Simulation studies of the e-beams for UED@ASTA
Renkai Li and Juhao Wu
5/20/2014
ALD UED@ASTA Review
2
Outline
• Mission
• Constrains and design consideration
• Baseline parameters
• Tolerance studies
• Summary
3
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
4
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
5
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
6
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)
7
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
8
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
9
Different bunch charge
FWHM bunch length at sample
rms spot size at detector 1
M at detector 1rms spot size at sample
i) constant initial charge density (10 fC / 100 μm rms)
ii) constant initial spot size (100 μm rms Gaussian)
10
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
11
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!