1. Crystal Simulations: the road from the SPS to the LHC CERN, 8 March 2005 Valery M. Biryukov Institute for High Energy Physics Protvino, Russia
2. We demonstrate that a channeling crystal can serve as a primary scraper for the collimation system of the Large Hadron Collider. It has been proven both experimentally and in Monte Carlo simulations that crystal as a scraper meets technical requirements imposed on the LHC collimation system. Crystal scraper works in efficient, predictable, reliable manner with beams of very high intensity over years (IHEP) . If used as a primary element in the LHC collimation system, crystal makes the machine cleaner by a factor of 10 due to channeling with efficiency of about 90% — the figure already demonstrated experimentally by IHEP at 70 GeV and in simulations for the LHC and Tevatron . Main message from this talk and from the research of IHEP group:
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4. Multi-pass issues in crystal extraction were on theory agenda long before the experiment; see e.g. V.M.Biryukov, M.D.Bavizhev, E.N.Tsyganov. SSCL-N-776 (1991) “On the influence of imperfect surface on the multiturn extraction efficiency” V.M.Biryukov. NIM B 53 (1991) 202 "On the theory of proton beam multiturn extraction with bent single crystal"
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6. Soon it was found that “poor-surface” option explains all the findings at SPS. Prediction [1] for new, U-shaped crystal, agreed with the measurement [2]. Figure taken from ref. [3]. [1] V.Biryukov CERN SL/Note-78 (1993) [2] RD22 2-nd Status Report (1994) CERN-DRDC-94-11 [3] F. Ferroni et al (RD22 coll.) NIM A351 (1994) 183 70 fwhm 70 microrad
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8. As a result, short crystal was realised at IHEP, not at CERN ! Predicting a boost in efficiency was not trivial: other models did not predict it ... J. Klem: only 15% rise from 4 2 cm Biryukov: factor 2-3 rise in effy from shortenedcrystal.
9. IHEP: collimation / extraction efficiency for 70-GeV protons. Measurements (*, , ) and MC predictions (o) for perfect crystal IHEP started new experiment in 1997 1997 2000 1998 Crystals shortened by factor of 20 from SPS and Tevatron cases. Efficiency increased by factor of 5. 70 GeV
10. It took several years in IHEP to approach the target set by theory. First IHEP crystal was a kind of strip, 7 mm along the 70 GeV beam. Then we turned to analog of U-shaped crystals of SPS; the required deflectors designed in IHEP were cut and polished in the optical workshop of PNPI chosen among other workshops because of their long experience in channeling. The decisive step was invention in IHEP of strip-type deflectors, very short – down to ~2 mm along the beam, without straight parts and uniformly bent. We produced in IHEP many strip deflectors from commercially available wafers. Crystal systems extract 70 GeV protons from IHEP main ring with efficiency of 85% at intensity of 10 12 . Today, six locations on the IHEP 70-GeV main ring are equipped by crystal extraction systems, serving mostly for routine applications rather than for research
11. Although the way to ~90% efficiency was predicted by theory, the dramatic boost in crystal efficiency is fully due to the breakthrough in bent crystal technology in IHEP (Yuri Chesnokov) Further success - at the SPS, Tevatron and LHC - depends on Yuri. IHEP: Basic idea is the use of anticlastic bending. The real know-how behind the idea was thoroughly developed and tested.
15. Average over 2003 RHIC run measured crystal effy 26%, theory predicted 32% for Au ions. [PAC 2003 Proceedings] The same crystal gave 42% effy for protons at IHEP. [Phys.Lett.B 435 (1998) 240] CERN SPS measured effy 4 to 11% for Pb ions. [PRL 79 (1997) 4182] world first crystal collimation for heavy ions, top efficiency
16. FNAL Tevatron: the nearest to the LHC in energy. Good agreement observed (1998) with Monte Carlo predictions (1995)
17. Simulations of LHC crystal collimation We applied the same computer model verifi ed at the IHEP, CERN SPS, Tevatron, and RHIC experiments in order to evaluate the potential effect of crystal collimation for the LHC. In the model, a bent crystal was positioned as a primary element at a hor iz ontal coordinate of 6 in the halo of the LHC beam, on the location presently chosen for an amorphous primary element of the LHC collimation system design. The LHC lattice functions were taken corresponding to this location.
26. Crystals of low-Z and high-Z material are available, e.g. diamond and Ge: they demonstrate efficiency similar to Silicon
27. Nanostructured channeling material could be used for primary scraper V.M. Biryukov and S. Bellucci. Nucl. Instrum. Meth . B 230 (2005) 619
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29. MC study of p ossible designs for crystal scraper: S-type versus O-type Strip type crystal performs better than O-type by a factor of 1.60±0.05 according to MC simulations for crystal scraping in Tevatron. This conclusion agrees with the IHEP experimental practice where typical inefficiency achieved with S-type crystal has been 0.15 (i.e. efficiency of 85%) while O-type crystals have shown inefficiency of 0.35-0.60 (i.e. efficiency of 40-65%), i.e. performance factor of 2-4 weaker than that of S-type crystal deflectors. Simulations and experiment have identified s trip-type crystal s as the choice for collimation in the LHC, SPS, Tevatron
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31. You are invited to International Workshop on Relativistic Channeling and Coherent Phenomena in Strong Fields Frascati, 25-28 July 2005 http://www.lnf.infn.it/~bellucci/RC05.html The 2004 edition of Relativistic Channeling is in press in NIM B . See also report in S.Bellucci and V.Biryukov. CERN Courier July 2004, pp.19-20. The Relativistic Channeling 2005 will be published in NIM B again. Chairs: Stefano Bellucci and Valery Biryukov
32. Actually, the absolute figures of efficiency for CERN SPS are well described by analytical theory without any fitting parameters like “inefficient layer”. See: V.Biryukov, EPAC 1998 Proc., p.2091 . Table RD
Editor's Notes
The bending scheme for the "strip" crystal
Photograph of the deflected (left) and incident (right) beams as seen downstream of the crystal. Prior to the test, the crystal was exposed in the ring to 50-ms pulses of very intense beam (about 10^{14} proton hits per pulse). No damage of crystal was seen in the test, after this extreme exposure.
