GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED

GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS R. CHEHAB (IPNL &LAL/IN 2 P 3 -CNRS), B. MOUTON, R. ROUX, A. VARIOLA, A. VIVOLI (LAL/IN 2 P 3 -CNRS), France R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 1

GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS INTRODUCTION n Polarized positrons are, now, considered for the positron source dedicated to the future linear collider (ILC). The first idea presented for VLEPP was based on photon generation in a long helicoidal undulator providing circularly polarized photons which created longitudinally polarized pairs in a thin target. The alternative method considered here consists in the generation of circularly polarized photons by Compton interaction between a circularly polarized Nd: Yag laser beam and an electron beam in a so-called Compton ring or in an ERL (Energy Recovery Linac). Some results obtained with this scheme are presented here. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 2

GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS… n n n PLAN Photon generation: the two schemes => Compton ring & ERL Positron converter Positron beam capture: Matching lenses (AMD or QWT) + L-Band linac Simulation results - Influence of the electron beam energy in Compton process - Comparison of the Compton ring and ERL - Comparison of two kinds of matching lenses: AMD & QWT - Optimisation in the linac Summary and conclusions R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 3

GENERATION AND TRANSPORT OF A POSITRON BEAM… n n n PHOTON GENERATION: THE COMPTON RING The installation is made of: * an electron ring in which energetic electrons (E= 1 to 2 Ge. V) collide with a circularly polarized laser beam in Fabry-Perot cavities of high finesse. The laser considered, here, is a Nd: Ya. G ( l=1. 064 mm). * a thin converter target (amorphous W; L=0. 4 Xo). * a capture section with a matching device; the preferred one is the Adiabatic Matching Device (AMD) with a magnetic field tapering slowly from a maximum value (here, 6 Tesla) to a minimum value (0. 5 Tesla). This device is followed by a solenoid coil with this field value imbedding some accelerating sections. These sections are L-Band to manage a large aperture. * a linac with standard quadrupole system brings the e+ to the DR R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 4

GENERATION AND TRANSPORT OF A POSITRON BEAM…: COMPTON RING R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 5

GENERATION AND TRANSPORT OF A POSITRON BEAM… n n n ENERGY RECOVERY LINAC (ERL) The ERL is made of a CW superconducting linac with f = 1. 3 GHz, having a maximum energy of 2 Ge. V and one return arc. ERL injector, using DC photocathode, works at a frequency of 20 MHz and delivers 20 ps RMS bunches. Bunch charge is 1. 5 n. C. Mean current is 30 m. A. A bunch compression, at the end of the injector, shortens the bunch to 1 ps RMS A Nd: Ya. G laser (l=1. 064 mm) associated to 10 Fabry-Perot cavities (500 m. J/cavity) provides the photon beam which crosses the electron beam at 8 degrees. The distance between the Compton interaction point and the conversion target is of 10 meters. R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 6

GENERATION AND TRANSPORT OF A POSITRON BEAM…: ERL R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 7

GENERATION AND TRANSPORT OF A POSITRON BEAM…: THE TARGET POSITRON CONVERTER n n The positron converter is a piece of amorphous tungsten 0. 4 Xo thick (1. 4 mm) n The target is inside the magnetic lens: the pairs are submitted to the maximum magnetic field at the target exit. n L= 0. 4 Xo g R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 L 8

GENERATION AND TRANSPORT OF A POSITRON BEAM: CAPTURE&ACCELERATION After the target an Adiabatic Matching Device (AMD) captures the positrons (and electrons) before acceleration in L-Band sections. The simulated pre-accelerator is comprising 5 cavities, each one providing an acceleration of 8 -9 Me. V. n For one application we shall consider a Quarter Wave Transformer (QWT) n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 9

GENERATION AND TRANSPORT OF A POSITRON BEAM…: MATCHING DEVICE ADIABATIC MATCHING DEVICE n The magnetic field is tapering from a maximum value (6 Tesla) to a minimum value (0. 5 Tesla) which corresponds also to the value of the magnetic field on the accelerating sections. The tapering length is 0. 5 m n The iris aperture radius is of 23 mm. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 10

GENERATION AND TRANSPORT OF A POSITRON BEAM…: MATCHING DEVICE QUARTER WAVE TRANSFORMER n The magnetic field has a (quasi) step-like shape. The maximum field (6 Tesla) extends on 10 cm. The transition to the lower field is on 5 cm. The lower field is of 0. 5 Tesla as for the solenoid on the accelerating sections. n Maximum and minimum field are similar for the AMD and QWT for easier comparison. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 11

GENERATION AND TRANSPORT OF A POSITRON BEAM. . : SIMULATION RESULTS n The results are concerning: * the comparison of the two schemes: Compton Ring & ERL n * the influence of the electron beam energy in the Compton ring n * the comparison between two matching systems: AMD and QWT n The simulation programmes used are: n - CAIN for the photon generation n - EGS for the pair creation n - PARMELA for the beam transport n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 12

GENERATION AND TRANSPORT OF A POSITRON BEAM. . : Compton Ring vs ERL COMPTON RING n The energy (top) and phase (bottom) distributions at the exit of the target are presented. n n Mean energy value is 19 Me. V RMS Energy value is 10. 7 Me. V The large phase distribution is due to the wide bunches (s=6 mm) in the Compton ring. RMS value is 8. 2 degree R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 13

GENERATION AND TRANSPORT OF A POSITRON BEAM. . : Compton Ring vs ERL n The energy (top) and phase (bottom) distributions are given at the exit of the target. n The narrow phase distribution is due to the narrow bunches delivered by the ERL. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 14

GENERATION AND TRANSPORT OF A POSITRON BEAM…: Compton Ring vs ERL COMPTON RING n The beam radius variation (cm) along the propagation axis z is presented (top) in the case of: - an e- beam in CR with E=1. 8 Ge. V and AMD lens * The beam length variation (ps) along z is presented (bottom), for the same hypotheses. n Transverse emittance at the end of solenoid: ex= 69 mm mrad ey= 73 mm mrad R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 15

GENERATION AND TRANSPORT OF A POSITRON BEAM…: Compton Ring vs ERL COMPTON RING n The variation of the relative energy spread (top) along the axis is given for the case of CR with AMD lens n The losses along the axis z (bottom) are given for the same hypotheses n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 16

GENERATION AND TRANSPORT OF A POSITRON BEAM…: Compton ring vs ERL n ERL The beam radius (top) and the beam length (bottom) variations along the propagation axis z are given. The case concerns: n * an electron beam energy of 1. 8 Ge. V in the ERL n * an AMD matching lens n n Transverse emittance at solenoid exit : ex= 68 mm mrad n ey= 70 mm mrad n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 17

GENERATION AND TRANSPORT OF A POSITRON BEAM…: Compton Ring vs ERL n The energy dispersion (top) and the losses (bottom) are given for the ERL case with E-=1. 8 Ge. V and the AMD as matching system. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 18

GENERATION AND TRANSPORT OF A POSITRON BEAM…: E-=1. 3 Ge. V in CR n Positron energy spread The positron energy distribution is given at the target exit for the case E-=1. 3 Ge. V in the Compton ring. n Mean value is: 11. 9 Me. V n RMS value is: 5. 5 Me. V n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 19

GENERATION AND TRANSPORT OF A POSITRON BEAM…: E-=1. 3 Ge. V in CR n n n POSITRON BEAM EMITTANCE AT TARGET We represent the positron beam emittances in the two planes. Emittance value is: ex=830 mm mrad ey=720 mm mrad The transverse beam distribution is represented also. R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 20

GENERATION AND TRANSPORT OF A POSITRON BEAM…: CR (1. 8 Ge. V) & QWT n n n BEAM EMITTANCE AT THE END OF THE FIRST PART OF PREACCELERATOR ~50 Me. V The emittance figures are given here: the emittance values are: ex=74 mm mrad ey=72 mm mrad The transverse beam distribution is also given. R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 21

GENERATION AND TRANSPORT OF A POSITRON BEAM…: CR(1. 8 Ge. V) & QWT COMPTON RING & QWT n The beam radius evolution along the z axis (top) as the beam length (bottom) along the same axis are represented. n Bunch lengthening is occuring rapidly in the first cm, where the magnetic field remains strong (6 Teslas) on 10 cm n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 22

GENERATION AND TRANSPORT OF A POSITRON BEAM…: CR(1. 8 Ge. V) & QWT COMPTON RING & QWT n The relative energy spread variation along the z axis is represented (top). The losses (bottom) are also presented. n Much part of the losses are occuring in the very first part of the matching system n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 23

GENERATION AND TRANSPORT OF A POSITRON BEAM…: SUMMARY & CONCLUSIONS n SUMMARY AND CONCLUSIONS Simulations have been carried out on the Compton scheme with no polarized positrons. The main results have shown that: n * Concerning the two possible schemes: Compton Ring and ERL, we got shorter positron bunches at the target, for the latter (1 ps vs 20 ps); the difference holds along the preaccelerator, however the bunch lengthening due mainly to the spiralization in the magnetic fields makes imperative in both case use of bunch compression. Lateral dimensions and emittances do not show differences, when compared. n * Concerning the electron beam energies in CR (1. 3 and 1. 8 Ge. V) , the positron spectra at the target are different, as expected; narrower spectrum is for the 1. 3 Ge. V case. Transport in the preaccelerator does not show any significant difference. n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 24

GENERATION AND TRANSPORT OF A POSITRON BEAM. . : SUMMARY & CONCLUSIONS n * Concerning the comparison of two matching systems (AMD & QWT), the differences are mainly in the accepted yields: the positron yield at the end of the solenoid, for the QWT case, represents less than half of the yield for the AMD. That was expected due to the larger momentum acceptance of the AMD. The emittances are quite close in both cases (around 70 mm mrad, at ~ 50 Me. V). Maximum emittance at the end of the solenoid depends essentially on the low magnetic field which is the same in both cases. Improvements(? ) * Use simulations with polarized particles * Optimize the matching systems in order to transmit maximum of polarized particles * Optimize the Compton scheme (bunch compression, …) n n R. Chehab/ R&D on positron sources for ILC/ Beijing, 2007 25