Compton based Polarized Positrons Source for ILC

V. YakimenkoBrookhaven National Laboratory

September 12, 2006

RuPAC 2006, Novosibirsk

ILC Source requirementsParameter Symbol Value Unit

Positrons per bunch np 2x1010 e+

Bunches per pulse Nb 2820

Bunch Spacing* b ~300 ns

Pulse rep. rate frep 5 Hz

Positron Polarization**

Pp ~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: CO2 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 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.

Compton Experiment at Brookhaven ATF

(record number of X-rays with 10 m laser) • More then 108 of x-rays per pulse were 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 ~30m was tested.

• Nonlinear limit (more then one laser photon scattered from electron) was verified. PRL 2005.

Real CCD imagesNonlinear 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 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 1012

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.

Ce

S

NNN

N,Ne and Nare 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 bunches @5 Hz to

100 bunches @150 Hz (1s @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.

Laser system for

PPS

ampl

ifier

24ns ring cavities (2 pulses x 12ns spacing) 1J / pulse sustained for 1.2 ms

IP#1 IP#10

CO2 oscillator

2 pulses, 5ps, 10mJ(YAG laser)

2 x 200ps Kerr generator

2 x 5ps1J

Regenerative amplifier

amplifier

2x5ps10mJ

2x300mJ

BS

TFPTFPPC

PC

2x 30mJ 5ps

2 x 1J 5ps

2x30mJ

2 x 1J

1x150ns

Ge optical switch

ampl

ifier

ampl

ifier

ampl

ifier

• 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

WH20 WH100 WH350 WH500Wavelength 9 – 11µm, Line TunableContinuous 20 Hz 100 Hz350 Hz500 HzRepetition RatePulse Energy 1.5 JMode Type MultimodeOptional: TEMoo, custom beam shapes, SLMBeam Size 13 x 13 mm2

Average Power 30 W 150 W 525 W 750 WPower Stability < 7 %

http://www.lightmachinery.com/SDI-CO2-lasers.html

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 s are R&D subjects.

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.