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ALICE Commissioning Part III :First energy recovery Yuri Saveliev

ALICE Commissioning Part III :First energy recovery Yuri Saveliev. ST1 ARC 1 ST2 & ST3 ARC 2 ST4 Energy Recovery. Seminar on 14-15/07/2008. Comments on setting ALICE after injector. Now, we are loosing BPMs until energy recovery

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ALICE Commissioning Part III :First energy recovery Yuri Saveliev

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  1. ALICE Commissioning Part III :First energy recovery Yuri Saveliev • ST1 • ARC 1 • ST2 & ST3 • ARC 2 • ST4 • Energy Recovery Seminar on 14-15/07/2008

  2. Comments on setting ALICE after injector • Now, we are loosing BPMs until energy recovery • Nominal Q (80pC or lower), low TrainLength ( ~ 1ms) • Linac must provide a specific energy tilt to match R56 in chicane • - in principle, it’s possible to do using zero-crossing technique but it seems • complicated (and unnecessary for now !) • Instead, we: • - crest both cavities in Main Linac • - set nominal 35MeV Notes : even at TrainLength ~1ms and Q~80pC, the energy drop is ~ 0.3% hence expect some horizontal “blurring” of the beam image in non-zero dispersion regions will we have enough light from OTR screens at ~1ms ???

  3. ST 1

  4. ST1 : setting the main Linac Mean beam energy (35MeV) : use DIP-01 ? may not have enough field no quads upstream  no way to ensure MIN b in the dump section hence : will use DIP-01 (ARC1) • Thread the beam to OTR-02 • - use HVCOR-01 • - remember 15mm dia. hole in OTR-02 • Crest linac cavities (coarse) • - LLRF based procedure • Set linac gradient to get 35MeV • - get the beam to DIP-01 (ARC1) – must be calibrated, and measure • the beam energy • Set beam on Linac axis • - similar to how it was done in the booster • - ideally, use HVCOR-01 only

  5. ST1: setting DIP-01 While setting the extraction chicane, DIP-01 must be set for the Injector energy (of 8.35MeV ) exactly this dipole will be used as a major diagnostic tool for establishing an energy recovery condition • Principle: • main linac cavities are switched OFF • booster is set to nominal beam energy (8.35MeV) • DIP-01 is set such that the beam is centred on BPM-01 (DMP) & OTR-01 (DMP) Question: assume - beam energy losses in a switched off linac are negligible – is it correct? The DIP-01 is now set and its setting must not be changed until the energy recovery is established.

  6. ST1: beamline setting • Thread the beam to OTR-03/04 • Set DIP-02 / 03 • - all quads OFF • - change DIP-02 and scan DIP-03 until the beam is centred on • OTR-03 and OTR-04 simultaneously • Dispersion suppression downstream DIP-03 • - nothing special here – (previous step may already established that) • - otherwise, set linac gradient ~3% below nominal and play with dipoles • as above • - should be easy anyway since Dx is not large (<0.1m) • Center the beam on the beamline axis – important for accurate electron energy • measurements • - check centering on OTR-03/04 and all quads • - try not to use HVCOR-01 too much

  7. FC ST1: Fine Linac setting • Fine crest the cavities • - use of DIP-01 (ARC1) and maximise the beam energy • Fine set the linac gradients • - use nominal (for 35MeV) field in DIP-01 (ARC1) • - centre the beam on OTR-01 (ARC1) • Measure the bunch charge (FC at the end of the ST1) Well… we now have 35MeV delivered to ARC1 entrance but the longitudinal phase space is not set yet (on crest !!)

  8. ARC 1 R56 = 0

  9. ARC1 setting (coarse) • 180 deg. bend condition • Symmetric optics in the arc • Cancellation of dispersion at the ARC1 exit • will not be easy without BPMs; • fine tuning may need to be left for later • R56 control (cancellation) • not now ! • Sextupole tuning • again, better be left for later Set the ARC1 in the middle of its motion range

  10. ARC1 setting (coarse) • Measure a and b at the entrance to ARC1 • - not clear how to do that for a though … • Thread the beam through the ARC1 • - no big deal: just centre the beam on OTRs, one by one • - use DIPOLES for horizontal and VCOR for vertical • steering • Check beam centring in quads • Measure a and b at the exit of ARC1 • - that’s easy ! • Refining ARC1 settings • Set symmetric optic • - compare a and b at the entrance and exit • * beam size – identical • * a – equal but of opposite signs • - compare beam sizes on OTR-01 and OTR-02 • * here the beam is ~ round with b ~ 2m • * but remember !!!: Dx ~ 0.9m at OTR-01 / 02 • hence the image will NOT be round • - adjust quads • * no clear strategy on how to do this ! • Estimate the residual dispersion on OTR-01 (ST2) • * should be < 0.05m

  11. ARC1 : Dispersion suppression • Not easy without BPMs • Not of huge importance though at the first energy recovery stage • Do not have a clear strategy for the moment …. • Some notes however … • Make sure Dx is identical at OTR-01 and OTR-02 • - Dx ~ 0.9m • - adjust Q-01, -02, and -03 (but not Q-04 !) and make sure: • - beam moves in the same direction on both OTRs • - and by the identical distance (equal Dx ~ 0.9m ) • Check dispersion at Q-04 • - this must have a specific value (=1.05m) to • ensure dispersion cancellation after DIP-03 • - vary beam energy and look at OTR-02 and OTR-01 (ST2) • - direction of beam motion on OTRs – indication of Dx value at Q-04 • (and of Q-04 strength at the same time – unfortunately) • (opposite signs = too high Dx; same sign = too low Dx) After energy recovery, using BPMs – this should be much easier !

  12. ST2 & ST3

  13. ST2 & ST3 setting (coarse) • wiggler will not be installed during the initial stage of commissioning • good but … • we are loosing three OTRs (or just wedges ??) over there • hence : a long ~8m beamline without viewers whatsoever • here, we do not aim to produce a short bunch (linac cavities crested !) • AIMS: • get the beam to OTR-01(ST3) • how to get the “ideal” trajectory in the chicane ? • minimise dispersion in ST3 • make some basic beam measurements • - dispersion at OTR-03 (ST2) • - energy spread at OTR-03 (ST2) • - Twiss parameters at the entrance to ARC2

  14. ST2 & ST3 setting (coarse) 1. Set beam on axis between OTR-01 and OTR-02 b = 3m (x) / 12m (y) on OTR-01 b = 12m (x) / 7m (y) on OTR-02 2. Get the beam to OTR-03 - ideally, set DIP-01 then vary DIP-02 and VCOR-02 only - set Dx at OTR-03 to ~0.4m (vary DIP-01 and return beam to OTR-03 with DIP-02) - check for beam clipping (vary HVCOR-02) We now get the beam to the centre of the chicane with a correct dispersion value at OTR-03 but not necessarily with a correct Dx(s) function behaviour (may not be symmetric)

  15. ST2 & ST3 setting (coarse) 3. Get the beam to OTR-01 (ST3) no viewers over ~8m distance !! beam divergence after the chicane is not too large – hence try to catch the beam on OTR-01 (ST3) without quads 3.1 Use VCOR-04 and DIP-04 to get beam on OTR-01 (ST3) 3.2 Center the beam at Q-05 - vertical correction: VCOR-03 - horizontal correction : simultaneously DIP-03 and DIP-04 (keep an eye on OTR-01(ST3))!! 3.3 Set all quads to nominal values - re-centre the beam on OTR-01 (ST3) with HVCOR-04 - check centring in quads 3.4 Set beam size on OTR-01 (ST3) b ~ 1m (x) and ~20m (y)

  16. ST2 & ST3 setting (coarse) 4. Check dispersion in ST3 - use OTR-01 5. Estimate an energy spread on OTR-03 (ST2) 6. Measure Twiss parameters at the entrance to the ARC2 - not clear about a here …

  17. ARC 2 R56 = 0.28m

  18. ARC2 setting (coarse) ARC2 is identical to ARC1 • 180 deg. bend condition • Symmetric optics in the arc • Cancellation of dispersion at the ARC2 exit • will not be easy without BPMs; • fine tuning may need to be left for later • R56 control (cancellation) • not now ! • Sextupole tuning • again, better be left for later Nothing special to mention for the coarse setting of the ARC2

  19. ST 4 Linac

  20. ST 4

  21. ST4 setting (coarse) • Beam: • 80pC, ~1us, low energy spread • AIMS: • get the beam to OTR-01 (ST1) and to OTR-02 (ST1) through the linac • minimise dispersion after DIP-03 DO NOT TOUCH !!! • NOTE: • OTR-01 and OTR-02 in ST1 have 15mm diameter holes in them ! • for now, we have no idea of what is the RF phase in the return beam • hence, after linac, expect any beam energy from 8 to 60MeV • DIP-03 is set for 8.35MeV beam from the Injector; do not touch it !!!

  22. ST4 setting (coarse) OTR-02 • 1. Get beam to OTR-2 • try it with all quads OFF (by~ 60m on OTR-02) • use VCOR-02 (ARC2) and DIP-03 (ARC2) • set beam size on OTR-02 : beta ~ 6m (x) and ~10m (y) • (quads to nominal; adjust if necessary) • 2. Get beam to OTR-01 (ST1) • do not touch DIP-03 • remember the hole in OTR-01 ! • centre the beam on OTR-01 (ST1) • - use HVCOR-02 • - note corrector settings for the beam symmetric wrt the OTR centre • - set the average current in the correctors • 3. Get beam to OTR-02 (ST1) • scan HVCOR-02 (ST4) – remember the hole ! • do not use HVCOR-01 (ST1) – it will affect both 35MeV and 8MeV beams !!!

  23. ENERGY RECOVERY

  24. Energy Recovery (coarse) • AIMS: • get 180o phase difference between the outcoming and incoming beams • catch the beam in the dump • HOW: • move the ARC1

  25. Possible methods to set energy recovery • Vary TrainLength • (no energy drop if energy recovery) • Monitor BPM-01 (ST1) raw signal • should vanish if 8MeV and 35MeV beams are exactly at opposite phases • Monitor LLRF signals • similar to that used in coarse cresting of the cavities • Catch the beam on OTR-01 (DMP) • it is there if the return beam exits Linac at 8 MeV energy • The last method seems to be the easiest. • It is facilitated by the fact that if: • DIP-01(ST1) is set for 8.35MeV • we see the return beam on OTR-02 (ST1) • the return beam is energy recovered (i.e. ~8.3MeV) • the vacuum apertures in ST1 and dump sections allow to see the beam on • OTR-01 (DMP) without any clipping

  26. Energy Recovery (coarse) ARC1: +/- 60mm  240mm (note : l=232mm) What is the maximal step in ARC1 motion ? (so that not to miss the beam …) Beam Dump line angle: 200. OTR-01 (DMP) screen: 180mm diameter (phosphor covered thick plate)  max angle error: ~20mrad  DE/E ~6% (at 8MeV)  max ARC1 step : ~1.1mm (near zero-crossing but could be larger when closer to the crest) • Set correct path length for the return beam • move ARC1 until the return beam is at the centre of OTR-01(DMP) • Confirm energy recovery • increase TrainLength to 50-100us : beam should not get lost anywhere

  27. Energy Recovery (coarse) • 1. Initial settings • confirm the beam is centred in OTR-01/02 (ST1) • - inside holes • - use HVCOR-02(ST4), - not HVCOR-01 • Confirm Injector beam (8.35MeV) and DIP-01 are set correctly • - switch main linac off and see the beam on OTR-01 (DMP) • 2. Set the path length • move ARC1 • ensure the beam never leaves holes in OTR-01/02 (ST1) • set ARC1 position such that the beam is at the centre of OTR-01 (DMP) • make beam size >20mm FWHM on OTR-01 (DMP) • - present design: tight focussing on this screen ( damage at higher TrainLength)

  28. Energy Recovery (coarse) • 3. Confirm energy recovery • confirm beam size >20mm FWHM on OTR-01 (DMP) • increase TrainLength to ~10s of ms • we should not see beam losses or beam movements on screens

  29. Call the big boss Open champagne

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