high harmonic generation off a tape drive as seed for the lpa-based fel physics and applications of...
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High Harmonic Generation off a Tape Drive as seed for the LPA-based FEL
Physics and Applications of High Brightness Beams: Towards a Fifth Generation Light Source
Monday 2013-03-25
Jeroen van TilborgLOASIS program, LBNL
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Acknowledgements
HHG experiments: Brian Shaw, Thomas Sokollik, Jeroen van Tilborg, and Wim Leemans
FEL concept & simulations: Carl Schroeder
Other LOASIS contributors: Sergey Rykovanov, Anthony Gonsalves, Kei Nakamura, Sven Steiniger, Nicholas Matlis, Eric Esarey, Csaba Tóth, Carlo Benedetti, and Cameron Geddes
CollaboratorsCEA Saclay: Sylvain Monchocé, Fabien Quéré, Arnaud Malvache, and Philip Martin LBNL, ALS: Eric GulliksonLBNL, Metrology: Valeriy Yashchuk, Wayne McKinney, and Nikolay Artemiev
LDRD
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Outline
Efforts at LOASIS/Bella Introduction to Coherent Wake Emission Experimental setup and data Influence of tape and laser parameters FEL calculations Comparison CWE details to model
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Each LOASIS/Bella system addresses unique challenges
Gonsalves et al. Nat. Phys 7 (2011)
Plateau et al. PRL 109 (2012)
High-quality LPA e-beams: compact coherent light source[energy, stability1, emittance2, (slice) spread3, charge]
1. Jet+Cap, Gonsalves et al. Nat. Phys 7 (2011)2. Betatron X-rays: Plateau et al. PRL 109 (2012)3. COTR: Lin et al. PRL 108 (2012)
Godzilla BellaTREX
Measured at LOASIS
Matlis, 10:50am
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Seeding the FEL has benefitsGoal: 53-nm LPA-driven seeded FEL
Schroeder et al., Proc. FEL (2006)Schroeder et al., Proc. FEL (2008)
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High-power lasers: trade-off scale-length and HHG divergence
200 mJ Laser Large spot (small HHG divergence)gas-based HHG
Small spot (large HHG divergence)
ROM HHG
Coherent Wake Emission I~1x1017 W/cm2
<2 meter delivery optics Target destruction: tape! Combiner, no transport Easy spatial overlap Quasi-linear regime
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Step 1 & 2: Electrons are pulled out of plasma into vacuum, and back into target
Step 1 Laser 45o on high-n target Ionization Brunel electrons into vacuum
Step 2 Restoring force turns electrons around into target “Ejection phase” determines return time and return velocity E-beam chirp leads to bunching
Heissler et al. Appl. Phys. B 101 (2010)
Hörlein, thesis MPQ (2008)
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Step 3: Electron beamlets drive wake and emit radiation at density step
At density step, e-beam creates plasma wave Light emitted at plasma frequency Gradient density emits broad spectrum Maximum frequency given by maximum density Every cycle Even and odd harmonics Atto-chirp present (high frequencies late)
electron beamPlasma
ωp
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Experimental Setup
Focal length=2m, θ=35 mrad (FWHM) P-polarization after 3” waveplate Change energy, zfocus, compression Mylar, VHS, Kapton tape. Glass plate Silicon Brewster plate (X~100) 100-nm-period transmission grating Double-stacked MCP
Borot et al. Opt. Lett. 36 (2011)
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Orders up the 18th observed,at divergences of 4-15 mrad
Shaw et al., submitted
Al foil
Table from Queré (CEA Saclay)
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15th
Dependence spectrum on intensity
VHS tape (“front”, iron oxide side) 15th and 16th only at higher intensities 15th harmonic, x225 over-critical Lower intensity density not high enough
300 mJ
150 mJ
70 mJ
15th
70 mJ 150 mJ15th
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Divergence depends on tape material
Same laser conditionsdifferent targetsdifferent divergences
Glass 3.9 mrad (rms)Kapton 7.4 mrad (rms)VHS & Mylar ~13 mrad (rms)
Roughness plays role?
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Gold627x470 μm 20 μm
Kapton627x470 μm
20 μm
Roughness more complex than just “sigma”
Power Spectral Density ~ FFT[ height distribution ]
k
Harvey et al. Opt. Eng. 51 (2012)
1/λ1/w0
ALS reflectometryMetrology
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Metrology reveals differences in roughness(correlated to divergence)
Glass 3.9 mrad (rms)Kapton 7.4 mrad (rms)VHS & Mylar ~13 mrad (rms)
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Quasi-linear CWE provides stability
30 mrad
VHS-front (iron-oxide on Mylar)
Pointing fluctuation0.2 mrad
Divergencefluctuation 2 mrad
Fluctuations total counts ~5%
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Concave reflective grating order-specific divergence
VHS-front (iron-oxide on Mylar)
Integrated over entire spectrum33 mrad (FWHM)
15 mrad (FWHM)17 mrad (FWHM) 11.5 mrad (FWHM)
15th 14th
13th
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Absolute flux calibration:megaWatts seed in 15th order
ALS CXRO beamline 6.3.2(http://cxro.lbl.gov/reflectometer)
Flux Circa 20% in 15th order 67 photons/count, 5x109 photons, 20 nJ Lose 40% Al foil, 35% Brewster plate 50 nJ in 20 fs, is ~2.5 MW Laser energy on target ~ 70 mJ CE for 15th is 7x10-7
Up to 250 mJ available Working on improvement
Borot et al. Opt. Lett. 36 (2011)
CWE
Easter et al. Opt. Lett. 35 (2010)
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Measured seed parameters & FEL model predict FEL gain
15 mrad
10 mrad
5 mrad
2 mrad
100 nJ
See
d st
reng
th a
s
Z [m]
Seed:15th harmonic60 nJ in 20 fsFocus 1 cm upstreamDivergence 5.7 mrad (rms)
Undulator & e-beam:4.4 kA peak current25 micron transverse sizeUndulator period 2.18 cmK=1.25Wavelength 53 nm (15th)Pierce parameter 0.012
FEL radiation
Phase electron
Energy electron
Model:Mono-energetic e-beam1d FEL radiationNot included: slippage, wavefront curvature
Shaw et al., submitted
€
θ ~ λ /πw0
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15th
15th
300 mJ
150 mJ
70 mJ
70 mJ 150 mJ15th
Further seed source improvement possible? Spectral details give insight
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Concentrate on 12th harmonic: higher intensity broadening & blue-shifting
150 mJ70 mJ
300 mJ
150 mJ
70 mJ
Energy scan
Focal scans
Always a red-shifted spectrumHigher intensity BroadeningHigher intensity Less red-shifting
driver 800nmorder 820nm/q
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x
Density n(x)
nc,ωL
nc,ωq
xω
Fundamental
Harmonic q
Use of a model to predict attochirp: dependent on intensity and density gradient
x=0
€
tω =xω
a0
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 3
Malvache et al., PRE 87 (2013)
Longer gradient longer delayHigher a faster e’s shorter delay
Leading edge: next cycle emits faster then previous one blue-shifting
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Energy and Focal scans: Model incomplete to match data
300 mJ
150 mJ
70 mJ
Energy scan
150 mJ
Focal scan
Model-No averaging over spot-size-No propagation to diagnostic
van Tilborg et al., in preparation (LBNL)
Red-shifting
Higher intensity-Broadening-Less red-shifting
Energy scan
Focal scan
No red-shifting
Higher intensity-Narrowing-No shifts
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Expand the model: include expanding plasma gradient
Increasing gradient length δ (distance ncr to ncr,q)
€
δ(t) = δ 0 +Cst
nmax
nq
Plasma expansion Saclay*: Pump 1e15 W/cm2 Cs=20 nm/ps We: Pump 3e17 W/cm2 Cs~100-1000 nm/ps
Warm plasma
Brunel orbits
Heissler et al., Appl. Phys. B 101 (2010)
x
Density n(x)
nc,ωL
nc,ωq
xω
Fundamental
Harmonic q
x=0
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300 mJ
150 mJ
70 mJ
Energy scan
150 mJ
Focal scan
Energy scan
Focal scan
Red-shifting
Higher intensity-Broadening-Less red-shifting
Energy and Focal scans: better agreement expanded model
Red-shifting
Higher intensity-Broadening-Less red-shifting
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Conclusion
Research towards compact (seeded) LPA-based FEL HHG from spooling tape Harmonics up to the 17th, 5-15 mrad divergence Tape roughness at micron-level is relevant MW-powers from VHS and Kapton FEL model predicts seed-induced bunching CWE model suggests plasma expansion relevant New round of CWE experiments planned
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ALS data reveals <13 nm on most samples(weak correlation divergence)
k
1/λ1/w0
ALS reflectometry
Glass 3.9 mrad (rms)Kapton 7.4 mrad (rms)VHS & Mylar ~13 mrad (rms)
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ξ=0 ξ=1 (red front)
ξ=-1 (blue front)
€
τ =τ0 1+ ξ 2
Laser chirp can compensate for CWE femtochirp
Blue-shifting Red-shifting
Borot et al. Opt. Lett. 36 (2011)
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Stable shot to shot performance
Experiment
Model
Scanparameter
Scanparameter
Experiment
Comparison Experiment to Model Insight in CWE physics Use insight for optimization
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Questions
-Sergey, what drives the electrons back into the target. The laser, or the restoring force of the plasma? If a density gradient exists, which electrons get pulled out? Where is the field supposed to be zero? Where does density gradient come from? Surface roughness? Plasma expansion into vacuum?-Thomas Strehl Ratio
e-beam
HHG drive laser
Tape Drive
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2 nJ2 mrad
Bottom line: deliver seed strength 10-6-10-
5 to undulator
15 mrad
10 mrad
5 mrad
2 mrad
100 nJS
eed
stre
ngth
as
Z [m]
FEL radiation
Phase electron
Energy electron
Seed:60 nJ in 20 fs
Model:1d-description FEL radiationNo wavefront effectsNo slippage
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Notes on Sequoia Scan
Divergence 4-15 mrad (rms)
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Notes on Compressor Data
-In-vacuum optimum compression is at comp4=-0.1mm.-Positive Comp4 Negative xi Blue front, red back Makes femtochirp worse Broad harmonics-Scan 33 on 2012-07-09 (CWE day 2). Transmission through Kapton (on fiber Hamamatsu).-Reflectometry on 2012-10-04 scan (VHS-front) Chromax -Also confirmed by 2012-06-28 (CWE day 1), compressor scan
€
τ =τ0 1+ ξ 2
Scan33, 2012-07-09Sequoia data and Grenouille data where taken and compared on 2012-09-05. By including temporal resolution, nice fitting for both diagnostics is retrieved
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Notes on spot size
-In-vacuum smallest spot is at z=+2 mm-Positive z focus downstream (more harmonics if focused at z=2mm, but smaller divergence at z=>3mm, see Day 2, scan 20)-Guppy scan on 2012-06-26 (scan 16) gives a FWHM at focus of 23 micron.-Guppy Strehl ratio experiments on 2012-07-18 give a FWHM of 23 micron (w0=19.5 micron), and a Strehl ratio of 0.73. -Use file “NotesSpotAveragedIntensity”. Based on 73%, we calculate a 100 mJ, 47.7 fs (I-FWHM), we find an Ipeak of 2.04e17 Wcm2.-We fitted the max-counts versus z to calculated intensity at other z’s.
2012096026, scan 16
€
Ipeak = 2.04 ×1017 ×ActualEnergy
100mJx
47.7 fs
ActualPulseDurationx
1
1+(z − 2 ×10−3)2λ2
π 2 19.5 ×10−6( )
4
€
Ipeak =2Ppeak
πr02 =
2Energy
(τ π /2)πr02
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Roughness more complex than just “sigma”
λ
Same Sigma, Different regimeCritical is the spatial frequencies
FFT[ h(x) ]
λ
k [nm-1]1/λ
FFT[ h(x) ]
k [nm-1]1/λ
AssumptionNevot-Croce“single σ“CXRO
grazing reflectometry
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Conclusion
Gradient length δ
€
δ(t) = δ 0 +Cst
Cs =kTi
M i
Function 1Vdelta=1e-5Time shift = 1e-5 ps per cycle, or 3nm per cycle, or 1100 nm/ps
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Intro to Laser Plasma Accelerators (LPA’s)
e- beamlaser LPA: Self injection + acceleration
Godzilla BellaTREX
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High-power lasers: trade-off scale-length and HHG divergence
General concept: More laser More harmonicsExample, 200 mJ of laser, 50 fs
Gas-based harmonicsRequirement: I~5x1014 W/cm2
Yields spotsize w0=0.7 mm, zR=1.9 mAt z=5 m: w0=1.9 mm, Fluence=1900 mJ/cm2 At z=10 m: w0=3.7 mm, Fluence= 470 mJ/cm2
ROM harmonicsRequirement: I~1x1019 W/cm2
Yields spotsize w0=5 μm, zR=100 μm, θ=50 mradTypically: Divergence harmonics ~ divergence laser
Coherent Wakefield Emission Intensities around I~1x1017 W/cm2
<20-mrad laser divergence <2 meter delivery optics CHALLENGE: Target destroyed every shot!
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Intensity regimes for Laser-produced Harmonics
Gas-based HHG Intensity ~ Ionization potential Laser on underdense plasma Phase matching (along z) important
Reflection off “relativistic mirror” Laser on overdense plasma a0>>1: longitudinal quiver motion
Coherent Wakefield Emission Laser on overdense plasma Quasi-linear motion of surface electrons
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ξ=1 (red front ξ=-1 (blue front)
€
τ =τ0 1+ ξ 2
Laser chirp can compensate for CWE femtochirp
Blue-shifting Red-shifting
Borot et al. Opt. Lett. 36 (2011)
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Coherent Wakefield Excitation: 3-step model for laser-plasma interaction
1. Laser (p-polarized) drives surface electrons out-of-target
2. Laser & plasma restoring force drive electrons back.
3. E-bunches travel through density gradient, emit radiation at the plasma frequency
Heissler et al. Appl. Phys. B 101 (2010)
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FEL simulation based on CWE source
Seed 50 nJ in the 15th
7 mrad (rms) divergence Source 1 cm from undulator 20 fs (FWHM duration)
Undulator Six 22-period sections (now three) K=1.25
Electron beam 307 MeV, λu=53 nm (15th) 25 pC (5 fs flat-top from LPA) Transverse size ~20 micron Ideal 0.5% dE/E, upto 4% dE/E Include beam decompression
x10 decompressionseeded FEL
no decompressionseeded FEL
Time
Ene
rgy
Decompression
Comments Optimize simulations Tapered undulator help Have energy up to 200 mJ available Seen 5-mrad (rms) divergence on VHS (Int) Kapton, integrated ~50% of VHS (Int) Optimization underway
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Repeats every laser cycle: odd and even harmonics
In a density ramp: Consider all n’s, each at specific location x Emission of continuous spectrum Low frequencies emitted first Attochirp
Happens every cycle: Even & odd harmonics
tL=2.67 fs
Hörlein, thesis MPQ (2008)