USPAS Course on 4th Generation Light Sources II ERLs and...

USPAS 4th Generation Light Sources II Krafft/Bazarov 20 October 2002CHESS / LEPPCHESS / LEPP

USPAS Course on4th Generation Light Sources IIERLs and Thomson Scattering

G. A. Krafft and I. V. BazarovJefferson Lab and Cornell University

Thomson Scattering Sources


Thomson Scatter Sources & Related Proposals

1. Thomson Scattering vs. Undulator Radiation2. Thomson Scattering Sources (TSS):

1. JLAB2. BNL3. Duke4. Berkeley5. Idaho Accelerator Center6. Naval Research Lab

3. Other proposals:1. Small-Angle Geometry2. Laser Electron Storage Ring

Sir J.J. Thomson


Thomson Scattering vs. Undulator Radiation

λp

N N

N

S

SS

γ/K θ

λ

Lλ

λe–

• Undulator radiation can be looked as Thomson scattering (and vice versa)

• Same signature: polarization, harmonic content when a0, K ≥ 1

cmkeBK

ep

0=20

0 cmeAa

e

=


Thomson Scattering Same as Short Undulator Radiation

)1(2

222 θγ

γλ

λ +≈ peptotph NNKN 2

, α≈For K ≤ 1:

1371

=e

erD

Scaling with undulator period of the formulas above:2~ γλp• to keep radiation wavelength the same:

• to keep the number of photons the sameif undulator length is kept the same: γ/1~0Bif number of periods is kept the same: 2

0 /1~ γB

B0 in undulator is limited to ~ Tesla, thus, electron energy is constrained to some multi-GeV, and undulator period to ~ cm for hard x-ray production.

~ (∆ω/ω)×(ε⊥/εph)2

useful fraction

πλ 4/


Different Thomson Scattering Geometries

Lλ

λ

e–

φθ

φθγ

γλλ

cos11)1(

222

2 −+≈ L

Θ′=Ω′

22 sineT r

ddσ

66538 2 == eT rπσ mbarn

• possible advantages include compact design (small γ), fs x-ray pulses

• we’ll look at different examples of TSS: φ = π/2, φ = π and φ << 1

φ ′φ

S S′

φcoskk z =yk yy kk =′

)cos( φβγ −−=′ kk z


Photons Wanted

kTh /~


Reality Check Against Photon “Starvation”From undulator:

Thomson scattered x-rays: eLphT

totph NNN ,, areaspot focused~ σ

mostlyincoherent

photons

204)µm 100(

2, 10~

mbarn 665~areaspot focused~

2

πσ

αT

pLph NKN

Laser pulse needed:

≈

~1

or ~ 10 J / pulse (TW laser)or:

)W/cm()µm(1085.0/ 29200 LLe IcmeAa λ−×≈=

218 W/cm102.2 ×≈LI1~0a for

• can be tough to match laser and electron beams in space-time, i.e. only a fraction of electrons or photons will participate in scattering

• rep rate is limited to ~ kHz (don’t need more for pump probe exp.)

eptotph NNKN 2, α≈


Outline


1. JLAB2. BNL3. Duke4. Berkeley5. Idaho Accelerator Center6. Japanese Tohuku Group



… …


…

peak

peak

0.1 % bw


Outline





Duke FEL γ-production


Machine Description


Measured γ-rays

Note: γ-ray polarization is ~ 100 %


Future Plans


Outline





…

…

h ≈ 4 eV·fskT ≈ 1/40 eV

?


…

…

…

…


…

…


Experimental ResultsR.W. Shoenlein et. al, Nature (274) 5285

e– beam50 MeV, 1.3 nC, 90 µm, 20 pslaser beam800 nm, 100 fs, 60 mJ, 30 µm

x-ray beam produced2 × 105 ph mm–2 mrad–2 s–1 in a 15% bandwidth


Outline





The Layout


Interaction Region


Indirect Laser Pulse Measurements


X-ray Measurements


Outline





…

… ≈ 3°


3°

6°


…

…

…

…

βx, y = 4 mm?

Note: state-of-the-art peak brightness from 3rd

generation light sources approaches 1024 photons s-1 mm-2 mrad-2 per 0.1% bw, and average fluxis 1015 photons s-1 per 0.1% bw.


f104 per 0.1% bw

per pulse

should be 1018

Note: different laser parameters were used for different Thomson scattering sources


Peak brightness alone can be misleading

Single photon peak brightness: 33 )2/()2(1hπ

=peakB

3o

24221 ))A(/(104.2bw) 0.1%per mradmmsphoton ( λ×=---peakB

or in conventional units:

peak brightness @ 1 Å when photon degeneracy is 1

SummaryThomson scattering sources can provide ~ 103-4 shorter pulses than that of 3rd

generation light sources, with peak brightness lower by at least ~ 103-4 and average flux lower by at least 109-10 assuming state-of-the-art rep rate for laser. Even if the rep rate of Thomson scattering event were somehow improved to some sub-GHz (c.f. next project), average flux would still be ~ 103-4 when compared to a 3rd generation light sources.


Outline





General Idea

phase space damping rate:

phase space excitation rate:

EPphε∝ 2

4

ρEPph ∝ due to bends alone, i.e. small

ringphph HENE

22

1 &∝


Damping From Thomson “Undulator”


Theoretical emittance & energy spread

Note: large initial emittancecan hinder the damping


Fabry-Perot resonator & Projected performance

laser peak power: 0.2 TW (transient)6 GW (steady)

rep rate: 0.1 GHz (steady)

laser power req.d: 200 W

USPAS Course on 4th Generation Light Sources II ERLs and...

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