Laser-Compton Scattering
Experiments at the ATF
J.Urakawa, KEK

1. Polarized e+ Source based on Comptonscattering,
2. CW laser wire results,
3. Pulsed laser wire development,
4. Polarized γ-ray generation,
5. Compact X-ray source,
6. Key components for photon beam source
based on laser-Compton scattering.

11. Polarized e+ Source based onCompton scattering

2Experiment at KEK-ATF
ATF: Accelerator Test Facility for ILC built at KEK
Collaborating institute: Waseda, TMU, KEK, NIRS, and AIST
T. Omori, M. Fukuda, T. Hirose, Y. Kurihara, R. Kuroda, M. Nomura, A. Ohashi, T. Okugi, K. Sakaue, T. Saito, J. Urakawa, M. Washio, and I . Yamazaki

3120 mExperiment@KEKi) proof-of-principle demonstrationii) accumulate technical informations:polarimetry, beam diagnosis, …
4

Compton Chamber5Nγ(design)= 1 x 107/bunch
2.2 x 107/bunch(achieved)
Ne+(design) = 3 x 104/bunchPol(expected) = 80%
Asym (expected) = 0.95%
6

2. CW laser wire results
CW Laser wire beam size monitor in DR

300mW 532nm Solid-state Laser
fed into optical cavity

14.7µm laser wire for X scan
5.7µm for Y scan
(whole scan: 15min for X,6min for Y)
7

Laser wire block
diagramFree spectral range:532nm/2=266nm
Line width=0.3nm

optical cavity resonance is kept by piezo actuator8

Beam profile by Laser wire

σe2 = σmeas2 - σlw2
εβ = σe2 – [η(Δp/p)]2

β:measured by Q-trim excitation
93. Pulsed laser wire development
Experimental results（Pulse Laser Storage）
Laser:
Mode Lock: Passive
SESAM
Frequency:357MHz
Cavity length:0.42 mPulse width: 7.3 p sec(FWHM)
Wave Length:1064 nmPower:~ 6W
SESAM: SEmi-conductor Saturable Absorber Mirrors

Ext. Cavity:
Cavity:Cavity length:
Mirrors:
Reflectivity:
Curvature:

Super Invar
0.42 m
99.7%, 99.9%
250 mm (ω0 = 180μm)

super invar
62φStorage of laser pulse
Resonance condition :Perfect resonance : Llaser = L cavity
The relationship withlaser and cavity :Imperfect Resonance : L laser ～ L cavity

The enhancement factor
is the function ofreflectivity, Δl and laser
pulse width.

Not resonance : L laser≠ L cavity

12・Finesse: R = 99.9%
Finesse =πτc/l
PDPBS

PBSP.C.

τ:decay time
c: light verocity
l: cavity length

Trans.

τ~ 3.0μsec
F ~ 6300(Preliminary)More than 3000
Times.
実際はOpticalMatchingやModeLocking noiseの問題があるので、現状1000倍程度にしてX線生成を優先した。
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Pulsed Laser and Electron Beam Collision to measure bunch length

Pulse Laser Wire
(Storage laser pulses in optical cavity ):The systems for New X-ray source & Newbunch length monitor at a storage ring
Hear Fukuda’s talk.
14EXPERIMENTAL SETUP : Optics
λ/2

Isolator

λ/4

Reflection

Injection mirrors
Transmission
CavityLaser head15

714MHz Cavity
Laser Repetition rate :357MHzlaser pulses

ScatteredGamma beam
Phase ScanCompton Scattering in every 357MHzElectron repetition rate :357MHzElectron bunches

As an X-ray source :An optical cavity stores higher peak power and getshigher flux X-ray with pulse laser than CW laser.
As Beam monitor :By scanning the laser pulse’s phase in the cavity andmeasuring the Compton signal count rate ; an electronbunch length profile is obtained.
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4. Polarized γ-ray generation using Optical
Stacking Cavity

17Experimental R/D in ATF
Hiroshima-Waseda-Kyoto-IHEP-KEK

Make a fist
prototype2-mirror cavity
Lcav = 420 mm

Put it in
ATF ring18

ame- bereslamabe19e - beamlaser beam

20

Non planer cavity with 4 mirrors in LAL

21

4-mirror-cavity--->
separate functions& confocal configuration22

2-mirror cavity

4-mirror cavity
R1=R2=L

R1=R2=L/2LL

concentric

confocal23

γ-ray Generation with Laser Pulse
Stacking Cavity (Hiroshima-Waseda-IHEP-KEK)
1. Achieve both highenhancement &small spot size
2. Establish feed backtechnology
3. Achieve smallcrossing angle
4. Get experinencewith e- beam

a- beeesla

Pulse stackingcavity
Lcav = 420 mmmmaebr
pulse stacking cavity
in vacuum chamber
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5. Compact X-ray source

43MeV end station to separate X-rayand e-beam. 33keV X-ray is deflectedby Crystals.
Pulsed laser stacking chamber25

Laser Undulator Compact X-ray (LUCX)
Project at KEK-ATF
43MeV Multi-bunch beam+ Super-Cavity = 33keV X-ray.

X-rayDetector

Multi-bunch
photo-cathodeRF GunS-band Acc. Structure

Beam size at
CP 60μm in σMulti-bunch e- beam 300nC at gun

Storage
Laser power
2.45kW,
next step :1MW

At present, laser waist size is 90μm inσ. We should reduce both beam size at CP
down to 40μm.
33keV X-ray generation based on inverse
Compton scattering will start from Oct.
2007 with Super-Cavity.
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6. Key components for photon beam source based onlaser-Compton scattering

World-Wide-Web of Laser Compton
ERL

4th Generation
light source
CLIC e+

ILC e+

Laser
ComptonX-raysource

OpticalCavity

LWmonitor

High powerlaser

e- source
ILC, ERLγγ collider
Medicalapplications
Industrialapplications

Polarimetry27

Re-use Concept

Compton ringElectron storage ringto main linac
Proposed by Posipol Group at Snowmass 2005.

28

positron stacking in main DR

laser pulse stacking cavities

One laser feeds 30 cavities in daisy chain

光共振器中点でサブミクロン、1mJ/pulse・10psecパルスレーザー
ビームを高繰り返し(357MHz)で安定に実現できる小型スーパー光共振器を製作して、その性能を実証する。
高繰り返し(357MHz)、短パルス(10psec, FWHM)、大強度レーザー(1mJ/pulase～100mJ/pulse)のパルスレーザービームをスーパー光共29振器で実現する。

New Idea by UK
Use a Misaligned Multipass Cavity
HR MIRRORLASER

λ/4 POCKELS CELL GATE

F/4 FOCUSING

POLARISING BEAMSPLITTER

e-BEAM
210mm (0.7ns)

500mJ/100ps @1.1μm
SINGLE PULSE

Mirror spacing determines the inter-pulse interval to match to 2.8ns
Slight mirror tilt from perfect auto-collimation or slight shear of one lens gives
scanning with equally spaced foci and a controllable spacing
PC gate switches pulse into cavity
Need to keep round-trip losses very low to ensure sufficient passes at sufficient
power
Other designs possible

30

Laser-wire at ATF-EXT By Grahame Blair (RHUL) et al.
6min 43s

40

Beam size (μm)

35
30
25
20
15
10
5060

65

70

75

80

85

QD4X (A)Modify opticallens to realize
sub-micron
laser waist size.
31