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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: http://loasis.lbl.gov/. plasma. d=2 mm. LWFA: two regimes for bunch production

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slide1

Temporal characterization of laser accelerated electron bunches using coherent THz

Wim Leemansand members of the LOASIS ProgramLawrence Berkeley National Laboratory

BIW 2006

May 1-4, 2006

Website: http://loasis.lbl.gov/

slide2

plasma

d=2 mm

  • LWFA: two regimes for bunch production
  • Large-energy-spread bunch (unchanneled)
  • Quasi-mono-energetic bunch (channeled)

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)

Sprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92); Esarey et al. (94); Mori et al. (94)

slide3

Tool: LOASIS multi-terawatt laser

100 TW-class Ti:sapphire

Shielded target room

10 TW Ti:sapphire

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 beam

250-300 mJ in 200-300 ps ===> heater beam

100-200 mJ in 50 fs ===> colliding beam

- T-REX:

2-3 J in 30-40 fs ===> capillary experiments

}

guiding

slide4

CCD

Phosphor

Phosphor

vacuum

Magnet

Laser beam

Gas Jet

e-beam on phosphor screen

Electron

beam

e-beam spectrum

Jet

Energy spectrum obtained with a magnetic spectrometer

Charge

Detector

~10 mrad

Magnet

OAP

Mid 90’s -2003: short pulse laser systems generate

electron beams with 100 % energy spread

LWFA experiments produce electrons with:

1-100 MeV, multi-nC,

~100 fs,

~10 mrad divergence

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)

how short are the bunches

Dominates if sz < l

How short are the bunches ?

Simulations predict 10-20 fs

Can we measure them? (Is the linac stable enough?)

Coherent emission

slide6

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 

slide8

CTR (THz) in spectral and temporal domain

Diffraction function

(boundary size )

  • 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

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

slide9

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

slide10

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

slide11

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

slide12

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

slide13

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, 114802 (2004)

slide14

Shot A

Spectrum A

Shot B

Spectrum B

Experiments show double THz pulse

Red curves are double-THz-pulse-based waveforms and spectra

  • Use GaP instead of ZnTe
  • Higher bandwidth
  • Observation
  • Temporal waveform: double pulse
  • Spectral modulation
  • Why?
  • Double bunch e-beam ?
slide15

Single-shot 2D EO imaging provides spatial profile of THz beam

Shot 1

=546 fs

5 mm

Shot 2

=796 fs

Shot 3

=1154 fs

7 mm

  • Measure 2 D THz profile
    • Focused THz beam
    • Collimated laser beam
    • Step laser beam in time

Van Tilborg et al., to be published

slide16

‘Ray Optics’ approach

to analyze spatio-temporal effects of coma

Shot 3

=1154 fs

slide17

with coma

t=0

t=-0.3

t=+0.3

t=-0.6

t=+0.6

Propagation of a single-cycle pulse through focus

no coma

t=0

t=-0.3

t=+0.3

t=-0.6

t=+0.6

slide18

No coma

With coma

‘Ray optics’ model for waveform and spectrum

slide19

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
plasma channel production hydrodynamic ignitor heater in h 2 gas jet

2w

probe

Cylindrical

Mirror

Main beam

<500mJ >50fs

Pre ionizing

Beam 20mJ

H, He gas jet

e-

Heater beam

100mJ 250ps

Interferometer

CCD &

Spectrometer

Plasma Channel Production: Hydrodynamic Ignitor-Heater in H2 Gas Jet

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).

at laser power of 8 9 tw e beam with level energy spread 0 3 nc 1 2 mm mrad
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

group velocity of laser speed of light causes particle dephasing which causes momentum bunching

Group velocity of laser < speed of light causes particle dephasing which causes momentum bunching

gv

Momentum

z-vgt

Phase

  • 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 Evolution and Dephasing Yield Low Energy Spread Beams in PIC Simulations

200

WAKE FORMING

Longitudinal

Momentum

0

Propagation Distance

200

INJECTION

Longitudinal

Momentum

0

Propagation Distance

200

DEPHASING

DEPHASING

Longitudinal

Momentum

0

Propagation Distance

Geddes et al., Nature (2004) & Phys. Plasmas (2005)

next step gev laser driven accelerator

1-2 GeV

e- beam

Next step: GeV laser driven accelerator

Capillary

L\'OASIS 100 TW laser

  • Lower density needed: capillary discharges

Increasing beam energy: cm-scale capillary discharge + 100 TW laser

Capillary

TREX

Laser

Plasma

injector

40-100 TW

40 fs

3-5 cm

slide26

Hydrogen based capillary discharge produces suitable density profile for guiding

  • Mach-Zehnder interferometer

CCD

  • 209 m diameter capillary
  • 85 mbar initial pressure
  • n0 = 8.5x1017 cm-3
  • 32 micron matched spot

A. Gonsalves et al., submitted to PRL

40 tw power guided over 3 cm

Input

Output

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

slide28

LOASIS GeV Spectrometer

Forward view: 0.16 - 1GeV

moderate resolution

- Maximum resolving energy: ~1.1 GeV

Yoke

- Large momentum acceptance (factor 25)

Pole

- High resolution (bottom: <1%, forward: 2~4%)

Chamber

Interaction point

Capillary

Beamline

1GeV

Mirror and cameras

Phosphor

40MeV

Bottom view: 40-160 MeV

high resolution(under const.)

160MeV

Chamber

Shielded mirror and cameras

slide29

Up to 1 GeV achieved with 40 TW laser pulses

25 TW

E<0.6 GeV

Q~50-300 pC

DATA UNDER PRESS EMBARGO

40 TW

E< 1.1 GeV

Q~50-100 pC

summary
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
slide31

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

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