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  • 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.
  • Biryukov

    1. 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. 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:
    3. 3. Monte Carlo simulations have been helpful and often decisive for the progress in c rystal channeling from the beginning <ul><li>Two examples - two clues found in simulations: </li></ul><ul><li>Real crystal extraction is multi-pass thing, not a single-pass </li></ul><ul><li>(this changed radically the requirements to crystal and expectations of efficiency and angular range) </li></ul><ul><li>For extraction, crystal must be shortened dramatically </li></ul><ul><li>(crystals were shortened from 40 mm to 2 mm, bringing efficiency up from ~20% to 85%; SPS, Tevatron  IHEP) </li></ul>
    4. 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 &quot;On the theory of proton beam multiturn extraction with bent single crystal&quot;
    5. 5. This is why the prediction (1992) for the upcoming CERN SPS RD22 experiment we did in two options: poor surface=15% effy ideal surface=40% effy At this early time, nobody believed the poor-surface option. Crystal was claimed perfect within 50 Angstrem, beam hits expected in micron range w.r.t. surface. <ul><li>RD22 1992 </li></ul><ul><li>Status Report </li></ul><ul><li>gave only </li></ul><ul><li>lower limits: </li></ul><ul><li>effy>2-3% </li></ul><ul><ul><li>CERN-DRDC-92-51 </li></ul></ul>
    6. 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
    7. 7. <ul><li>Reminding the second clue found in simulations: </li></ul><ul><li>For extraction, crystal must be shortened dramatically </li></ul><ul><li>(crystals were shortened from 40 mm to 2 mm, bringing efficiency up from ~20% to 85%; SPS, Tevatron  IHEP) </li></ul>
    8. 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. 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. 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. 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.
    12. 12. World first demonstration of crystal collimation: IHEP (1998) <ul><li>Background </li></ul><ul><li>measured </li></ul><ul><li>downstream </li></ul><ul><li>of the scraper </li></ul><ul><li>(detectors 1,2) </li></ul><ul><li>vs crystal angle: </li></ul><ul><li>Factor of 2 gained </li></ul><ul><li>due to channeling </li></ul><ul><li>with 50% effy </li></ul>
    13. 13. IHEP: crystal collimation studied over full energy range 1 to 70 GeV
    14. 14. <ul><li>The crystal exposed to 50-ms pulses of very intense beam (10 14 proton hits per pulse). No damage seen. </li></ul>Part of IHEP research toward LHC: The IHEP crystal survives an instant dump of 1000 bunches of the LHC. IHEP crystals channel ~10 12 protons (up to 3·10 12 in some runs) in a spill of 0.5-1s. Let us illustrate it in the following way. Suppose, all the LHC store of 3·10 14 protons is dumped on our single crystal in 0.2 hour. This makes a beam of 4·10 11 proton/s incident on the crystal face. In IHEP, this is just routine work for crystal , practiced every day.
    15. 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. 16. FNAL Tevatron: the nearest to the LHC in energy. Good agreement observed (1998) with Monte Carlo predictions (1995)
    17. 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.
    18. 18. Simulations of LHC crystal collimation
    19. 19. Simulations with smaller bending, 0.1 mrad
    20. 20. Two bending options compared: 0.2 and 0.1 mrad
    21. 21. Efficiency vs bending angle. The corresponding range of crystals, 1 to 10 mm, is already produced and tested in IHEP.
    22. 22. Background suppression factor vs crystal bending
    23. 23. Channeling efficiency as a function of the crystal orientation angle. The orientation curve has FWHM = 7  rad at top energy.
    24. 24. Requirement on crystal surface at the LHC : weak dependence on it
    25. 25. Similar dependences were earlier found for the SPS: only weak dependence on surface quality
    26. 26. Crystals of low-Z and high-Z material are available, e.g. diamond and Ge: they demonstrate efficiency similar to Silicon
    27. 27. Nanostructured channeling material could be used for primary scraper V.M. Biryukov and S. Bellucci. Nucl. Instrum. Meth . B 230 (2005) 619
    28. 28. What kind of tests at the IHEP, SPS, Tevatron would be most valuable? Two kinds of experiment: collimation or extraction, and two (different) approaches can be pursued, focusing on either <ul><li>most efficient channeling: </li></ul><ul><li>location: smaller angle! ~1 mrad </li></ul><ul><li>take machine optics into account </li></ul><ul><li>find best crystal in simulations </li></ul><ul><li>produce crystal state of the art </li></ul><ul><li>b) modeling the LHC case as close as possible : </li></ul><ul><li>more difficult to identify the criteria! </li></ul><ul><ul><li>fix crystal angle? or crystal size? </li></ul></ul><ul><ul><li>model machine optics? </li></ul></ul><ul><ul><li>verify computer models in tests. </li></ul></ul>
    29. 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
    30. 30. Conclusion ( from IHEP research for LHC ) <ul><ul><ul><li>Crystal will be very efficient in the LHC environment. The expected </li></ul></ul></ul><ul><ul><ul><li>efficiency figure, ~90%, is already experimentally demonstrated by IHEP </li></ul></ul></ul><ul><ul><ul><li>and confirmed by simulations for the Tevatron. This will make the LHC </li></ul></ul></ul><ul><ul><ul><li>10 times (up to ~40 times) cleaner. </li></ul></ul></ul><ul><ul><ul><li>Monte Carlo model successfully predicts the crystal work in the circulating beam, as </li></ul></ul></ul><ul><ul><ul><li>demonstrated recently in crystal collimation experiments at IHEP and RHIC, and in </li></ul></ul></ul><ul><ul><ul><li>crystal extraction experiments at up to 900 GeV (the Tevatron). </li></ul></ul></ul><ul><ul><ul><li>Crystal works efficiently at very high intensities (~10 12 ), actually much higher than </li></ul></ul></ul><ul><ul><ul><li>the LHC requires, with a lifetime of many years. </li></ul></ul></ul><ul><ul><ul><li>Crystal survives the abnormal dump of the LHC beam with ~100-fold safety margin </li></ul></ul></ul><ul><ul><ul><li>(i.e. survives the instant dump of 1000 LHC bunches or ~10 14 protons) </li></ul></ul></ul><ul><ul><ul><li>as demonstrated experimentally at 70 GeV (IHEP). </li></ul></ul></ul><ul><ul><ul><li>The same crystal scraper works efficiently over full energy range , from injection </li></ul></ul></ul><ul><ul><ul><li>through ramping up to top energy, as demonstrated experimentally by IHEP from 1 </li></ul></ul></ul><ul><ul><ul><li>through 70 GeV and as seen in simulations for the LHC. </li></ul></ul></ul><ul><ul><ul><li>Bent crystals of low-Z and high-Z material are available, e.g. diamond and germanium, </li></ul></ul></ul><ul><ul><ul><li>and they demonstrate the efficiency similar to that of silicon. </li></ul></ul></ul><ul><ul><ul><li>Even when a crystal is misaligned, nonchanneling, it still works as an amorphous </li></ul></ul></ul><ul><ul><ul><li>scatterer so the collimation system returns to its traditional scheme. This makes it safe . </li></ul></ul></ul>
    31. 31. You are invited to International Workshop on Relativistic Channeling and Coherent Phenomena in Strong Fields Frascati, 25-28 July 2005 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. 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