Status of halo excitation studies at CERN R. Bruce, D. Banfi, M. Buzio, J. Barranco, O. Bruning, X. Buffat, R. Chritin, R. de Maria, M. Fitterer, M. Giovannozzi, W. Höfle, S. Redaelli, T. Pieloni, H. Thiesen, CERN G. Stancari, A. Valishev, Fermilab R. Bruce,

Halo removal Halo depletion is a possible way to mitigate spurious beam dumps at fast orbit oscillations When do we need it in the operational cycle? – Most important during the squeeze and collision preparation, before beams are brought into collision (Run 1). Application to stable beams for increased protection during crab cavity failure Possible methods under study for halo removal: – electron lens (studied by G. Stancari et al.) – Tune modulation – Transverse damper (ADT) narrow-band excitation

R. Bruce, Halo removal Timeline: – Nothing available for 2015 startup as operational tool. Hollow electron lens: not before LS2. What can we do before? This talk: Brief introduction to the alternatives to electron lens Ongoing and planned studies

R. Bruce, Tune modulation Idea: By modulating tunes at a fixed frequency, resonance sidebands are introduced around the existing resonance lines (Bruning, Willeke PRL 76:3719) Use detuning with amplitude of the beam With right modulation frequency: could put a resonance line on the halo, while leaving the beam core unaffected Some uncertainties on how to use present LHC hardware – Under investigation – see later

R. Bruce, ADT excitation Instead of modulating the tune with a quadrupole, we could use the transverse damper (electric field kicker) to make a narrow-band excitation Rely on detuning with amplitude. Simplest approach: Knowing the fractional tune of the halo Q h apply kick in resonance at frequency More advanced ideas: colored noise to excite selectively only some amplitudes Hardware-wise, no modifications needed

R. Bruce, Alternative methods Both tune modulation and ADT excitation relies on – Detuning with amplitude – Good knowledge, stability, and reproducibility of the tune in the machine Theoretical validation – Both concepts to be validated in simulation studies – Allows also to fine-tune parameters – Ongoing work

R. Bruce, Tune footprints and resonances Showing tune footprints for different bunches Resonance lines shown up to 5 th order Introducing a tune modulation Studying the resonance lines when modulation frequency is increased Thanks to the beam-beam team for help and input!

R. Bruce, Tune footprints and resonance lines – all footprints, 5 th order Thanks to the beam-beam team for help and input!

R. Bruce, Tune footprints and resonance lines – all footprints, 5 th order Zoom on collision tunes – separated and colliding beams, 6.5 TeV, b*=55cm HO IP1/5/8 LR IP1/5 one side only HO IP8 No HO

R. Bruce, Needed frequency Depending on tune, which tune footprint we want to hit, and which resonance we want to use: big spread of possible frequencies Hz – different bunches have different footprints depending on where they collide. Need to be careful... when hitting the halo in some bunches, we risk to hit the core in others!

R. Bruce, LHC hardware for tune modulation Cold magnets: risk for problems with quench protection system Most suitable magnets in the LHC: warm trim quadrupoles MQWB in IR3/7. 8 units installed Measurements carried out to verify power converter capabilities – Applying modulation to 4 magnets in IR7 in parallel

R. Bruce, Power converter measurements Data at max achieved amplitude for different frequencies Thanks to H. Thiesen

R. Bruce, Synchronization Signals by default not synchronized, but could be using timing

R. Bruce, Max achieved amplitudes Saturation of power converter limits the achievable amplitude Previous guess (O. Bruning, H. Schmickler in 2013 collimation review): dQ of 1e-4 needed – If no other damping, we would have sufficient current at injection, but not at 7 TeV – Needed amplitude to be confirmed by ongoing FMA simulations Preliminary

R. Bruce, Magnetic measurements So far discussed only power converter Risk for additional loss in amplitude due to attenuation in magnet and vacuum chamber Ongoing activity to set up magnetic measurements with full magnet to measure transfer function (collaboration with M. Buzio, R. Chritin et al.) Measurements of power converters combined with transfer function through magnet and beam screen should give hardware limits

R. Bruce, For discussion: Ideas for beam tests for 2015 We could aim at some first tests in 2015 with both tune modulation and ADT Start simple: Injection energy – easy re-fill if beam is lost - not unlikely to hit also the core during trials – Larger available tune amplitudes for tune modulation with MQWB Ideal measurement procedure – Estimate population in halo and core – Apply halo cleaning – Estimate population in halo and core – Re-populate halo – Repeat…

R. Bruce, For discussion: Measurement challenges Estimation of halo population – LHC instrumentation for profile measurements has insufficient resolution – Possible method: BLM signals during collimator scraping Disadvantage: destructive! – Could be used anyway if we have reproducible way to re-populate halo Halo re-population – Best option - smooth ADT blowup? Disadvantage: blows up also the core. To what level is the halo population reproducible in repeated (distructive) measurements? – Alternative: Re-injecting fresh bunches? Checking that halo cleaning does not affect core – Need to disentangle core blowup from halo repopulation and halo cleaning

R. Bruce, Pros and cons Tune modulation affects in the LHC both beams and planes simultaneously due to the powering of the magnets ADT can act on a single beam and plane, even on single bunch ADT and tune modulation: need good knowledge of tune and detuning with amplitude. Risk to hit the core ADT and tune modulation rely on existing hardware Hollow e-lens – completely independent on the tune. Acts only on particle amplitude – Can act on individual trains but not bunches – New hardware needed. Not available in LHC until Run 3

R. Bruce, Summary Tune modulation and ADT narrow-band excitation are alternatives to hollow e-lens for halo excitation – Using existing hardware ADT: existing hardware should be sufficient Tune modulation: Ongoing measurement campaign on MQWB warm quadrupoles – Several magnets can be used synchronized Hope for first LHC measurements to show effect in 2015 – Several challenges to be carefully thought about Ongoing simulation studies for validation of parameters