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
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
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
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)
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Measured at LOASIS
Matlis, 10:50am
Seeding the FEL has benefitsGoal: 53-nm LPA-driven seeded FEL
Schroeder et al., Proc. FEL (2006)Schroeder et al., Proc. FEL (2008)
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
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)
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
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)
Orders up the 18th observed,at divergences of 4-15 mrad
Shaw et al., submitted
Al foil
Table from Queré (CEA Saclay)
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
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?
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
Metrology reveals differences in roughness(correlated to divergence)
Glass 3.9 mrad (rms)Kapton 7.4 mrad (rms)VHS & Mylar ~13 mrad (rms)
Quasi-linear CWE provides stability
30 mrad
VHS-front (iron-oxide on Mylar)
Pointing fluctuation0.2 mrad
Divergencefluctuation 2 mrad
Fluctuations total counts ~5%
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
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)
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
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θ ~ λ /πw0
15th
15th
300 mJ
150 mJ
70 mJ
70 mJ 150 mJ15th
Further seed source improvement possible? Spectral details give insight
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
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
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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
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
Expand the model: include expanding plasma gradient
Increasing gradient length δ (distance ncr to ncr,q)
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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
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
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
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)
ξ=0 ξ=1 (red front)
ξ=-1 (blue front)
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τ =τ0 1+ ξ 2
Laser chirp can compensate for CWE femtochirp
Blue-shifting Red-shifting
Borot et al. Opt. Lett. 36 (2011)
Stable shot to shot performance
Experiment
Model
Scanparameter
Scanparameter
Experiment
Comparison Experiment to Model Insight in CWE physics Use insight for optimization
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
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
Notes on Sequoia Scan
Divergence 4-15 mrad (rms)
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
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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
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
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Ipeak = 2.04 ×1017 ×ActualEnergy
100mJx
47.7 fs
ActualPulseDurationx
1
1+(z − 2 ×10−3)2λ2
π 2 19.5 ×10−6( )
4
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Ipeak =2Ppeak
πr02 =
2Energy
(τ π /2)πr02
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
Conclusion
Gradient length δ
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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
Intro to Laser Plasma Accelerators (LPA’s)
e- beamlaser LPA: Self injection + acceleration
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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!
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
ξ=1 (red front ξ=-1 (blue front)
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τ =τ0 1+ ξ 2
Laser chirp can compensate for CWE femtochirp
Blue-shifting Red-shifting
Borot et al. Opt. Lett. 36 (2011)
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)
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
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)