[R. Alemany] [CERN BE/OP] [Engineer In Charge of LHC] Lectures at NIKHEF (21.03.2013 )

1. [R. Alemany] [CERN BE/OP] [Engineer In Charge of LHC] Lectures at NIKHEF (21.03.2013) The Large Hadron ColliderLHC Operation II: pushing the limits UFOsSEUse-cloud25 ns operation Beam-beam effects

2. High intensity beam issues Pushing the limits does not come for free. The 2011 and 2012 operation encountered issues related to the increased storage intensity >2 x 1014 p+: • Vacuum pressure increase • Heating of machine elements (BSRT mirrors, injection kickers, collimators) by the beam induced high order modes • Losses due to dust particles falling into the beam (UFO) • Single Event Upsets (SEU) • Beam losses due to tails • Beam instabilities blowing up the beams The last two issues dumped 35 physics fills in 2012

3. UFOs (Unidentified Flying Objects) Unforeseen sudden losses appearing around the ring on the ms time scale  interaction of macro-particles, of sizes estimated to be 1-100 µm, with the proton beams. • UFOs can give losses over the dump threshold and dump the beams. • UFO rate = f(beam intensity) up to few hundred of nominal (1.5· 1011p) bunches • UFO rate ~ kte for higher beam intensities

4. UFOs in LHC during 2011 & 2012 • The UFO rate (h-1) decreases with operation time  cleaning effect • 2011 ~ 10 UFO/h  2 UFO/h • After a major machine intervention, like the Xmas shut downs, the UFO rate increases • In 2012 initially about 2.5xUFO rate of 2011 • 25 ns scrubbing (2012) showed 10xUFO rate of 2012 • Tobias Baer, Evian Workshop December, 19th 2012

5. UFOs: extrapolation to 7 TeV 91 21 Additionally (not considered): UFOs around IRs until cell 11, at collimators/movable devices and Ufinos in experiments. • Tobias Baer, Evian Workshop December, 19th 2012

6. Single Event Upset (SEU) SEU is caused by a very high energy deposition in a small volume of the electronics chip data is lost or device destroyed M. Calviani R2E Review 21 Nov 2011 • Radiation sources at LHC: • 20 MeV hadrons and • thermal neutrons • Coming from: • IP1,5&8  luminosity • IR3&7 collimator losses • DS  leakage from luminosity, collimator losses and beam gas • ARC  beam gas

7. Single Event Upset (SEU) • 2011 237 events detected. 22% of STABLE BEAMS were dumped by SEU. Cryogenics and Quench Protection Systems most affected. 2012 50 10-20 Expected Dumps Crucial  mitigation measurements: equipment relocation outside high radiation areas, use radiation hard electronics, shielding

8. e-cloud vacuum chamber wall p+ bunch e- e-cloud @injection energy  ionization of gas molecules by p+ e-cloud @higher energy  photoelectrons from synchrotron light (44 eV photons = critical energy for photoemission yield from cooper(beam screen))

9. e-cloud mechanism at injection • e- from ionization: • ~eV slow motion still inside the beam screen when the next proton bunch passes • accelerated to ~100 – 1000 eV by the Coulomb field of the next bunch • before arrival of the next bunch, strike the wall, yielding one or more secondary electrons.

10. e-cloud mechanism at injection secondary electron yield δ=emitted e-/incident e- If δ >1  re-generative process and the ambient electron density will grow exponentially. Beam screen (copper) δ ~1.1 to ~1.7

11. e-cloud mechanism at injection Re-generative process

12. e-cloud: effects on collider operation Transverse mode-coupling instability (TMCI), coupled-bunch instabilities, head-tail motion within the proton bunch, tune spread, beam loss and incoherent emittance growth Injection tests with 48bunches trains (26.08.2011) • Beam unstable right after the injection (beams dumped due to losses) • Probably triggered by e-cloud in the main dipoles • Observed vertical motion in the trailing bunches • Beam stable with high chromaticity settings Q’=15 (while normally 2) Courtesy of W. Hofle, D. Valuch

13. e-cloud: effects on collider operation e-cloud desorbs gases from the walls of the beam screen  • Pour beam lifetime • Important emittance growth • Preassure bumps instabilities G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

14. e-cloud: effects on collider operation Energetic electrons heat the surfaces that they impact heat load could exceed installed refrigeration capacity for 25 ns bunch spacing. Total beam intensity Heat load G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

15. e-cloud: scrubbing runs Inject a high current beam to induce e-cloud  many gas molecules trapped inside the beam pipe metal released. Then pump 2011 B1 2100b B2 1020b Secondary electron yield   from 2.2 down to 1.52 G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

16. e-cloud: scrubbing runs B1 2748b 2012 B2 2748b Secondary electron yield   from 1.55 down to 1.45 G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

17. G. Arduini, H.Bartosik, G. Iadarola, G. Rumolo, Evian Workshop 2012

18. e-cloud observations at 4 TeV • After Scrubbing Run machine studies with 25ns beams at 4TeV were possible. Main observations: • The heat-load strongly increases during the ramp since the EC is enhanced by the photoelectronsdue to synchrotron radiation  This violent transient on the heat load in the arcs limits the number of bunches which can be accelerated • Despite the larger number of electrons, at high energy the beam becomes less affected by EC  the beam quality achievable at collisions is determined by the EC effects at 450GeV

19. 25 ns operation (from 2015) • 25 ns operation is a request from the experiments  less pile-up  less computational resources needed  cleaner event reconstruction • But it is a challenge for the machine. First suspect?  e-cloud

20. Beam-beam interactions at LHC • Two counter rotating beams made of a large number of p+ bunches interact at the IPs. • When these two density of charge particles come close together  electromagnetic interaction  beam-beam interaction • Head-on (HO)  unavoidable if we want to do physics • Long-range (LR)  pseudo-unavoidable  we need a crossing angle to avoid more than one HO

21. Crossing angle

22. Beam-beam interactions at LHC • Each beam represents an electromagnetic potential to the other beam: • Acts like a non-linear electromagnetic lens at the location of the interaction (adding additional very non-linear multipoles in the IP) • Localized, periodic beam force HO: linear force quadrupole like amplitud independent tune shift Courtesy of W. Herr Courtesy of M. Schaumann LR: non-linear force amplitud dependent tune shift Courtesy of W. Herr

23. Long range interactions  tune spread • Number of LR interaction depends on spacing and length of common part • In LHC 15 LR interactions (for 25 ns) on each side of the IP  4x2x15 =120! • Effects depend on separation  (for large enough d) Large effects for largest amplitudes where non-linearities are strong The size of the effect depends on d  for small d  problems The tune spread is very asymmetric since all the non-linear part of the beam-beam force curve is scanned. Courtesy of W. Herr

24. Long-range interactions  closed orbit effects For d >> σ Maclaurin series Amplitude independent kick  dipole quadrupole sextupole octupole

25. Pacman bunches • Orbit can be corrected, but only global corrections are possible • Pacman bunches will always be overcorrected they’ll no have the optimum position The difference in orbit kick before and after the IP is cancelled for bunches in the core of the train, but for Pacman bunches not!

26. vertical Long range effects in ATLAS IP1 Effect arising from missing LR interactions in the vertical plane of IP1 Courtesy of M. Schaumann Different history of LR encounters for head and trail bunches responsible for the asymmetry Courtesy of M. Schaumann

27. Horizontal effect in ATLAS arises from LR in horizontal plane in CMS! And propagates to IP1

28. LR effects when reducing the crossing angle

29. Beam losses = f(number HO) HO IP1,5,8 HO IP8

30. Beam losses = f(number HO) HO IP8 HO IP1,5,8

31. Lead ion beam production Small sliver of solid isotopically pure 208Pb is placed in a ceramic crucible that sits in an "oven" The accelerator chain consumes about 2 mg of lead every hour – a tiny amount, but 10 g costs some SwFr 12,000 The metal is heated to around 800°C and ionized to become plasma. Ions are then extracted from the plasma and accelerated.

32. II. LHC Operational cycle: Injection B2 Dump 5 RF LBDS B1 Dump 6 4 Collim (p) Collim (beta) 3 7 Pb82+ SPS 8 2 TI8 Top energyCircumference(m) LINAC3 4.2 MeV/u ~10 LEIR72 MeV/u 78 CPS 4.2 GeV/u 628 = 4 PSB SPS 157 GeV/u 6911 = 11 x PS LHC 2760/u 26657=27/7xSPS 1 TI2 Strip foil LEIR Pb54+ CPS Strip foil LINAC 3 Ion source Pb29+ (2.5 keV/u)