1. Temporal characterization of laser accelerated electron bunches using coherent THz
Wim Leemans and members of the LOASIS Program Lawrence Berkeley National Laboratory
BIW 2006
May 1-4, 2006
Website:

2. Laser wakefield acceleration
Ionization of gas by laser
Ponderomotive push of plasma electrons
Restoring force from due to charge separation
Density oscillation: strong electric fields (100 GV/m)
d=2 mm
plasma
LWFA: two regimes for bunch production
Large-energy-spread bunch (unchanneled)
Quasi-mono-energetic bunch (channeled)
Sprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92); Esarey et al. (94); Mori et al. (94)

3. } Tool: LOASIS multi-terawatt laser 10 TW Ti:sapphire
100 TW-class Ti:sapphire
Shielded target room
LOASIS laser system
Three main amplifiers (Ti:sapphire,10 Hz):
- Godzilla:
J in fs (10-15 TW) ===> main drive beam (to date)
- Chihuahua:
20-50 mJ in 50 fs ===> ignitor beam
mJ in ps ===> heater beam
mJ in 50 fs ===> colliding beam
- T-REX:
2-3 J in fs ===> capillary experiments }
guiding

4. e-beam on phosphor screen
Mid 90’s -2003: short pulse laser systems generate
electron beams with 100 % energy spread
Jet
Laser beam
Electron
beam
Magnet
Phosphor
OAP
Gas Jet
Charge
Detector
vacuum
CCD
e-beam spectrum
Energy spectrum obtained with a magnetic spectrometer
LWFA experiments produce electrons with:
1-100 MeV, multi-nC,
~100 fs,
~10 mrad divergence
~10 mrad
e-beam on phosphor screen
Modena et al. (95); Nakajima et al. (95); Umstadter et al. (96); Ting et al. (97); Gahn et al. (99); Leemans et al. (01); Malka et al. (01)

5. How short are the bunches ?
Simulations predict fs
Can we measure them? (Is the linac stable enough?)
Coherent emission
Dominates if sz < l

6. Diagnostic relies on coherent transition radiation
from the plasma-vacuum boundary
Laser-Wakefield Accelerator
Schematic for Transition Radiation
Leemans et al., Phys. Rev. Lett. (2003);
Schroeder et al., Phys. Rev. E (2004);
Van Tilborg et al., Phys. Rev. Lett. (2006)
Diagnostic implementation:
• Use radiated field
• Couple out of vacuum chamber
Boundary size 

7. In detail: CTR from Plasma-vacuum boundary

8. CTR (THz) in spectral and temporal domain
Diffraction function
(boundary size )
Intense THz source
MV/cm at focus (up to 10’s of J in THz pulse)
‘traditional’ laser-based sources deliver <100 kV/cm
Form factor
Single electron TR
CTR spectrum
CTR in time
Schroeder et al., Phys. Rev. E (04)
van Tilborg et al., Laser Part. Beams (04)
van Tilborg et al., Phys. Plasmas, submitted

9. Temporal THz measurement: electro-optic sampling
Valdmanis (82); Yariv (88); Gallot (99); Yan (00); Fitch(01); Wilke(02); Berden(04); Cavalieri(05)
Phase shift  is proportional to THz field

10. Electro-Optic measurement of Coherent Transition Radiation yields information on laser accelerated electron beam: < 50 fs bunches
W.P. Leemans et al., PRL2003
C.B. Schroeder et al., PRE2004
J. Van Tilborg et al., Laser and Particle Beams 2004; PRL 2006

11. Choice of EO-material affects temporal resolution
• CTR based on 50 fs (rms) Gaussian electron bunch
• ZnTe vs GaP:
• ZnTe cutoff ~ 4 THz
• GaP cutoff ~ 8 THz

12. Scanning technique provides bunch duration:
Resolution limited by crystal properties
Scanning technique
(takes 1.5 hours)
< 50 fs bunches
Synchronization
Charge and bunch stability
Van Tilborg et al., PRL2006, Phys. Plasmas06

13. G. Berden et al., Phys. Rev. Lett. 93, 114802 (2004)
Single-Shot Technique for EO detection of THz pulses:
Information on every bunch
3 ps
50 fs
< 50 fs bunches
peak E-field of ECTR≈150 kV/cm
J. van Tilborg et al., submitted to PRL
G. Berden et al., Phys. Rev. Lett. 93, (2004)

14. Experiments show double THz pulse
Red curves are double-THz-pulse-based waveforms and spectra
Spectrum A
Shot A
Use GaP instead of ZnTe
Higher bandwidth
Observation
Temporal waveform: double pulse
Spectral modulation
Why?
Double bunch e-beam ?
Spectrum B
Shot B

15. Single-shot 2D EO imaging provides spatial profile of THz beam
Measure 2 D THz profile
Focused THz beam
Collimated laser beam
Step laser beam in time
5 mm
7 mm
Shot 2
=796 fs
Shot 1
=546 fs
Shot 3
=1154 fs
Van Tilborg et al., to be published

16. to analyze spatio-temporal effects of coma
‘Ray Optics’ approach
to analyze spatio-temporal effects of coma
=1154 fs
Shot 3

17. Propagation of a single-cycle pulse through focus
no coma
t=0
t=-0.3
t=+0.3
t=-0.6
t=+0.6
t=0
t=+0.3
t=+0.6
t=-0.3
t=-0.6
with coma

18. ‘Ray optics’ model for waveform and spectrum
No coma
With coma

19. 2004 Results: High-Quality Bunches
Large spot size, no channel (ZR order of gas jet length)
RAL/IC: (Mangles et al.)
No Channel: 21019 cm-3
Laser: 12 TW, 40 fs, 0.5 J, 2.51018 W/cm2, 25 m
E-bunch: 1.4108 (22 pC), 70 MeV, E/E=3%, 87 mrad
LOA: (Faure et al.)
No Channel: 0.5-2x1019 cm-3
Laser: 30 TW, 30 fs, 1 J, 18 m
E-bunch: 3109 (0.5 nC), 170 MeV, E/E=24%,10 mrad
Controlled laser guiding with channel
LBNL: (Geddes et al.)
Plasma Channel: 1-4x1019 cm-3
Laser: 8-9 TW, 8.5 m, 55 fs
E-bunch: 2109 (0.3 nC), 86 MeV, E/E=1-2%, 3 mrad

20. Plasma Channel Production: Hydrodynamic Ignitor-Heater in H2 Gas Jet
CCD &
Spectrometer 2w
probe
Interferometer
Cylindrical
Mirror
Heater beam
100mJ 250ps e-
H, He gas jet
Main beam
<500mJ >50fs
Pre ionizing
Beam 20mJ
Plasma channel
Ti:sapphire *
P. Volfbeyn, E. Esarey and W.P. Leemans, Phys. Plasmas 1999
C.G.R. Geddes et al., Nature 431, p. 543 (2004), Phys.Rev.Lett. (2005).

21. Unguided Guided Charge~100 pC
At laser power of 8-9 TW: e-beam with %-level energy spread, 0.3 nC, 1-2 mm-mrad
Beam profile
Spectrum
Unguided
Guided
2-5 mrad divergence
Charge~100 pC
C.G.R. Geddes et al., Nature 431 (2004); PRL (2005); Phys. Plasmas 2005

22. Group velocity of laser < speed of light causes particle dephasing which causes momentum bunchinggv
z-vgt
Phase
Dephasing distance:
Control via density and a0 (laser intensity)
Optimum acceleration requires Lacc = Ldeph: channel or large ZR

23. Geddes et al., Nature (2004) & Phys. Plasmas (2005)
Wake Evolution and Dephasing Yield Low Energy Spread Beams in PIC Simulations
WAKE FORMING
200
Longitudinal
Momentum
Propagation Distance
INJECTION
200
Longitudinal
Momentum
Propagation Distance
200
DEPHASING
DEPHASING
Longitudinal
Momentum
Propagation Distance
Geddes et al., Nature (2004) & Phys. Plasmas (2005)

24. Next step: GeV laser driven accelerator
Increasing beam energy: cm-scale capillary discharge TW laser
Capillary
L'OASIS 100 TW laser
Lower density needed: capillary discharges
Capillary
TREX
Laser
e- beam
1-2 GeV
Plasma
injector TW
40 fs
3-5 cm

25. Capillary channel guiding: set-up

26. Hydrogen based capillary discharge produces suitable density profile for guiding
209 m diameter capillary
85 mbar initial pressure
n0 = 8.5x1017 cm-3
32 micron matched spot
Mach-Zehnder interferometer
A. Gonsalves et al., submitted to PRL
CCD

27. 40 TW power guided over > 3 cm
Output
Input
P = TW in 40 fs, 10 Hz
wx,in=wy,in= 26 m
wx,out=wy,out= 33 m

28. high resolution(under const.)
LOASIS GeV Spectrometer
- Maximum resolving energy: ~1.1 GeV
Yoke
- Large momentum acceptance (factor 25)
Pole
- High resolution (bottom: <1%, forward: 2~4%)
Chamber
Capillary
Interaction point
Beamline
1GeV
Mirror and cameras
Phosphor
40MeV
Bottom view: MeV
high resolution(under const.)
160MeV
moderate resolution
Forward view: GeV
Chamber
Shielded mirror and cameras

29. Up to 1 GeV achieved with 40 TW laser pulses
E<0.6 GeV
Q~ pC
DATA UNDER PRESS EMBARGO
40 TW
E< 1.1 GeV
Q~ pC

30. Summary
Single shot EO-based methods of CTR THz radiation measures < 50fs laser-wakefield accelerated e-bunches
Single cycle THz detected, 0.4 MV/cm
Spatio-temporal coupling from aberrations in imaging can lead to apparent double bunches
GeV electron beam generated in 3.3 cm with 40 TW laser pulses
THz based bunch profile measurements underway
Novel diagnostics needed with fs and sub-fs resolution for slice energy spread and emittance
Next steps are on staging modules towards 10 GeV

31. Scientists and Techs of LOASIS team
Staff:
Exp’t: C. Geddes, W. Leemans, C. Toth
Theory: E. Esarey, C. Schroeder, B. Shadwick,
Postdocs:E. Michel*, P. Michel, B. Nagler
Students: K. Nakamura, J. van Tilborg, G. Plateau,T. Wolf
Techs: D. Syversrud, N. Ybarrolaza
Collaborators:
D. Bruhwiler, D. Dimitrov, J. Cary--TechX Corp
T. Cowan, H. Ruhl -- University of Nevada, Reno*
S. Hooker, A. Gonsalves--Oxford University, UK
R. Ryne, J. Qiang--AMAC, LBNL
R. Huber, R.Kaindl, J. Byrd, M. Martin--LBNL
W. Mori--UCLA
D. Jaroszynski-University of Strathclyde, UK
M. Van der Wiel-TUE, Eindhoven, NL
G. Dugan--Cornell University
D. Schneider, B. Stuart, C. Barty, C. Bibeau--LLNL