Biryukov

CERN 8 March 2005 V.M. Biryukov: Crystal Simulations 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


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:


Monte Carlo simulations have been helpful and often decisive for the progress in crystal channeling from the beginning

Two examples - two clues found in simulations:

a) Real crystal extraction is multi-pass thing, not a single-pass (this changed radically the requirements to crystal and

expectations of efficiency and angular range)

b) For extraction, crystal must be shortened dramatically (crystals were shortened from 40 mm to 2 mm, bringing

efficiency up from ~20% to 85%; SPS, Tevatron IHEP)


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"


This is why the prediction (1992) for the upcoming CERN SPS RD22 experiment we did in two options:poor surface=15% effyideal 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.

RD22 1992Status Reportgave only lower limits:effy>2-3%

CERN-DRDC-92-51


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

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SPS angular scan with U crystal

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Biryukov (1993)

fwhm 70microrad


Reminding the second clue found in simulations:

b) For extraction, crystal must be shortened dramatically (crystals were shortened from 40 mm to 2 mm, bringing

efficiency up from ~20% to 85%; SPS, Tevatron IHEP)


As a result, short crystal was realised at IHEP, not at CERN !

Predicting a boost in efficiency was not trivial:Predicting a boost in efficiency was not trivial:other models did not predict it ...other models did not predict it ...

J. Klem:only 15% rise from42 cm

Biryukov: factor 2-3rise in effy fromshortenedcrystal.

SPS extraction, 120 GeV

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SPS (1995)


70 GeV

IHEP: collimation / extraction efficiency for 70-GeV protons.

Measurements (*, , � ) and MC predictions (o) for perfect crystal

IHEP started new experiment in 1997IHEP started new experiment in 1997

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Crystalsshortenedby factorof 20 fromSPS andTevatroncases.Efficiencyincreasedby factorof 5.


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 1012. 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


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 ofanticlastic bending.The real know-how behindthe idea was thoroughlydeveloped and tested.


World first demonstration of crystalcollimation: IHEP (1998)

Backgroundmeasureddownstreamof the scraper(detectors 1,2)vs crystal angle:

Factor of 2 gaineddue to channelingwith 50% effy


IHEP: crystal collimation studied over full energy range 1 to 70 GeV


• The crystal exposed to 50-ms pulses of very intense beam (1014 proton hits per pulse). No damage seen.

Part of IHEP research toward LHC:

The IHEP crystal survives an instant dump of 1000 bunches of the LHC.

IHEP crystals channel ~1012 protons (up to 3·1012 in some runs) in a spill of 0.5-1s.

Let us illustrate it in the following way. Suppose, all the LHC store of 3·1014 protons is dumped on our single crystal in 0.2 hour. This makes a beam of 4·1011 proton/s incident on the crystal face. In IHEP, this is just routine work for crystal, practiced every day.


Average over 2003 RHIC runmeasured 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 effy4 to 11% for Pb ions.[PRL 79 (1997) 4182]

world first crystal collimation for heavy ions, top efficiency


FNAL Tevatron: the nearest to the LHC in energy. Good agreement observed (1998) with Monte Carlo predictions (1995)


Simulations of LHC crystal collimation

We applied the same computer model verified 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 horizontal 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.


Simulations of LHC crystal collimation

LHC crystal collimation (0.2 mrad)

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Simulations with smaller bending, 0.1 mrad

LHC crystal collimation (0.1 mrad)

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Two bending options compared: 0.2 and 0.1 mrad

LHC crystal collimation (7 TeV)

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Efficiency vs bending angle. The corresponding range of crystals, 1 to 10 mm, is already produced and tested in IHEP.


Background suppression factor vs crystal bending


Channeling efficiency as a function of the crystal orientation angle. The orientation curve has FWHM = 7 rad at top energy.

LHC crystal collimation, 7 TeV

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Requirement on crystal surface at the LHC: weak dependence on it


Similar dependences were earlier found for the SPS:only weak dependence on surface quality

SPS, 120 GeV

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SPS, U crystal


Crystals of low-Z and high-Z material are available, e.g. diamond and Ge: they demonstrate efficiency similar to Silicon

LHC collimation, 7 TeV

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Nanostructured channeling material could be used for primary scraperV.M. Biryukov and S. Bellucci. Nucl. Instrum. Meth. B 230 (2005) 619


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

a) most efficient channeling:• location: smaller angle! ~1 mrad• take machine optics into account• find best crystal in simulations• produce crystal state of the art

b) modeling the LHC case as close as possible: more difficult to identify the criteria!

• fix crystal angle? or crystal size?• model machine optics?• verify computer models in tests.


MC study of possible 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 strip-type crystals as the choice for collimation

in the LHC, SPS, Tevatron


Conclusion (from IHEP research for LHC)Crystal will be very efficient in the LHC environment. The expected efficiency figure, ~90%, is already experimentally demonstrated by IHEP and confirmed by simulations for the Tevatron. This will make the LHC 10 times (up to ~40 times) cleaner. Monte Carlo model successfully predicts the crystal work in the circulating beam, as demonstrated recently in crystal collimation experiments at IHEP and RHIC, and in crystal extraction experiments at up to 900 GeV (the Tevatron).Crystal works efficiently at very high intensities (~1012), actually much higher than the LHC requires, with a lifetime of many years.Crystal survives the abnormal dump of the LHC beam with ~100-fold safety margin (i.e. survives the instant dump of 1000 LHC bunches or ~1014 protons) as demonstrated experimentally at 70 GeV (IHEP).The same crystal scraper works efficiently over full energy range, from injection through ramping up to top energy, as demonstrated experimentally by IHEP from 1 through 70 GeV and as seen in simulations for the LHC.Bent crystals of low-Z and high-Z material are available, e.g. diamond and germanium, and they demonstrate the efficiency similar to that of silicon.Even when a crystal is misaligned, nonchanneling, it still works as an amorphous scatterer so the collimation system returns to its traditional scheme. This makes it safe.


You are invited to

International Workshop on Relativistic Channelingand 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


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

Biryukov

Technology

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