In this article we perform a general study of possible positron source for linear collider TESLA based on the HERA electron ring. 30 GeV electrons passing 10 T wiggler can produce intense synchrotron radiation with characteristic energy of photons of about 6 MeV. High energy photons are converted to few-MeV positrons in a suitable target. The ability of such a source to fit for the TESLA requirements is discussed. 14 refs., 3 figs

Kiselev, A.V.

Shiltsev, V.D.

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F Fermi National Accelerator LaboratoryFERMILAB-Conf-97/112Proposed Intense Positron Source for TESLA Based on HERA-eA.V. KiselevINP630090, Novosibirsk, RussiaV.D. ShiltsevFermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510April 1997Presented at the Particle Accelerator Conference PAC ’97,Vancouver, Canada, May 12-16, 1997Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of EnergyDisclaimerThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any oftheir employees, makes any warranty, expressed or implied, or assumes any legal liability orresponsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specic commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United States Government or any agencythereof. The views and opinions of authors expressed herein do not necessarily state or reectthose of the United States Government or any agency thereof.DistributionApproved for public release; further dissemination unlimited.Proposed Intense Positron Source for TESLA Based on HERA-e A.V. Kiselev, Budker INP, 630090, Novosibirsk, Russia,and V.D. Shiltsev, FNALy , M.S.221, P.O.Box 500, Batavia, IL 60174AbstractIn this article we perform a general study of possiblepositron source for linear collider TESLA based on theHERA electron ring. 30 GeV electrons passing 10T wig-gler can produce intense synchrotron radiation with char-acteristic energy of photons of about 6 MeV. High energyphotons are converted to few-MeV positrons in a suitabletarget. The ability of such a source to fit for the TESLA re-quirements is discussed.1 INTRODUCTIONLinear collider TESLA [1] intends to operate with some1100 bunches in electron and positron beam each populatedwith about 3:7  1010 particles. Thus, the linac cycle of5Hz yields the production requirement of 2  1014 particlesper second. While one expects no severe problems with theelectron production, several types of the positron source op-tions are now under consideration (see e.g. [2, 3]) whichhave to fit in requirements of the TESLA linac and damp-ing ring [4]. In particular, one scheme assumes conversionof electrons from a powerful electron linac into positronsin a target [5], and the second concept is based on tar-get conversion of high-energy gamma quanta produced bya high-energy electron beam in short-period helical undu-lator [6]. Several more sophisticated positron productionschemes were discussed at Sources’94 Workshop [7].This paper presents a brief consideration of another pos-sible positron production option with use of existing highcurrent HERA electron storage ring - see general layoutin Fig.1. 30 GeV electrons pass some 10T magnetic wig-gler (presumably, superconducting) and produce powerfulsynchrotron radiation (SR) with the characteristic energyof photons of about 6 MeV. These photons are convertedinto positrons in a target and captured by special optics forfurther acceleration and injection into the TESLA dampingring.As the mean electron current in the storage ring is muchhigher than in electron linacs to be used for positronproduc-tion, then the ring-based scheme may have an advantage inobtaining the required positron production rate.2 STORAGE RING-BASED POSITRON SOURCEThe idea of positron production with use of SR from astorage ring is not new. For example, ring-based sourceswork is supported in part by DESY, Hamburgy Operated by Universities Research Association, Inc., under contractwith the U.S. Department of Energy                                                                       TargetpositronsBypassHERA-e orbitSRmagnet10TCapture opticsFigure 1: General layout of the proposed e+ source.of slow positrons for technological applications were pro-posed in Refs.[8, 9]. Few MeV positrons are much moresuitable for the Linear Collider sources. Below we discussseveral aspects of production of such particles.2.1 SR from Superconducting WigglerLet us estimate the rate of photon production in a wigglerwith a high magnetic field. We assume the undulator pa-rameter of the device K = max to be much bigger than1, therefore, the known formula of the photon flux densitycan be applied [10, 11]:dNdt d[photonssmrad]  3:96  1016E[GeV ] I[A]S("="c)"";(1)where I is the electron beam current, " is the photon energy,S("="c) is the spectral function [11] defined asS(x) =9p3x8Z1xK5=3(y)dy;Z10S(x)dx = 1: (2)The characteristic photon energy is"c = 2:218[keV ]E3[GeV ]R[m]; (3)the curvature radius R of electron trajectory in magneticfield B is equal toR[m] =3:33E[GeV ]B[T ]: (4)For B = 10T and energy of HERA-e E = 30 GeV,the radius is equal to R = 10m and characteristic photonenergy "c = 6 MeV that is much bigger than the e e+pair production threshold "th = 2m0c2  1:022MeV , orxth = ("th="c)  0:16.The integral ofR1xthS("="c) ' 1, thus, for the storagering with I = 50 mA the total flux of photons which canproduce electron-positron pairs is equal todNdt d[photonss mrad ]  6  1016: (5)The corresponding power of the SR is about 57kW=mrad. If we assume the that the total electron currentof 50 mA is distributed uniformly between 10 bunches,then each bunch produces some 1:3 1011 =mrad per sin-gle passage through the magnet with the HERA revolutionfrequency of 47 kHz.2.2 Positron ProductionThe process of positron production and interaction with thetarget material involves several physical phenomena (paircreation, ionization losses, scattering, etc.). Some analyt-ical estimations of the conversion efficiency can be foundelsewhere (see, e.g [8, 2, 9] and references there). Our fur-ther consideration is based on computer simulations [12].All the results below are obtained for tungsten target (3.5mm radiation length) under the following parameters: thedistance from the 8T magnet to the target is 10 m, the widthof the target is w =10 mm (therefore, horizontal angularsize of the target is  = 1mrad), its height is 0.3mm, thethickness of the target is 10 mm. In a good accordance withEq.(1), the code estimates the photon flux on the target of5 1016 photons per second in the photon energy range from1.022 MeV to 20 MeV. The conversion efficiency is foundto be of the order of Ne+=N ' 0:11. For SR from 10Tmagnet this value arises up to '0.13.Fig.2 and Fig.3 present kinetic energy and angular distri-bution of the positrons, correspondingly. One can see thatthe mean particles energy is about 2 MeV while the FWHMof the energy distribution is some 3 MeV. Positrons leavethe target in orthogonal plane to the orbit of HERA-e withbroad spread of angles ' =2 rad.2.3 Capture Efficiency and Injection SchemeThe efficiency of the capture optics and acceptance of fur-ther acceleration are key issues for the source. Major prob-lems to solve are to capture as many particles as possibleand, at the same time, to avoid unacceptable pulse length-ening (initially the rms length of the HERA-e bunch is about1 cm) when the pulse can not be accelerated as a whole.For the considered source, a two stage system could be pro-posed which consists of initial part with low frequency RF(e.g., 500 MHz) acceleration up to 20-30 MeV with largetransverse and longitudinal acceptance, weak longitudinalFigure 2: Kinetic energy distribution for positrons emittedfrom tungsten target 0.31010 mm under impact of SRphotons from HERA-e 8T wiggler.magnetic field in the section, as high as possible pulsedmagnetic field at the target position and/or other measureto focus positrons leaving the target. Then the bunch hasto be compressed in -magnet or other magnetic structurefor further acceleration to some 3 GeV in the high gradientS-band accelerating structure.We will characterize the capture efficiency by a factor of = N e+accelerated=Ne+produced. Assuming the angular widthof the target equal to , each of 10 HERA-e bunches passingthe wiggler with NP periods gives outcome of:Ne+ = 1:3 10110:13  NP  = 1:7 1010NP : (6)The TESLA damping ring requires bunch population of3:7  1010 which can be obtained with multi- or single-turninjection. At present time, the multiturn injection into thering does not look very attractive because due to limitedlinac shot cycle of 200 ms the damping time have to bereduced down to some 5–10 ms (compare with the designvalue of 37 ms [4]) that needs several times longer damp-ing wiggler and more powerful RF system.Thus, the better choice is single-turn bunch-by-bunch in-jection. Comparison of the design bunch population with(6) yields the requirement: NP    2:2: (7)Taking as an optimistic estimate the value  = 0:25, weget for the single pole wiggler   8:7 mrad. As an exam-ple, the target could be 2 cm wide and set at the distance of2.3 m from the wiggler. Corresponding wiggler pole lengthhas to be more than R = 8:7 cm. The total radiated SRFigure 3: Distributionover  for positrons from 1-20 MeVSR photons, tungsten target size 0:3  10  10 mm, allpositrons are emitted from target.power is equal to 0.5 MW which requires rotating or liquidtarget as it is assumed in other e+ sources for LC [2].If (7) is satisfied, then filling of the TESLA DR withuse of 10 bunches in HERA-e will take 110 revolutions, orabout 2.3 ms. It is much less than 200 ms of the TESLAlinac repetition time and allows to use the bypass with thewiggler in the pulsed regime while all other time the HERA-e ring can work for high energy physics experiments.2.4 Effects on the Electron BeamThe vertical tune shift and distortion of the amplitude func-tion due to installation of the wiggler are estimated to bey ' 0:001 and (y)=y  1 % that could be com-pensated by tuning the strength of nearby quadrupoles.While some 10 MeV energy loss of 30-GeV electrons inthe 9-cm long 10T wiggler is much less than the total lossesof 70 MeV/turn in HERA-e, than the effects on the elec-trons’ damping times, energy spread and emittances doesnot look drastic, although not negligible. Certainly, theseeffects are weak if the bypass will operates in the pulsedmode.Other important issues such as changes of dynamic aper-ture due to the wiggler installation and the electron beamstability in the presence of the SR absorbers, collimatorsand other discontinuities of vacuum chamber have to bechecked numerically as soon as more detailed design of thesource will appear.3 DISCUSSION AND CONCLUSIONSThe proposed scheme of positronproduction – through con-version of energetic photons of synchrotron radiation from30 GeV electrons of HERA in the target material – looksvery promising and fits into the TESLA requirements, al-though further detailed studies of the positron productionefficiency, the target, the capture optics and accelerationsystems optimization are necessary. The efficiency of thepositron capture looks the most important parameter to in-vestigate.An obvious disadvantage of the source is that it needs op-eration of the HERA-electron ring (although with a smallduty factor).As the 5 mA single bunch current looks to be an upperlimit for HERA-e [13], we would note a possibility of fur-ther increase of positron production with use of several-poles wiggler or a wiggler with higher field. An experiencewith constructionand exploitationof ”state-of-the-art” highfield magnets (see e.g. [14]) has shown a good field qualityin them.We would like also note that with the use of elliptical wig-gler the proposed scheme allows to obtain polarized pho-tons and therefore, polarized positrons. This topic needsfurther thorough numerical studies.Authors are thankful to P.Ivanov, G.Kulipanov, E.Saldin,R.Brinkmann, K.Flöttmann, N.Holtkamp, J.Rossbach, G.-A.Voss, N.Walker, H.Weise for useful discussions and valu-able suggestions.4 REFERENCES[1] R. Brinkmann, Proc. of IEEE PAC’95, Dallas (1995),see also DESY Internal Report M-95-08 (1995), pp.57-59.[2] K. Flöttmann, DESY 93-161 (1993).[3] J. Rossbach, NIM, A309 (1991), p.25.[4] V. Shiltsev, TESLA Print 96-02, DESY (1995).[5] F.Bulos, et. al, SLAC-PUB-3635 (1985).[6] V.E. Balakin and A.A. Mikhailichenko, Preprint INP 79-85,Novosibirsk (1979); see also in Proc. of XII Int. Conf. onHigh Energy Accelerators, Fermilab (1983), p.127.[7] R. Wanzberg, ed., Proc. of Int. Workshop on e+e  Sourcesand Preaccelerators for Linear Colliders, Schwerin, Ger-many (1994).[8] V.N. Belyaev et. al, Preprint MEPhI 026-92, Moscow,(1992).[9] A. Ando et. al, paper submitted to NIM A, (1995).[10] S. Krinsky, M.L. Perelman and R.E. Watson, in Handbookon SynchrotronRadiation, Vol.I, Horth Holland Publ.(1983).[11] H.Wiedemann, Particle Accelerator Physics II, Springer-Verlag, Berlin-Heidelberg (1995).[12] A.V. Kiselev, e+ production simulation code (Novosibirsk).[13] R. Brinkmann, private communication.[14] A. Grudiev,et. al, NIM A359 (1995), p.101.

Proposed intense positron source for TESLA based on HERA-e

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Proposed intense positron source for TESLA based on HERA-e

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