Simulation studies of the e-beams for UED@ASTARenkai Li and Juhao Wu

5/20/2014ALD UED@ASTA Review

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Outline

• Mission

• Constrains and design consideration

• Baseline parameters

• Tolerance studies

• Summary

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Mission

• Support design, commissioning and experiment

• Find a set of practical beam/operation parameters

• Compatible with existing hardware and upgrades

• Tolerance requirements on hardware

• Fulfill constrains (temporal resolution, sample size)

To deliver high-quality data with <100 fs temporal resolution

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Constrains

• temporal resolution ≤ 100 fs

• e-beam spot size at sample ≤ 500 μm diameter- sample size usually < 500 μm

- available laser energy for uniform pump up to a few tens of mJ/cm2

temporal resolution

pump laser length

probe e-beam length

velocity mismatch

TOA jitter

1𝑚𝐽2𝜋 (0.1𝑐𝑚)2

=16𝑚𝐽 /𝑐𝑚2

602+602+602=982 𝑓𝑠

0

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Design consideration

solenoid

sample

detector

Rhkl

• Rhkl : radius of the diffraction ring

• ΔR : width of the diffraction ring / spot size of the direct beam

• M = Rhkl /ΔR is good measure of the pattern quality

• Consideration for UED@ASTA simulation and optimization- e-beam pulse length ≤ 60 fs FWHM

- time-of-arrival (TOA) jitter ≤ 60 fs FWHM

- e-beam spot size ≤ 500 μm diameter

- maximize M for better pattern quality

ΔR

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Layout of the UED@ASTA beamline

cathode (z=0)

sample (z=1.08 m)

detector 1 (z=2.98 m) detector 2 (z=4.48 m)

solenoid (z=0.19 m)

collimator 1 (z=0.57 m)

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Baseline parameters

M=10M=10.5

assume shkl=0.43 Å-1

sample size ≤ 0.6 mm

60 fs FWHM

gun gradient 100 MV/m

gun phase 40 deg

solenoid B0 0.181 T

UV pulse length 60 fs fwhm

UV spot size 100 μm rms

intrinsic emittance 50 nm

bunch charge 10 fC

beam energy γ 10.2

beam emittance 50 nm

energy spread < 2×10-4

(B0 = 0.181 T)(B0 = 0.178 T)

(B0 = 0.181 T)(B0 = 0.178 T)

Aluminum sampleM ≈ 9

P. Zhu, X. J. Wang, et al. at SDL BNL

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Gun phase / rf focusing effect

rms spot size at sample

rms spot size at detector 1 M=R/ΔR

rf focusing effects

• beam emittance stay constant

• larger spot size at sample leads to smaller divergence

• more solenoid lens can improve flexibility (future upgrade)

• Velocity bunching cavity (future upgrade)

rf compression

FWHM bunch length at sample

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Different bunch charge

FWHM bunch length at sample

rms spot size at detector 1 M at detector 1 rms spot size at

sample

i) constant initial charge density (10 fC / 100 μm rms)

ii) constant initial spot size (100 μm rms Gaussian)

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rf phase and amplitude jitter

• the rf phase and amplitude jitter may be partially correlated

• requires ~100 fs timing and ~2×10-4 power stability for TOA

• negligible effects on beam spot size

rf phase jitter rf field amplitude jitter

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Summary

• Identified a set of practical baseline parameters

• Understood the trend/scaling of some main parameters

• Require short and small spot UV on the cathode

• Require ~100 fs rf-to-laser synchronization and ~2×10-4 rf power stability

• Simulation will support commissioning and experiment

Thank you!