temporal characterization of laser accelerated electron bunches using coherent thz website: wim...
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Temporal characterization of laser accelerated electron bunches using coherent THz
Website: http://loasis.lbl.gov/
Wim Leemansand members of the LOASIS Program
Lawrence Berkeley National Laboratory
BIW 2006May 1-4, 2006
Laser wakefield acceleration
Sprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92); Esarey et al. (94); Mori et al. (94)
d=2 mm
plasmaLWFA: two regimes for bunch production • Large-energy-spread bunch (unchanneled) • Quasi-mono-energetic bunch (channeled)
1. Ionization of gas by laser2. Ponderomotive push of
plasma electrons3. Restoring force from due to
charge separation4. Density oscillation: strong
electric fields (100 GV/m)
10 TW Ti:sapphire 100 TW-class Ti:sapphire Shielded target room
Tool: LOASIS multi-terawatt laser
LOASIS laser system
Three main amplifiers (Ti:sapphire,10 Hz):- Godzilla:
0.5-0.6 J in 40-50 fs (10-15 TW) ===> main drive beam (to date)- Chihuahua:
20-50 mJ in 50 fs ===> ignitor beam250-300 mJ in 200-300 ps ===> heater beam100-200 mJ in 50 fs ===> colliding beam
- T-REX:2-3 J in 30-40 fs ===> capillary experiments
} guiding
Mid 90’s -2003: short pulse laser systems generate electron beams with 100 % energy spread
~10 mrad
e-beam on phosphor screen
100
101
102
103
104
0 10 20 30 40 50
E[MeV]
Detection Threshold
e-beam spectrum
Energy spectrum obtained with a magnetic spectrometer
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)
LWFA experiments produce electrons with:
1-100 MeV, multi-nC, ~100 fs, ~10 mrad divergence
Jet
Laser beam
Electron beam
Magnet
PhosphorPhosphor
OAP
Gas Jet
ChargeDetector
Magnet
vacuum
CCD
How short are the bunches ?
•Simulations predict 10-20 fs
•Can we measure them? (Is the linac stable enough?)
• Coherent emission
€
I total(ω)= N+N(N−1)g(k) 2{ }I e(ω)
g(k)= ρ(z)eikzdz−∞
∞
∫
Dominates if z <
Diagnostic relies on coherent transition radiationfrom the plasma-vacuum boundary
Schematic for Transition Radiation
Boundary size
Leemans et al., Phys. Rev. Lett. (2003);Schroeder et al., Phys. Rev. E (2004);Van Tilborg et al., Phys. Rev. Lett. (2006)
Laser-Wakefield Accelerator
Diagnostic implementation:• Use radiated field• Couple out of vacuum chamber
CTR (THz) in spectral and temporal domain
Schroeder et al., Phys. Rev. E (04)van Tilborg et al., Laser Part. Beams (04)van Tilborg et al., Phys. Plasmas, submitted
Intense THz source• 0.01-10 MV/cm at focus (up to 10’s of J in THz pulse)• ‘traditional’ laser-based sources deliver <100 kV/cm
Diffraction function(boundary size )
Form factor
CTR spectrum CTR in time
Single electron TR
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
Electro-Optic measurement of Coherent Transition Radiation yields information on laser accelerated electron beam: < 50 fs bunches
W.P. Leemans et al., PRL2003C.B. Schroeder et al., PRE2004J. Van Tilborg et al., Laser and Particle Beams 2004; PRL 2006
Choice of EO-material affects temporal resolution
• ZnTe vs GaP: • ZnTe cutoff ~ 4 THz • GaP cutoff ~ 8 THz
• CTR based on 50 fs (rms) Gaussian electron bunch
Scanning technique(takes 1.5 hours)
• < 50 fs bunches
•Synchronization
• Charge and bunch stability
Scanning technique provides bunch duration: Resolution limited by crystal properties
Van Tilborg et al., PRL2006, Phys. Plasmas06
Single-Shot Technique for EO detection of THz pulses:Information on every bunch
J. van Tilborg et al., submitted to PRLG. Berden et al., Phys. Rev. Lett. 93, 114802 (2004)
• < 50 fs bunches
• peak E-field of ECTR≈150 kV/cm
3 ps
50 fs
Experiments show double THz pulse
Red curves are double-THz-pulse-based waveforms and spectra
Spectrum AShot A
Spectrum BShot B
Use GaP instead of ZnTe• Higher bandwidth
Observation
•Temporal waveform: double pulse
•Spectral modulation
Why?
• Double bunch e-beam ?
Single-shot 2D EO imaging provides spatial profile of THz beam
5 mm
7 mmShot 2=796 fs
Shot 1=546 fs
Shot 3=1154 fs
Van Tilborg et al., to be published
•Measure 2 D THz profile
• Focused THz beam
• Collimated laser beam
• Step laser beam in time
Propagation of a single-cycle pulse through focus
t=0t=+0.3
t=+0.6t=-0.3
t=-0.6
no coma
t=0t=+0.3
t=+0.6
t=-0.3t=-0.6
with coma
Large spot size, no channel (ZR order of gas jet length)
• RAL/IC: (Mangles et al.)• No Channel: 21019 cm-3
• Laser: 12 TW, 40 fs, 0.5 J, 2.51018 W/cm2, 25 m
• E-bunch: 1.4108 (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: 3109 (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: 2109 (0.3 nC), 86 MeV, E/E=1-2%, 3 mrad
2004 Results: High-Quality Bunches
Plasma Channel Production: Hydrodynamic Ignitor-Heater in H2 Gas Jet
*
P. Volfbeyn, E. Esarey and W.P. Leemans, Phys. Plasmas 1999C.G.R. Geddes et al., Nature 431, p. 543 (2004), Phys.Rev.Lett. (2005).
Plasma channel
CCD &Spectrometer
2 probe
Interferometer
CylindricalMirror
Heater beam100mJ 250ps
e-
H, He gas jet
Main beam<500mJ >50fs
Pre ionizingBeam 20mJ
Ti:sapphire
At laser power of 8-9 TW: e-beam with %-level energy spread, 0.3 nC, 1-2 mm-mrad
UnguidedBeam profile Spectrum
Guided
Charge~100 pC
2-5 mrad divergence
C.G.R. Geddes et al., Nature 431 (2004); PRL (2005); Phys. Plasmas 2005
-1.000
-0.500
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
-4 -3.50 -3.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00
z-vgt
vM
om
en
tum
Phase
Group velocity of laser < speed of light causes particle dephasing which causes momentum bunching
• Dephasing distance:
• Control via density and a0 (laser intensity)
• Optimum acceleration requires Lacc = Ldeph: channel or large ZR
Wake Evolution and Dephasing Yield Low Energy Spread Beams in PIC Simulations
WAKE FORMING
INJECTION
DEPHASINGDEPHASING
Propagation Distance
Lon
gitu
dina
l M
omen
tum
200
0
Propagation DistanceL
ongi
tudi
nal
Mom
entu
m
200
0
Propagation Distance
Lon
gitu
dina
l M
omen
tum
200
0
Geddes et al., Nature (2004) & Phys. Plasmas (2005)
Next step: GeV laser driven accelerator
•
Increasin
g beam
energy:
cm-scale
capillary
discharg
e + 100
TW laser
CapillaryL'OASIS 100 TW laser
€
Wd[GeV] ~ I[W/cm2] n[cm-3]
• Lower density needed: capillary discharges
Plasma injector
3-5 cm
e- beam
1-2 GeVLaser
40-100 TW40 fs
CapillaryTREX
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
40 TW power guided over > 3 cm
P = 0.1-40 TW in 40 fs, 10 Hz
wx,in=wy,in= 26 m
wx,out=wy,out= 33 m
Output
Input
LOASIS GeV Spectrometer
1GeV
160MeV
40MeV
moderate resolution
Forward view: 0.16 - 1GeV
Interaction point
Yoke
Phosphor
Bottom view: 40-160 MeVhigh resolution(under const.)
Mirr
or a
nd c
amer
as
Capillary
Chamber Shielded mirror and cameras
- Large momentum acceptance (factor 25)
- Maximum resolving energy: ~1.1 GeV
- High resolution (bottom: <1%, forward: 2~4%)Chamber
Beamline
Pole
Up to 1 GeV achieved with 40 TW laser pulses
25 TW
E<0.6 GeV
Q~50-300 pC
40 TW
E< 1.1 GeV
Q~50-100 pC
DATA UNDER PRESS EMBARGO
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
Scientists and Techs of LOASIS team
Staff:
Exp’t: C. Geddes, W. Leemans, C. TothTheory: E. Esarey, C. Schroeder, B. Shadwick,Postdocs:E. Michel*, P. Michel, B. NaglerStudents: K. Nakamura, J. van Tilborg, G. Plateau,T. Wolf Techs: D. Syversrud, N. Ybarrolaza
Collaborators:
D. Bruhwiler, D. Dimitrov, J. Cary--TechX CorpT. Cowan, H. Ruhl -- University of Nevada, Reno*
S. Hooker, A. Gonsalves--Oxford University, UKR. Ryne, J. Qiang--AMAC, LBNLR. Huber, R.Kaindl, J. Byrd, M. Martin--LBNLW. Mori--UCLAD. Jaroszynski-University of Strathclyde, UKM. Van der Wiel-TUE, Eindhoven, NLG. Dugan--Cornell UniversityD. Schneider, B. Stuart, C. Barty, C. Bibeau--LLNL