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14th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-018 (CERN)
Minutes of the 14th WP2 Task Leader Meeting held on 27/08/2013
Present: G. Arduini, O. Brüning, R. De Maria, P. Fessia, M. Giovannozzi, J. Jowett, R. Kersevan, A. Valishev, A. Wolski
Minutes of the last meeting and follow-up of actions
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The minutes have been approved without changes
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D. Missiaen suggested contacting K. Artoos concerning data on ground motion in the triplet/D1 area. Paolo suggested contacting H. Prin, too. Action: Gianluigi
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Sasha confirmed that the results of the benchmarking of the Lifetrack/SixTrack for the cases identified in the meeting held on 8/7 can be given on 10/9.
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Paolo and Oliver reminded that the issue of the radiation at the crab cavities should be followed-up. Action: Riccardo will contact Francesco or Luigi to get the estimated values of the radiation at the cavities.
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Oliver suggested adding as an action the study of a remotely controllable TAN should be considered as a mean of reducing the tolerances at the TAN position. Riccardo added that it might be good to consider two versions of the TAN tailored for a horizontal or vertical crossing angle. Action: Gianluigi.
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The preparation of the Daresbury meeting is on-going. Gianluigi has circulated a couple of DRAFT versions and received feedback from the task leaders and from S. Redaelli and F. Cerutti. He is waiting for feedback from WP3 (E. Todesco and G. Sabbi have been contacted).
Features of Heavy Ions Operations for HL-LHC (J. Jowett)
John gave an overview of the modes of operation and requirements in terms of integrated luminosities for the ion programme. After LS1 the peak luminosity is expected to reach 2-3 times the nominal value (1027 cm-2s-1) and levelling for ALICE (by separation) will be required. Key upgrades for ALICE will take place in LS2 and more precisely ALICE would like to implement them in 2018. After these upgrades ALICE will be capable to accept 6 to 7 times the nominal luminosity. There is no request for ions other than protons and lead ions. Proton ion operation will require proton bunch populations of approximately 5x1010 p/bunch (i.e. 3 times the bunch population achieved in 2013). Oliver noted that the protection provided by the TAN might not be sufficient and should be addressed. Action: Francesco to check the radiation level for the ion-ion and proton-ion operation before freezing the design of the TAN for the proton high intensity operation.
The required hardware upgrades in the LHC for the ion upgrade programme are:
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Dispersion Suppressor collimator in IR2 (implying 11 T magnets) in LS2 (or when the ALICE upgrade will take place)
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Dispersion suppressor collimators might be required in LS3 for IR5, IR1, IR3 and IR7
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Stochastic cooling is being studied and would require space in LSS4
Oliver and Paolo noted that several installations are planned in LSS4 in the frame of the HL-LHC (800 MHz system, electron lens) and integration aspects should not be neglected. Action: John to inform Paolo on the space requirements for the stochastic cooling system once these are defined.
The kicker for the stochastic cooling system might have a non-negligible impedance even when retracted for high intensity operation. The design of such a system will be carried out at BNL. Impedance aspects should not be neglected. Action: John to inform Elias when a design of the system is available so to evaluate the impact in terms of impedance.
John reminded that the ion beam delivered by the injectors during the HL-LHC era will have a physical emittance equivalent to the nominal proton beam emittance and therefore the aperture at injection should be compatible with that. Gianluigi and Massimo reminded that this should be the case for the proton operation too as scrubbing runs will be required also during HL-LHC operation and blow-up must be expected.
Dynamic Aperture: Simulations vs. Experiments and first DA results in the LHC (Massimo)
Massimo gave an introduction of dynamic aperture definition and direct and indirect numerical calculations. In particular inverse logarithmic laws are identified as promising tool to analyse numerical simulations and experimental data. The scaling law can be also used to find direct relation to intensity lifetime with the benefit of getting simple scaling laws. Experimental data on lifetime are provided from Tevatron and more recently from the LHC. In particular for the LHC an analysis of several stable beams intensity evolution data show reproducibility of the method and consistency with the collimator settings that were set to approximately 4 nominal sigmas in 2012. Gianluigi suggested looking at 2011 data when the collimator opening was larger to verify the consistency of the measurements.
Recent MD in 2011 and 2012 to measure dynamic aperture were discussed. Two approaches were used:
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Blow up the beam until slow losses are observed. Record evolution of beam intensity and fit it with proposed models. Compare with fit parameters from numerical simulations of DA.
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Kick the beam in order to push it towards high amplitudes until large losses are obtained and compare amplitude of beam losses with numerical simulations of DA.
For both methods the collimator aperture is increased and for the first one the dynamic aperture artificially reduced by acting on corrector circuits (e.g. MCO).
Measurements have been performed in 2011 and 2012 with both methods and comparison with simulation data is on-going. In particular for the second method the comparison between measurements and simulations seem to indicate that the measured dynamic aperture is 9 sigmas to be compared with 10.5 sigmas from numerical simulations.
Massimo reminded that a safety margin of a factor 2 with respect to the simulation results was considered for the LHC design and the preliminary results seem to indicate that this was too pessimistic.
Andy asked whether the longitudinal dynamics is taking into account. Massimo replied that synchrotron motion is taken into account and the tracked particles are conducting synchrotron oscillations with amplitude corresponding to the maximum energy offset of 2/3 of the bucket height. Andy noted that the momentum distribution should be taken into account.
Andy and Sasha pointed out that frequency map analysis is also a promising DA indicator.
First results on Dynamic Aperture including D2 errors (Massimo)
The main aim of the simulations is to determine the values of the D2 field errors that lead to a reduction of the dynamic aperture below that determined by the triplet errors:
The simulations have been done in the following conditions:
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15cm beta* round optics.
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IT field quality as specified by task 2.3.
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D2 field errors are reported as fraction of the values in the error table transmitted by Ezio (corresponding to a 5 T magnets with apertures of 105 mm diameter). The field errors are expected to decrease by a factor 4 when the magnetic field is reduced from 5 to 3.5 T and the aperture is reduced from 105 mm to 95 mm diameter.
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For Q4 some very preliminary field error table (not yet official) has been assumed
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For Q5 MQY field errors have been assumed
In the best case (errors reduced by a factor 4) the dynamic aperture decreases with a
The results show that already at 25% the DA reduces by 3.5 sigmas.
b3 has the strongest impact on dynamic aperture followed by b4. The impact of higher order field errors is negligible as it is negligible the effect of the Q4 and Q5 field errors. b2 errors appear to have a beneficial effect in spite of the fact that the machine is re-matched to keep the tune constant. This remains to be understood.
Next steps:
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Determine the maximum value of b3 and b4 for which the impact of D2 field quality on dynamic aperture is negligible w.r.t that of the triplet field errors
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Include feed-down effects due to the sagitta of the proton beam trajectory in the D2 aperture.
In the meantime WP3 is working on the optimization of the field quality. Options could include:
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Shaping of the iron yoke
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Spool pieces
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Asymmetric coils
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Single large aperture warm magnet joke with superconducting coils (TAN could be integrated in the aperture)
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Single large aperture warm D2 magnet (TAN could be integrated in the aperture)
An update on the optimization of the field quality for D2 is expected by the end of September by Ezio. Action: Ezio.
The option of using the b3 correctors of the triplets to correct the b3 component in D2 has been mentioned. Compensation of the b4 errors would not be possible for both beams at the same time because the b4 (and all even components) have opposite sign in the two D2 aperture and the triplets’ correctors have a single aperture.
Reported by: Gianluigi and Riccardo
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