Agenda

1. LTEX report at LMC:Status of impedance studies of the LHC forward detectors planned to be upgraded during LS1 (in particular ALFA and TOTEM) Olav Berrig (BE-ABP), Fritz Caspers (BE-RF), Sune Jakobsen (ATLAS-ALFA), Oleksiy Kononenko (BE-RF), Mauro Migliorati (visitor BE-ABP), Nicola Minafra (TOTEM), Serena Persichelli (BE-ABP),Benoit Salvant (BE-ABP), Antonello Sbrizzi (ATLAS-AFP) for the impedance team Acknowledgments: LEB and LTEX members, ATLAS-AFP, ATLAS-ALFA, CMS-HPS and TOTEM collaborations. Many thanks to Andre Braem (ATLAS-ALFA), Mario Deile (TOTEM), Patrick Fassnacht (ATLAS-ALFA), Daniela Macina (ATLAS-AFP), Elias Métral, Julien Migne (ATLAS-AFP), Nicolas Mounet, Alexandre Seletskiy (ATLAS-AFP), Anne-Laure Perrot, Dorothea Pfeiffer, Sorina Popescu (CMS-HPS), Giancarlo Spigo (ATLAS-AFP).

2. Agenda Request from forward detector community Why should we worry about the impedance of the forward detectors? What could we do about it? Outlook

3. Request of forward detectors for after LS1 • Request to move closer (~1 to 4 mm) to the beam with high luminosity beams • ATLAS-ALFA upgrade  installation plans during LS1 • TOTEM upgrade installation plans during LS1 • New AFP Hamburg pipe (ATLAS)  installation delayed to winter stop 2015-2016 • New HPS Hamburg pipe (CMS)  installation delayed to winter stop 2015-2016 • Challenging design in terms of impedance with high luminosity beams. • Impact of these requests on machine protection and other aspects than impedance are not discussed here • Independently from these requests the following issues should be solved: • the beam induced heating issue experienced by ATLAS-ALFA in parking position before LS1 • potentially the vacuum issues of Q6R5 (if it is linked to TOTEM ferrite heating and outgassing : studies are ongoing).  from the LHC machine point of view, need to reduce RF heating and upgrades are needed on existing pots (for ATLAS-ALFA and maybe also TOTEM)

4. Agenda Request from forward detector community Why should we worry about the impedance of the forward detectors? What could we do about it? Outlook

5. Context: minimizing the beam impedance of the LHC • LHC optimized for low impedance and high intensity beams From the design phase, the LHC has been optimized to cope with high intensity beams and significant effort and budget were allocated to minimize the impedance of many devices and mitigate its effects • Some examples: • Tapers (11 degrees) and RF fingers for all collimators • Conducting strips for injection kickers MKI • Dump kickers MKD outside of the vacuum pipe • RF fingers to shield thousands of bellows • Wakefield suppressor in LHCb • Avoid sharp steps between chambers and limit tapers to 15 degrees • ferrites and cooling in all kinds of devices (ALFA, TOTEM, TDI, BSRT, etc.) • Consequence: together with other improvements, small LHC impedance allowed maximization of luminosity to the experiments before LS1 • For comparison:

6. Context: impact of beam impedance on performance Round beam pipe (radius 40 mm) Round beam pipe with Roman pot(at 1 mm from the beam) • Strong perturbation of the electromagnetic fields by the Roman pots during (short range wake fields) and after (long range wakefields) the passage of the bunch • When a beam of particles traverses a device which • is not smooth • or is not a perfect conductor, it will produce wakefields that will perturb the following particles  resistive or geometric wakefields (in time domain) and impedance (in frequency domain). • These wakefields are perturbations to the guiding EM fields to keep the beam stable and circulating.

7. Context: impact of beam impedance on performance • Need to study in detail the 3 components of the impedance (real and imaginary parts)as a function of frequency (short range and long range) to identify threats to LHC operation • These perturbations are usually decomposed into longitudinal and transverse wakefields • longitudinal wakefieldslead to energy lost from the particle and dissipated in the walls of the neighbouring devices  heating of beam surrounding  temperature interlocks or degradation of machine devices  limits the LHC intensity and luminosity • longitudinal wakefieldslead to perturbation of the synchrotron oscillations  can excite longitudinal instabilities degrades longitudinal emittance  limits the LHC intensity and luminosity • Transverse wakefieldslead to perturbation of the betatron oscillations  can excite transverse instabilities  degrades transverse emittance  limits the LHC intensity and luminosity

8. Context: impact of forward detectors on impedance?  Not really reassuring for forward detectors  Requires careful design to minimize impedance  Requires robust cooling design to absorb the heat load • From the impedance point of view, the potential threats from forward detectors closer to the beam are: • (1) The resistive material very close to the beam (~1 mm for horizontal pots and ~4 mm for vertical pots) • (2) The sharp step transitions very close to the beam • There are resistive materials close to the beam (collimators) in the LHC, and they indeed represent a very large contribution of the LHC impedance  But there is no such abrupt step so close to the beam in the machine (tapers).  The 2 other abrupt steps that we are aware of are: • TDI collimator (5 mm from the beam at injection)  damaged in 2011 by beam induced heating, and very large contributor to the transverse impedance • BSRT mirror for emittance measurement (20 mm from the beam)  damaged in 2012 by beam induced heating

9. Agenda Request from forward detector community Why should we worry about the impedance of the forward detectors? What could we do about it? Outlook

10. What could we do about it? Abrupt transition With taper Keep the detector as far as possible from the beam Use a coating with a good conductor for the portion facing the beam and not bare stainless steel Avoid the abrupt transition for the beam fields at the location of the beam passage (taper) Try to push resonant frequencies as high as possible (beyond 2 GHz) or damp them with ferrite or coupler Reduce the number of transitions  reduce the number of devices

11. Improvements achieved (1/3) Initial design Less pots and smaller Coppercoating Taper design New design A factor 10 gained in power loss is expected with the new geometry compared to the initial design, to be checked with bench measurements and coupled to thermo-mechanical simulations (with EN-MME). Significant common effort between impedance team and all experiments (ATLAS-ALFA, AFP, CMS-HPS, TOTEM) to assess and minimize the impedance of these forward detectors since mid-2012. Example: AFP power loss with detector at 1 mm from the beam

12. Improvements achieved (2/3) Current design Upgraded design Parking position • Reduction of factor ~5 of RF heating to ATLAS-ALFA expected with the new cylindrical filler • with simulations, to be checked soon with bench measurements on the prototype and thermo-mechanical simulations (with EN-MME) •  Upgrade needed to avoid limiting the LHC luminosity after LS1 (even in parking position) Example: expected RF heating for ALFA in parking position

13. Improvements achieved (3/3) Current design current TOTEM pot (with ferrites) upgraded TOTEM (with ferrites) New cylindrical design • Reduction of factor ~8 of RF heating to TOTEM is expected with the new cylindrical pot • with simulations, to be checked with bench measurements and coupled with thermo-mechanical simulations. Example: expected TOTEM beam induced RF heating

14. Agenda Request from forward detector community Why should we worry about the impedance of the forward detectors? What could we do about it? Outlook

15. Outlook • Common studies are on-going with very useful contribution of experiments • From simulations, most of the impedance contributions seem now more reasonable (studies under way) • However the following potential issues were identified: • The longitudinal impedance would increase by around 1% of the full LHC impedance per detector close to the beam, which would decrease the thresholds for longitudinal instabilities  BE/RF-BR says that there is some margin for this threshold  but it should not be decreased without a very good reason • Ferrites or RF contacts are needed to damp low frequency resonant modes, and the risks associated to these mitigation strategies should be accounted for. • RF fingers: mechanical issues over the operation, potential UFOs • Ferrites: difficulty to dissipate the heat (needs serious studies in collaboration with EN/MME), vacuum issues at high temperatures • Another solution would be mode couplers to extract the power, but these would require a lot of R&D to deliver robust solutions • R&D tools and procedures: possibility that our simulations and procedures are not accurate (e.g. numerical errors, oversimplification)  need careful benchmark with several codes and methods as well as crosschecks with bench measurements • Roman pots are so close to the beam that the impedance is not linear anymore and surprises are possible  the “constant” wake term leads to a differential dipolar kick between bunches of different intensities (on the order of nrad for TOTEM  should not be a concern from the optics point of view)  differential sextupolar kick?

16. Outlook Thank you for your attention • It is important to note that these potential issues are expected to: • Be smaller than before LS1 when the upgraded pots remain in the parking position during physics. • decrease if the insertion of the pots is performed later in the fill, or if the distance of the detector to the beam is increased

18. Pros and cons from impedance point of view Hamburg pipe Vs roman pots(current view) With these considerations, no clear conclusion now as to whether Roman pot or Hamburg pipe is a better solution from the impedance point of view. The main objective for us is to give you more quantitative arguments in the very near future.

19. Draft figures,crosschecks and optimizations underway

20. Effect of 2 ideal Hamburg pipes at injection (preliminary computation with the wall at 25 mm from the beam) Effect of 2 Hamburg pipes at top energy (preliminary computation with the wall at 1 mm from the beam)  There is hope

21. Preliminary numbers of the TOTEM upgrade (at 1 mm)  The low beta function at the location of the pot helps reduce the transverse impedance Most contributions are being minimized, but the abrupt geometry change at ~ 1.5 mm from the beam can not be easily suppressed in the Roman pot geometry (no space available for tapering)  the longitudinal effective impedance is still in the “1% of full LHC” range. Besides, strong non-linear terms close to the wall make it hard to estimate the impact on beam dynamics

22. Preliminary numbers of the ALFA upgrade  The high beta function at the location of the pot (300 to 400 m) increases the effective transverse impedance  the transverse effective impedance is in the “1% of full LHC” range.