Compton based Polarized Positrons Source for ILC

V. Yakimenko, I. PogorelskyBNL

Collaboration meeting,Beijing, January 29-February1,

2006

Outline:

• Numbers and issues.• Target and conversion efficiency• Laser test:

– Laser spot size – First results from laser cavity tests

• Plans

Polarized Positrons Source (PPS for ILC)Conventional Non-Polarized Positrons:

In the proposal • Polarized -ray beam is generated in Compton backscattering

inside optical cavity of CO2 laser beam and 6 GeV e-beam produced by linac.

• The required intensities of polarized positrons are obtained due to 10 times increase of the “drive” e-beam charge (compared to non polarized case) and 5 to 10 consecutive IPs.

• Laser system relies on commercially available lasers but need R&D on a new mode of operation.

• 5ps, 10J CO2 laser is operated at BNL/ATF.6GeV 1A e- beam 60MeV

beam

30MeV e+ beam

to e+ conv. target

~2 m

5-ps, 1-JCO2 laser

Linac Compton Source (LCS): Numbers

e- beam energy 6 GeV

e- bunch charge 10nC

RMS bunch length (laser & e- beams)

3 ps

beam peak energy 60 MeV

Number of laser IPS 10(5)

Total N/Ne- yield (in all IPs) 10(5)

Ne+/N capture 2% (4%)

Ne+/Ne- yield 0.2

Total e+ yield 2nC

# of stacking No stacking

Proposal numbers are in black, Optimistic numbers are in Red

Compton Experiment at Brookhaven ATF

(record number of X-rays with 10 m laser) • More than 108 of x-rays were generated in the

experiment NX/Ne- ~0.35.

• 0.35 was 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

LCS: Issues to be checked

• Conversion target and capture efficiency optimization (nearly done)

• Laser beam generation and injection into the cavity (very encouraging first results)

• Laser cavity detailed design and tests at low and nominal repetition rate (no funding yet)

• Electron beam source and IPs optics (ongoing at BNL for different project)

• Cost and reliability

LCS: Conversion target and capture efficiency optimization

• The proposal relies on 2% conversion efficiency of the beam (produced in the Compton backscattering) into captured polarized positrons.

• Approximate analytical model developed for quick parameter optimization predicts up to 4% efficiency.

• Detailed computer simulation at ANL confirmed 2% parameter set efficiency.

• Further work is needed to confirm a case with sicker target and 4% efficiency.

LCS: Conversion target and capture layout

• Target: Ti - 0.3 rl• AMD: 60 cm long 10T to 2T AMD pulsed• Linac: Pulsed L-band with 30MV/m• 2% gamma to captured e+ beam

efficiency predicted by analytical model and detailed computer modeling with ~70% (?) polarization

Photon collimator

Target AMD Pre-accelerator and solenoid

• ~50% of the gammas with energies 30-60MeV can be selected by collimator with ~1.

E_ 4 2

El

1 4 2

El

Ee

22

0 1 2 30

20

40

60

80

E_ ( )

MeV

.

1.2 Energy spectrum:

E_ 4 2

El

1 4 2

El

Ee

22

d E if E Emax 0 d E

0 20 40 600

2 10 18

4 10 18

6 10 18

8 10 18

1 10 17

d E( )

E

MeV

.

Energy cross-section (top) and angular dependence (bottom) for the Compton backscattering

Step1. Energy filtering of the gamma beam

Step2: Gamma to pair conversion in target

• ~30% of the 30-60 MeV gammas will generate e+e- pair in X/3 target.

• ~25% of them will have combined energy in the 30-60 MeV range.

f x( ) x2 1

1 x2

0.20206 0.0369x2 0.0083x

4 0.002x6

X0A

Z716.408

gm

cm2

Z ln 184.15Z

1

3

f Z

ln 1194 Z

2

3

1

X01

6.168 103m2

kg

X0

3.67 10

6 kgs2

G x( ) if x 1 0 x2

1 x( )2

2

3x 1 x( )

x 1 x( )

9 ln 1833Z

dp E Ep L A L 20 X0

GEp

E

0 20 40 60 800

0.2

0.4

0.6

0.8

dp 30MeV Ep0.3X0

dp Emax Ep0.3X0

Ep

MeV

.

Differential cross-section of the pair production as a function of positron energy

Step 3: Positron energy selection

• ~50% of the positrons will have 30-60MeV energies.

• They will loose ~15% of energy in X/3 target on average due to bremsstralung.

• New energy range 25-60MeV.• Total efficiency up to this point: 50%

x 30 % x 25% x 50 % = 2%.• Computer simulations predict

~2.2%.

Step 4: Capturing efficiency• All positrons in with

the 25-60MeV energies can be captured.

P 2 LAD

RF1

1

1mc2

P c

2

12

B0

Bs1ln

B0

Bs

0 10 20 30 40 50 6020

15

10

5

0

P 0 deg( )

deg

P 10 deg( )

deg

P 15 deg( )

deg

P 20 deg( )

deg

P c

MeV

0 5 10 15 20 25 3050

40

30

20

10

0

5MeV

c

deg

10MeV

c

deg

40MeV

c

deg

deg

2 Ep L( )14MeV

Ep

2L X0

.

10 20 30 40 50 600

0.5

1

1.5

2

max P( )

max P( ) k P( )

2 P c 0.3X0

P c

MeV

.

0deg 1deg 30deg

Angular acceptance is shown on the left and longitudinal phase slippage is shown on the right graphs

Step 5: Polarization

• The positron longitudinal polarization goes from 0.5 at half the gamma energy to 1 at the full gamma energy for polarized gammas. Integrating from positron energy 50% to 100% of the gamma energy gives a polarization of about 0.8.

IP#1 IP#5

2x30mJ

CO2 oscillator

10mJ 5ps from YAG laser

200ps

1mJ 5ps

10mJ5ps 300mJ 5 ps

TFPPC PC

150ns Ge

1J

e-

LCS: CO2 laser system

• pulse length 5 ps• energy per pulse 1 J• period inside pulse train 12

ns• total train duration 1.5 s• train repetition rate 150

Hz

LCS: LDRD support at BNL

• LDRD at the level of $110K supports this effort

• Cavity simulations and tests, injection design are the main goals of the LDRD

• Available at BNL/ATF CO2 Laser hardware is used to supports this effort.

• PostDoc with CO2 laser experience will be soon hired to work on this project.

• Funding is nearly sufficient to complete the main LDRD goals on a two-year scale.

LCS: Laser focus characterization

0 50 100 1500

0.5

1

X1 r( )

r

0 50 100 1500

0.5

11

0

X1 r( )

1500 r

0 50 100 1500

0.5

11

0

X1 r( )

1500 r m

75 m 100 m 150 m

mw 7520

mw 650

Gaussian approximation

Tra

nsm

itte

d e

nerg

y75 m

d=75 m 100 m 150 m no pinhole

• High power (~3J, 5ps) CO2 laser spot size of =32m was demonstrated using F#~4 parabola with a hole for e-beam transmission.

• Proposal assumes =40m.

• Laser is circular polarized as is required for ILC.

LDRD – cavity tests• Has a potential to increase average intra-

cavity power ~100 times at 10.6 microns.

Purpose of the test:• Demonstration of 100-

pulse train inside regenerative amplifier that incorporates Compton interaction point.

• Demonstration of linear-to-circular polarization inversion inside the laser cavity.

• Test of the high power injection scheme

4-atm CO2 amplifier

/4-waveplates

parabolic mirrors

YAG (14 ps)

200

ps

Ge

“~100 times increase of the average intra-cavity power at 10 m”

• The required laser train format / repetition rate /average acting power at each IP: 100 pulses x 150Hz x 1J = 15 kW.

• Efficient interaction with electron beam requires short (~5ps) and powerful (~1-2J) laser beam.

• Such high-pressure laser does not exist. Non-destructive feature of Compton scattering allows putting interaction point inside laser cavity.

• We can keep and repetitively utilize a circulating laser pulse inside a cavity until nominal laser power is spent into mirror/windows losses.

• Assuming available 0.5 kW CO2 laser and 3% round-trip loss, 1-J pulse is maintained over 15,000 round trips/interactions (100 pulses x 150 Hz). Thus, 0.5 kW laser effectively acts as a 15 kW laser.

• Equivalent solid state ( 1m) laser producing the same number of gamma photons should be 150 kW average power with ~10J, 5ps beam.

3-atmCO2 amplifier

parabolic mirrors

vacuum cell

detector

YAG (14 ps)200 ns

200 ps

Ge

LCS LDRD: Simplified test setup

First observations:• Optical gain over 4 s• Misbalanced gain/loss

regime results in lasing interruption by plasma

• Single seed pulse amplification continues to the end

LCS LDRD: Simplified test setup• Balancing gain/losses and

plasma threshold results in continuous amplification with two characteristic time constants : – ~200 ns due to CO2 inversion

depletion– ~2 s due to N2-CO2 collisional

transfer.• Early injection of seed pulse

before the gain reaches maximum allows to control train envelope

3% over 1 s

The best train uniformity achieved

• Very encouraging results obtained with simplified cavity test setup: ~200 ps pulse of the order of 100 mJ circulated for >1 s.

• Further test would require pulse length monitoring and high pressure or isotope mixture based amplifier (to sustain 5 ps beams).

LCS: Laser cavity detailed design and tests at low and nominal repetition

rate• The key point – not funded• There is a detailed $250K/2 year proposal

from SDI to study amplifier for the cavity at 5 Hz ($100K laser + modifications & R&D ~ $150K)

• Not modified 150Hz laser price is ~ $450K • 1 additional FTE at BNL is needed to support

amplifier work and to design, simulate and test laser cavity

• ~$350K/year – 3 years is needed to test laser cavity at 5 Hz

LCS: Electron beam source and IPs optics

• Photoinjector gun is simulated at CAD/BNL with the required beam parameters

• Detailed design is needed and estimated 1FTE-year is expected

• Depend on the number of laser IPs. Design uses 10 IPS. (potentially can be reduced to 3-5 IPs)

• Should be delayed till laser cavity parameters (required number of cavities) are verified

Acknowledgments:• W. Gai (ANL), W. Liu (ANL), V.

Litvinenko (BNL), W. Morse (BNL) and I. Pavlishin (BNL) helped with analytical and computational analysis of the conversion process and optical cavity work