Akbari, H

Elsener, K

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Results on Proton Extraction from the CERN-SPS with a bent CrystalH. Akbari et al. (RD22 collaboration) [1]presented by K. ElsenerCERN SL-Division, CH-1211 Geneva 23, SwitzerlandAbstractExtraction of protons from a high energy accelerator bymeans of a bent crystal could be an attractive tool to providean external beam at hadron colliders. A test experiment at theCERN-SPS, using a 120 GeV coasting proton beam and a3 cm long bent silicon crystal, has been performed. Diffusionof protons onto the crystal has been created by exciting anelectrostatic device with bandlimited white noise. First resultsshowed an extraction efficiency of about 10%. Recent devel-opments using a new crystal with improved geometricalproperties are discussed, and future experiments outlined.1.  INTRODUCTIONThe possible extraction of high energy proton beams fromthe future TeV hadron colliders by means of channeling in abent crystal has been discussed for several years [2,3]. Thecrystals would be placed in the halo of a circulating beam, andthe extraction process be entirely parasitic to collideroperation. The space needed for such an extraction is minimal,and the strong fields in the crystal allow deflection angles ofthe order 1 mrad even at multi-TeV proton energies.Channeling of high energy protons and beam deflection inbent silicon crystals have been studied in detail at high energybeams, and deflection efficiencies of up to 50% have beenfound for a parallel beam of 450 GeV protons [4,5].Extraction experiments using crystals have been successfullyperformed at lower energies [6,7]. Here, we report on anexperiment with a 120 GeV coasting proton beam and a bentsilicon crystal at the CERN-SPS. The aim of this experimentis an understanding of the extraction process, its dependenceon machine parameters and crystal properties and the meas-urement of the properties of the extracted beam as well as theextraction efficiency.2.  EXPERIMENTA schematic view of the experimental arrangement isshown in Fig. 1. Two bent silicon crystals, which can be usedalternatively, are installed in a vacuum tank on goniometersallowing the adjustment of the angle of the crystal with thebeam. For a first series of measurements, the crystals (shownin Fig. 2) were 18 mm high, 1.5 mm thick and 30 mm long inbeam direction, bent on a "Serpukhov-type" bending device[6]. The bend angles are adjusted with a differential screw andare set to be 8.5 mrad. The crystals are cut parallel to the(110) planes with an accuracy better than 200 µrad, thenpolished and chemically etched to remove the damagefrom the cutting. Finally, the crystals were mechano-chemi-cally polished (Syton technique) to obtain an optically flatsurface.The protons are extracted horizontally towards the centerof the SPS and detected 20 m downstream of the crystal (cf.Fig. 1). A scintillating screen (CsI) equipped with a CCDcamera is used to get a two dimensional view of the extractedbeam on a video screen in the SPS control room. A scintillatorhodoscope with 32 horizontal and 32 vertical strips (1 mmwide) is used for monitoring of the beam profiles, while twosets of microstrip gas chambers (MSGC) with a pitch of 200µm, placed 1 m apart, are used to measure the divergence andthe profiles of the extracted beam. Three scintillation countersare used in coincidence as a trigger for these detectors.Hodoscope  8.5 mrad22 m120 GeV/cCrystals3 cm SiFIGURE 1 : Schematic view of the experimental apparatus used inthe SPS extraction experiment with a bent crystal.  C1 and C2 aretwo crystals mounted on goniometers. The detectors for the extractedbeam are located 22 meters downstream of the crystals. For details,see text.For the measurements, a 120 GeV coasting beam with anintensity of about 5·1011 protons was used. The SPS showshardly any non-linear effects at this energy and little naturaldiffusion, resulting in a beam lifetime of more than hundredhours. The normalized beam emittances (2V values) are 6-8mm·mrad horizontally and vertically for an unperturbed beam.The crystal was placed at 10 mm from the closed orbit,corresponding to 6 - 10 times the RMS transverse beam size,where only a few halo particles are found when no noise isapplied. The beam is excited horizontally with band limitedwhite noise induced on a pair of condenser plates. Randomkicks producing a typical deflection of about 0.001 µrad(RMS) are applied. The horizontal beam size slowly increasesand protons diffuse towards the crystal. The horizontalemittance is typically 60 mm·mrad when the crystal is reachedby the particles with large amplitudes, while the verticalemittance remains unchanged (<10 mm·mrad). The statisticalnature of this process allows reliable calculations and asimulation of the diffusion process. Mean impact parametersof the protons on the crystal are  estimated to be in themicrometer rangeFIGURE 2 : The silicon crystal  bent on the "Serpukhov type"bending device. The bending is adjusted with a differential screwbefore mounting in the vacuum tank3.  INITIAL RESULTSFirst measurements were performed with bent crystals ofthe type shown in Fig. 2. The results are reported in detail inref. [8]. For protons to be channeled and extracted, the crystalhas to be aligned with the beam axis to within ± 14 µrad [4]. Itwas therefore a surprise to find some extracted beam over anangular range of about 200 µrad (FWHM). Moreover, asshown in Fig. 3, the profiles of the extracted beam showeddouble peaks, when the crystal angle was set to be away fromthe maximum of extracted beam intensity. Both effects can bequalitatively explained: The mounting of the crystals, asshown in Fig. 2, produces an unwanted curvature in the ver-tical plane, i.e. transverse to the plane of beam extraction.Moreover, the direction of the (110) planes at both ends of thecrystal varies as a function of the vertical position (anticlasticbending). The observed profiles can also be reproduced bysimulations [8,9].FIGURE 3 :  Beam profiles of the extracted beam as measured withthe hodoscope of scintillation counters, (a) at the maximum extrac-tion efficiency and (b) off-maximum.In order to measure the extraction efficiency for the opti-mum crystal alignment, the beam has to reach a "steady state"diffusion mode, i.e. when the extracted beam intensity doesnot vary with time. The efficiency is determined as the ratio ofprotons lost from the SPS for a given time interval. Thenumber of protons in the circulating beam and the proton fluxonto the crystal is obtained form intensity monitors and beamlifetime measurements. The rate of protons extracted isdetermined by integrating the beam profiles measured by thehodoscope after subtracting the background. The combinedefficiency of hodoscope and trigger is found to be about 80%.Typically, for three different crystals of the type shown inFig. 2, about 6·105 protons have been extracted per second,corresponding to an extraction efficiency of about 10%. Fur-ther details are given in ref. [8].4. RECENT RESULTSFollowing the measurements with the silicon crystals in the"Serpukhov type" bending devices (Fig. 2), a new crystal wasproduced and installed in the SPS vacuum. This so called "U-shaped" crystal is shown in Fig. 4. It is cut out of a block ofsilicon, with "feet" at both ends, allowing to bend the siliconcrystal with two differential screws. The stiff ends avoid theunwanted curvature at the entrance and exit faces of thecrystal, and careful adjustment of the two screws allowsbending to 8.5 mrad without any longitudinal twist. Thecrystal surface used for extraction is cut parallel to the (110)plane within less than 25 µrad.Differential screws5 mm1.5 mm40 mmFIGURE 4 : Schematic drawing of the new "U-shaped" siliconcrystal, cut out of a block of silicon, including the "feet" needed tobend it. Bending is achieved with two differential screws.First results with this new crystal are encouraging: theangular scans have become considerably narrower (FWHM of70 µrad) and the double peaks in the beam profiles havecompletely disappeared (Fig. 5). The extraction efficiencyfound is again about 10%. Further experiments will be per-formed in order to fully explore the properties of this newcrystal, and to understand the spurious instabilities in thebehavior of the extracted beam profiles and intensities.5.  FUTURE PLANSThe extraction of a 120 GeV proton beam being achievedwith a good efficiency, and the initial problems of crystal ge-ometry being solved, it is now essential to address the basicquestion of the extraction mechanism leading to the observedbeam profiles and the 10% extraction efficiency. Basically,two mechanism are being discussed: first-pass extraction andmulti-pass extraction.FIGURE 5 : Horizontal and vertical beam profiles measured with thehodoscope for the "U-shaped" crystal, cf. Fig. 3.First-pass extraction would correspond to the experimentalsituation in an external beam [4], the only difference being thesmall impact parameters of the protons onto the crystal, thusthe importance of the surface quality (see Fig. 6). Note that anefficiency of about 40% would be expected for pure first-passextraction and a perfect crystal surface.Multi-pass extraction is inherent to the situation in a cir-cular accelerator: protons which are not channeled during thefirst passage of the crystal will undergo multiple scattering,usually with small angles, and will thus have "further chances"at later turns to be channeled and extracted when passing thecrystal inside the critical angle for channeling. This extractionmechanism has been predicted and may be inferred from someof the observations in the present experiment – no directexperimental proof for the existence of multi-pass extractionhas yet been found.bb'scatteredchanneledBeamimpingingimpact parameter : bimpact angle : b'FIGURE 6 : Importance of the surface quality for first-pass channel-ing and extraction.Very recently, another crystal has been installed in the SPSvacuum tank. It is a crystal bent as shown in Fig. 2, which hadalready been successfully used in this experiment to extractprotons. Now, an amorphous silicon oxide layer of about30 µm thickness has been evaporated onto the crystal surfaceused for extraction. Therefore, given the small impactparameters of typically 1 µm in our experiment, this crystalwould not be useful for first-pass extraction. Hopefully in thenear future, an experiment with this crystal should showwhether multi-pass extraction exists and what is its relativecontribution to the extraction efficiency of 10% observed ear-lier.Apart from the possibly larger extraction efficiency[10,11], the importance of multi-pass extraction would be two-fold. First, the size of the impact parameters of the protons be-comes irrelevant: mutiple scattering will occur whenever theprotons reach the crystal. Therefore, for a prediction of theextraction probability with a bent crystal at future accelerators,a detailed knowledge of the diffusion mechanism would nolonger be important, and no artificial control of the diffusionwould be needed. Second, the requirements on the crystalsurface quality become less stringent — the surface is used asa scatterer rather than for channeling and extraction.Finally, simulations have shown that the machine opticscan be adapted in order to optimize the multi-pass extractionefficiency: for a given tune value Q, an optimal E can befound [11]. Clearly, once multi-pass extraction is established,this would be the next step to pursue experimentally.6.  CONCLUSIONSThe first experiments on proton extraction from theCERN-SPS with a bent crystal have been successful. An ef-ficiency of about 10% has been regularly achieved. For a moredetailed understanding and further optimization of theextraction process, the first hardware changes have beenmade: a crystal with an amorphous layer should allow ameasurement of the contributions from multi-pass extraction.More experiments and simulations will be needed in order tostudy the feasibility of extracting beam with a crystal fromLHC or other high energy accelerators.7.  REFERENCES [1] H. Akbari, X. Altuna, S. Bardin, V. Biryukov, B. Dehning,K. Elsener, G.F. Ferioli, A. Ferrari, G.P. Ferri, G. Fidecaro,R. Guinand, M. Gyr, W. Herr, A. Hilaire, J. Klem, V. Mertens,S. Péraire, M. Placidi, W. Scandale, R. Schmidt,B. Vettermann, E. Weisse, S. Weisz, S.P. Møller, E. Uggerhøj,A. Freund, R. Hustache, A. Calcaterra, R de Sangro,O. Palamara, R. Bellazzini, A. Brez, G. Carboni, F. Costantini,M.M. Massai, P.F. Vita, F. Ferroni, S. Morganti,R. Santacesaria, M.P. Bussa, L. Busso, F. Tosello,  L. Lanceri,G. Vuagnin and K. Maier.[2] R.A. Carrigan et al., Nucl Instr Meth  B48 (1990) 167.[3] W. Scandale, Experimental insertion for the LHC, in:  LargeHadron Collider Workshop (Aachen, 1990), CERN 90-10,ECFA 90-133, Vol. III, p. 760.[4] S.P. Møller et al., Nucl. Instr. Meth. B84 (1994) 434,and references therein.[5] K. Elsener et al., these proceedings.[6] A.A. Asseev et al., Nucl. Instr. Meth. A330 (1993), 39.[7] A.A. Asseev and E.A. Myae, these proceedings.[8] H. Akbari et al., Phys. Lett. B313 (1993) 491.[9] V. Biryukov, these proceedings.[10] A.M. Taratin et al., Nucl. Instr. Meth. B58 (1991) 103.[11] W. Herr, CERN SL/92-53 and S. Bardin, SL/Note 92-52 (AP).

Results on proton extraction from the CERN-SPS with a bent crystal

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