Compton based Polarized Positrons Source for ILC

1. Compton based Polarized Positrons Source for ILC V. Yakimenko Brookhaven National Laboratory September 12, 2006 RuPAC 2006, Novosibirsk

2. ILC Source requirements * The length of the bunch train in ILC is 2820x300ns = 0.85 ms or 250 km. Bunch spacing has to be reduced in the dumping ring. ** Polarization level defines conversion/capture efficiency of polarized g rays into polarized positrons. 60% level corresponds to ~1.5% efficiency.

3. Polarized Positron Production: Compton Ring Scheme: CO2 Version (Omori, et al.)

4. Polarized Positrons Source (PPS for ILC) Conventional Non-Polarized Positrons: In this proposal • polarized g-ray beam is generated in the Compton back scattering inside optical cavity of CO2 laser beam and 4-6 GeV e-beam produced by linac. • The required intensities of polarized positrons are obtained due to 5 times increase of the e-beam charge (compared to non polarized case) and 10IP CO2 laser system. • Laser system relies on the commercially available lasers but need R&D for the new mode of operation • 5ps 10J@0.05 Hz CO2 laser is operated at Brookhaven ATF.

5. Compton Experiment at Brookhaven ATF(record number of X-rays with 10 mm laser) • More then 108 of x-rays per pulsewere generated in the experiment PR ST 2000. NX/Ne-~0.1. • (0.35 as of April 2006- limited by laser/electron beams diagnostics) • Interaction point with high power laser focus of ~30mm was tested. • Nonlinear limit (more then one laser photon scattered from electron) was verified. PRL 2005. Real CCD images Nonlinear and linear x-rays

6. Choice of parameters Ng ,Ne and Nfare the numbers of g-rays, electrons and laser photons, S is the area of the interacting beams and sC is the Compton cross sections • ~40 mm laser focus is set by practical considerations of electron and laser beams focusing and requires ~5 ps long laser pulses • Nonlinear effects in Compton back scattering limit laser energy at ~1-2J • Pulse train structure of 2820 bunches @ 5 Hz is set by main linac. We change it to 100 bunches at 150Hz. This mode is more natuaral for warm RF and lasers. • ~300ns bunch spacing in the main linac will be changed in the dumping ring in any design. 6-12 ns bunch spacing is selected for optimal current in the drive linac and to match the inversion life time of the laser 12ns *100 bunches = 1.2 ms. • Train of ~10 nC electron bunches is required to produce 1012 polarized gammas per bunch. (~1 g-ray per 1 electron per laser IP) • Conversion efficiency of polarized gammas into captured polarized positrons is assumed at ~1.5% and is subject of optimization.

7. Compton based PPS with CO2 laser • No positron accumulation is needed: • Efficient head-on collision due to higher divergence of CO2 beam. • 10 mm CO2 beam has 10x number of photons per laser energy. • 750 W average power industrial laser. • Easier target and efficient positron capture: • Beam format changed in the injector from 3000 bunches @5 Hz to 100 bunches @150 Hz (1ms @150 Hz is more natural for warm RF and laser). • 40MV/m gradient in post target linac is possible. • Efficient collimation due to strong energy / divergence correlation of the gamma beam in the Compton scattering. • Doable laser: • Commercially available components (designed for 750W@500Hz; needed 750W@150Hz). • Low repetition rate model (10Hz) is operational at ATF as an amplifier of 5ps beam (laser cavity mode with 5ps pulse is needed). • Can be add-on option for non-polarized source linac. • 2 Step R&D is needed: • Test of the laser cavity at low repetition rate with ATF laser; optimize target and capture for this scheme. • Assemble and test seed system and one laser cavity at full power.

8. Kerr generator 2 x 200ps CO2 oscillator Laser system for PPS 1x150ns Ge optical switch 2 x 5ps 1mJ 2 pulses, 5ps, 10mJ (YAG laser) Regenerative amplifier PC TFP TFP 2x5ps 10mJ PC amplifier 2x300mJ BS 2x30mJ 2x 30mJ 5ps • Train of 2 pulses spaced by 12 ns and 5 ps long sliced with a YAG beam from a 150 ns CO2 oscillator pulse • This train is seeded inside a regenerative amplifier cavity that has a round-trip time (12ns x 2=24 ns) • Amplified 2 pulses are dumped from the regenerative cavity with a Pockels cell and, after amplification, split with partial reflectors in 10 beams. • After amplification to 1 J/pulse, each 2-pulse train is injected into a ring cavity individual for each IP • An intracavity amplifier serves just to compensate optical losses during 1.2 s time interval needed for interactions with 100 electron bunches. amplifier amplifier 2 x 1J 5ps 2 x 1J 24ns ring cavities (2 pulses x 12ns spacing) 1J / pulse sustained for 1.2 ms amplifier amplifier IP#1 IP#10

9. Lasers from SDI http://www.lightmachinery.com/SDI-CO2-lasers.html WH20 WH100 WH350 WH500 Wavelength 9 – 11µm, Line Tunable Continuous 20 Hz 100 Hz 350 Hz 500 Hz Repetition Rate Pulse Energy 1.5 J Mode Type Multimode Optional: TEMoo, custom beam shapes, SLM Beam Size 13 x 13 mm2 Average Power 30 W 150 W 525 W 750 W Power Stability < 7 %

10. Laser system for PPS • Optical slicing and amplification of 5 ps CO2 pulses has been demonstrated and utilized in routine ATF operation for user experiments. • CO2 oscillator and initial amplifiers are commercially available lasers from SDI and operate at rep. rate up to 500Hz. • Final intracavity amplifiers shall operate at average power 750W (100 bunches x 1J x ~5% intracavity loss x 150Hz). • High pressure CO2 laser is available at 750W average power at 500Hz. • Operation of laser in nonstandard regime at high rep. rate, injection and uniform enregy during 1.2 ms are R&D subjects.

11. Conclusion • We proposed Polarized Positron Source based on Compton back scattering inside optical cavity of CO2 laser beam and 4-6 GeV e-beam produced by linac. • The proposal utilizes commercially available units for laser and accelerator systems. • The proposal requires high power picosecond CO2 laser mode of operation developed at ATF.