Longitudinal parameters and beam induced heatin g

2. Longitudinal parameters and beam induced heating Juan F. Esteban Müller, Benoit Salvant, Elena Shaposhnikova Thanks to: P. Baudreghien, G. Iadarola, M. Kuhn, N. Mounet, T. Pieloni Evian’14 Workshop

3. Outline Evian’14 Workshop • Introduction • Longitudinal parameters • Landau damping • Beam lifetime • Synchrotron radiation • Intra Beam Scattering • Luminosity levelling • Beam induced heating • Issues during LHC Run 1 and mitigation measures during LS1 • Flat bunches • Bunch length reduction impact at 6.5 TeV • Proposed strategy • Conclusions

4. Longitudinal parameters At the end of LHC run 1 (2012)  Lower emittance at injection than in DR But bunch length at 4 TeVhad to be increased in several steps due to beam induced heating • For LHC Run 2: Aim to reduce bunch length to profit from low β* (only for 25 ns)  Side-effects should be taken into account to optimize luminosity Evian’14 Workshop

5. Landau damping Evian’14 Workshop • LHC beam: similar threshold at 6.5 TeV as at 4 TeV for VRF=12 MV if bl = 1.25 ns • MD in 2012  Nth ~ 1.0 x 1011 for 1 eVs • Scaling to 6.5 TeV, 12 MV: • 2.8 eVs (1.25 ns)  Nth ~ 7.2 x 1011 • 1.9 eVs (1.0 ns)  Nth ~ 2.7 x 1011 • 1.15 x 1011 εth ~ 1.35 eVs (0.85 ns) Margin on stability

6. Beam lifetime • Beam-beam: maximum 1.3 ns at 4 TeV in 2012 (longitudinal shaving) RF losses Dependence on Δp?  ε and VRF could be reduced In both cases, loss rate increases with bunch length! Courtesy of G. Papotti, see her talk Evian’14 Workshop Two loss mechanisms:

7. Synchrotron radiation 4 deg Evian’14 Workshop • At 6.5 TeV, SR energy loss per turn = 5 KeV • Lost particles behavior different compared to 4 TeV  All will move in the same direction

8. Intra Beam Scattering Courtesy of M. Kuhn Evian’14 Workshop • Higher horizontal emittance growth rate for shorter bunch length • No vertical emittance growth from IBS simulations • High dependence on initial transverse emittance: • Very small growth for 3.75 μm • Effect on luminosity???

9. Luminosity levelling Evian’14 Workshop • Acceleration with 6 MV constant or ramp to 8 MV (if needed for stability) • Voltage slowly increased to 16 MV during collisions to reduce bunch length by ~20 % • From 1.25 ns (2.0 eVs) to 1.0 ns Luminosity increases by ~15% Effect of lower fs (Qs) on transverse instability should be checked!

10. Beam induced heating  Very significant effort by all groups to protect their equipment Evian’14 Workshop Issues during LHC Run 1 and mitigation measures during LS1

11. Beam induced heating • Different beam: 25 ns bunch spacing, lower bunch intensity (1.15 x 1011) • Heating on ALFA roman pot: • Improved with new design  Less heating even for shorter bunches • Bunch length reduction  Heating increases • Expected behavior in most of the equipment Old design before LS1 New design after LS1 Evian’14 Workshop

12. Flat bunches Evian’14 Workshop Possible mitigation measure if heating is again an issue Tested in 2012 MD, as expected from simulation  More tests needed in 2015 Beneficial for devices with impedance below 1.2 GHz Possibility to improve bunch shape in RF operation (see P. Baudrenghien talk)

13. Bunch length reduction impact at 6.5 TeV Evian’14 Workshop

14. Proposed strategy Evian’14 Workshop • Injection at 6 MV • Start with “safe parameters”: bunch length 1.25 ns and 12 MV • Two options: • Reduce ε Reduce bunch length (to nominal) • Reduce ε and VRF Potential for luminosity levelling Reduction of the blow-up target must be done in small steps to check the effect on heating

15. Conclusions Evian’14 Workshop Shorter bunches (1.0 ns) are probably feasible: • Improvements in heating • Flat bunches as possible mitigation • Margin in longitudinal stability Lower emittances are tolerable: • Possibility of luminosity levelling • Improved beam lifetime

16. Spare slides Evian’14 Workshop

17. Longitudinal parameters for different scenarios Evian’14 Workshop

18. Beam induced heating: one of the LHC performance limitations Evian’14 Workshop Machine elements with heating problems before LS1

19. Beam induced heating Evian’14 Workshop Beam induced heating issues during LHC Run 1: • Damage to equipment (RF fingers of VMTSA double bellow modules, BSRT mirror, TDI beam screen, TDI jaw deformation, damage came a few degrees close for ALFA detector) • Beam dumps (due to interlock on TCP and TCTVB collimator temperatures, and maybe also vacuum interlock next to TOTEM pot) • Delay to re-inject (MKI temperature) • Believed to have affected temperature regulation of Q6R5 standalone (due TOTEM pot heating) => Bunch length was increased a few times to reduce heating

20. Beam induced heating  Very significant effort by all groups to protect their equipment Evian’14 Workshop Mitigations put in place by equipment groups before and during LS1: • VMTSA double bellows were all removedin 2012  TE-VSC • All non conform RF fingers were repairedduring LS1  TE-VSC and LRFF task force (also working on new design) • 2-beam-collimators TCTVBs were all removed (half in 2012, half in LS1) EN-STI • TCP.B6L7.B1 that was heating was exchanged during LS1 (investigations to know what happened will be performed in September with EN-STI to allow sufficient radiation cooldown) EN-STI • New design of the BSRT mirror during LS1to reduce heating was installed  BE-BI • The TDI beam screen was stiffened and more support was installed during LS1  EN-STI/TE-ABT • Copper coating on TDI jaw was planned but had to be abandoned at the last moment due to technical issues  EN-STI/TE-ABT • Installed shielding on ATLAS-ALFA and TOTEM detectors during LS1 are planned to reduce heating, however TOTEM plans to approach high luminosity beams may increase heating • MKI screening was significantly improvedand the two non-conform magnets that were causing heating problems were repaired (MKI8C and in particular MKI8D)  TE/ABT • Interesting to decrease bunch length (25 ns beams only)?