.

Warm orbit correctors : Circuit time constants ~ 1 s. PC could run well beyond 10 Hz ! Too few of them in the cleaning section to build a closed correction !

would need warm (or super-power cold) correctors in the coldsection of the machine !

Cannot profit from their speed – we could consider slowing them down to remove this source of fast orbit movements !

Controls : All PCs accept real-time input @ up to 100 Hz. Each PC can only be controlled by a SINGLE feedback loop !


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Orbit acquisitionOrbit acquisition

Per ring and plane : 500 orbit measurements ~ @ every quadrupole.

The real-time orbit acquisition will run at 10 Hz. For a good FB performance :

sampling frequency ≥ 20 x (fastest perturbation to stabilize)

FB limited ~ 0.5 Hz !

SPS tests in 2002 on 4 BPMs equipped with LHC readout:

Transmission delays over standard SPS network are OK for 10Hz CO.

Very good electronics performance. CO resolution < 20 m for nominal

intensity.

Extr. flat top

10 Hz sampling of the LHC beam cycle in the SPS averaged over 2 hours

Start of ramp


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Feedback performanceFeedback performance

To improve the performance towards higher frequencies orbit sampling of 20 Hz or more !

Delay of 1 period (100 ms). Limitations due to the correction

strategy not included !

2 period delay (200 ms) may be more conservative for initial operation… reduced gain.

Gain = 10 @ 0.1 Hz

Gain = 1 @ 1 Hz

Feedback gain (not ultimate performance !)


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Complications, complications…Complications, complications…

Ramp : Energy tracking.

Squeeze : Orbit response matrix must be updated to track optics changes. Reference orbit must be updated (crossing scheme…).

LHC energy stabilization at injection with horizontal orbit correctors : The same correctors are also used by orbit FB.

FB also responsible for energy ? Energy trims not via real-time inputs since very slow changes !

Ring 1 – Ring 2 coupling in IRs 1,2,5 & 8 : Handle rings individually or in common ? Individual ring handling will NOT work well for the squeeze.

Trims : Must allow some form of manual corrections (bumps, Xing angles …).

Post-mortem diagnostics


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Feedback StrategiesFeedback Strategies

Global correction / feedback : By definition such a FB affects the orbit in (at least) one entire ring.

Local correction / feedback : Uses a subset of monitors and correctors. Provides a LOCAL correction, i.e. does not affect the orbit outside its

‘working’ range. Requires a buffer region to enforce the closure.

Collimation IR

This is NOT really what we want (for protection…) !


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Marrying local & global FB loops

The classical approach (Light sources) : frequency de-coupling Very fast local loops (> 50 Hz), sampling rate ~ kHz. One slow global loop (0.1 Hz).

Does not work (well) @ LHC due to the ‘slow’ sampling and large perturbations during snapback and squeeze.

A single global loop with chained corrections : Can apply both global & local corrections – complete info available ! Very flexible & easy to (re)configure. Avoids correction weighting – tricky to tune. Total correction = corrections

Input Orbit

Predicted Orbit

Predicted Orbit

Corrected Orbit…

GlobalCorr.

LocalCorr. # 1

LocalCorr. # 2

LocalCorr. # n


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Centralized feedback architectureCentralized feedback architecture

Global correction as “workhorse” – good to satisfy most requirements entire CO information available. can be made rather insensitive to bad monitors. can be easily configured and adapted. numerical problems are more complex. large amount of network connections to front-ends.

Local corrections ensure tight constraints in local sections… (very) sensitive to faulty monitors.

FBData transfer first tests OK ! lightweight ‘protocols’ please !


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Ground motion correction in collisionGround motion correction in collision

Simple global correction : “Conservative” correction strategy – insensitive to isolated faulty BPMs. Decouple rings (i.e. common beam pipe elements not used).

IP1Primary Coll.

Residual orbit shifts after ~ few hours of coast / 1 beam

=10 m = 17 m

Note the very large residual drift @ IP1 despite a 100 x smaller correction strategy !


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Entirely local feedback architectureEntirely local feedback architecture

FB

FB

FB

FB

FB

FB

FB

FBreduced # of network connections. numerical processing simpler.less flexibility.not ideal for global corrections.coupling/X-talk between loops is an issue.problem with boundary areas to ensure

closures.

Example of an aggressive solution…the Swiss Light Source…


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SLS : global correction with local loops !SLS : global correction with local loops !

yA ~

One can cast the solution of the orbit problem in the form of a matrix multiplication ( = kicks, y = input orbit)

All non-zero elements are very close to the diagonal

Each local FB loop receives a piece of the matrix to perform a global orbit correction(+ needs to talk to its neighbor !).

Equivalent to a MICADO correction using ALL AVAILABLE orbit correctors of the machine – every “bad” monitor kills you !

A =

LHC matrix


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BPM reliability in critical areasBPM reliability in critical areas

Cleaning Section : Stabilization to the required accuracy with a local correction can only be

achieved throughout the cleaning sections if the BPMs are reliable at the level of 50 m or better.

To detect systematic errors at the level of 100 m or less is not simple ! Those BPMs are installed in a very difficult area (radiation).

Triplets – inner IR region : The directional couplers in the common beam tube have a tough job to

separate the beams. This is a critical region with * = 0.5 m – aperture ! Experience will show how much we can trust them. Fortunately we start with 75 ns bunch spacing OK !


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Summary & outlookSummary & outlook

• Stabilization requirements for protection & collimation Tough @ 7 TeV + squeezed – but no show-stoppers. The squeeze is likely to be the most delicate phase.

• Architecture & correction strategiesMore systematic simulations & tests required to :

choose implementation – local / global… check ring decoupling and strategies.

• Fast orbit movements or failures cannot be avoided by any orbit feedback interlocks on beam movement / beam position.

• SPS tests in 2004 Test of a closed local orbit FB based on 6 BPMs equipped with standard

LHC electronics good test bed & milestone.

end 2003

05.03.2003chamonix 03 / presentation 5.5 / j. wenninger1 orbit control for machine operation and...

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