Minutes of the 55 th WP2 Task Leader Meeting held on 25/09/2015
Participants: G. Arduini, R. Bruce, F. Cerutti, B. De Girolamo, I. Efthymipoulos, M. Fitterer, M.
Giovannozzi, E. Metral, Y. Papaphilippou, R. Tomas, S. Valishev.
Minutes, Follow-up of Actions, General Information (Gianluigi)
The approval of the last minutes has been postponed.
Gianluigi thanked the task leaders for the TDR and the people involved. Chapter 2 has been sent to
Isabel. The Annex is being updated for beam parameters and magnet parameters. Riccardo asked if beam screens are mentioned in some of the chapters, Francesco added it could be in the chapter describing the vacuum system.
There was a meeting on triplet powering chaired by Felix. See Task 2.2 report.
8b+4e replaced the 50 ns as back-up scenario; the former gives 20% less performance at constant LIU performance with respect to 25 ns options.
For ion, the requirements for injectors have been defined by John and will be included in the PLC table.
The Q4 design will be reviewed and it will be based on a double-layer coil to increase the inductance and reduce the sensitivity to ripple. The Q4 coil aperture will be kept at 90 mm.
At the last PLC, Paolo Fessia and Serge Claudet presented the latest status on layout and cryogenics. For cryogenics, it was agreed to make an upgrade of the existing system and develop a mobile refrigerator to cope with possible peak requirements and tests. Stefano Redaelli presented the new baseline for the collimation system. Ion collimation might not require the installation of 11 T dipoles if the solution based on bumps in the dispersion suppression area is validated by experiments in 2015. This would require the installation of a collimator in the empty cryostat in the dispersion suppressor region in Point
2. Quench tests with protons and ions are necessary to confirm this decision and to define whether dispersion suppressor collimators based on 11 T dipoles are needed. An 11 T unit should be ready for
LS2.
LHCb requested to develop scenarios for levelled operation at 1-2 × 10 34 cm -2 s -1 with a shift of the IP of
3.74 m 5n the direction of IR7 and an additional spectrometer. There are several implications for beam dynamics (optics, beam-beam), energy deposition. The idea is not to change magnetic elements to provide optics. First analysis shows that only 5% less integrated luminosity will be received by ATLAS and
CMS. A redesign of TAS and TAN is needed and the IP shift requested by the experiments is mandatory.
LHCb asked to take data with ion runs.
Francesco asked if flat optics should be considered for energy deposition studies for the proton optics in
IR1 and IR5 and Gianluigi replied that is the case.
HL-LHC/LIU workshop and programme (G. Arduini)

Gianluigi presented the schedule, scope and purpose of the meeting. The agenda and the abstracts of the talks have been described. In particular Gianluigi will have give a summary of the updated HL-LHC baseline, triplet vibrations and the impact of no ion physics in Run 5/6.
HL-LHC V1.2 Update (R. De Maria)
Riccardo gave a summary of the status of the new optics and layout V1.2, which is currently being validated by the collimation team and used for energy deposition studies (limited to IP to D1 for the time being). The integration study is being completed in these days: the position and length of the magnetic elements are fixed, however other elements are still readjusted. There was some discussion about the procedure for the release of the layout and of the optics that for the moment is not defined clearly.
Action: Riccardo, will freeze HLLHCV1.2 sequence and continue to develop the further updates, if needed, in unreleased versions till the next HLLHCV1.3. Gianluigi and Massimo, will discuss with Paolo
Fessia to define a proposal and bring the issues to the next PLC/TC.
Besides the integration studies, the IR6 optics is being updated (no change of layout is expected in the following versions) with the new constraints from ABT. He reminded that following the decision on the
Q4 aperture there is sufficient flexibility for the optics in the region of D2 and Q4 which can be optimized either for crab cavities operation or for the long range wire compensation. One of the main limitations in the flexibility is posed by the maximum strength of Q7.
As next point Riccardo summarizes the latest aperture studies for the new optics and layout V1.2 including an explanation of the calculation of the aperture margins, a summary of the aperture in IR1/5 and the orbit deviation due to the different orbit knobs, the latest aperture limits from collimation, the survey tolerances and an update of the aperture margins for the different optics of the V1.2.
Regarding the orbit deviation due to the different knobs (crossing and separation, crab cavity beam based alignment and offset at the IP), the offset knob with a reference of +/-2 mm results in a large orbit in particular in the matching section and thus reduction of the aperture in the same. The orbit resulting form the offset knob is very sensitive to optics variation and orbit corrector margins, so variations of the order of mm in the matching section to what has been presented could emerge. Riccardo also clarified, that the H, V full gap presented in the slides is the nominal aperture reduced to take into account the mechanical tolerances (i.e. straightness and shape tolerances). The reduction resulting from the orbit knobs is not included. Roderik asked whether the offset knob resulted from an explicit request of the experiment endorsed by the PLC and Gianluigi confirmed it was the case.
Based on the positive Run I experiences and adding safety margins for the unknowns, the target aperture at collision is at 12 σ if protected by a TCT and 18 σ if not protected by the TCT. Riccardo highlights that the value of 18 σ for the protected aperture in the arcs should be confirmed by WP5 and
WP14. In addition, also the value of 9 σ at injection has to be confirmed by WP5 and WP14.
Using these latest aperture margins and adding the different contributions one by one, the triplet and
D1 do not comply with the aperture target of 12 σ for round optics (β*=15 cm). This implies that the target of β*=15 cm can be met only if the optics is very well corrected and machine components are built with tighter tolerances and are very well aligned. Furthermore the possibility of running with lower crossing angles should be evaluated. Roderik comments that about 1 sigma could be possibly gained in

the protected aperture by using low-impedance materials in the secondary collimators, which would allow to reduce the IR7 hierarchy margin. Relaxing β* to 20 cm, all aperture tolerances are met. For flat optics with β*=30/0.75 cm, the bottleneck occurs at the triplet, D1 and TAXN. Relaxing β* to 40/10 cm the requirement of 12 sigma is approached.
As next topic, Riccardo presents the studies aiming at the optimisation of the optics for the beam-beam compensation wire (ratio of 2 between βx and βy at the location of the wire), minimisation of the crab cavity voltage and minimisation of the impedance of the crab cavities. In general, the optimum case for minimizing the impedance is to have the minimum number of cavities running at their maximum voltage. To find the optimal optics for the different scenarios, the βx and βy at Q4 has been scanned.
The triplet gradient only depends weakly on the β in Q4 and always stays below the maximum gradient.
Furthermore, the ratio βx / βy at the wire increases with the crab cavity voltage, where the optimal ratio is achieved around 12.0 MV compared to the minimal achievable voltage of around 11.5 MV. Except for the extreme cases, the impedance is almost independent in respect to the crab cavity voltage due to optics constraints.
In addition, the impact of a possible movement of Q4 and Q5 has been studied. Moving Q4 towards the
IP increases the optics flexibility and aperture for alignment optics, albeit it results in a higher crab cavity voltage and loss in aperture in Q4/Q5 (but margin is available). Moving Q4 towards the arc has the opposite effect. However, the most recent results of the integration study showed, that there is only little space left between D2 and Q4 due to increased space allocated between D2 and Q4 (crab cavities, instrumentation, magnet connections). A movement of Q5 has in general only little impact on the optics.
Roderik stressed that moving Q4 towards the IP, have also the benefit of liberating space for TCT in case an alternative TCT layout is considered. In summary the present optics constraints allow only the freedom to choose the βx / βy ratio at one location, while the product is about constant, which implies that the optics solutions cannot be adapted to minimize also the impedance for a given number of cavities. Only a stronger Q7 would open up this possibility.
Following a recent request of LHCb for a higher luminosity of 1-2 · 10 34 cm -2 s -1 for HL-LHC implying a shift of the IP by 3.74 m towards Point 7 and an insertion of a spectrometer dipole with opposite field in front of the present LHCb spectrometer, Riccardo summarizes the first optics studies performed aiming at reducing β* at IP8. In an earlier study by B. Schmidt and S. Fartoukh β*=3 m have been reached for a shift of 7.5m, however without an ATS squeeze. By taking into account only quadrupole strengths, but not the aperture, the target value of β*=0.5 m is extremely challenging without a shift of the IP and impossible with an IP shift. Without a shift of the IP, a minimum of 70 cm could be reached. With a shift of the IP, only 2 m have been reached. Massimo warned that, for a given polarity, without IP shift, β* is limited by 2 m due to aperture constraints. In the case of a shift of the IP, an increase of the Q1 trim power converter range might be required. Riccardo stressed that at injection energy the IP cannot be shifted due to aperture constraints, with impact if collision data have to be taken at injection energy.
Beniamino will make sure the information is passed to the experiments. These first studies have then to be followed by developing a new spectrometer bump, a check of the crossing and separation bumps at injection, the triplet powering, gradient and aperture, aperture of the experimental beam pipe. Action:
Riccardo.
For IR8 Roderik comments that if a TCT is present upstream, the triplet should be protected. Q4/Q5 will not be protected by the TCT and the impact has to be checked. Action: WP5
In addition, also the TCL in IR1/5 has to be checked.
Gianluigi notes that a first estimate of the impact on beam-beam has to be performed starting with the current layout and crossing angle and only reducing β*. Ilias mentions that also the aperture of the TAN

has to be checked. Gianluigi summarizes that these first studies should be followed by aperture checks, an evaluation of the effect on beam-beam and energy deposition studies for the case without IP shift followed by a similar study with an IP shift. Action Yannis, Sasha: impact of reduction of β* in IR8 on beam-beam.
Report from Task Leaders:
Task 2.2-2.3 (Massimo): Massimo attended a meeting organized by Felix on triplet powering. Progress is reported on the protection scheme (4 CLIQ modules). The issues evidenced by Jean-Paul Burnet at the last TC concerning the installation of a resistor in parallel to the magnets have been solved by replacing it with diodes. The EPC team prefers the option with all the quadrupole magnets in series, although a fast ramp-down solution needs to be found and some R&D is required. The option of powering D1-D2 in series was not discussed at the meeting.
Following the presentation at the HL-LHC TC on 30 th of July, Ezio mentioned that assumed Q4 and D2 mechanical tolerances might have been be too pessimistic. In order to review this he suggested iterating with vacuum experts to obtained more realistic estimates of the tolerances. For the time being, however, there is no news on this front.
Rogelio and team are progressing with the study of the correction scheme for the coupling. Yuri
Nosochkov is studying the sensitivity on the quality of triplet corrections.
Task 2.4 (Yannis): Andrei progressed with beam-beam wire compensator studies for the LHC. Javier
Alabau is working on reproducing the simulations from Sasha. SixTrack beam-beam module update is in progress.
Task 2.5 (Elias): Niccolò B. will report next week on the status of the studies of the impact of crab cavities impedance.
Reported by Gianluigi, Miriam and Riccardo