Laser-Plasma Accelerators as Sources of Electron Beams at 1 MeV
to 1 GeV Eric Esarey FLS Workshop, March 1 -5, 2010
http://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
Slide 2
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)
Slide 3
Laser Wakefield Accelerator (LWFA) B.A. Shadwick et al., IEEE
PS. 2002 Standard regime (LWFA): pulse duration matches plasma
period Ultrahigh axial electric fields E z > 10 GV/m, fast waves
Ultrashort plasma wavelength p ~ 30 m (100 fs) Plasma channel:
Guides laser pulse and supports plasma wave Tajima, Dawson (79);
Gorbunov, Kirsanov (87); Sprangle, Esarey et al. (88)
Slide 4
Basic design of a laser-plasma accelerator: single-stage
limited by laser energy laser E z wake Laser pulse length
determined by plasma density k p z 1, z ~ p ~ n -1/2 Wakefield
regime determined by laser intensity Linear (a 0 1) Determines
bunch parameters via beam loading Ex: a 0 = 1 for I 0 = 2x10 18
W/cm 2 and 0 = 0.8 m Accelerating field determined by density and
laser intensity E z ~ (a 0 2 /4)(1+a 0 2 /2) -1/2 n 1/2 ~ 10 GV/m
Energy gain determined by laser energy via depletion* Laser:
Present CPA technology 10s J/pulse * Shadwick, Schroeder, Esarey,
Phys. Plasmas (2009)
Slide 5
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 laser pulse Plasma density wave
Sprangle 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 Mirror CCD e - beam -40-2002040 -0.75 -0.50 -0.25 0.00 0.25
0.50 0.75 Detection Threshold Energy Distribution Red: 1000 psi He
Blue: 500 psi He Leemans et al. (02) Few TW 10 20 cm -3 nCs
Slide 6
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: 1-4x10 19 cm -3 Laser: 8-9 TW, 8.5 m, 55 fs
E-bunch: 2 10 9 (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
10 19 cm -3 Laser: 12 TW, 40 fs, 0.5 J, 2.5 10 18 W/cm 2, 25 m
E-bunch: 1.4 10 8 (22 pC), 70 MeV, E/E=3%, 87 mrad LOA/France:
Faure et al. No Channel: 0.5-2x10 19 cm -3 Laser: 30 TW, 30 fs, 1
J, 18 m E-bunch: 3 10 9 (0.5 nC), 170 MeV, E/E=24%,10 mrad Channel
allows higher e-energy with lower laser power Breakthrough Results:
High Quality Bunches
Slide 7
GeV: channeling over cm-scale Increasing beam energy requires
increased dephasing length and power: Scalings indicate cm-scale
channel at ~ 10 18 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 3 cm e - beam 1 GeV Laser: 40-100 TW, 40 fs 10 Hz Plasma
channel technology: Capillary
Slide 8
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
Slide 9
Wake Evolution and Dephasing Yield Low Energy Spread Beams in
PIC Simulations WAKE FORMING INJECTION DEPHASING Propagation
Distance Longitudinal Momentum 200 0 Propagation Distance
Longitudinal Momentum 200 0 Propagation Distance Longitudinal
Momentum 200 0 Geddes et al., Nature (2004) & Phys. Plasmas
(2005)
Slide 10
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 spread If
dephasing length: monoenergetic Wake Excitation
TrappingAcceleration: L accel ~L dephase 142-3 z-v g t vv Momentum
Phase Dephasing distance :
Slide 11
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
Slide 12
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[m] X[m] 5 -5 8002000 Transverse motion
Slide 13
Basic physics of downramp injection Bucket length ~ 1/n Phase
velocity drop enables trapping 13 nene x10 18 5 1 z [mm] 0 -0.5
0.51.030.0 Laser
Slide 14
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~10 17 -10 18 cm -3 n z Plasma ramp
injector: 1mm; 10 19 cm -3 laser Laser 10TW e-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 at Ramp Exit #/P z (MeV/c) P
1.5MeV/c P 200keV/c Energy Spectrum at 3mm P z (MeV/c) P 20MeV/c P
200keV/c
Slide 15
Gas Jet Nozzle Machined Into Capillary Can Provide Local
Density Perturbation laser e - beam 1mm Laser-machined gas jet Axis
of the capillary 0.2mm Measured surface profile Density profile in
jet region
Slide 16
Jet Improves Beam Stability Input Parameters: P jet 145psi, N e
2x10 18 cm -3,a 0 1 (25TW), Laser pulse length 45 fs laser NB: Both
data sets show subsequent shots Pointing 0.8 mrad Divergence 1mrad
Energy 300MeV 7MeV E/E 6% 0.7% Q 7.3pC 1.7pC Stability with jet
Best stability without jet Pointing 1.8 mrad Divergence 1mrad
Energy 440MeV 95MeV E/E 4% 2% Q 2.6pC 2.0pC Input Parameters: no
jet in cap, N e 2x10 18 cm -3,a 0 1 (25TW), Laser pulse length 45
fs
Slide 17
Colliding pulse allows control of injection -1.5 -0.5 0 0.5 1.0
1.5 2.0 Untrapped Wake Orbit Trapped + Focused Wake Orbit Beat Wave
Separatrices -20 1 23 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 pulses 2. control injected charge: laser
intensities and pulse durations 3. control beat phase velocity:
different laser frequencies 2-pulse version: Pump + backward
[Fubiani et al., PRE (2004)] kpzkpz 3-pulse Colliding Pulse
Injection
Slide 18
k p (z-ct) laser trapped orbits untrapped orbits 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-e- Leemans et al. AAC (2002); (2004);
Faure et al. Nature (2006); Rechatin et al. PRL (2009); Kotaki et
al. PRL (2009) Experiment: laser a=0.35 a=1.2 Gas jet: 7x10 18 cm
-3 Pump laser (drives wake) Colliding laser 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:
Slide 19
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
Slide 20
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 Q ICT ~
O[40pC] Phosphor Charge vs. collision timing e-beam image
Simulation at a=0.5 Geddes et al., ongoing
Slide 21
Ionization-induced trapping using high-Z gas (nitrogen) laser z
Injection region,
Laser-plasma accelerators: Summary Self-modulated LWFAs: Status
Prior to 2004 100% energy spread, max energy > 100 MeV, nCs of
charge LWFAs: High quality e-beam production at 100 MeV-level
(2004) Narrow energy spread, small divergence, 100 MeV, 100s pC
LWFAs: High quality e-beam production at 1 GeV-level (2006) Narrow
energy spread (few %), small divergence (few mrad), 1 GeV, 10s 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), 100s 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)