Minutes of the 29^th WP2 Task Leader Meeting held on 06/06/2014

Present: G. Arduini, M. Giovannozzi, E. Métral, T. Pieloni, A. Valishev, A. Wolski, Y. Papaphilippou

Minutes, Follow-up of Actions, General Information (Gianluigi)

Minutes of the last meeting:

• Elias mentioned that after discussion with G. Rumolo and G. Iadarola the estimate on the instability threshold in the presence of electron cloud in the triplets, matching sections and main quadrupoles assuming that cryogenics can cope with the heat load will be completed for the end of April 2015.
• Elias suggested adding “low impedance” as a criterion to be added in the table describing the degree of compliance of the different BPMs in the triplet. Gianluigi agreed that this was important information and asked Elias to provide the list of BPMs in order of increasing impedance. Action: Elias.
• Following a remark form Massimo Gianluigi noted that for the moment there is no reason to reduce the number of BPMs in the triplet but the criteria defined and the prioritization will help in case of issues arising in the design.

Follow-up of actions:

The actions for Riccardo for the position of the BPMs and the beam characteristics at the BPM positions will be provided once the layout HLLHCv1.1 is finalized (This should be at the end of the month).

Ezio confirmed that the cycle-to-cycle reproducibility in the transfer functions of the triplets and of the orbit correctors of D2 and Q4 are expected to be in the range of 1-2x10^‑4 (provided that pre-cycle rules are respected).

General Information:

• Gianluigi thanked Elias, Sasha and Tatiana for having prepared the milestone reports 30 and 31.

US-LARP (and recent) meetings Highlights / Issues (G. Arduini)

Gianluigi presented a short summary of the outcome of two recent meetings:

• US-LARP Collaboration meeting (7-9 May) preceded by Crab-Cavity and Magnet specific workshops
• Extended Steering committee meeting (27/5)

He started thanking the contributors to the presentations during the US-LARP meeting and highlighting the progress and decisions concerning crab cavities, triplet design and prototyping and the discussions concerning halo monitoring and control, beam-beam long range compensation.

The following points/items have been raised and require follow-up:

• TAS dimension: the project management has decided to opt for the smaller aperture TAS (27 mm radius as compared to the proposed 28.5 mm radius) under the agreement that:
  • The need for that reduction is justified by WP8 studying failure scenarios
  • No need to increase the aperture is necessary as a result of aperture considerations at injection. Action: Riccardo.
  • Energy deposition estimates will take into account the aperture and assess the implications.
• Beam-beam:
  • PACMAN effects and their impact need to be estimated. Action: Tatiana.
  • The impact of the magnetic multipolar errors on the dynamic aperture in the presence of beam-beam should be studied with the new version of the layout to be delivered soon to help in steering the magnetic field quality. Action: Sasha, Tatiana.
  • Develop and further benchmark luminosity/intensity evolution to provide additional criteria/checks in addition to Dynamic Aperture for validating the HL-LHC configurations. Action: Sasha (for LIFETRAC), Yannis (for Sixtrack).
  • Strong-strong simulations to guide in the estimation of stability in the presence of damper, noise, crab cavities. Action: Sasha and Tatiana.
  • The interplay between beam-beam and impedance remain one major issue that needs to be understood.
  • Crab-cavities might seriously contribute to machine impedance and reduce the threshold for instabilities. The SPS tests could provide useful information in that respect and it would be important to evaluate the impact of the SPS crab-cavities on the LHC beam to benchmark simulations and possible cures. Action: Elias.
  • The beam-beam long range compensator could allow running with equal crossing planes in IR1 and IR5. This could allow fixing the crossing plane and in particular a vertical crossing plane would reduce the thermal load on the matching section. Action: Sasha.
  • The design of the components and the beam dynamics simulations should assume a total luminosity (for a 25 ns beam) of 7.5x10³⁴ cm^-2s^-1 corresponding to a pile-up level of approximately 200. Action: Task leaders to determine the implications of that choice.

Gianluigi reminded the Task Leaders that the deadline for delivering the deliverable reports is the end of October. The draft reports should be sent to Gianluigi before 15^th October and possibly earlier as we will have the Hi-Lumi general meeting in Japan from 17 to 21 November.

Update on the tracking simulations with HL-LHCV1.0 – M. Giovannozzi

Massimo presented the results of the tracking studies with the optics/layout version HLLHCV1.0 and the comparison with those obtained with the optics/layout version SLHCV3.1b after a brief recap of the error tables considered at collision (7 TeV) and at injection (450 GeV) energies so far, as provided by E. Todesco for the Inner Triplet, D1, D2and including some error reductions required as a result of the tracking studies. It is worth noting that the error tables for Q4 and Q5 have been prepared by S. Fartoukh on the basis of the measured field quality of MQY magnets. The field errors are specified at a radius corresponding to 2/3 of the aperture radius with the exception of Q5 – MQY - for which the standard LHC definition at 17 mm has been used.

The minimum dynamic aperture for the optics/layout version SLHCV3.1b at collision (7 TeV and b*=15 cm) is 9.9 s(assuming an emittance of 3.75 mm) while at injection the dynamic aperture is 10.2 s (for a b*=5.5 and 11 m). For the LHC the target dynamic aperture at injection was 12 sigmas.

The reduction in dynamic aperture observed at injection with respect to that estimated for the LHC is not the result of the field quality of the new insertion elements, but it is likely due to the vanishing arc cell phase split. An increase of the dynamic aperture of 0.5 sigmas can be obtained by reducing the horizontal and vertical tunes by 0.01. A change of the integer tune split is possible. Although smaller than the target value for the LHC at injection the dynamic aperture at injection should be sufficient taken into account that:

• No evident dynamic aperture issues have been revealed by the Machine Development sessions with ATS optics and this can be verified during Run II if the ATS optics is implemented in operation.
• The preliminary results of the benchmark between measurements of dynamic aperture and simulations performed seem to indicate that the simulations are overestimating the dynamic aperture by no more than 20-30 % (see M. Giovannozzi’s presentation on 27 August 2013) and progress is being done in correlating dynamic aperture estimates with intensity lifetime and emittance blow-up.

Tracking with the layout/optics HLLHCV1.0 in collision (7 TeV and b*=15 cm) indicate a further reduction of the dynamic aperture from 9.9 to 8.8 sigmas. The main difference between layout/optics HLLHCV1.0 and SLHCV3.1b lies in the layout of the triplets and it has induced an increase in the maximum beta functions (by 7 %) in the triplets due to the smaller gradient and the Q1 and Q3 split in two quadrupoles. The phase advance between IP1 and IP5 has also been changed.

The optimization of the field quality for the new elements of the insertion aims at keeping their impact on dynamic aperture in the shadow of the triplets and at this stage of the optimization (not including yet beam-beam effects) to keep the dynamic aperture at values larger than 10 s. The latest magnetic field error table for D2 (v5), which is the outcome of the optimisation of the experts in INFN Genoa, reduces the impact of the D2 field errors and satisfies the above criteria although some cancellation effects of the D1 and D2 field errors have been observed and are not satisfactory.

Next steps include:

• Finalization of the study for the D2 field quality and update of the D2 target error table.
• Study of the dependence of the dynamic aperture at injection/collision on the IP1/5 phase advance
• Assess the impact of the new expected error table for triplet quadrupoles including the change in the systematic multipoles (reduction of b₆ and increase of b₁₀ and b₁₄).

The possible impact of the triplet quadrupole orientation on dynamic aperture in collision has been studied, but no clear sign of improvement has been observed.

Massimo highlighted that an update of the Q4 (MQYY) error table based on magnetic field simulations would be required in order to evaluate its impact on dynamic aperture. Action: Ezio.

Updates from Task Leaders – M. Giovannozzi, E. Métral, T. Pieloni, A. Valishev

Task 2.2 and 2.3 (Massimo)

Nothing to report.

Task 2.4 (Elias)

Elias reported that Nicolas Mounet will leave ABP group towards the end of this month before the end of his contract (31/1/2015). His activities for HL-LHC will be redistributed within the Impedance and Collective Effects Section (in particular N. Biancacci who has joined as HL-LHC fellow and K. Li).

Task 2.5 (Sasha & Tatiana)

Tatiana has been preparing the Evian workshop and she reminded the criteria used for the HL-LHC dynamic aperture in the presence of beam-beam. Yannis asked whether the option of having crossing angles at 45 degrees in 1 and 5 has been considered. Gianluigi suggested that we review this option.

Next meeting will take place regularly on Friday 27^th June 2014 at 16:00.

Reported by Gianluigi.