Pol. positron geneｒation scheme for ILC KEK Junji Urakawa 2005.6.21, at RHU Summary of Compton Scattering Experiment for Pol. Positron generation at ATF Scheme for ILC positron generation How to get enough positron yield. How to generate ILC positron beam. Conclusion

High Pol. Beam Generation, Low Cost Beam Source, Flexible Beam Source Tuning But, there are many challenges. ~100MeV Accumulator Ring Pol. positron To 5GeV Ring 1.3GeV S-band Linac To 5GeV Ring 180m Laser-Compton Ring ~100MeV Accumulator Ring Pol. electron

Almost 100% Pol. g-ray 107 to about 90% ～ 104 e+

Experiment@KEK-ATF i) proof-of-principle demonstrations ii) accumulate technical informations: polarimetry, beam diagnosis, …

Accelerator Test Facility@KEK 120 m

Measured Number of g-rays Ng = 2.1x107/bunch in 31 p sec

Measured Asymmetry A= -0.93±0.15 % A= 1.18± 0.15 % laser pol. = - 79 % This is old data obtained by old laser. Now we measured almost 100% Pol. g-ray by new Laser.

Pol. g-ray Production Pol. e+ Production Ng ≈ 1 x 107 /bunch DT(rms) = 31 psec Pol. : g = ~100 % (measure Eg > 50 MeV?) Pol. e+ Production Ng = 2 x 107 /bunch Ne+ = 6 x 104/bunch? (Ee+ = 25 to 45 MeV) Pol. e+ = 77 %----~90%? DT(rms) = 31 psec Can NOT measure each e+ Polarimetry ?

Measure e+ polarization : use Bremsstrahlung g-ray polarized e+ Pb conveter

Scheme for ILC positron generation S-band Linac with Multi-bunch Photo-Cathode RF Gun to generate 200 bunch train. (Norm. emittance<5x10-6 mrad) Small Ring for Laser Compton Scattering Collimation of g-ray , Target, Separator Pre-acceleration to positron storage (collector) ring S-band Linac to accelerate 200 bunch train upto energy of 3km damping ring.

Multi-bunch electron beam generation with 2 Multi-bunch electron beam generation with 2.8nsec bunch spacing 200 bunches/train 12.5Hz operation is possible. 940 mJ/100 pulses with 2.8nsec spacing, UV 266nm, 7psec（FWHM) Farady Cup to measure multi-bunch Current, 400nC/100 bunches

Small Ring for Laser Compton Scattering 185.16m (=199x2.8ns+60ns) small ring with two 50 m long straight sections for laser Compton scattering and beam inj./ext. & RF acceleration. Racetrack ring. Many IP’s are possible with one target. 12.5Hz fast kicker with flat-top 300ns and rise/fall time<60ns is manufactured by SLAC.-maybe 600nsec flat-top possible. 50% operation of this ring will be devoted to Pol. positron generation. Other 50% is for use as g-factory or pol. positron and electron application.

Collimation of g-ray , Target, Separator We need good design as total system. Thermal effect is not severe comparing the undulator scheme. Since the design is not difficult, I only say you should learn ATF pol. positron setup as not good example in which we want to tight selection of pol. Positron and the total system for the undulator scheme from TESLA TDR. Anyway, we can obtain pol. ~90% positrons which corresponds to 0.1% of g-yield in the range from 0.9 max. energy to max. energy g as number of positrons. Max. g energy is important parameter.

Pre-acceleration to positron storage (collector) ring Pre-acceleration from about 10MeV to 100MeV are necessary by DC super conducting linac (RF frequency is maybe 714MHz or more.). DC super-conducting Linac is necessary to boost the energy of 9x103 positrons/bunch beam upto the energy of the accumulator ring. I donot take care of de-polarization at present. Very Fast Multi-turn Injection to accumulate positron beam. I can use very fast kicker and synchrotron tune resonant injection. Accumulator ring has to accept large synchrotron oscillation. Bunch length of injected positron beam is very small, 10psec (rms). RF acceleration system of the accumulator ring is 357MHz. Laser induced radiation cooling is helpful. What energy of the accumulator ring is optimum?

Very Fast Kicker Experiment for this scheme and 3km damping ring (strip line kicker) 3MHz operation is OK. 2.8nsec bunch spacing beam with 60nsec train gap will be extracted stably but 357MHz injection is problem which requests Fourier Series kicker under development by my group.

How to get enough positron yield. Recalculation with electron energy of 1.28GeV, See Klaus Monig-san’s report. Collimated g-ray energy range: 25.6-28.5MeV 6.2x1015 photons/sec per IP, need 6 IP’s Pulse energy 20mJ is OK. If we make 30 IP’s,we have to produce 20mJ with 357MHz. 42cm Optical Cavity with mode-lock laser

Experimental results（Pulse Laser Storage） Mode Lock: Passive SESAM Frequency: 357MHz Cavity length: 0.42 m Pulse width: 7.3 p sec (FWHM) Wave Length: 1064 nm Power: ~ 6W SESAM: SEmi-conductor Saturable Absorber Mirrors

Ext. Cavity: Cavity: Super Invar Cavity length: 0.42 m Mirrors: Reflectivity: 99.7%, 99.9% Curvature: 250 mm (ω0 = 180μm) super invar 62φ

・Finesse: R = 99.9% Finesse =πτc/l P.C. τ~ 3.0μsec τ:decay time c: light verocity l: cavity length PD PBS PBS P.C. Trans. τ~ 3.0μsec F ~ 6300 (Preliminary) More than 3000 Times.

Plused Laser and Electron Beam Collision to measure bunch length Pulse Laser Wire (Storage laser pulses in optical cavity ):

New Project by JSPS from 2005 to 2009 To make 1mm(rms) focusing at IP with small crossing angle.

7 % energy acceptance small ring(16m). Test ring for positron collection from ATF=DR Laser Compton

How to generate ILC positron beam. We already demonstrate very fast strip-line kicker with 3MHz repetition rate. 3km damping ring was designed with train gap. There are many instability issues to solve in this design but the damping time is short. I already mentioned the scheme at 1st ILC workshop.

Beam Injection/ Extraction Schemes Train gap is necessary to cure fast ion instability.

Example, Large acceptance Design of Storage Ring Example, Large acceptance Energy [GeV] 1.98-1.3 Number of bunches 340-200 Circumference [m] 299.792-185 Natural emittance [nm] 0.5-0.22 Horizontal emittance (Full C.)[nm] 0.25-0.11 Vertical emittance (Full C.) [nm] Momentum compaction 1.388x10-3 RF frequency [MHz] 714 Rms energy spread 0.0975%-? Natural Bunch Length [mm] 5.49-3.0?

Conclusion My conclusion is almost same as Klaus’s talk. Necessary Devices R&D’s are on going. Thank you.