Compton based Polarized Positrons Source for ILC
description
Transcript of Compton based Polarized Positrons Source for ILC
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