Progress in Crystal Extraction and Collimation
V.Biryukov,A.Afonin, V.Baranov, V.Chepegin,Y.Chesnokov, V.Kotov, V.Terekhov,
E.Troyanov,IHEP-Protvino, Russia;V.Guidi, G.Martinelli, M.Stefancich, D.Vincenzi,Ferrara Un., Italy;
Yu.Ivanov, PNPI, Russia;D.Trbojevic, BNL,USA;W.Scandale, CERN;M.Breese, Surrey Un., UK
Abstract
Recent IHEP Protvino experiments show efficienciesof crystal-assisted slowextraction and collimation of
85.32.8%, at the intensities of thechanneled beam on
the order of 10 12 proton per spillof 2 s duration.Theobtained experimental data wellfollows the theory predictions. We compare the measurements against theoryand
outline the theoretical potential for further improvement intheefficiency of the technique. This success is important
for the efficient useof IHEP accelerator and for implementation of crystal-assisted collimationat RHIC and slow extraction from AGS onto E952, now in preparation.Futureapplications, spanning in the energy from order of 1 GeV(scraping inSNS, slow extractio from COSY and medicalaccelerators)to order of 1 TeV and beyond (scraping inTevatron, LHC, VLHC), can benefit from these studies.

1 INTRODUCTION
Two major applications of crystal channeling in modern
hadronaccelerators are slow extraction and halo collimation(see e.g. [1] and refs therein).The benefits of crystal
extraction are fourfold.In hadron colliders this mode of extraction can be made compatiblewith the colliding mode ofoperation.The time structure of the extracted beam is practically flat,since the extraction mechanism is resonancefree.The size of the extracted beam is smaller.Finally polarized beams can be extracted withoutdetrimental effectson the polarization.The benefits of crystal-assisted scraping we discuss in the next section.These applications canbe exploited in a broad range of energies,from sub-GeV
cases (i.e. for medical accelerators) to multi-TeV machines(for high-energy research). Indeed,several projects
are in progress to investigate them.Crystal collimation is
being studiedat RHIC (100-250 GeV) [2],for the Tevatron(1000 GeV) [3]and the LHC [4],for the Spallation Neutron
Source (1 GeV) [5],whilstcrystal-assisted slow extraction
is consideredfor COSY (1-2 GeV)and AGS (25 GeV) [6].

2CRYSTAL AS A SCRAPERClassic two-stage collimation system for loss localisationin
accelerators typically uses a small scattering target as aprimaryelement and a bulk absorber as secondary element [7].The role of the primary element is to give a substantial angular kickto the incoming particles in order toincrease the impact parameteron the secondary element,
which is generally placed in the optimum positiontointercept transverse or longitudinal beam halo.Naturally, anamorphous target scatters particles in all possible directions.Ideally, one would prefer a ”smart target” that kicks
all particles inonly one direction: for instance, only in radial plane, only outward,and only into the preferred angularrange corresponding to the center ofabsorber (to excludeescapes).Bent crystal is the first idea for such a smart target:it traps particles and conveys them into the desireddirection.In physics language, we replace the scattering on
single atomsof amorphous target by the coherent scattering
on atomic planes ofaligned monocrystal.

3CHANNELING FFICIENCY

It’s been long argued theoretically[8]that a breakthroughin crystal efficiency can be due tomultiple character ofparticle encounters with a crystalinstalled in a circulating beam.To clarify this mechanism an extraction experiment was started at IHEP Protvinoat the end of 1997(see Ref.[9, 10]and refs therein).In the last two years, wedemonstrated crystal channeling with50% efficiency.We
also showed that these crystals couldbe efficiently used as
primary collimators, thereby reducingby a factor two theradiation level measured downstream of the collimation region of U-70[9, 10].To continue our investigations,we installed and tested in U-70 ring several new crystalsproduced by different manufacturers with a new shape.Theazimuthal length of the Si(111) crystals was only 1.84.0 mm,bending angle 0.8 to 1.5 mrad.The advantagesof ”new-generation” crystals are threefold:(a) they can bemade shorter than a usual bulk crystal,(b) they have nostraight ends, since the bending mechanism is continuous,and (c) they have no amorphous material close to thebeam.The ne technology allows us to control precisely thecrystallength and bending radius.Two crystals were assembled in Protvino:one 2 mm long was bent by 0.9 mrad, theother 4 mmlong was bent by 1.5 mrad.The third crystal 1:8mm long bent by 0.8 mrad was built and polishedin theUniversity of Ferrara (Italy).The two Russian crystals wereused in extraction mode,whilst the Italian one wastested asa primary collimator.The three crystals were exposed to 70
GeV proton beams andused to channel and extract halo particles.
Fig.1 illustratesthe beneficial effect of crystalswhen usedas primary collimators.We present beam profiles in the radial planedownstream of the crystal itself,recorded with theprofile-meter of Ref.[9]. The coordinateR represents theradial displacement referredto the crystal edge.Four cases
are reported.In first one, an amorphous collimator is used as
primary targetwhilst the close-by crystal is kept outside of
the beam envelope.As expected, the beam profile is peaked

I
100
8060
40
20Extraction efficiency (%)

(a)

100
8060
40
20400c??c cc__|__|60?
__|__|(b)40
2000 2.5 5 7.5 1012.5 R(mm)Iccccc c c800 2.5 5 7.5 1012.5 R(mm)I100(c)300

35674Crystal length (mm)Figure 2: Crystal extraction efficiency as measured for70-GeV protons.Recent results ( , strips 1.8, 2.0, and 4mm),1999-2000 (|____| , O-shaped crystals 3 and 5 mm),and
1997 ( , strip 7 mm).Also shown (o) is Monte Carlo prediction [7]for a perfect crystal.
12?

200
100
0 2.5 5 7.5 1012.5 R(mm)

I(d)

200
100
0 2.5 5 7.5 1012.5 R(mm)Figure 1: Beam profiles measured at the collimator entryface:(a) crystal out;(b) crystal in, but misaligned;(c) crystal
in the beam, aligned;(d) bea kicked by magnet.

at the collimator edge (Fig. 1(a)).In the second case (Fig.
1(b)) thecrystal is used as the primary scraper,whilst theamorphous target is retracted.No care is taken toalign crystal with respect to the beam direction,hence its action on theincoming protons is very similar to thatof an amorphoustarget.When properly aligned (see Fig. 1(c)), the crystal
channels mostof the incoming beam and displaces their distribution byabout 10 mm inside the crystal edge.In the lastcase (see Fig. 1(d)),the beam is simply kicked by a magnet
towards the secondary collimator,whilst the primary target
is retracted.
The channeling efficiency is given bythe ratio of the extracted bea intensity, as measured inthe external beam line,
to all the beam loss, as measured in the entire ring;seethe diagnostics part of the experiment describedin refs.
[9, 10].We obtained very high channeling efficiencies ineach of thethree new crystals: namely, both the 1.8 an 2

mm long crystals reached85% efficiency, whilst the 4mmlong crystalreached 68% efficiency.In Fig.2 we plot theexpected (the prediction published in[10]) and the measured channelingefficiencies together with data relative toan old O-shaped crystal.The agreement between measurements and simulations is excellent.Fig.2 shows the theoretical potential for channeling efficienciesof 90-95% when
we manage a crystal deflector with the size optimalfor ourset-up.These unprecedented results were indeed obtainedin a steady mannerover many runs. In particular, the 2 mmlong crystal was regularlyfunctioning to extract beams witha channelling efficiency of 85.32.8%.

4 HIGH-INTENSITY TESTS
Beside the channeling efficiency, also important arestanding a high beam intensity andcrystal lifetime.Crystals located in the region upstream of the U-70 cleaning area
wereirradiated with the entire circulating beam, spilled out
in rathershort time durations to simulate very dense halo
collimation.We can measure preciselythe beam intensity
intentionally damped intothe crystal. However, we canonly estimate with computer simulationsthe total amountof particle hits during a spill, sinceunchanneled protons
are simply scattered andmay continue to circulate in thering hitting the crystal many times.The number of hits per
primary particle can vary from a fewto more than hundred.Such analysis has shown thatour crystals were irradiated up to2  1014 particlesper spill of 1 s duration.When
averaged over machine cycles,the irradiation rate was ashigh as2  1013 proton hits/s.Notice that this irradiation
ratealready exceeds the expected beamloss rate at the Spallation Neutron Source. Indeed,the SNS Accumulator Ring
should generate a 1 GeVproton fluxof 60210 14 per second. At the expected rate ofbeam loss of 0.1% the halo flux
will be1.210 13 protons/s.Several crystals in use in U-70

have been exposed to high intensity beamsfor months.Afterthe irradiation of 10 20p/cm2 the initial channelling efficiency was practically unaffected.

N1.3 GeV

300
200
100

0

2.55

7.5

1012.5R (mm)Figure 3: Beam profile as measured on the collimator entry
facewith 1.3 GeV protons.In black is shown the simulated
profile of channeled protons.

5COLLIMATION AT 1.3 GEV

On the same location in U-70with the same 1.8-mm crystal of Si(111)positioned 20 m upstream of the ring collimator,we have repeated the crystal collimation experiment at the injection flattop of U-70,proton kinetic energy of 1.3 GeV.With the crystal aligned to the incoming
halo particles,the radial beam profile at the collimator entry faceshowed a significant channeled peak far from theedge, Fig.3.The expected width of the channeled peakis
about 5 bins in the profileof Fig.3, in agreement with observations.About half of the protons intercepted by the collimator jaw,have been channeled there by a crystal; i.e.,crystaldoubled the amount of particles intercepted by thejaw.As only part (about 34%) of all particles scattered
off the crystalhave reached the jaw, we estimate thecrystal deflection efficiency as 15-20%.The observedfigure ofefficiency could be well reproduced in computer simulations.This figure is orders of magnitude higher than prevous world datafor low-GeV energy range.It is remarkablethat the same crystal was efficiently channelingboth at 70
GeV and at 1.3 GeV,thus demonstrating to be operational
in a very wide energy range.

6CONCLUSION

The crystal channeling efficiency has reached unprecedented high valuesboth at top energy and at injection energy.The same 2 mm long crystal was used to channel 70
GeV protonswith an efficiency of 85.32.8% during several weeks of operationand 1.32 GeV protons with an efficiency of 15-20% during some test runs.Crystals with asimilar design were able to stand radiation dosesover 10 20

proton/cm 2and irradiation rates of 2  10 14 particlesincident on crystal in spills of 2 s durationwithout deterioration of their performances.The efficiency results wellmatch the figures theoretically expectedfor ideal crystals.
As simulations show, extractionand collimation with channeling efficiencies over 90-95% is feasible.The obtained
high figures providea crucial support for the ideas to apply this technique in beam cleaningsystems, for instance in
RHIC and Tevatron.Earlier Tevatron scraping simulations[3] have shown that crystalscraper reduces acceleratorrelated background in CDF and D0experiments by a factor of 10.This year, first experimental data is expectedfrom RHICwhere crystal collimator [2] is installed.Thetechnique presented here is potentially applicable also inLHC forinstance to improve the efficiency of the LHC
cleaning system by embeddingbent crystals in the primary
collimators[4].This work is supported by INTAS-CERN
grant.

7

REFERENCES[1] V.M.Biryukov, Yu.A.Chesnokov, and V.I.Kotov,CrystalChanneling and its Application at High EnergyAccelerators.
(Springer, Berlin: 1997)[2] D.Trbojevic, V.Biryukov et al.,in EPAC Proceedings (Stockholm, 1998)[3] V.M.Biryukov, A.I.Drozhdin, N.V.Mokhov.1999 PAC (NewYork).Fermilab-Conf-99/072 (1999).
[4] CERN-INTAS project ”Crystal Technique for Halo Cleaning
in the LHC”approved 2001.
[5] N.Catalan-Lasheras,
A.Fedotov,D.Gassner,J.Wei,V.Biryukov,Y.Fedotov,V.Terekhov.EPAC’2000
Proceedings (Vienna)[6] J.W.Glenn, K.A.Brown, V.M.Biryukov, PAC’01[7] J.B.Jeanneret, Phys. Rev. ST Accel. and Beams 1, 081001(1998)[8] V.Biryukov.Nucl.Instr.Meth. B 117 (1996) 463and refs
therein.
[9] A.G.Afonin, V.M.Biryukov, V.N.Chepegin, et al.,1999 PAC(New York).CERN LHC 99-2 (MMS).
[10] V.I.Kotov, A.G.Afonin, V.I.Baranov, et al.,EPAC’2000 Proceedings (Vienna).
[11] More papers on the subject of crystal collimation/extractionare available at http://beam.ihep.su/ b̃iryukov/