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