Compton Linac for Polarized Positrons

V. Yakimenko, I. Pogorelsky,M. Polyanskiy, M. Fedurin

BNLCERN, October 15, 2009

Polarized positron source: the concept

CO2amplifier

Interactionpoint

Metallictarget

- A picosecond CO2 laser pulse circulates in a ring cavity

- At each pass through the cavity the laser pulse interacts with a counter-propagating electron pulse generating γ-quanta via Compton scattering

- Optical losses are compensated by intracavity amplifier

- The λ-proportional number of photons per Joule of laser energy allows for higher γ-yield (compared to solid state lasers)

Pulse duration: 5 psPulse energy: 1JRepetition: 3-12 nsPulses/bunch: 100

Polarized positron sourceConventionalNon-Polarized Positrons:

3-atmCO2 amplifier

parabolic mirrors

vacuum cell detector

YAG (14 ps)200 ns

200 ps

Ge

3% over 1 s

First tests of the laser cavity

Polarized γ-ray beam is generated in the Compton back scattering inside optical cavity of CO2 laser beam and 6 GeV e-beam produced by linac.

6GeV e- beam 60MeV beam

30MeV e+ beam

to e+ conv. target

~2 m

Linac Compton Source (LCS): Numbers

Positron beam requirement

ILC CLIC

2 1010/3 nc 4 109/0.6 nc

2656@5Hz 312@50Hze- beam energy 4 / 6 GeVe- bunch charge 15 / 10 nC 6 / 4 nC

RMS bunch length (laser & e- beams) 3psNumber of laser IPS 10 5

Total N/Ne- yield (in all IPs) 10 5Ne+/N capture 2 / 3 %Ne+/Ne- yield 20 / 30 % 10 / 15%Total e+ yield 3 nC 0.6 nC# of stacking No stacking

Normalized e+ emittance 6 / 4 mm rad 3 / 2 mm rad

Computer simulations: Model

Pumping(slow time-scale)

Vibrational relaxation(slow time-scale)

Amplification &Rotational relaxation(fast time-scale)

Beam Propagation(diffraction, optics, losses)

Spectra(amplification band)

Boltzmann equations(discharge energy distribution)

Discharge dynamics

Using data fromHITRAN2008

Simulation results

Natural CO2 O16:O18 = 50:50

Pulse energy dynamics

Simulation results

Natural CO2 O16:O18 = 50:50

Pulse energy dynamics

Gain spectra

Simulation results

Natural CO2 O16:O18 = 50:50

Pulse energy dynamics

Pulse spectra(initial and after reaching 1 J)

Simulation results

Natural CO2 O16:O18 = 50:50

Pulse energy dynamics

Pulse spectra(initial and after reaching 1 J)

Temporal pulse profile(initial and after reaching 1 J)

Pulse diagnostics

Total bandwidth <=> Individual pulsesub-ps resolution

Individual lines <=> Trainresolution improvement needed

“Streak camera”

:) Single-shot

:( Low resolution (~10 ps )

:) Train measurements

10μm

Laser diode0.9 μm

0.8 μm

Mixingcrystal Filter

CO2

Streak camera

time

25 psMeasuredSimulated

“Interferometer”“Spectrometer”

Pyrocamera

Δt = 0 5 ps 10 ps

PyrocameraDiffractivegrating

wavenumber

1.3 cm-1

Fourier transformMeasuredSimulated

:) Single-shot

:) Simple = reliable

:) Indiv. pulse measurements

... Train measurements (?)

:( Indirect method

Fourier transform

:( Multiple-shot

:) Indiv. pulse

measurements

:) Train measurements

:( Complicated data

analysis

25 ps 50 ps 75 ps

Indi

vidu

alpu

lse

Trai

n

Laser system

Possible configuration with 5 IPs and 1 laser amplifier

~15 cm

~1m

Wall plug power consideration

• ILC: – 3 1014 positrons/second; – 2% - > e+ efficiency for 60 MeV => 150 kW beam

• Wall plug to for warm linac/CO2 is expected ~5-10%

Cross section for Pair production

0 0.2 0.4 0.6 0.80

5 10 28

1 10 27

1.5 10 27

2 10 27

2.5 10 27

d 240 MeV ZW AW x d 120 MeV ZW AW x d 60 MeV ZW AW x d 30 MeV ZW AW x d 15 MeV ZW AW x

x

.

Positron generation efficiencyNp E Z A n L 1 exp n L

0.5

1xd E Z A x

d

0.01 0.1 11 10 4

1 10 3

0.01

0.1

1

Np 240MeV ZW AW nW x XW Np 120MeV ZW AW nW x XW Np 60MeV ZW AW nW x XW Np 30MeV ZW AW nW x XW Np 15MeV ZW AW nW x XW

x

Angular spread of positron beam'scat Ep L_X0 13.6MeV

Ep 1me c2

Ep

2

L_X0 0.555

0.01 0.1 11 10 3

0.01

0.1

1

0.5'scat 240 MeV x( )

0.5'scat 120 MeV x( )

0.5'scat 60 MeV x( )

0.5'scat 30 MeV x( )

0.5'scat 15 MeV x( )

x

Positron beam size at the target exit Ep E L_X0 X0 E

0

1x'scat Ep L_X0 1 x( ) L_X0 X0 x

d

0.01 0.1 11 10 5

1 10 4

1 10 3

0.01

240 MeV 240 MeV x XW 120 MeV 120 MeV x XW 60 MeV 60 MeV x XW 30 MeV 30 MeV x XW 15 MeV 15 MeV x XW

x

Normalized emittance at the target exit

N E L_X0 X0 2

0.5

1

xx E

me c2 x E E L_X0 X0

12

'scat x E L_X0

d

0.01 0.1 11 10 5

1 10 4

1 10 3

0.01

0.1

N 240 MeV x XW N 120 MeV x XW N 60 MeV x XW N 30 MeV x XW N 15 MeV x XW

x

Positron generation efficiency normalized by emittance

ff E x Np E ZW AW nW x XW 0.1mm

N E x XW

0.01 0.1 11 10 4

1 10 3

0.01

0.1

1

ff 240MeV x( )

ff 120MeV x( )

ff 60MeV x( )

ff 30MeV x( )

ff 15MeV x( )

x

Positron generation efficiency normalized by emittance and gamma beam power

ff E x Np E ZW AW nW x XW 0.1 mm

N E x XW

240MeV

E

0.01 0.1 11 10 3

0.01

0.1

1

ff 240MeV x( )

ff 120MeV x( )

ff 60MeV x( )

ff 30MeV x( )

ff 15MeV x( )

x

Positron generation efficiency normalized by transverse phase space

ff E x Np E ZW AW nW x XW 0.1mm

N E x XW

2

0.01 0.1 11 10 6

1 10 5

1 10 4

1 10 3

0.01

0.1

1

ff 240MeV x( )

ff 120MeV x( )

ff 60MeV x( )

ff 30MeV x( )

ff 15MeV x( )

x

Positron generation efficiency normalized by transverse phase space and gamma beam power

ff E x Np E ZW AW nW x XW 0.1 mm

N E x XW

2

240MeV

E

0.01 0.1 11 10 5

1 10 4

1 10 3

0.01

0.1

1

ff 240MeV x( )

ff 120MeV x( )

ff 60MeV x( )

ff 30MeV x( )

ff 15MeV x( )

x

Conclusion• Polarized positron beam requirement for CLIC can be

satisfied with Compton CO2/LINAC based gamma source• Higher energy gamma beam is preferential for the

thermal load on the target• Shorter target is preferential when low emittance after

target is needed (CLIC, LeHC …)• Total power consumption should be part of optimization

for high positron demands (LeHC)• Amplification in Isotope mixture will be tested shortly at

ATF• Seed pulse generation using solid state laser will be

tested at ATF in ~year• There is no funding/activity for regenerative cavity test