Laser-Plasma Accelerators as Sources of Electron Beams at 1 MeV to 1 GeV

Eric Esarey

FLS Workshop, March 1 -5, 2010http://loasis.lbl.gov/

W. Leemans, C. Geddes, C. Schroeder, S. Toth,A. Gonsalzas, J. van Tilborg, K. Nakamura, M. Chen and

others

LOASIS Program Lawrence Berkeley National Laboratory

Laser-plasma accelerators: Outline

• Self-modulated LWFAs: Status Prior to 2004

• LWFAs: High quality e-beam production at 100 MeV-level (2004)

• LWFAs: High quality e-beam production at 1 GeV-level (2006)

• Downramp injection at 1 MeV-level (2008)

• Integrated gas jet+capillary structure (2009)

• Colliding pulse injection at 100 MeV-level (2006, 2009)

• Ionization injection at 100 MeV-level (2008, 2010)

Laser Wakefield Accelerator (LWFA)

B.A. Shadwick et al., IEEE PS. 2002

Standard regime (LWFA): pulse duration matches plasma period

Ultrahigh axial electric fieldsEz > 10 GV/m, fast waves

Ultrashort plasma wavelengthλp ~ 30 µm (100 fs)

Plasma channel: Guides laser pulseand supports plasma wave

Tajima, Dawson (79); Gorbunov, Kirsanov (87); Sprangle, Esarey et al. (88)

Basic design of a laser-plasma accelerator: single-stage limited by laser energy

laser

Ez wake

• Laser pulse length determined by plasma density– kp σz ≤ 1, σz ~ λp ~ n-1/2

• Wakefield regime determined by laser intensity– Linear (a0<1) or blowout (a0>1)

– Determines bunch parameters via beam loading

– Ex: a0 = 1 for I0 = 2x1018 W/cm2 and λ0 = 0.8 µm

• Accelerating field determined by density and laser intensity– Ez ~ (a0

2/4)(1+a02/2)-1/2 n1/2 ~ 10 GV/m

• Energy gain determined by laser energy via depletion*– Laser: Present CPA technology 10’s J/pulse

*Shadwick, Schroeder, Esarey, Phys. Plasmas (2009)

State-of-the-Art Prior to 2004:Self-Modulated Laser Wakefield Accelerator (SM-LWFA)

Self-modulated regime:• Laser pulse duration > plasma period• Laser power > critical power for self-guiding• High-phase velocity plasma waves by

• Raman forward scattering• Self-modulation instability

kp (z − ct)

a0

δρ

laser pulse

Plasma density waveSprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92);

Esarey et al. (94); Mori et al. (94)

SM-LWFA experiments routinely produce electrons with: 1-100 MeV (100% energy spread), 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)

Gas jet

Laser beam

Parabolic mirror

MirrorCCDe- beam

-40 -20 0 20 40

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

Detection Threshold

Energy Distribution

Red: 1000 psi HeBlue: 500 psi He

Leemans et al. (02)

Few TW

1020 cm-3

nC’s

30 Sep 2004 issue of nature:Three groups report production of high quality e-bunches

• Approach 1: Plasma channel• LBNL/USA: Geddes et al.

• Plasma Channel: 1-4x1019 cm-3

• Laser: 8-9 TW, 8.5 µm, 55 fs• E-bunch: 2×109 (0.3 nC), 86 MeV, ∆E/E=1-2%, 3 mrad

• Approach 2: No channel, larger spot size• RAL/IC/UK: Mangles et al.

• No Channel: 2×1019 cm-3

• Laser: 12 TW, 40 fs, 0.5 J, 2.5×1018 W/cm2, 25 µm• E-bunch: 1.4×108 (22 pC), 70 MeV, ∆E/E=3%, 87 mrad

• LOA/France: Faure et al. • No Channel: 0.5-2x1019 cm-3

• Laser: 30 TW, 30 fs, 1 J, 18 µm• E-bunch: 3×109 (0.5 nC), 170 MeV, ∆E/E=24%,10 mrad

• Channel allows higher e-energy with lower laser power

Breakthrough Results: High Quality Bunches

GeV: channeling over cm-scale

Increasing beam energy requires increased dephasing length and power:

Scalings indicate cm-scale channel at ~ 1018 cm-3 and ~50 TW laser for GeV

Laser heated plasma channel formation is inefficient at low density

Use capillary plasma channels for cm-scale, low density plasma channels

Capillary

∆W[GeV] ~ I[W/cm2] n[cm-3]

3 cm

e- beam

1 GeV

Laser: 40-100 TW, 40 fs 10 Hz

Plasma channel technology: Capillary

GeV Beams in 3cm

40TW laser Capillary discharge 1 Tesla magnetic

spectrometer Optical diagnostics

(not shown)

Divergence(rms): 1.6 mrad Energy spread (rms): 2.5% Resolution: 2.4%

3cm

Leemans et al., Nature Physics 2006

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

WAKE FORMING

INJECTION

DEPHASINGDEPHASING

Propagation Distance

Long

itudi

nal

Mom

entu

m200

0

Propagation DistanceLo

ngitu

dina

l M

omen

tum

200

0

Propagation Distance

Long

itudi

nal

Mom

entu

m200

0

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

LWFA: Production of a Monoenergetic Beam

1. Excitation of wake (e.g., self-modulation of laser)2. Onset of self-trapping (e.g., wavebreaking)3. Termination of trapping (e.g., beam loading)4. Acceleration

If > dephasing length: large energy spreadIf ≈ dephasing length: monoenergetic

Wake Excitation Trapping Acceleration: Laccel ~Ldephase

1 42-3

z-vgt

γv

Mom

entu

m

Phase

Ldph ≈ λp3 /λ2( )∝ ne

−3 / 2

• Dephasing distance:

GeV Beams Repeatable but not Stable –Available Controls not Sufficient

Accelerator performance

Laser energy, pulse width, plasma density, discharge delay, plasma channel density, depth, and length, degree of ionization

But optimizing injection does not optimize guiding (accelerating structure)Need to separate injection and acceleration

Reducing energy spread and emittance requires controlled injection

Self-injection experiments have been in bubble regime:

Cannot tune injection and acceleration separately

Emittance degraded due to off-axis injection and high transverse fields.

Energy spread degraded due to lack of control over trapping

⇒ Use injector based on controlled trapping at lower wake amplitude and separately tunable acceleration stage to reduce emittance and energy spread

Y[µm

]

X[µm]

5-5

800 2000

Transverse motion

Basic physics of downramp injection

Bucket length ~ 1/√n Phase velocity drop

enables trapping

13

n ex1

018

5

1

z [mm]0-0.5 0.5 1.0 30.0

Laser

Down-ramp Injector Demonstrated: Simulations Show Injector Coupled to Low Density Accelerator

Produces Low Energy Spread Beams

Inject low ∆E: ∆E conserved during acceleration so as E ↑, ∆E/E ↓

Geddes et al., PRL V 100, 215004 (2008) *Nieter et al., JCP 2004

Accelerator:3 - 50 cm; n~1017 -1018 cm-3

n

z

Plasma ramp injector:1mm; 1019 cm-3

laser

Laser10TW

e-

Jet

Laser focused on down-ramp of gas jet density profile

MeV beam produced with Low divergence (20 mrad) Good stability Central energy (760keV/c ± 20keV/c rms) Momentum spread (170 keV/c ± 20keV/c rms) Beam pointing (1.5 mrad rms)

Laser transmission 70% and mode still good for driving wakefield

Energy Spectrum atRamp Exit

#/P

z(M

eV/c

)

P 1.5MeV/c∆P 200keV/c

Energy Spectrum at3mm

Pz (MeV/c)

P 20MeV/c∆P 200keV/c

Gas Jet Nozzle Machined Into Capillary Can Provide Local Density Perturbation

laser

1mm

Laser-machined gas jet

Axis of the capillary

0.2m

m

Measured surface profile

Density profile in jet region

Jet Improves Beam Stability

Input Parameters: Pjet≈145psi,Ne ≈ 2x1018 cm-3,a0≈1 (25TW),Laser pulse length≈ 45 fs

laser

NB: Both data sets show subsequent shots

Pointing ± 0.8 mradDivergence 1mradEnergy 300MeV ± 7MeVΔE/E 6% ± 0.7%Q 7.3pC ± 1.7pC

Stability with jet

Best stabilitywithout jet

Pointing ± 1.8 mradDivergence 1mradEnergy 440MeV ± 95MeVΔE/E 4% ± 2%Q 2.6pC ± 2.0pC

Input Parameters: no jet in cap,Ne ≈ 2x1018 cm-3,a0≈1 (25TW),Laser pulse length≈ 45 fs

Colliding pulse allows control of injection

γβz

ψ = kp z − ct( )

-1.5

-1.0

-0.50

0.5

1.0

1.5

2.0

UntrappedWake Orbit

Trapped + Focused

Wake Orbit

Beat Wave Separatrices

-2 -1 0 1 2 3

injψ

Phase Space

• Add two counter-propagating laser pulses• Collision produces laser beat wave with slow phase velocity• 3-pulse colliding pulse [Esarey et al. PRL (1997), Schroeder et al., PRE (1999)]

1. control of injection position: delay between pump and trailing pulses2. control injected charge: laser intensities and pulse durations3. control beat phase velocity: different laser frequencies

• 2-pulse version: Pump + backward [Fubiani et al., PRE (2004)]

kpz

γβz

apump

a1

a2

∝ a1a2

βp

3-pulse Colliding Pulse Injection

γ

kp (z-ct)

laser∆γ

trapped orbits

untrappedorbits

laser

Controlled injection via colliding laser pulses improves beam quality

Esarey et al. PRL (1997); Schroeder et al. PRE (1999); Fubiani et al. PRE (2004);

e-

Leemans et al. AAC (2002); (2004);Faure et al. Nature (2006);Rechatin et al. PRL (2009);Kotaki et al. PRL (2009)

Experiment:

laser lasera=0.35a=1.2

Gas jet: 7x1018 cm-3

Pump laser (drives wake)

Collidinglaser pulse

3 mm

Rechatin et al. Phys. Rev. Lett. (2009)

LOA (France): Faure et al., Nature (2006)

Experimental demonstration (2-pulse):

1% FWHM energy spread

Theory:

Colliding pulse experiments at LBNL

Colliding pulse experimental setup online

Experimental plan:

Step1: demonstrate reliable injector

Step 2: accelerator/laser control for high energy

Step 3:tune for high quality beam

12 TW system

Colliding pulses produce stable, reproducible beam

Scan timing of collider

Charge measured on phosphor screen, ICT

Timing window as expected from simulation

~20% rms charge stability QICT ~ O[40pC]

Phosphor

Charge vs. collision timing

e-beam image

Simulationat a=0.5

Geddes et al., ongoing

Ionization-induced trapping using high-Z gas (nitrogen)

laser

z

Injection region, <mm, n~1018 cm-3

(mixture of H2 + high Z gas)

n

H2

Ionization-induced trapping using integrated gas jet + capillary (LBNL)

a

eEz mcω p

kp (z − ct)

pz mc

trapped e- orbit

• Trapped charge determined by length of injection region (<< dephasing length) and probability of ionization

kp (z − ct)

Umstadter et al. PRL (1996); M. Chen et al. JAP (2006);Schroeder et al. (in progress)

Theory:

A. Pak, K.A. Marsh, et al., PRL (2010)

Ionization injection experiments

Oz et al. PRL (2007); Rowlands-Rees et al. PRL (2008); Pak et al. PRL (2010)

Experiments:

2-mm gas jet: He/Nitrogen (10%)

PIC simuation: a=2.0, wr=15.0, f=75λ, Gaussian pulse, L=0.4λp , ne=0.001,

Ionization injection: improve beam quality with gas jet+capillary

nH H80%, N20% H

20 20 20 30 20x/λ

laser

M. Chen, C. Schroeder et al. (in progress)

10 GeV laser-plasma accelerator with BELLA (40 J laser)

a0 =1.5P/Pc = 1.1UL=40 JTL=130 fsrL=63 micron

1.1×1017 cm-3

79 cmλp = 110 µm

10 GeV X-ray FEL

low density plasmas (~1017 cm-3)

long plasma channels (~m)

Lacc ~ λp3 λL

2 ∝ n−3 / 2

W ~ mcω p e( )Lacc ∝ n−1

Accelerator length:

Energy gain:

Laser energy/power:

Ulaser ∝ n−3 / 2more laser energy (~10 J)

Plaser ∝ n−1

Plasma density scalings:

Plasma channel

Laser-plasma accelerators: Summary

• Self-modulated LWFAs: Status Prior to 2004– 100% energy spread, max energy > 100 MeV, nC’s of charge

• LWFAs: High quality e-beam production at 100 MeV-level (2004)– Narrow energy spread, small divergence, 100 MeV, 100’s pC

• LWFAs: High quality e-beam production at 1 GeV-level (2006)– Narrow energy spread (few %), small divergence (few mrad), 1 GeV, 10’s pC

– Few-cm long plasma channel guiding (capillary discharge)

• Downramp injection at 1 MeV-level (2008)– Good stability, narrow absolute momentum spread (170 keV/c), 100’s pC

• Integrated gas jet+capillary structure (2009)– Improved stability, few % energy spread, 0.5 GeV, few pC (ongoing)

• Colliding pulse injection at 100 MeV-level (2006, 2009)– Good stability, narrow energy spread (1%), 180 MeV, 10 pC

• Ionization injection at 100 MeV-level (2008, 2010)