high harmonic generation off a tape drive as seed for the lpa-based fel physics and applications of...

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)