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High Harmonic Generation off a Tape Drive as seed for the LPA-based FEL

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## High Harmonic Generation off a Tape Drive as seed for the LPA-based FEL

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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 Tilborg LOASIS 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 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**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**Matlis, 10:50am Measured at LOASIS 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 TREX Bella Plateau et al. PRL 109 (2012) Gonsalves et al. Nat. Phys 7 (2011)**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**gas-based HHG Large spot (small HHG divergence) 200 mJ Laser ROM HHG Small spot (large HHG divergence) • 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 Heissler et al. Appl. Phys. B 101 (2010) • 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 Hörlein, thesis MPQ (2008)**Step 3: Electron beamlets drive wake and emit radiation at**density step electron beam 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)**Experimental Setup**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**Orders up the 18th observed,at divergences of 4-15 mrad**Shaw et al., submitted Al foil Table from Queré (CEA Saclay)**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 70 mJ 150 mJ 300 mJ 15th 15th 150 mJ 70 mJ 15th**Divergence depends on tape material**Same laser conditions different targets different divergences Glass 3.9 mrad (rms) Kapton 7.4 mrad (rms) VHS & Mylar ~13 mrad (rms) Roughness plays role?**Roughness more complex than just “sigma”**Harvey et al. Opt. Eng. 51 (2012) 1/λ 1/w0 Metrology ALS reflectometry k Gold 627x470 μm Kapton 627x470 μm 20 μm 20 μm Power Spectral Density ~ FFT[ height distribution ]**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**VHS-front (iron-oxide on Mylar) 30 mrad • Pointing fluctuation • 0.2 mrad • Divergence • fluctuation • 2 mrad • Fluctuations total counts ~5%**Concave reflective grating **order-specific divergence VHS-front (iron-oxide on Mylar) Integrated over entire spectrum 33 mrad (FWHM) 13th 14th 15th 11.5 mrad (FWHM) 17 mrad (FWHM) 15 mrad (FWHM)**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**Seed: 15th harmonic 60 nJ in 20 fs Focus 1 cm upstream Divergence 5.7 mrad (rms) 100 nJ Seed strength as 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 2 mrad 5 mrad 10 mrad 15 mrad Z [m] Model: Mono-energetic e-beam 1d FEL radiation Not included: slippage, wavefront curvature Shaw et al., submitted Phase electron Energy electron FEL radiation**Further seed source improvement possible? Spectral details**give insight 70 mJ 150 mJ 300 mJ 15th 15th 150 mJ 70 mJ 15th**Concentrate on 12th harmonic: higher intensity broadening &**blue-shifting 70 mJ 150 mJ Focal scans Always a red-shifted spectrum Higher intensity Broadening Higher intensity Less red-shifting 70 mJ 150 mJ Energy scan driver 800nm order 820nm/q 300 mJ**Use of a model to predict attochirp: dependent on intensity**and density gradient Density n(x) Longer gradient longer delay Higher a faster e’s shorter delay Leading edge: next cycle emits faster then previous one blue-shifting Harmonic q Malvache et al., PRE 87 (2013) nc,ωq Fundamental nc,ωL xω x x=0**Energy and Focal scans: Model incomplete to match data**• No red-shifting • Higher intensity • Narrowing • No shifts 150 mJ Focal scan Energy scan • Model • No averaging over spot-size • No propagation to diagnostic van Tilborg et al., in preparation (LBNL) 70 mJ • Red-shifting • Higher intensity • Broadening • Less red-shifting 150 mJ Energy scan Focal scan 300 mJ**Expand the model: include expanding plasma gradient**Increasing gradient length δ (distance ncr to ncr,q) Density n(x) nmax Warm plasma nq Harmonic q • Plasma expansion • Saclay*: Pump 1e15 W/cm2 Cs=20 nm/ps • We: Pump 3e17 W/cm2 Cs~100-1000 nm/ps nc,ωq Heissler et al., Appl. Phys. B 101 (2010) Brunel orbits Fundamental nc,ωL xω x x=0**Energy and Focal scans: better agreement expanded model**• Red-shifting • Higher intensity • Broadening • Less red-shifting 150 mJ Energy scan Focal scan 70 mJ • Red-shifting • Higher intensity • Broadening • Less red-shifting 150 mJ Energy scan Focal scan 300 mJ**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) ALS reflectometry 1/λ 1/w0 k Glass 3.9 mrad (rms) Kapton 7.4 mrad (rms) VHS & Mylar ~13 mrad (rms)**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**Stable shot to shot performance**Experiment Experiment Scan parameter Model • Comparison Experiment to Model • Insight in CWE physics • Use insight for optimization Scan parameter**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 Tape Drive HHG drive laser**Bottom line: deliver seed strength 10-6-10-5 to undulator**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 2 nJ 2 mrad Z [m] Phase electron Energy electron FEL radiation**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 Sequoia data and Grenouille data where taken and compared on 2012-09-05. By including temporal resolution, nice fitting for both diagnostics is retrieved Scan33, 2012-07-09**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**Roughness more complex than just “sigma”**FFT[ h(x) ] FFT[ h(x) ] Same Sigma, Different regime Critical is the spatial frequencies 1/λ k [nm-1] Assumption Nevot-Croce “single σ“ CXRO grazing reflectometry λ λ 1/λ k [nm-1]**Conclusion**Gradient length δ Function 1 Vdelta=1e-5 Time shift = 1e-5 ps per cycle, or 3nm per cycle, or 1100 nm/ps**e- beam**laser Intro to Laser Plasma Accelerators (LPA’s) LPA: Self injection + acceleration Godzilla TREX Bella**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!**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**Laser chirp can compensate for**CWE femtochirp Blue-shifting Red-shifting ξ=-1 (blue front) ξ=1 (red front Borot et al. Opt. Lett. 36 (2011)**Coherent Wakefield Excitation: 3-step model for laser-plasma**interaction Heissler et al. Appl. Phys. B 101 (2010) • 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**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 Energy Decompression • Undulator • Six 22-period sections (now three) • K=1.25 Time • 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 no decompression seeded FEL x10 decompression seeded FEL**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