Action of Task 2.5: Beam-beam effects

The team

Staff: Yannis (50%), Guido, Gianni (50%)

Ilias (luminosity model), Hannes (PhD supervision, modeling of noise in circuits), Frank (modeling of noise in circuits), Stéphane (optics), Davide (seismic noise), Xavier (BB coherent effects)

Fellow: Sofia

PhD: Axel (wire compensation), Kostas (e-cloud but also lifetime/BB), Natalia (noise and CC)

Thanks to Emilia, Androula, Michele, Kyriacos, Nikos, Stefania and Lily for their work!

EPFL is willing to contribute: $\beta$ -beating studies (14 week student on TP).
Discussion with TRIUMF (PhD student)

BB-WS simulation tools

1.a Mask of B4 with BB: prepare the mask [end of 2019]. CLOSED

Good progress in strong and fundamental collaboration with Gianni and Frederik. See https://github.com/lhcopt/lhcmask.

1.b DA of B4 with BB: assess possible asymmetry in DA [Q1 2020]. CLOSED

Tool are there. First presentation of Sofia on 181th HiLumi WP2 Meeting.
Consolidation of B4 with BB is our FIRST priority.

2. Define BB driven tolerances on bbb charge and $\epsilon_n$ (also H/V differences). This will require development of new tools [longer term].

The new (pythonic) mask has been conceived also to address this problem. Proof of principle in Q4 2020 but manpower is needed to conduct a full study.

3. Include bbb orbit effects in the BB simulations [longer term].

We propose to maintain this this activity as low priority.

4. Ion-ion/proton-ion collisions, i.e., taking into account the Z and A scalings…Evaluate the possibility to simulate BB effects with other colliding species with SIXTRACK (need to test the mask, discuss with John/Roderik to inherit tools developed in the past for benchmarking) [end of 2020].

It is our SECOND priority. We contacted John and Roderik. We confirm our target (end of 2020).

BBLR compensation for HL-LHC

1. Flat optics simulations for configurations proposed by Rogelio and documented in a paper describing the performance of nominal and alternative operational scenarios, simulations for the flat optics scenario presented at the HL-LHC CM 2018 should be done also for nominal chromaticity (+15) and ultimate scenario [low priority, to be revised in 2020].

We propose to consider the flat optics still as low priority.

2. Evaluate the improvement on performance of round optics and wire (with/without CCs). CLOSED

The improvement on performance of a scenario with round optics and wire (with/without CCs) was presented at the HL-LHC CM 2019.

3. Evaluate the impact of the wire on lifetime [lower priority to be revised in 2020]

Activity in stand-by.

Present reliable HW solutions to HL-LHC project [end 2019]. CLOSED

Kyriacos’s work presented HL-LHC CM 2019 showed the potential of the wire compensation in HL-LHC within the tertiary limit. EN-MME is now validating the proof of concept of the Mo wire on the ceramic support. We are following it closely but EN-MME is the driving force.

BB-WS and noise for flat optics

1. Performance for flat optics with CC (after round optics studies are completed) [longer term action, update end of 2020].
2. Need to update the estimations on the impact of noise on DA, lifetime and blow-up/tail generation with and without beam-beam on the basis of the tables produced by EPC for flat optics [longer term action, update end of 2020].

We propose to consider all the flat optics related actions as low-priority.

BB-WS actions

1. Given the estimated non closure of the crab-bump (limitations in the measurements f the closure – provided by Androula, BB effects provided by Xavier) estimate impact on DA or estimate the maximum acceptable non-closure (i.e., crabbing leaking) from DA considerations.

In pipeline: it is our first priority expected to be presented in 184th HiLumi WP2 Meeting (tbc).

2. Following the presentation on the BPM calibrations on 60th HiLumi WP2 Task Leader Meeting, we should also define the required accuracy in the definition of the crossing angle [end of 2019 with available information]. CLOSED

Covered in the presentation of Nikos on 164th HiLumi WP2 Meeting.

3. Define the deltap to be used for DA simulations without and with beam-beam (see presentation on 126th HiLumi WP2 Meeting). Two values should be quoted, those corresponding to the DA for the maximum $\Delta$ p/p used so far (corresponding to 2/3 of the bucket height) and one corresponding to the $\Delta$ p/p providing the weighted (on the momentum distribution) average DA. Similar study should be done with beam-beam.

Initial work done by Dario on this aspect. The convolution of the effective DA is in the pipeline.

New operational scenarios:

4a. Estimate DA for the parameters indicated in the DRAFT tables (include coupling, etc.). CLOSED

See presentation of Sofia for HL-LHC V1.5 in 181th HiLumi WP2 Meeting.

4b. Include the effect of field errors and coupling (see specs in the operational scenario – see also discussion at WP2 meeting on 156th HiLumi WP2 Meeting. In particular study the dependence on the a4 and b5 errors and determine DA for non-perfect correction by the triplet correctors and in the absence of triplet correctors [Q4 2019]. CLOSED

See presentation of Nikos in 164th HiLumi WP2 Meeting.

4c. Pros and cons of operating below or above the coupling resonance all through the cycle.

Interesting topics but we propose to consider it as lower priority.

4d. crossing angle at 45 degrees? Do we have simulations for that case? It could allow changing regularly the orientation to minimize radiation. Advantages/disadvantages for machine protection? Impact on field quality requirements? Excluded from other considerations (stability)?

Interesting topics but we propose to consider it as lower priority.

6. Can we run simulations for Pb beams? Can we accept a half crossing angle at IP2 of 100 urad for the 25 ns beam? See also presentation by Roderik on 174th HiLumi WP2 Meeting.

This is connected to the previous point about the tools. We suggest merging the two actions. This is our second priority.

7. Suppress the installation of MS in Q10 in IR1 and 5 (see 158th HiLumi WP2 Meeting and 168th HiLumi WP2 Meeting). So far the most appealing solution appears the NO MS10 solution with phase advance optimization. Feedback from BB WS simulation studies to be provided with new operational scenario. Review of the situation without MS10 at the beginning of collisions and at the end of the leveling with the right intensity and crossing angle for positive octupole polarity. Optimization of the phase advance with beam-beam effects included checking also interactions with PACMAN effects.

In the pipeline: this is our third priority.

From DA simulation to losses simulation.

8a. Repeat simulations for beam losses using realistic beam profile, tunes and collimator settings in the presence of beam-beam. Action: Kostas, Gianni. CLOSED

Significant progress in this direction. See Kostas’s presentation at 178th HiLumi WP2 Meeting.

8b. Parametric study of the parameters affecting losses in the presence of beam beam [longer term, person to be identified]

Person still to be identified. To be discussed again in 2021.

Noise effects and emittance blow-up in the HL-LHC era

Emittance blow-up

1. Include elastic and diffractive part and effect of burn-off in SIRE for observing the impact on distributions (not expected to be important, growth rate in the shadow of radiation damping) [Lower priority. To be discussed whether other options exist]

The suggested solution is to combine SIRE with scripting glue to take into account the effect.

Impact of noise

Crab cavities

2. Effect of crab cavity noise throughout the cycle before going in collision and in collision. Q4-2019. During the WP2 meeting on 140th HiLumi WP2 Meeting the issue of the non perfect counterphasing (5 degrees error) and uncertainty on the voltage (10%) have been raised. The impact on the beam at injection assuming 0.25 MV/ cavity should be studied with beam-beam.

Studies ongoing with PhD student (Natalia, timeline 2020-2020): the moment focus in bench-marking SPS experimental results (see BB meeting 29th May 2020). The HL-LHC CC noise simulations will follow.

Power converters

3a. Issue with the ramp dependence that does not fit with the observations for the moment (under investigation). CLOSED

We show a clear interplay with the ADT hence we do not expect to have anymore a naive ramp dependence.

3b. Noise impact on the emittance growth and comparison with analytical models.

At the moment, studies concentrated on losses and lifetime.

3c. Add the effects of the beam screen shielding and the damper in the noise studies.

Discussion are on-going for warm measurement of LHC dipole and dedicated ADT MD.

3d. Check the UPS contribution to the noise. CLOSED

See presentations at 110th HL-LHC TCC, Special Joint HiLumi WP2/WP5 Meeting and 401st LMC.

3e. Verify if the assumption of the same phase between the 50 Hz PC harmonics for the different dipole PCs is correct

This should be partly addressed with the warm measurements.

3f. Check which beam distribution is to use in the simulations.

Discussions with Hector following his presentation a 177th HiLumi WP2 Meeting.

Vibrations

4. Follow-up of the low frequency noise (vibrations) experiments and comparison with simulations [Q4 2020].

Davide is progressing in the 10 Hz dumps analysis.

General topic

A table with the various expected source of noise, their characteristics, their origin and mitigation measures has been prepared by Guido (174th HiLumi WP2 Meeting and http://noisestudies.web.cern.ch/). Establish tolerances and order of priority for addressing the corresponding sources of noise and document in a note. The impact on DA and on emittance blow-up and halo generation should be studied.

Most efforts concentrated on the UPS and PCs investigations.

Thank you for your attention.