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20th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-018 (CERN)
Minutes of the 20th WP2 Task Leader Meeting held on 25/10/2013
Present: G. Arduini, H. Burkhardt, R. De Maria, L. Esposito, S. Fartoukh (part-time), P. Fessia, A. Lechner, E. Métral, T. Pieloni, S. Redaelli (part-time), A. Valishev, A. Wolski
Minutes of the last meeting and follow-up of actions
Update on energy deposition in the matching section – L. Esposito
Luigi reviewed the results presented at the last meeting. With the exception of slide 3 (where the ultimate case with a b* of 10 cm and a crossing angle of 720 mrad is considered all the simulations refer to the nominal case. For the ultimate optics the TCL in front of D2 implies an increase of the dose at the D2.
Luigi showed the results of the simulations for round optics 15cm/590 mrad with the following modifications:
The combined effect of tungsten masks and TCL shows that D2, Q4 and Q5 peak power density and doses are now under control. There is no need of additional shielding on the beam screen for D2 and therefore some clearance might be available between the beam screen and the D2 cold bore. Stephane commented that in the dipoles, the clearance of the beam screen is due to the end spacer as communicated by Ranko at the time of the Phase I project. Action: Paolo will check with Ezio if it is the case for the present D2.
Paolo noted that the chosen position of the masks might imply that these are installed inside the cryostat. Luigi should check the available space and positions with Paolo such that the masks installed in the warm part in front of the magnets. Action: Luigi, Paolo.
Stephane noted that for the considered scenario with two MQYY magnets the strength is larger than the MQYL baseline (4.8 m MQY operated at 1.9 K) and it might limit some of the optics solutions requiring low currents. Riccardo replied that the small Q5 strength limits will be considered in the evaluation of MQYY choice.
Adding a mask in Q5 has the effect of further reducing the energy deposition on Q5. The peak observed in Q7 could be reduced by adding a TCL in front of Q7. Increasing the TCL4 aperture to 20 sigma increases the radiation on D2. Paolo noted that the integration of the mask in front of Q7 should be studied with respect to the position of the DFB.
Stephane asked whether a TAN placed much closer to the TCL could be evaluate, assuming the that TCT of the other beam line will be placed either in front of Q4 or in between D1 and D2.
The energy deposition as a function of azimuthal angle shows the effect of the TCL4 and the fact that for Q7 the losses are mostly vertical. In terms of dose the values are mostly below 10 MGy with a peak in D2 at 20 MGy that can be cured by closing the TCL4.
Stephane asked how much power goes in the arc, Luigi replied 20 W at 5e34 luminosity.
The following assumptions should be made for the future:
The present configuration is very good from the point of view of energy deposition.
The integration issues pointed out by Paolo should be addressed by Francesco, Luigi, Paolo, Stefano and a new iteration in the energy deposition studies should be made after addressing these issues. A possible optimization of the mask thickness in front of D2 could be made as its effect is very close to that of a perfect shielding with 100% efficiency. The radiation on the crab cavities should be estimated. Action: Francesco, Luigi, Paolo, Stefano.
Update on crossing angle scaling with bunch intensity – T. Pieloni, A. Valishev
For the LHC the target dynamic aperture in the presence of beam-beam and imperfections was 8 nominal (i.e. corresponding to 3.75 mm emittance) sigmas to be compared with a collimator aperture of 6 nominal sigmas. Riccardo asked whether the MD of last year can answer the question addressing the luminosity lifetime, rather than dynamic aperture or intensity lifetime.
The simulations using only beam-beam interactions using SLHC3.1b optics showed that:
Gianluigi concluded that one should consider the bunch populations at the end of the leveling phase when the nominal beam-beam separation is reached. The difference observed between round and flat beams should be understood. Action: Sasha, Tatiana. For the criteria to be used for the simulated dynamic aperture one should look at the 2012 experience to determine the link between the core diffusion, relevant for luminosity lifetime, to the observables accessible with simulations.
Tatiana noted that IP8 was switched off for the time being in all simulations as no significant effect was observed when the head-on collisions in IP8 were considered. In all the simulations crab-cavities were off. This implies a smaller head-on tune spread.
Riccardo asked about the impact of PACMAN effects. Tatiana replied that this needs to be evaluated. Tatiana and Gianluigi commented that perhaps crab cavity can act at the bunch by bunch level to compensate the orbit differences at the expenses of an increased effect of the RF curvature.
Update on stability limit and collimator impedance upgrade needs (E. Metral)
The construction of the HL-LHC Impedance model is underway together with the understanding of the present one. The LHC model is still evolving to include geometric contribution to collimator impedance being evaluated by M. Zobov (INFN-LNF). For the HL-LHC the new ingredients are new optics, new settings and new material. 10cm optics is used and can be pessimistic for in the high beta location but optimistic for the tune spread induced by the Landau Octupoles as a result of the largest beating in the arcs. Riccardo observed that the beta function in the arcs have a tunable range independently from the final beta that could be exploited if needed.
HL-LHC behavior is similar to LHC. Simulation of growth rates considers the effect of the damper. The damper effectiveness should be checked a 7TeV. Action: Elias to check the expected damping time for 7 TeV.
The analysis is based on scaling from past experience with 50ns to the most critical mode for 25 ns. The scaling is performed linearly and including the full interactions of modes. If the damper works well with ATS optics, all scenarios are stable for both coupled bunch and single bunch instabilities. The metallic collimators reduce the impedance except at low frequencies (<few tens of kHz). Damping of the low frequency coupled bunch modes is therefore mandatory.
In the future the impedance model will be completed with new equipment.
Gianluigi concluded that the metallic collimators will increase the stability margin in the same way from PIC to US2 therefore they will be proposed as part of the PIC scenario.
Reported by Gianluigi and Riccardo.
Present: G. Arduini, H. Burkhardt, R. De Maria, L. Esposito, S. Fartoukh (part-time), P. Fessia, A. Lechner, E. Métral, T. Pieloni, S. Redaelli (part-time), A. Valishev, A. Wolski
Minutes of the last meeting and follow-up of actions
- The minutes of the joint WP2-WP5 meeting have been issued.
- The heat load estimated for electron cloud and impedance in the triplets have been transmitted to Laurent Tavian.
- The simulations for the new D1 and matching section and the present triplets in IP2 and 8 will continue together with the possible implementation of clearing electrodes.
- An update on the beam-beam simulations for the RLIUP scenarios will be given today.
Update on energy deposition in the matching section – L. Esposito
Luigi reviewed the results presented at the last meeting. With the exception of slide 3 (where the ultimate case with a b* of 10 cm and a crossing angle of 720 mrad is considered all the simulations refer to the nominal case. For the ultimate optics the TCL in front of D2 implies an increase of the dose at the D2.
Luigi showed the results of the simulations for round optics 15cm/590 mrad with the following modifications:
- D2 and its orbit correctors with 100 mm aperture
- Clearance cold bore/beam screen reduced from 4.5 mm to 2.0 mm
- Tungsten masks in front of D2, Q4, Q5, Q6 (for both beams)
- Q5 composed by MQYY (Q4) modules
- TCL4 opening set at 20σ
- D2 mask centered
- Mask material: IMER180 (ρ=18.0 g cm-3) as for TCT and new TCL
- TCL4, TCL5, TCL6 installed (only on B1).
The combined effect of tungsten masks and TCL shows that D2, Q4 and Q5 peak power density and doses are now under control. There is no need of additional shielding on the beam screen for D2 and therefore some clearance might be available between the beam screen and the D2 cold bore. Stephane commented that in the dipoles, the clearance of the beam screen is due to the end spacer as communicated by Ranko at the time of the Phase I project. Action: Paolo will check with Ezio if it is the case for the present D2.
Paolo noted that the chosen position of the masks might imply that these are installed inside the cryostat. Luigi should check the available space and positions with Paolo such that the masks installed in the warm part in front of the magnets. Action: Luigi, Paolo.
Stephane noted that for the considered scenario with two MQYY magnets the strength is larger than the MQYL baseline (4.8 m MQY operated at 1.9 K) and it might limit some of the optics solutions requiring low currents. Riccardo replied that the small Q5 strength limits will be considered in the evaluation of MQYY choice.
Adding a mask in Q5 has the effect of further reducing the energy deposition on Q5. The peak observed in Q7 could be reduced by adding a TCL in front of Q7. Increasing the TCL4 aperture to 20 sigma increases the radiation on D2. Paolo noted that the integration of the mask in front of Q7 should be studied with respect to the position of the DFB.
Stephane asked whether a TAN placed much closer to the TCL could be evaluate, assuming the that TCT of the other beam line will be placed either in front of Q4 or in between D1 and D2.
The energy deposition as a function of azimuthal angle shows the effect of the TCL4 and the fact that for Q7 the losses are mostly vertical. In terms of dose the values are mostly below 10 MGy with a peak in D2 at 20 MGy that can be cured by closing the TCL4.
Stephane asked how much power goes in the arc, Luigi replied 20 W at 5e34 luminosity.
The following assumptions should be made for the future:
- D2 and its orbit correctors with 105 mm aperture
- IMER180 (ρ=18.0 g cm-3)masks in front of D2, Q4, Q5, Q6 (for both beams)
- TCL4, TCL5, TCL6, TCL7 installed (only on B1).
The present configuration is very good from the point of view of energy deposition.
The integration issues pointed out by Paolo should be addressed by Francesco, Luigi, Paolo, Stefano and a new iteration in the energy deposition studies should be made after addressing these issues. A possible optimization of the mask thickness in front of D2 could be made as its effect is very close to that of a perfect shielding with 100% efficiency. The radiation on the crab cavities should be estimated. Action: Francesco, Luigi, Paolo, Stefano.
Update on crossing angle scaling with bunch intensity – T. Pieloni, A. Valishev
For the LHC the target dynamic aperture in the presence of beam-beam and imperfections was 8 nominal (i.e. corresponding to 3.75 mm emittance) sigmas to be compared with a collimator aperture of 6 nominal sigmas. Riccardo asked whether the MD of last year can answer the question addressing the luminosity lifetime, rather than dynamic aperture or intensity lifetime.
The simulations using only beam-beam interactions using SLHC3.1b optics showed that:
- for PIC one needs around 400 mrad to obtain DA at the level of ~7 sigma (for a normalized emittance of 2.5 mm);
- for US1 one needs the beam-beam wire compensator to bring the crossing from more than 14 sigmas to 10 sigma (for a normalized emittance of 2.5 mm);
- for US2 a dynamic aperture of ~7 sigmas (for a normalized emittance of 2.5 mm) is available with the nominal crossing angle for the bunch populations that are typically reached at the end of the levelling.
Gianluigi concluded that one should consider the bunch populations at the end of the leveling phase when the nominal beam-beam separation is reached. The difference observed between round and flat beams should be understood. Action: Sasha, Tatiana. For the criteria to be used for the simulated dynamic aperture one should look at the 2012 experience to determine the link between the core diffusion, relevant for luminosity lifetime, to the observables accessible with simulations.
Tatiana noted that IP8 was switched off for the time being in all simulations as no significant effect was observed when the head-on collisions in IP8 were considered. In all the simulations crab-cavities were off. This implies a smaller head-on tune spread.
Riccardo asked about the impact of PACMAN effects. Tatiana replied that this needs to be evaluated. Tatiana and Gianluigi commented that perhaps crab cavity can act at the bunch by bunch level to compensate the orbit differences at the expenses of an increased effect of the RF curvature.
Update on stability limit and collimator impedance upgrade needs (E. Metral)
The construction of the HL-LHC Impedance model is underway together with the understanding of the present one. The LHC model is still evolving to include geometric contribution to collimator impedance being evaluated by M. Zobov (INFN-LNF). For the HL-LHC the new ingredients are new optics, new settings and new material. 10cm optics is used and can be pessimistic for in the high beta location but optimistic for the tune spread induced by the Landau Octupoles as a result of the largest beating in the arcs. Riccardo observed that the beta function in the arcs have a tunable range independently from the final beta that could be exploited if needed.
HL-LHC behavior is similar to LHC. Simulation of growth rates considers the effect of the damper. The damper effectiveness should be checked a 7TeV. Action: Elias to check the expected damping time for 7 TeV.
The analysis is based on scaling from past experience with 50ns to the most critical mode for 25 ns. The scaling is performed linearly and including the full interactions of modes. If the damper works well with ATS optics, all scenarios are stable for both coupled bunch and single bunch instabilities. The metallic collimators reduce the impedance except at low frequencies (<few tens of kHz). Damping of the low frequency coupled bunch modes is therefore mandatory.
In the future the impedance model will be completed with new equipment.
Gianluigi concluded that the metallic collimators will increase the stability margin in the same way from PIC to US2 therefore they will be proposed as part of the PIC scenario.
Reported by Gianluigi and Riccardo.
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