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

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.

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

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 Hz CO 2 laser is operated at Brookhaven ATF.

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 N X /N e- ~0.1. (0.35 as of April 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 Real CCD images Nonlinear and linear x-rays

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 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 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 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

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 Hz to 100 Hz (1  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 needed –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.

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.

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 %

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.

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.