1. 1 CC & MP - CC10 - CERN 16.12.2010 Crab Protection @ LHC J. Wenninger CERN Beams Department for the LHC Machine Protection Panel

2. Beam excursions 16.12.2010 CC & MP - CC10 - CERN 2  The principle of the CC is to kick head and tail of the bunches to enhance the luminosity. The more effective the CC is, the larger the kicks, the larger the excursions of head and tail. Local crabs : SLHC-V3 Assuming a full length of  2  s s x In case of ‘failures’ (cavity trips, control errors etc) the resulting particle excursions should not lead to component damage, and if possible not even to quenches (efficiency). Performance gain and criticality of machine protection issues are strongly correlated !

3. Stored Energies 16.12.2010 CC & MP - CC10 - CERN 3 The LHC : a new regime for Machine Protection. Even the beam halo can be dangerous ! LHC 2010 LHC 2011 Already today MP largely drives progress, MDs etc (unfortunately)

4. Tails @ LHC 2010 16.12.2010 CC & MP - CC10 - CERN 4  y = 0.27 mm ~ 5.7  ~ 4.7  ~ 3.7  3.5% of beam within 1.5   Tail populations of 1% to 3% were measured with primary collimator scans (end of fills) in the ‘last’ sigma: For a nominal beam this would represent around 3-10 MJ ! R. Assmann

5. MPS & Collimation 16.12.2010 CC & MP - CC10 - CERN 5 Courtesy C. Bracco Although the primary design goal of the collimators is beam cleaning, they also play an essential role for MP.  Collimators define the machine aperture. But phase space coverage is not complete !  The large majority of failures leads to a primary particle impact at one of the collimators. o Collimators are robust to survive limited beam impact. o Collimators must cope with beam impact from CC failures. o Beware of (massive) quenches downstream of collimation sections !

6. LHC beam dump delays 16.12.2010 CC & MP - CC10 - CERN 6 Achievable response time ranges between 100  s and 270  s. >> Triggering a dump is not the end of the story, must be able to survive up to another 3 turns without damage. User System process a failure has been detected… beam dump request Beam Dumping System waiting for beam gap max 89μs Signals send to LBDS t2t2 t3t3 Beam Interlock system process max 100 μs t1t1 > 10μs Kicker fired t4t4 all bunches have been extracted max 89μs

7. KEKB CC trip 16.12.2010 CC & MP - CC10 - CERN 7 1. D11-F klystron out 2. D11-F cavity voltage 3. D11-F cavity tuner phase 4. HER DCCT RF off Beam Abort Y. Morita Complete cavity trip in less than one LHC turn ! Machine Protection

8. Ultra-fast failures 16.12.2010 CC & MP - CC10 - CERN 8 ~ 1 turn failures are too fast for the MPS to react => protection requirements for ultra-fast failing objects:  ‘Harmless’ => no damage and/or  ‘Never occur’ => really rare “consequences” x “rate” must compatible with SIL3. (less than 1 ‘catastrophic’ event in 1000 years) SIL3 systems at the LHC : BLMs, beam interlock system, beam dump system.

9. Protection of the LHC against CC failures 16.12.2010 CC & MP - CC10 - CERN 9 Options to limit the machine protection footprint of CCs :  Make failures ‘slow’ and provide fast interlock signals.  Reduce the effect of a trip.  Reduce the density of tails.  … Reduce movement (loss) to a fraction of a beam sigma / turn Detailed info on failure modes is required !

10. Failure mitigation: divide and conquer ! 16.12.2010 CC & MP - CC10 - CERN 10 Possible scenario for CC would be to split them into a sufficient number of INDEPENDENT sub- systems (power+control+cavity). o Failure of one cavity / unit becomes acceptable. o Impedance? o Watch out for common-cause failures that break the independence ! - What about mains failures??

11. Failure mitigation: get rid of the tails! 16.12.2010 CC & MP - CC10 - CERN 11 Assuming primary collimation set to ~6 sigma, one could try to deplete the amplitudes between 3-4 sigma and primary collimation: o Those particles do not contribute significantly to luminosity. o If they are ‘no’ particles out there, failures may be acceptable. Use CCs in combination with a hollow electron lens acting as tail scrapper? o CC operation would be coupled to e-lens. o How to verify that tail population is acceptable?

12. Safe(r) CCs 16.12.2010 CC & MP - CC10 - CERN 12  Limit losses/movements per turn to a fraction of beam sigma equivalent (~per mill of total beam).  This will probably require some optimization: o failure modes (time constants), o no. cavities (effect of trip), o tail control (?), o collimation (upgrades), o …  Do not forget: o Commissioning time (also MP aspects !), o Availability (trip rate !!).

13. 16.12.2010 CC & MP - CC10 - CERN 13

14. Times scales 16.12.2010 CC & MP - CC10 - CERN 14 Best failure detection time = 40 us = half turn NC magnet circuit Quench Kickers Human beings Failures Interlocks UFOs