1. 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
Jeroen van Tilborg
LOASIS program, LBNL

2. Acknowledgements LDRD
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
Collaborators
CEA Saclay: Sylvain Monchocé, Fabien Quéré, Arnaud Malvache, and Philip Martin
LBNL, ALS: Eric Gullikson
LBNL, Metrology: Valeriy Yashchuk, Wayne McKinney, and Nikolay Artemiev
LDRD

3. 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

4. Each LOASIS/Bella system addresses unique challenges
Godzilla
Bella
TREX
Matlis, 10:50am
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)
Measured at LOASIS

5. Seeding the FEL has benefits Goal: 53-nm LPA-driven seeded FEL
Bunching starts sooner
More stable
Longitudinal coherence
Goal: 50 nm seeded FEL
Schroeder et al., Proc. FEL (2006)
Schroeder et al., Proc. FEL (2008)

6. 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

7. Step 1 & 2: Electrons are pulled out of plasma into vacuum, and back into target
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)

8. Step 3: Electron beamlets drive wake and emit radiation at density step
Plasma ωp
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)

9. Experimental Setup Focal length=2m, θ=35 mrad (FWHM)
Borot et al. Opt. Lett. 36 (2011)
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

10. Orders up the 18th observed, at divergences of 4-15 mrad
Shaw et al., submitted
Al foil
Table from Queré
(CEA Saclay)

11. Dependence spectrum on intensity
15th
70 mJ
VHS tape (“front”, iron oxide side)
15th and 16th only at higher intensities
15th harmonic, x225 over-critical
Lower intensity  density not high enough
150 mJ
300 mJ
15th
70 mJ
150 mJ

12. Divergence depends on tape material
Same laser conditions
different targets
different divergences
Glass mrad (rms)
Kapton mrad (rms)
VHS & Mylar ~13 mrad (rms)
Roughness plays role?

13. Roughness more complex than just “sigma”k
Harvey et al. Opt. Eng. 51 (2012)
1/λ
1/w0
ALS reflectometry
Metrology
Gold
627x470 μm
20 μm
Kapton
627x470 μm
20 μm
Power Spectral Density ~ FFT[ height distribution ]

14. Metrology reveals differences in roughness (correlated to divergence)
Glass mrad (rms)
Kapton mrad (rms)
VHS & Mylar ~13 mrad (rms)

15. Quasi-linear CWE provides stability
VHS-front (iron-oxide on Mylar)
30 mrad
Pointing fluctuation
0.2 mrad
Divergence
fluctuation
2 mrad
Fluctuations total counts ~5%

16. Concave reflective grating  order-specific divergence
VHS-front (iron-oxide on Mylar)
Integrated over entire spectrum
33 mrad (FWHM)
15 mrad (FWHM)
17 mrad (FWHM)
11.5 mrad (FWHM)
15th
14th
13th

17. Absolute flux calibration: megaWatts seed in 15th order
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
ALS CXRO beamline 6.3.2 (
Borot et al. Opt. Lett. 36 (2011)
CWE
Easter et al. Opt. Lett. 35 (2010)

18. Measured seed parameters & FEL model predict FEL gain
15th harmonic
60 nJ in 20 fs
Focus 1 cm upstream
Divergence 5.7 mrad (rms)
Undulator & e-beam:
4.4 kA peak current
25 micron transverse size
Undulator period 2.18 cm
K=1.25
Wavelength 53 nm (15th)
Pierce parameter 0.012
100 nJ
Seed strength as
2 mrad
5 mrad
10 mrad
15 mrad
Z [m]
FEL radiation
Phase electron
Energy electron
Model:
Mono-energetic e-beam
1d FEL radiation
Not included: slippage, wavefront curvature
Shaw et al., submitted

19. Further seed source improvement possible? Spectral details give insight
70 mJ
150 mJ
300 mJ
15th
70 mJ
150 mJ
15th

20. Concentrate on 12th harmonic: higher intensity broadening & blue-shifting
150 mJ
70 mJ
300 mJ
Energy scan
Focal scans
Always a red-shifted spectrum
Higher intensity  Broadening
Higher intensity  Less red-shifting
driver 800nm
order 820nm/q

21. Use of a model to predict attochirp: dependent on intensity and density gradient
Density n(x)
Malvache et al., PRE 87 (2013)
Longer gradient  longer delay
Higher a  faster e’s  shorter delay
Leading edge: next cycle emits
faster then previous one  blue-shifting
Harmonic q
nc,ωq
Fundamental
nc,ωL xω x
x=0

22. Energy and Focal scans: Model incomplete to match data
Energy scan
Focal scan
No red-shifting
Higher intensity
Narrowing
No shifts
Model
No averaging over spot-size
No propagation to diagnostic
van Tilborg et al., in preparation (LBNL)
300 mJ
150 mJ
70 mJ
Energy scan
150 mJ
Red-shifting
Higher intensity
Broadening
Less red-shifting
Focal scan

23. Expand the model: include expanding plasma gradient
Increasing gradient length δ (distance ncr to ncr,q)
Density n(x)
nmax nq
Plasma expansion
Saclay*: Pump 1e15 W/cm2  Cs=20 nm/ps
We: Pump 3e17 W/cm2  Cs~ nm/ps
Warm plasma
Harmonic q
nc,ωq
Brunel orbits
Heissler et al., Appl. Phys. B 101 (2010)
Fundamental
nc,ωL xω x
x=0

24. Energy and Focal scans: better agreement expanded model
Red-shifting
Higher intensity
Broadening
Less red-shifting
Energy scan
Focal scan
300 mJ
150 mJ
70 mJ
Energy scan
150 mJ
Red-shifting
Higher intensity
Broadening
Less red-shifting
Focal scan

25. 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

27. ALS data reveals <13 nm on most samples (weak correlation divergence)
1/λ
1/w0
ALS reflectometry
Glass mrad (rms)
Kapton mrad (rms)
VHS & Mylar ~13 mrad (rms)
Could be cancelled!

28. Laser chirp can compensate for
CWE femtochirp
ξ=1 (red front)
ξ=-1 (blue front)
ξ=0
Borot et al. Opt. Lett. 36 (2011)
Blue-shifting
Red-shifting

29. Stable shot to shot performance
Experiment
Experiment
Scan
parameter
Model
Comparison Experiment to Model
Insight in CWE physics
Use insight for optimization
Scan
parameter

30. 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

31. Bottom line: deliver seed strength 10-6-10-5 to undulator
2 nJ
2 mrad
Seed:
60 nJ in 20 fs
Model:
1d-description FEL radiation
No wavefront effects
No slippage
100 nJ
Seed strength as
2 mrad
5 mrad
10 mrad
15 mrad
Z [m]
Phase electron
Energy electron
FEL radiation

32. Notes on Sequoia Scan
Divergence 4-15 mrad (rms)

33. 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 (CWE day 2). Transmission through Kapton (on fiber Hamamatsu).
-Reflectometry on scan (VHS-front) Chromax
-Also confirmed by (CWE day 1), compressor scan
Scan33,
Sequoia data and Grenouille data where taken and compared on By including temporal resolution, nice fitting for both diagnostics is retrieved

34. 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 (scan 16) gives a FWHM at focus of 23 micron.
-Guppy Strehl ratio experiments on 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.
, scan 16

35. Roughness more complex than just “sigma”
FFT[ h(x) ]
FFT[ h(x) ]
Same Sigma, Different regime
Critical is the spatial frequencies λ
1/λ
k [nm-1]
Assumption
Nevot-Croce
“single σ“
CXRO
grazing
reflectometry λ
1/λ
k [nm-1]

36. Conclusion Gradient length δ Function 1 Vdelta=1e-5
Time shift = 1e-5 ps per cycle, or 3nm per cycle, or 1100 nm/ps

37. Intro to Laser Plasma Accelerators (LPA’s)
e- beam
laser
LPA: Self injection + acceleration
Show cap
Godzilla
Bella
TREX

38. High-power lasers: trade-off scale-length and HHG divergence
General concept: More laser  More harmonics
Example, 200 mJ of laser, 50 fs
Gas-based harmonics
Requirement: I~5x1014 W/cm2
Yields spotsize w0=0.7 mm, zR=1.9 m
At z=5 m: w0=1.9 mm, Fluence=1900 mJ/cm2
At z=10 m: w0=3.7 mm, Fluence= 470 mJ/cm2
ROM harmonics
Requirement: I~1x1019 W/cm2
Yields spotsize w0=5 μm, zR=100 μm, θ=50 mrad
Typically: 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!

39. Intensity regimes for Laser-produced Harmonics
Coherent Wakefield Emission
Laser on overdense plasma
Quasi-linear motion of surface electrons
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

40. Laser chirp can compensate for
CWE femtochirp
Blue-shifting
Red-shifting
ξ=1 (red front
ξ=-1 (blue front)
Borot et al. Opt. Lett. 36 (2011)

41. Coherent Wakefield Excitation: 3-step model for laser-plasma interaction
Laser (p-polarized) drives surface electrons out-of-target
Laser & plasma restoring force drive electrons back.
E-bunches travel through density gradient, emit radiation at the plasma frequency
Heissler et al. Appl. Phys. B 101 (2010)

42. 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)
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
Time
Energy
Decompression
Undulator
Six 22-period sections (now three)
K=1.25
x10 decompression
seeded FEL
no decompression
seeded FEL
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

43. 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
Hörlein, thesis MPQ (2008)
tL=2.67 fs