POSITRON SOURCES FOR FUTURE COLLIDERS

POSITRON SOURCES FOR FUTURE COLLIDERS
Robert CHEHAB LAL-Orsay With collaboration of i.Chaikovska and H.Guler(lal), X.Artru, M.Chevallier (IPNL) and P.Sievers (CERN) R.Chehab/positrons/Frascati

INTRODUCTION The challenges for future positron sources: * Needs for powerful positron sources for e+e- colliders * Importance of the low positron emittance * High reliability required example of the SLC source * Importance of heat load and thermal shocks  A presentation of the different ways to produce polarized and unpolarized positrons will be given. Emphasis will be put on a particular way, using channeling radiation in crystals, on which LAL,IPN and BINP have been working since many years. Examples of positron sources considered for e+e- colliders ILC and CLIC will be shown. Some applications to future μ+μ- colliders will be briefly considered. R.Chehab/positrons/Frascati

PLAN A-* Photon generation for positron sources A-a Polarized photons A-b Unpolarized photons B -* Photon generators considered for future e+e- colliders C- *Applications to future μ+μ- colliders ( some words only ) D-* Summary and conclusions R.Chehab/positrons/Frascati

A-a-1- PHOTON GENERATION FOR POLARIZED POSITRON SOURCES 1-1 The magnetic helical undulator The circularly polarized photons are emitted in an angle ~ 1/γ. The photon wavelength  is given by: = (u/2γ2 )(1 + K2) Where K =eBu/2mc is the undulator strength, with u, the undulator period. For u = 1 cm, we need an e- beam energy of 150 GeV to get 30 MeV photons useful to create e+e- pairs in a conversion target.  Necessity to reach very high incident electron energies due to the usual ~cm value of the undulator period. R.Chehab/positrons/Frascati

A-a-1-2 COMPTON BACKSCATTERING With λ = (λu/2).[-2 +(Θ)2]/(1 + cos) The electrons with relative energy γ are colliding with a circularly polarized laser ( ) The Compton photons may have 30 to 60 MeV as average energy if the e- beam has 1.3 to 1.8 GeV. This beam is provided by: a ring, a linac or an ERL. As the Compton cross section is low, to increase the positron intensity we may: - use a multi-cavity optical system (5 to 10) - improve the stacking of e+ bunches in the DR by reducing the longitudinal emittance prior to the injection in the DR. R.Chehab/positrons/Frascati

A-a--1-3 POLARIZED BREMSSTRAHLUNG: Within a high Z target, longitudinally polarized e- radiate circularly polarized photons. Within the same target, circularly polarized photons create longitudinally polarized e+e- Such possibility has been studied by A.P.Potylitsin and A.A.Mikhailichenko and E.G.Bessonov Experiment has been operated at JLAB : a strained AsGa PK was illuminated by a polarized laser. Polarized electron beam of 8.5 MeV was, then, impinging on a 1 mm amorphous W target > Asymmetries were measured with a Compton polarimeter. e+ maximum polarization level was: ~ 82% for a 85% polarized electron [E.Voutier et al/JLAB: Proceedings of IPAC 2013, Shanghai, China]. AsGa PK Amorphous target Positron pre-accelerator LINAC Production of polarized positrons via bremsstrahlung laser R.Chehab/positrons/Frascati

A-a--1-4 POLARIZED POSITRONS FROM THE COLLISION OF 2 REAL PHOTONS A circularly polarized laser photon is colliding with an unpolarized high energy photon. The hard photon may be produced by the radiation of high energy electrons in a crystal. A threshold condition is: ωh.ωs ≥ m2 With a Nd:YaG laser (green) hard photons of ~120 GeV are needed. In turn, the electron energy must be higher. Such a scheme has been considered but not deeply analyzed. [ see V.N.Baier et al. NIMA 338 (1994)156] R.Chehab/positrons/Frascati

A-b- UNPOLARIZED POSITRON SOURCES A-b-2-1 CONVENTIONAL SOURCES USING BREMSSTRAHLUNG The impinging particle on the target is an electron; it initiates an e-m shower with a continuous photon spectrum, from soft to hard photons. Needs for an intense positron beam leads to intense incident electron beam and thick target of high Z number (W, for instance). Positron beam emittance related to important multiple scattering and thermal problems are, then, of concern. We present two examples; Multiple thin targets  easier escape for low energy e+ R.Chehab/positrons/Frascati

A-b-2-2 SEPARATE TARGETS The radiator may be an amorphous target or an oriented crystal. In the last case, photons due to channeling radiation, coherent bremsstrahlung and ordinary bremsstrahlung generate e-e+ pairs in the amorphous converter. This concept is leading to the so-called hybrid source where the radiator is an axially oriented crystal and the converter an amorphous tungsten material. Such a device has been studied by LAL-IPNL group with collaborations from BINP, Tomsk U, KEK and CERN, since many years. R.Chehab/positrons/Frascati

A-b-3 PHOTONS FROM CHANNELING IN ORIENTED CRYSTALS  The average potentials in the neighborhood of crystal atoms when an incident electron penetrates the crystal at glancing incidence to the rows/planes makes the electron “trapped” in the potentials and it radiates quite intensively (Kumakhov). The radiation becomes much more intense than ordinary bremsstrahlung at energies > 1 GeV, for W. Channeled electrons are then a powerful source of photons The W crystal with <111> axial orientation has been chosen. Experiments made at Orsay (1992), CERN ( ) and KEK have validated the simulations. Enhancements in photon and positron production as high as 4 (w.r.t. amorphous targets of the same thickness) have been obtained with 4 mm thick crystals at E-=10 GeV. At CERN (WA 103) R.Chehab/positrons/Frascati

A-B-3-AXIAL CHANNELING OF RELATIVISTIC ELECTRONS  Axial potentials are generally 5 to 10 times stronger than planar potentials. Moreover,the radiated energy is proportional to the square of the channeling field; that explains why axial channeling is preferred for γ-radiation in a positron source. In the case of W, one of the most used crystal for this purpose, the potential depth is of 1 kV at normal temperature. Provided the angle of incidence of the e- on the atomic rows is smaller than the Lindhard critical angle,  Ψc =[2U/E]1/2 ,  where U is the potential depth and E, the electron energy, the e- will develop a rosette motion and as in a magnetic wiggler, will radiate. The frequency of the radiated photon is given approximately by:  ω =2γ2Δ ET  where γ is the Lorentz factor and ΔET is the transverse energy loss between channeled states. Typically, ΔET is of some eV and for 1 GeV e- beam, we can obtain 40 MeV photons, which represents an interesting energy to produce e+e- pairs When the incident electron energy on the crystal is moderately high (some GeV) the pair creation process in the crystal is determined by Bethe-Heitler mechanism. For very high energies (E> 20 GeV, for W), pair creation in strong field is occurring leading to very strong enhancements (CERN, A.Belkacem et al in NIMB 13 (1986)9-14) R.Chehab/positrons/Frascati

A-b-3 ADVANTAGES OF CHANNELING RADIATION FOR POSITRON SOURCES Channeling radiation is providing a large number of soft photons: much more than with bremsstrahlung in the same conditions (E-, kind of target material, thickness,..).These soft photons are, consequently, generating soft positrons much easily captured in known matching devices. An example showing the enhancement is given here. With the following conditions: E-= 8 GeV; Crystal thickness: 1 mm we observe a strong enhancement in the region of soft photons. The vertical scale is expressed in E.(dN/dE) BS radiation is almost constant as following 1/E behaviour. R.Chehab/positrons/Frascati

A-b-4 Some features of channeling radiation used in hybrid e+ sources To illustrate some peculiarities of channeling radiation we use the results of a study made by S.Dabagov et al (Journal of Surface investigations, X-ray, synchrotron and neutron techniques 2016, Vol.10, no1, p ) with the SPARC parameters: The comparisons between different incident e- energies and between CR and Bremsstrahlung show: * e+ spectrum is softer for CR than for Brems. At 200 MeV: the major part of the yield, for CR, is made of (1-3 MeV) e+, whereas it represents only 10% for Brems. Case . This result would also be confirmed at higher incident energies if we consider soft photons with an extended interval. * At 200 MeV, the Yield(Brems) > Yield (CR); at 800 MeV, the yield obtained with CR becomes higher than that from Brems. Such result is confirming the predictions of V.N. Baier et al; they showed that the energy radiated by channeling is becoming larger than for Bremsstrahlung at MeV for W crystal. Using different crystals as Ge, C(d) or Si, this threshold should be higher. R.Chehab/positrons/Frascati

A-B-5 COHERENT BREMSSTRAHLUNG
 At angles larger than the Lindhard critical angle, the electrons crossing planes or strings of atoms at regular steps, emit radiation with interference effects. These photons a have larger energy than that due to channeling (the period of oscillation is ~a/θ, where a is the inter-atomic distance and θ, the incidence angle; it is, typically, 10 times smaller than for channeling). The photons can , in turn, generate e-e+ pairs in the same crystal or in an associated amorphous converter. R.Chehab/positrons/Frascati

COHERENT BREMSSTRAHLUNG: STRING OF STRING ORIENTATION If the direction of the incident particle (e-) : has a small angle with a cristallographic axis * is parallel with the plane formed by the atomic strings along the chosen axis,  String of String (SOS) orientation. The obtained radiation has the following features: # production of harder photon spectrum than CB due to successive scattering off the axial potential which is stronger than the planar potential. # production of a soft part in the spectrum (Planar) The strong enhancement in high energy photons may be interesting for the generation of e+e- pairs and μ+ μ- pairs. From V.Tikhomirov (Medenwalt) R.Chehab/positrons/Frascati

B-1-PLANAR UNDULATORS Unlike the helical undulator for which the length must be important enough for the yield and, especially, for the polarization, the planar undulator (linear polarization) may have some meters only (depending on the wished yield). Polarization is not considered for the associated positron source. R.Chehab/positrons/Frascati

PHOTON SOURCES FOR FUTURE COLLIDERS
B-1 -2 PHOTON GENERATORS CONSIDERED FOR THE FUTURE COLLIDERS 1- ILC: THE MAGNETIC HELICAL UNDULATOR A SC helical undulator is made of two SC wires wound around a tube with currents flowing in opposite directions. The undulator period and the electron orbit period are the same: λu. Circularly polarized photons are emitted in a conical angle θ ~1/γ around the e- direction of motion . The number of photons is proportional to the number of periods and the polarized photons are near the axis. collimation needed. ILC: L=147 m, λu = 11.5 mm; beam aperture: 5.85 mm; e+ target at 400 m from undulator; Incident electron energy: 15o/250 GeV Undulator strength K: 0.92/0.45 photon energy is : 10/42 MeV (1st harm.) Photon beam Power: 63/42 kW R.Chehab/positrons/Frascati

ILC POSITRON SOURCE BASELINE Number of e-/bunch: ; Number of bunches/pulse: 1312; Repetition rate; 5 Hz Target (Ti) thickness; 0.4 Xo (1.4 cm) Incident photon spot size on target; 1.4 mm (150 GeV e-) Number of positrons at IP: /bunch PEDD in target: from J/g depending on incident E- R.Chehab/positrons/Frascati

B-1-3 POSITRON SOURCES USING CHANNELING: THE HYBRID SOLUTION Though channeling radiation and pair creation (BH) could be realized in a whole crystal, it is really interesting to separate the two processes, taking a thin crystal for the radiator and an amorphous disk for the conversion into pairs. The advantage is to limit the heating in the crystal (higher potentials at low temperature) and to decrease the energy density in the converter by using a distance between the two targets and a magnet to sweep off the charged paricles (e+e-) coming out from the crystal Such a solution has been chosen for future colliders; it is the baseline for CLIC e+. E-: 5 GeV; crystal thickness 1.4 mm; amorphous W converter: 10 mm thick; distance between targets: 2 meters R.Chehab/positrons/Frascati

B-1-3 OPTIMIZATION OF THE HYBRID SOURCE: THE GRANULAR CONVERTER Instead of a compact amorphous converter, a granular target made of small tungsten spheres 1-2 mm diameter arranged in staggered rows has a lot of advantages: - better heat dissipation ~ surface/volume of the ball ; improves with decrease of radius - lower energy deposition density Such a system was previously proposed for neutrino factories (Targets submitted to high power proton beams) by P.Sievers et al. A systematic study started with the PhD work of Chenghai XU at LAL-Orsay Example: we got for W targets giving almost he same yield For E-= 10 GeV; 6 layers ; W crystal (1 mm) Deposited energy: compact; 520 MeV/e- Granular ( 1 mm radius spheres): 400 MeV/e- PEDD: compact: 2.20 GeV/cm3/e- Granular (1 mm radius) : 1.4 GeV/cm3/e- photons R.Chehab/positrons/Frascati

ADVANTAGES OF THE GRANULAR VS COMPACT CONVERTER A comparison has been operated between granular and compact converters giving the same yield. Elementary equal volumes (spheres-granular ; cylinders-compact) were chosen to determine the energy deposition density in the “smallest” volumes. The figure is showing the advantage of the granular converter. Such quantity expressed in GeV/cm3/e- are providing the actual density with the chosen incident electron beam expressed in J/g which value might not overcome 35J/g (for W), after the analysis of SLC target breakdown. R.Chehab/positrons/Frascati

AN HYBRID POSITRON SOURCE FOR CLIC The hybrid positron source parameters for CLIC are presented on the figure (right) The equivalent solution with a granular converter requires 6 layers of staggered W spheres layers. Simulations have been worked out on this scheme. Total yield is 8 e+/e-; deposited energy is 250 MeV/e- and the peak Energy Deposition Density PEDD is 0.7 GeV/cm3/e- The advantage of such scheme is to get soft photons, which in turn generate soft positrons more easily captured in known matching systems. R.Chehab/positrons/Frascati

SIMULATION RESULTS FOR CLIC Number of layers cumulated Accepted: 1e+/e- after 200 MeV using an AMD capture system after the converter. R.Chehab/positrons/Frascati

AN HYBRID POSITRON SOURCE FOR ILC The main problem for ILC is the very high intensity of the beam with a large duration (~ 1 ms). A solid target will not survive with such a beam choice of a radiator (undulator) associated to an amorphous converter. QUESTION: is an hybrid source applicable for ILC? To address this point: necessity to modify the beam pulse duration creating, inside the nominal pulse, Macropulses with minitrains inside them in order to manage relaxation periods between the minitrains , avoiding thermal shock effects. Such a solution is represented here: in a 5 Hz configuration, the e+ macropulses of 40 ms duration are injected in a DR where they are stacked R.Chehab/positrons/Frascati

R.Chehab/positrons/Frascati

TEST OF THE HYBRID-GRANULAR TARGET AT KEK 4 granular converters with W spheres 1.1 mm radius have been built at LAL and sent to KEK. The experiments using a 7 GeV e- beam of KEKB linac started during autumn Thermocouples were fixed on some of the spheres at the exit face of the converter (Al grids with 1.8 mm holes) R.Chehab/positrons/Frascati

TEST OF THE HYBRID-GRANULAR TARGET ON KEKB LINAC The 7 GeV e- beam is sent on a 1 mm W crystal; sweeping magnet and collimator let only the  beam on the amorphous-granular W converter. Positrons are detected with Cherenkov counter. Analysis is done for MeV e+. Photons are not detected R.Chehab/positrons/Frascati

SOME RESULTS OF THE KEK TESTS Data taken by Iryna, Hayg and KEK colleagues R.Chehab/positrons/Frascati

TEMPERATURE RISE DETECTED ON THERMOCOUPLES Position of the thermocouples on the exit face of the converter 1 Hz R.Chehab/positrons/Frascati

APPLICATION TO THE GENERATION OF μ+μ- PAIRS A- Generation with a photon beam on a target  + A  A’ + μ+μ- as the cross section is ~ Z2 (material) high Z material like W are more effective (thickness limitation is linked to multiple scattering). We need also a high energy photon beam to allow the muons to have already high energies..  Use of crystal effects to produce a powerful photon beam B- Generation by a positron beam on a target The positron annihilates with the rest electron from the target. In that case, the positron energy threshold is given by 2 mμ = [2E+.me]1/2 A positron beam of 44 GeV is needed R.Chehab/positrons/Frascati

POSITRON BEAM FOR FUTURE COLLIDERS
PHOTON BEAM GENERATION Channeling radiation The channeling radiation provides a lot of soft photons ; in order to get a higher μ yield, it would be better to use harder photons (b) Coherent Bremsstrahlung The coherent Bremsstrahlung is providing harder photons than channeling (much shorter period in the crystal). Moreover, if we use String of String (SOS) orientation , we should benefit of a hard part of the spectrum In a pratical way, as we need a large number of photons giving rise to a large number of μ pairs, we can lower the energy deposition and the Peak Energy Deposition Density (PEDD) by substituting a granular amorphous target to the compact one  from channeling/Coh.Brems μ+ μ- e- R.Chehab/positrons/Frascati

GENERATION OF HIGH ENERGY-HIGH INTENSITY POSITRON BEAM A high intensity positron beam can be generated using an hybrid source based on channeling radiation . Referring to the studies made for CLIC and ILC, we could expect, using a 10 GeV incident e- beam, a total yield of 12 e+/e- and accepted yield of 2-3 e+/e-. Such beam can be accelerated up to 45 GeV and impinge on a fixed target to produce muons via annihilation with the atomic electrons of the target. Is it interesting to use a crystal associated to the 45 GeV beam to generate the muons? If the positrons are sent with glancing angles w.r.t. the atomic strings, they will meet a very restricted e- population  annihilation not efficient The positrons must be sent to the crystal with a non-zero angle, close (slightly lower) to the critical angle (Lindhard) in order to have a better annihilation efficiency and to benefit from channeling effects. R.Chehab/positrons/Frascati

SUMMARY AND CONCLUSIONS
* Photon generation methods providing polarized or unpolarized photons have been presented together with the associated conversion devices. Emphasis has been put on photon generation using oriented crystals effects. This method developed since many years and validated in experiments at Orsay, CERN and KEK is considered for e+e- colliders like CLIC and under study for ILC. Advantages of this method have been underlined. * An hybrid positron source using channeling in axially oriented crystals can be considered for the 45 GeV positron beam dedicated to μ+μ- generation via e+ annihilation. * Crystal effects may be used for the powerful  source needed for μ+μ- generation * Referring to the available results, a powerful positron source using crystal effects may be considered for the needed positron beam impinging on the μ+μ- converter. * R.Chehab/positrons/Frascati

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POSITRON SOURCES FOR FUTURE COLLIDERS

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