HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study - - PDF document

CERN-ACC-SLIDES-2014-0069

HiLumi LHC

FP7 High Luminosity Large Hadron Collider Design Study

Presentation Recommendations (from Collective Effects Considerations) for the LHC 2015 Run

M´ etral, E (CERN)

25 March 2014

The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. This work is part of HiLumi LHC Work Package 2: Accelerator Physics & Performance.

The electronic version of this HiLumi LHC Publication is available via the HiLumi LHC web site <http://hilumilhc.web.cern.ch> or on the CERN Document Server at the following URL: <http://cds.cern.ch/search?p=CERN-ACC-SLIDES-2014-0069>

CERN-ACC-SLIDES-2014-0069

Elias Métral, LBOC, 25/03/2014 /19 1

RECOMMENDATIONS (FROM COLLECTIVE EFFECTS CONSIDERATIONS) FOR THE LHC 2015 RUN

Elias Métral for the HSC section and collaborators (many thanks!)

 Reminder of the recommendations for 2012  Experience from 2012  Recommendations for 2015

§ Plan A § Plan B

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RECOMMENDED VALUES FOR 2012 VS. ONES OF 2011

Used at the end of 2011 Proposed value for 2012 Chromaticities ~ + 2 ~ + 1 (≥ 0, as low as possible) Octupoles’s current [A]: IoctD = - IoctF 200 ~ 450 Rms bunch length [cm] (4- sigma bunch length [ns]) 9 (1.2) ~ 10 (~ 1.35)

After we reached the same performance as last year with the same bunch length (for RF heating reason with MKI mainly) As the complex tune shift with the tight collimators’ settings will be ~ 2.3 times higher Minimizes the amount of octupoles needed and better for beam lifetime + reduced ADT gain (as much as possible to avoid unnecessary noise)

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EXPERIENCE FROM 2012 (1/9)

=> Instability at the End-Of-Squeeze => Instability in Adjust Led to the decision to change the sign of the octupoles EOSI could not be cured until end of the Run I

Measurements

“MYC” = LOF > 0 as of 07/08/12 (fill # 2926) with ~ 500 A + high chroma (~ 15-20) + higher damper gain in the vertical plane (~ 50 turns) WHY?

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EXPERIENCE FROM 2012 (2/9)

 Why beam instabilities? The beam (some bunches) can cross

regions of reduced (minimum) Landau damping area due to the interplay between the Landau octupoles (sign important) and BBLR and/or BBHO => Several strategies possible 1) Try and avoid the regions of reduced Landau damping area => Led to the idea to change the sign of the LOF (see StephaneF, LMC#141, 11/07/2012) and use LOF > 0 2) Try and go as fast as possible through critical areas => Led to the idea to improve the collision process (see XavierB, LMC#146, 29/08/2012). Can we do more? 3) Try and increase the instability rise-time => Optimization of the collimators positions (see R. Bruce, LBOC, 25/02/14) & materials (long term) and optics (ongoing) => Optimization of the ADT (bunch-by-bunch, done) and chromaticities (not yet done)

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EXPERIENCE FROM 2012 (3/9)

 Chromaticity (should) play a significant role in the instabilities

 Instability rise-time ~ 1.4 s

• 50 ns
• 1.7E11 p/b
• No Landau
• H-plane
• 50 turns ADT

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EXPERIENCE FROM 2012 (4/9)

• 25 ns
• 1.7E11 p/b
• No Landau
• H-plane
• 50 turns ADT

 Instability rise-time ~ 1.4 s

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§ New (bbb – flat gain) ADT

• Chroma. of ~ 2 units

good again…

§ Old ADT EXPERIENCE FROM 2012 (5/9)

 Reminder on AlexeyB’s past results

Q’

damper gain

1.4  50 turns

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EXPERIENCE FROM 2012 (6/9)

What were the exact / real values?

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EXPERIENCE FROM 2012 (7/9)

 Effects on chromaticity

=> … + Landau octupoles + BB Measurements Simulations => A good measurement and control of the (real, total) chromaticities will be needed to achieve the highest performance in the LHC

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EXPERIENCE FROM 2012 (8/9)

 Some more detail about the change of the LOF polarity

Similar situation for the most critical cases => This change alone could not (and did not) solve the EOSI End of squeeze Re (ΔQ)

• Im (ΔQ)

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EXPERIENCE FROM 2012 (9/9)

Adjust Better here => This change should improve the situation in Adjust (depends also on the instability rise time and time spent there)

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RECOMMENDATIONS FOR 2015 (1/8)

 There are 3 main phases in the LHC currently (without e-cloud!)

1) 1-beam regime 2) 2-beam regime through BBLR (during the squeeze) 3) 2-beam regime through BBHO (& BBLR) 1) 1-beam regime

§ LOF < 0 produces the largest area of Landau damping § Opening of collimators and low chromaticity minimize the

amount of LOF needed

§ Some cases in 2012 where only ~ - 20 A in LOF sufficient

2) 2-beam regime through BBLR (during the squeeze)

§ Depends critically on beam and machine parameters § Leads to variations between the different bunches § Where we had the main pb in 2012 and the instability could

not be cured

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RECOMMENDATIONS FOR 2015 (2/8)

3) 2-beam regime through BBHO (& BBLR)

§ BBHO produces the largest source of Landau damping § OK once in HO and the impedance of collimators could even

be increased there (by putting the collimators closer to the beam after HO)

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RECOMMENDATIONS FOR 2015 (3/8)

 The proposed strategy (PLAN A) is to maximize the LHC

performance (to reach the highest brightnesses): Collide and squeeze as already proposed / discussed in the past => Go directly from 1-beam regime to BBHO-regime and have the BBLR-regime afterwards, i.e. replace sequence 1-2-3 by 1-3-2 => Proposition: 1) LOF < 0 and low chromaticities (~ 1-2 units) IF controllable 2) Squeeze until β* ≈ 3m (to have no BBLR => Works also if emittance 2 times larger) 3) Collide (as fast as possible) with the collimators opened as much as possible 4) Continue the squeeze until the required β*

ddrift ≈ ϑ Fullc.a. γ β * εnorm

To be decided after MDs => Maybe safer to start with the 2012 high chromas (~ 15-20 units)

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RECOMMENDATIONS FOR 2015 (4/8)

 BCMS beam

• 500 A

+ 500 A Speed of the separation bump in 2012: ~ 16 µm / s => ~ 0.5 σ / s (using BCMS parameters) => This should be checked carefully and updated /

• ptimized for 2015

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RECOMMENDATIONS FOR 2015 (5/8)

 STANDARD beam

• 500 A

+ 500 A

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RECOMMENDATIONS FOR 2015 (6/8)

 Requirements

§ Open the collimators as much as possible before colliding § Control the full beam separation within ~ 0.5 σ => In fact in HO

• ne could also optimize the octupoles’ value and sign if needed

§ No non-colliding bunches or optimization of them (low

intensity, large emittance, etc.)

§ Damper => Bunch-by-bunch (flat gain) and 50 turns damping

 In case of problem remaining during Adjust (due to area with

reduced Landau damping) => Possibility to go in collision 1st in IP1 (or IP5) and then IP5 (or IP1) to profit from the sharing of Landau damping between the 2 transverse planes through nonlinear BB => Stability diagrams analysis (XavierB) confirmed by simulation (SimonW) - without the effect of the transverse damper

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RECOMMENDATIONS FOR 2015 (7/8)

 PLAN B => If collide and squeeze not possible / desired, LOF > 0

and ~ as in 2012 (trying and optimizing the collimators positions and the chromaticities, even more than for LOF < 0)

§ Could be ~ OK but less performance reach than with LOF < 0

for 1-beam regime => Maybe enough for the brightnesses discussed for 2015

§ EOSI not cured in 2012 => What will happen in 2015?

 Reminder / summary why PLAN A vs. PLAN B:

1) Better for 1-beam (even if maybe not needed) 2) Avoid BBLR (without BBHO) during squeeze where instability could not be cured in 2012 3) Profit from BBHO to put the collimators the closer to the beam as BBHO provides the largest source of Landau damping

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RECOMMENDATIONS FOR 2015 (8/8)

 Next steps

§ A good measurement and control of the (real, total)

chromaticities will be needed to achieve the highest performance in the LHC in the future => What can we do?

§ Required MDs

• Optimization of the collimators positions (the farther away

from the beam before BBHO the better) => Collimation team

• The 1-beam instability rise-time vs. chromaticity should be

measured to check our predictions and optimize the chromaticities

§ Other MDs could be discussed in the future to try and

understand better the 2-beam instability, etc.

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APPENDIX A