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

2. ILC Source requirements ParameterSymbolValueUnit Positrons per bunchnpnp 2x10 10 e+e+ Bunches per pulseNbNb 2820 Bunch Spacing* bb ~300ns Pulse rep. ratef rep 5Hz Positron Polarization**PpPp ~60% * 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  rays into polarized positrons. 60% level corresponds to ~1.5% efficiency.

3. Polarized Positron Production: Compton Ring Scheme: CO 2 Version (Omori, et al.)

4. Polarized Positrons Source (PPS for ILC) Conventional Non- Polarized Positrons: In this proposal polarized  -ray beam is generated in the Compton back scattering inside optical cavity of CO 2 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 CO 2 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 CO 2 laser is operated at Brookhaven ATF.

5. Compton Experiment at Brookhaven ATF (record number of X-rays with 10  m laser) More then 10 8 of x-rays per pulse were generated in the experiment PR ST 2000. N X /N e- ~0.1. (0.35 as of April 2006- limited by laser/electron beams diagnostics) Interaction point with high power laser focus of ~30  m 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 ~40  m 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  s. Train of ~10 nC electron bunches is required to produce 10 12 polarized gammas per bunch. (~1  -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. N , N e  and N   are the numbers of  -rays, electrons and laser photons, S is the area of the interacting beams and  C is the Compton cross sections

7. Compton based PPS with CO2 laser No positron accumulation is needed: –Efficient head-on collision due to higher divergence of CO2 beam. –10  m 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 (1  s @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. Laser system for PPS amplifier 24ns ring cavities (2 pulses x 12ns spacing) 1J / pulse sustained for 1.2 ms IP#1IP#10 CO2 oscillator 2 pulses, 5ps, 10mJ (YAG laser) 2 x 200ps Kerr generator 2 x 5ps 1  J Regenerative amplifier amplifier 2x5ps 10mJ 2x300mJ BS TFP PC 2x 30mJ 5ps 2 x 1J 5ps 2x30mJ 2 x 1J 1x150ns Ge optical switch amplifier 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.

9. Lasers from SDI WH20WH100WH350 WH500 Wavelength 9 – 11µm, Line Tunable Continuous 20 Hz100 Hz350 Hz500 Hz Repetition Rate Pulse Energy1.5 J Mode Type Multimode Optional:TEMoo, custom beam shapes, SLM Beam Size13 x 13 mm 2 Average Power30 W150 W525 W750 W Power Stability < 7 % http://www.lightmachinery.com/SDI-CO2-lasers.html

10. Laser system for PPS Optical slicing and amplification of 5 ps CO 2 pulses has been demonstrated and utilized in routine ATF operation for user experiments. CO 2 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  s are R&D subjects.

11. Conclusion We proposed Polarized Positron Source based on Compton back scattering inside optical cavity of CO 2 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.