Prospects for generating high brightness and
low energy spread electron beams through
self-injection schemes
Xinlu Xu*, Fei Li, Peicheng Yu,
Wei Lu, Warren Mori and Chan Joshi
University of California, Los Angeles
Tsinghua University
*xuxinlu04@gmail.com
Oct 15, 2015
Plasma-based acceleration
• Plasma-based acceleration1:
 High gradient: ~ 100 GV/m
 High efficiency: ~ 30%
 High energy gain: ~ 10s GeV
 Beam quality ?
Emittance
Brightness
current
energy spread
1T.
Hard X-FEL at LCLS2
~0.4×1017
A/m2/rad2 (~3 kA,
~400 nm)
< ρ ~ 0.001
Tajima and J. M. Dawson, Phys. Rev. Lett., 43, 267 (1979); P. Chen et al., Phys. Rev. Lett., 54, 693
(1985); I. Blumenfeld, et al., Nature 445, 741-744 (2007); W. P. Leemans, et al., Phys. Rev. Lett., 113,
245002 (2014); M. Litos, et al., Nature 515, 92-95 (2014); and many other papers.
2P. Emma et al., Nature Photonics 4, 641-647 (2010).
Experimental results
Question:
How to generate an electron beam with both
much higher brightness (>1017 A/m2/rad2) and
much lower energy spread (<1%) in plasmabased accelerators?
Injection schemes
• What is injection?
The electron is in the acceleration phase.
Injection schemes
• External Injection1: the electrons from other
sources, e.g., RF-based accelerators
• Self Injection: the electrons from the plasma
 Ionization Injection2
 Downramp Injection3
 Others
1C.
E. Clayton et al., Phys. Rev. Lett. 70,37 (1993); M. Everett et al., Nature 368, 527 (1994).
Oz et al., Phys. Rev. Lett. 98, 084801 (2007); A. Pak et al., Phys. Rev. Lett. 104, 025003 (2010).
3C. G. R. Geddes et al., Phys. Rev. Lett. 100, 215004 (2008); A. J. Gonsalves, et al., Nature Phys. 7,
862-866 (2011); A. Buck, et al., Phys. Rev. Lett. 110, 18506 (2013).
2E.
Ionization injection – Different Scenarios
Ionization injection- The key physics
Transverse Phase Mixing
Longitudinal Phase Mixing
 Transverse phase mixing happens in
each slice.
 The slice energy spread is determined
by the injection distance, ~ several MeV.
X. Xu et al., Phys. Rev. Letts. 112, 035003 (2014).
Ionization injection by transverse
colliding pulses injection
 Peak brightness: 1.7×1019 A/m2/rad2
 Slice energy spread: 0.012 MeV
F. Li et al., Phys. Rev. Letts. 111, 015003 (2013).
Downramp injection
• “Accordion effect”:
S. Bulanov, et al., and H. Suk et al., studied the injection
process using 1D analysis. We are studying downramp injection:
 In the 3D blowout regime
 With the aim of generating unprecedented beam quality
~ 1020 A/m2/rad2 + ~1 MeV !
T. Katsouleas, Phys. Rev. A 33, 2056 (1986); S. Bulanov, et al., Phys. Rev. E 58, R5257 (1998); H. Suk, et
al., Phys. Rev. Lett. 86, 1011 (2001);
Downramp Injection – The phase
velocity
• We focus on:
 3D blowout regime
 Gradual density ramp:
 Injection condition:
• The phase velocity
W. Lu et al., PRL 96, 165002 (2006)
1.5 np0
np0
Downramp injection – transverse
motion
• The transverse force for the particle at r=rb
Defocusing
Focusing
Downramp injection – General
features of the injected electrons
Downramp injection – Beam Quality
 Emittance εn:
determined by σri
 Slice energy spread:
determined by σzi
 Current:
How to generate a beam with high
brightness?
• Scaling:
One stage Laser driver: np ~ 1019 cm-3 and
higher
How to generate a beam with high
brightness?
• Staging: ~ 0.8×1021 A/m2/rad2, < 2 MeV
50 um: 3.3×1018cm-3  2.6×1018cm-3
16 um: 2.3×1020cm-3  1.4×1020cm-3
How to generate a beam with low
projected energy spread?
initial energy chirp
the chirp of acceleration gradient
How to generate a beam with low
projected energy spread?
• Laser driver case: at zplasma = 0.17 mm
How to further reduce the relative
projected energy spread?
The ramp length L is increased from
17 c/ωp to 150 c/ωp.
The beam duration is mainly
determined by the density difference
(np,h - np,l).
 Increase the ramp length to
increase the initial energy chirp κ !
How to further reduce the relative
projected energy spread?
• Generate an ultrashort beam.
The plasma density is decreased
from 1.05 np to np in 2 c/ωp.
1018 cm-3: ~20 nm
Other issues - Matching
X. Xu, et al., arXiv:1411.4386 (2014)
• Parameters:
Profile of the matching section
1017 cm-3
• OSIRIS results:
Conclusions
• We studied the density downramp injection in
the 3D blowout regime driven by charged particle
or laser pulses.
• The phase space dynamics of the injection beams
are studied using 3D simulations and theoretical
analysis.
• The current, emittance, beam length and initial
energy chirp can be controlled by adjusting the
driver intensity and the ramp parameters.
• Finally, electron beams with both high brightness
and low energy spread can be generated.
Thanks for your attention!
Downramp injection – The particle
velocity
• In a uniform plasma: with the quasi-static approximation1:
The distribution of Ψ1:
P. Mora and T. M. Antonsen Jr, Physics of Plasma 4, 217 (1997).
Downramp injection – The particle
velocity
• In a uniform plasma:
Downramp injection – General
features of the injected electrons