The first proof-of-principle experiment on "crystal collimation" was performed with 70-GeV protons on IHEP accelerator. A bent crystal installed in the ring as a primary element upstream of a collimator has reduced the radiation levels downstream in the accelerator by a factor of two. The measurements agree with Monte Carlo predictions

Afonin, A.G.

Biryukov, V.M.

Breese, M.B.H.

Chepegin, V.N.

Chesnokov, Yu.A.

Fedotov, Yu.S.

Ivanov, Yu.M.

Kotov, V.I.

Scandale, W.

Terekhov, V.I.

Troyanov, E.F.

CERN Document Server

arXiv:hep-ex/0111045v1  13 Nov 2001Crystal Collimation Experiment on 70-GeV ProtonAcceleratorA.G.Afonin, V.M.Biryukov, V.N.Chepegin, Yu.A.Chesnokov,V.I.Kotov, V.I.Terekhov, E.F.Troyanov, Yu.S. FedotovInstitute for High Energy Physics, Protvino, RussiaYu.M.Ivanov, Nuclear Physics Institute, St.Petersburg, RussiaW.Scandale, CERN, Geneva; M.B.H.Breese, U. of Surrey, UKProc. of 7th ICFA mini-workshop (Lake Como 1999), pp.127-130AbstractThe first proof-of-principle experiment on ”crystal collimation” was per-formed with 70-GeV protons on IHEP accelerator. A bent crystal installedin the ring as a primary element upstream of a collimator has reduced theradiation levels downstream in the accelerator by a factor of two. The mea-surements agree with Monte Carlo predictions.1 IntroductionBent-crystal technique is well established for extracting high energy beams from accel-erators. It was successfully applied at the energies up to 900 GeV[1], and simulationswere able to predict the results correctly. Recent experiments at IHEP Protvino[2] havedemonstrated that this technique can be quite efficient: 50-70% of the beam have beenextracted using a thin (3-5 mm) Si channeling crystal with bending of 0.5-1.5 mrad,with intensity of the extracted 70-GeV beam up to 6×1011 protons per spill. At thisintensity, no cooling measures were taken and no reduction in the efficiency observed.At IHEP Protvino this technique has been routinely used since 1987 to deliver a 70GeV beam to particle physics experiments. One of the IHEP crystals did extract 70GeV protons over 10 years since 1989 without replacement and without any degradationseen! It was shown in the experiments at BNL AGS and at CERN SPS that radiationdamage in channeling crystals is sizable only at over (2-4)×1020 proton/cm2.The theory of crystal extraction is based mainly on detailed Monte Carlo simulationstracking the particles through a curved crystal lattice and the accelerator environmentin a multipass mode. Our code CATCH was successfully tested in the extraction exper-iments at CERN, FNAL, and IHEP in 1992-99[3]. Monte Carlo predictions, suggestinga ”multipass” mode of crystal extraction where efficiency is dominated by the multi-plicity of particle encounters with a short crystal, have lead to the breakthrough in theextraction efficiency demonstrated at IHEP Protvino[2].1Crystal can channel a charged particle if it comes within so-called critical angle θc,about ±150 µrad/√pv(GeV ) in silicon. This restricts crystal efficiency in divergentbeams. However, if a crystal is installed in a circulating beam, particle may scatter ininefficient encounters and have new chances on later turns. To benefit from the ”multi-pass” channeling, the crystal must be short enough to reduce beam losses in multipleencounters with it.It should be promising to apply this bent-crystal technique for a beam halo scrapingin accelerators and storage rings[4, 5]. A bent crystal, serving as a primary element,should bend halo particles onto a secondary collimator. A demonstration experimentof this kind was performed at IHEP where for the first time a significant reduction inthe accelerator background was obtained with a bent crystal incorporated into beamcleaning system[2].A crystal collimation system for a gold ion beam is now being installed at RHIC incollaboration with IHEP[6], and –upon success– it will serve there on permanent basis.2 Crystal DeflectorBending a short crystal to be installed in the accelerator vacuum chamber is not easy.The first crystal used in the course of our experiment of 1997-1999 was of Si(111) typeand performed as a short plate of a big height, 0.5×40×7 mm3 (thickness, height, andlength along the beam direction, respectively). It was bent transversally with a metalholder which had a hole of 20 mm size for beam passage, and gave the channeled protonsa deflection of 1.7 mrad. Despite an angular distortion (a ”twist”) in that design,encouraging results on beam extraction were obtained in our first run in December1997, Figure 1. The peak extraction efficiency reached about 20% and the extractedbeam intensity was up to 1.9×1011 [7]. Here and later on in the paper, the extractionefficiency is defined as the ratio of the extracted beam intensity as measured in theexternal beamline to all the beam loss in the accelerator.To further increase the extraction efficiency, further crystals (without twist) weremade from a monolithic Si piece in a shape of ”O” at the Petersburg Nuclear PhysicsInstitute, as described in Ref. [8]. The crystals Si(110) used in our recent runs had thelength along the beam direction of only 5 mm. The bent part of the crystal was just 3mm long, and the straight ends were 1 mm each.Such a crystal, with bending angle of 1.5 mrad, was successfully tested in March1998 and has shown extraction efficiencies over 40% [8]. In the mean time we havechanged the crystal location in order to use another septum magnet (with partitionthickness of 2.5 mm instead of 8 mm as in the old scheme) where a smaller bendingangle is required from a crystal. This change was also motivated by the intention to testeven shorter crystals (two of them, 2.5 and 3.0 mm long, are already undergoing tests).The crystal used in this location was new, but of the same design and dimensions asearlier described[8]. The bending angle used in this run was 0.65 mrad.2✉✉✉✉❡❡❡❡25 50 7520406080100F(%)∆I(%)Figure 1: Spill-averaged efficiency of extraction as measured with 5-mm crystal 0.65mrad bent (•), December 1998; 5-mm crystal 1.5 mrad bent (+), March 1998; 7-mmtwisted crystal 1.7 mrad bent (o), December 1997; plotted against the beam fractiontaken from the accelerator.3 Study of Crystal Work in Slow-Extraction ModeExperiments on crystal-assisted slow extraction and scraping are very similar on thepart of crystal component, the only difference being the target of the channeled deflectedbeam — is it an external beamline or beam absorber. This is why we were able to studythe crystal work first in the conditions of slow extraction where we could measure theamount and characteristics of the channeled beam more easiely.The general schematics of beam extraction by a crystal is shown in Ref.[8]. Asthe small angles of deflection are insufficient for a direct extraction of the beam fromthe accelerator, a crystal served as a primary element in the existing scheme of slowextraction. Crystal was placed in straight section 106 of the accelerator upstream of aseptum-magnet of slow-extraction system. The accuracy of the crystal horizontal andangular translations was 0.1 mm and 13.5 µrad, respectively. The horizontal emittanceof the circulating proton beam was about 2π mm×mrad, and the beam divergence atthe crystal location was 0.6 mrad. A local distortion of the orbit by means of bumpwindings in magnets moved the beam slowly toward the crystal. To obtain a uniformrate of the beam at crystal, a monitor for close loop operation based on a photomultiplierwith scintillator was used to automatically adjust the orbit distortion. We used alsofunction generator to control current in bump windings.The beam deflection to the septum and its transmission through the beam line ofextraction were supervised with a complex system of beam diagnostics, including TVsystem, loss monitors, profilometers, intensity monitors[8]. All the diagnostics devices3were firstly tested in fast-extraction mode and calibrated with beam transformers. Thebackground conditions were periodically measured with and without crystal. Accordingto the measurements, the fraction of background particles (e.g. elastically scatteredprotons) together with the apparatus noise did not exceed 4% of the useful signal level.This background was subtracted from the efficiency figures shown in the paper. Thefraction of the beam directed to the crystal was defined as the difference between themeasurements of the circulating beam intensity done with beam transformers before andafter the beam extraction, with the systematic error of 1%. The extraction efficiencywas evaluated in every cycle of acceleration.4 Crystal EfficiencyThe accelerator beam intensity during the experiment was about 1.3×1012 protons percycle. The fraction of the circulating beam incident on the crystal ∆I was varied from20 to 90%. The spill duration of the channeled beam in the feedback regime was on theorder of 0.5 s. The plateau of the IHEP U-70 accelerator magnet cycle is 2 s long whilethe overall cycle of the machine is 9.6 s. Figure 1 shows the efficiency of extractionaveraged over the spill, as measured in our three experiments of 1997-98. In the lastone, the efficiency was about 50% even when all the accelerator beam was directed ontothe crystal. The spill-averaged efficiency figures were reproducible with 1% accuracyfrom run to run. The dependence of the extracted beam intensity on orientation of thecrystal was about the same as in Ref.[8] and not shown here. The highest intensity ofthe extracted beam, for 1.15×1012 protons incident at the crystal in a cycle, was equalto 5.2×1011.As the beam moves radially toward the crystal, the proton incidence angle driftsat the crystal. For this reason the extraction efficiency varies in time during the spill,especially for a large beam fraction used. The peak extraction efficiency in a spillwas always greater than 60%. The absolute extraction efficiency as obtained in ourMonte Carlo simulations agree with the measurements to accuracy of about 5% forspill-averaged figures.5 Crystal Collimation ExperimentBent crystal, situated in the halo of a circulating beam, can be the primary element in ascraping system, thus serving as an ’active’ collimator. In this case, the only differencefrom extraction is that channeled particles are bent onto a secondary collimator insteadof the extraction beamline. The bent particles are then intercepted (with sufficientlybig impact parameter) at the secondary element and absorbed there.We have performed the first demonstration experiment on crystal-assisted collima-tion. A bent crystal, with the same dimensions as the extraction crystals describedabove and with bending angle of 1 mrad, was positioned upstream of a secondary colli-mator (stainless steel absorber 4 cm wide, 18 cm high, 250 cm long) ”FEP” and closerto the beam in the horizontal plane. As the horizontal betatron tune is 9.73 in ouraccelerator, it was most convenient to intercept the bent beam at FEP not immediately4⋄⋄⋄•••◦◦◦⋆⋆⋆∗ ∗ ∗0 5 10 1501234567I (rel.)Distance from collimator (m)Figure 2: Radiation levels as monitored at three places along the ring in the vicinityof FEP, for different cases (bottom up): ∗ - beam kicked onto absorber by a kickermagnet; ⋆ - aligned crystal as primary; ◦ - FEP works as primary; • - misalignedcrystal as primary; ⋄ - Si target downstream of FEP is primary.on the first turn, but after 3 turns in the accelerator. In this case the deflection angleof 1 mrad transforms into more than 20 mm horizontal offset, and so the bent beamenters the FEP collimator at some ∼15 mm from the FEP edge. The optimal horizontalposition of the crystal w.r.t. the FEP edge was found to be ∼10 mm.Radiation levels were monitored at three places along the ring in the vicinity of FEP,from ∼2 to ∼10 meters downstream of the backward edge of the collimator. Severaldifferent cases have been studied.• The whole accelerator beam was kicked into the middle of the FEP face by akicker magnet. That’s an ideal case for a beam interception and absorption, so theresulting radiation levels (nonzero due to escape of some primary and secondaryparticles from the FEP body) can be considered as a kind of pedestal for theresults obtained then with several actual scraping methods. These lowest levelsare shown in Figure 2 by (∗) marks.• When FEP was a primary element scraping the beam halo continously, the haloparticles were entering FEP very close to its edge (at sub-micron depths) so theescape of particles from FEP body because of outscattering was very important.This resulted in higher radiation levels (◦) as shown in Figure 2.• A bent silicon crystal was introduced then about 60 cm upstream of the forwardedge of FEP. Crystal served as a primary element of the scraping system, beingcloser to the circulating beam than FEP, with the offset of about 5-15 mm in theradial plane. When the crystal was misaligned, it was acting as an amorphoustarget scattering particles. The collimator downstream could intercept some ofthe scattered particles. The radiation levels measured (•) in this setting were notso different from the preceding case of direct (by FEP) scraping of the beam halo.• When the crystal was aligned to the best angle w.r.t. the incident beam, a sub-stantial number of halo particles was channeled and deflected into the depth of5FEP for best absorption. The monitored radiation levels with aligned crystalserving as primary element are shown (⋆) in Figure 2. One can conclude thatabout one half of the halo was extracted and forwarded to a safe place (i.e. themiddle of FEP face) for absorption, reducing the radiation background in the ringcorrespondingly.• Finally, another case studied was a silicon target (amorphous) positioned down-stream of FEP as a primary element. In this case the machine was not pro-tected from the scattered particles originating in the target, so the radiation levelsachieved (⋄) were the highest.1 ⋆⋆⋆⋆⋆⋆⋆⋆⋆⋆⋆⋆⋆⋆3 ssssssssssssssssssss2 ❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝❝–0.5 0 0.5302010I(rel.)Angle (mrad)Figure 3: Measured irradiation in detectors 1, 2, 3 as function of crystal angle.Figure 3 shows how the radiation level depends on the angular alignment of thecrystal. At the best crystal angle, preferable for channeling, the radiation levels decreaseby up to factor of ∼two in the places of monitoring. This is explained by the fact that∼50% of the incident beam is channeled by the crystal and deflected to the depth ofFEP where absorbed. In the case when crystal was out and the beam was scrapeddirectly by FEP, the radiation at the monitors was at about the same level as in thecase of disaligned crystal.We were able to check the crystal efficiency figure by alternative means, measuringthe profile and intensity of the particles incident at the FEP entry face. The channeledbeam had a narrow profile and was well distanced from the FEP edge, as shows Figure4 where this profile is shown in comparison with the profile of the accelerator beamdeflected onto FEP by a kicker magnet. From comparison of the two profiles, fromcrystal and from kicker, we again derived the crystal efficiency, which was found to beabout 50%, in agreement with the radiation monitoring figures and with the earliershown figures of extraction efficiency with crystal in straight section 106.60 10 20 30051015N(rel.)x (mm)Figure 4: Profiles measured at FEP entry face: the channeled beam (thick line) andthe beam (thin line) deflected by kicker magnet.6 ConclusionsThe crystal-assisted method of beam steering (for scraping or slow extraction) demon-strates peak efficiencies in the order of 60-70% and shows reliable, reproducible andpredictable work. Crystal can channel at least 5-6×1011 ppp with no cooling measurestaken and no degradation seen.In our experiment this technique was for the first time demonstrated for scraping ofthe beam halo. Such application has been studied by computer simulation for severalmachines, notably RHIC [6] and Tevatron [9]. We have shown that radiation levels inaccelerator can be significantly decreased by means of channeling crystal incorporatedinto beam cleaning system as a primary element.We continue tests with crystals as short as down to 1 mm, where Monte Carlopredicts 80-90% efficiency of steering. We study different techniques to prepare bentcrystal lattices with required size, one of the most interesting approaches is describedin Ref.[10].7 AcknowledgementsThe author thanks very much the conference Organizers, Nikolai Mokhov and WeirenChou, for kind hospitality and support.References[1] Nucl. Instr. and Meth. B 119 (1996), 231 C. T. Murphy et al. [E853 Collaboration],“First results from bent crystal extraction at the Fermilab Tevatron,” Nucl. Instrum.Meth. B 119, 231 (1996).[2] A.G.Afonin, V.M.Biryukov, V.N.Chepegin et al., ”New projects of crystal extractionat IHEP 70-GeV accelerator”. Particle Accelerator Conference Proceedings (NewYork, 1999); CERN LHC 99-2 (MMS), Geneva 1999. [arXiv:hep-ex/0111028].7[3] V.M. Biryukov, Yu.A. Chesnokov and V.I.Kotov, ”Crystal Channeling and its Ap-plication at High Energy Accelerators”. Berlin: Springer (1997) V. M. Biryukov,V. I. Kotov and Yu. A. Chesnokov, Phys. Usp. 37, 937 (1994).[4] M.Maslov, N.Mokhov, I.Yazynin, ”The SSC Beam Scraper System”, SSCL-484(1991)[5] V.M.Biryukov, Near Beam Physics Symp. Proceedings (Fermilab 1997). Fermilab-Conf-98/166, pp.179-184 (1998). [arXiv:hep-ex/0110074].[6] D. Trbojevic et al., “A study of RHIC crystal collimation,” EPAC Proceedings(Stockholm, 1998), p.2146 [arXiv:hep-ex/0111021].[7] A. G. Afonin et al., JETP Lett. 67, 781 (1998) [Pisma Zh. Eksp. Teor. Fiz. 67, 741(1998)].[8] A. G. Afonin et al., Phys. Lett. B 435, 240 (1998).[9] V.M.Biryukov, A.I.Drozhdin, N.V.Mokhov. ”On Possible Use of Bent Crystal toImprove Tevatron Beam Scraping”. PAC Proceedings (New York, 1999). Fermilab-Conf-99/072 (1999).[10] M.B.H.Breese, NIM B 132 (1997) 540[11] More papers on the subject of crystal collimation/extraction are available athttp://beam.ihep.su/crystal.html8

Crystal collimation experiment on 70-GeV proton accelerator

Crystal collimation experiment on 70-GeV proton accelerator

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