Visualising the Dynamics of a Plasma-Based Electron Accelerator Malte C. Kaluza Institute of Optics and Quantum Electronics, FSU Jena, Germany Helmholtz-Institute Jena 1 1
Thanks to All Collaborators! A. Sävert, M. B. Schwab, M. Leier, M. Reuter, M. Schnell, A. Kawshik, D. Ullmann, O. Jäckel, F. Ronneberger, B. Beleites, C. Spielmann, G. G. Paulus, M. Zepf Visualising Plasma-Based Acceleration Institute of Optics and Quantum Electronics, Friedrich-Schiller-University Jena, Helmholtz-Institute Jena A. Buck, K. Schmid, C.M.S. Sears, J. M. Mikhailowa, F. Krausz, L. Veisz Max-Planck-Institute of Quantum Optics, Garching S. P. D. Mangles, K. Poder, J. Cole, Z. Najmudin Imperial College London, UK E. Siminos, S. Skupin Max-Planck-Institute of the Physics of Complex Systems, Dresden 2 2 2
Motivation and Outline • Compact laser-driven plasma-electron accelerators: o plasma formed and modulated by high-intensity laser pulse o electrons accelerated by fields of laser-generated plasma wave Visualising Plasma-Based Acceleration („wakefield“) o electron pulse parameters determined by details of interaction o generation and evolution of this wakefield? o acceleration dynamics? • High relevance for future beam-driven plasma-electron accelerators: o research programs started or planned e.g. at SLAC and DESY o first experimental results • Pump-probe geometry well suited for investigation: o accelerator driven by main pulse (“pump pulse”), o can be characterized (“probed“) using synchronized probe pulse • Generate synchronized electro-magnetic probe pulses: o investigate details of interaction with high temporal and spatial resolution 3 3 3
Conventional Particle Accelerators High-energy particle accelerators CERN • for protons, • heavy ions, Visualising Plasma-Based Acceleration • electrons – linacs, • electrons – synchrotrons GSI are well established. However, they are large because of limited acceleration field strength to avoid break-through or ionization. SLAC/LCLS Diamond DESY 4 4 4
Conventional Particle Accelerators High-energy particle accelerators CERN • for protons, • heavy ions, Visualising Plasma-Based Acceleration • electrons – linacs, • electrons – synchrotrons GSI are well established. However, they are large because of limited acceleration field strength to avoid break-through or ionization. SLAC/LCLS JETI Diamond ⇒ use plasma as the medium, high-intensity laser or electron pulse as the driver! 5 5 5
What are „High Intensities“? Laser intensity I L ≥ 10 19 W/cm 2 Intensity of sun @ earth ≈ 10 3 W/m 2 Visualising Plasma-Based Acceleration Earth’s cross section ≈ 10 14 m 2 Total power of the sun reaching the earth ≈ 10 17 W 6 6 6
What are „High Intensities“? Laser intensity I L ≥ 10 19 W/cm 2 Intensity of sun @ earth ≈ 10 3 W/m 2 Visualising Plasma-Based Acceleration Earth’s cross section ≈ 10 14 m 2 Total power of the sun reaching the earth ≈ 10 17 W Focussingthis power ? to (1 cm) 2 : I L = 10 17 W/cm 2 to (1 mm) 2 : I L = 10 19 W/cm 2 to (0.1 mm) 2 : I L = 10 21 W/cm 2 7 7 7
What are „High Intensities“? JETI @ FSU Jena and LWS 20 @ MPQ Garching Visualising Plasma-Based Acceleration 10...30-TW Ti:Sapphire Laser Multi-TW OPCPA Laser pulse duration: 85 ... 35 fs pulse duration: 8.5 fs pulse energy: 750 mJ pulse energy: 65 mJ focus diameter: <3 µm focus diameter: <3 µm >1 × 10 20 W/cm 2 max. intensity: >1 × 10 20 W/cm 2 max. intensity: 8 8 8
Plasma Wakefield Acceleration Principle of the acceleration process • Plasma wave excited by F pond of high-intensity laser pulse ≡ modulation of n e against ion background ( v ph,plasma = v gr,laser ) Visualising Plasma-Based Acceleration ⇒ longitudinal E-fields (~ 0.1…1 TV/m) Image courtesy of A.G.R. Thomas A. Pukhov et al., APB (2002) • injection of electrons into the wave (e.g. by wave breaking or externally) ⇒ quasi-monoenergetic, ultra-short electron pulse 9 9 W. Leemans et al., PRL (2014) 9
Plasma Wakefield Acceleration Principle of the acceleration process • Plasma wave excited by F pond of high-intensity laser pulse ≡ modulation of n e against ion background (v ph,plasma = v gr,laser ) Visualising Plasma-Based Acceleration ⇒ longitudinal E-fields (~ 0.1…1 TV/m) Image courtesy of A.G.R. Thomas • injection of electrons into the wave (e.g. by wave breaking or externally) ⇒ quasi-monoenergetic, ultra-short electron pulse ⇒ relativsiticelectron current ⇔ azimuthal B-fields 10 10 M. Litos et al., Nature (2014) 10
Electromagnetic Probe Pulses Probe-pulse generation • Generation ofsynchronized optical probe pulses: o split off part of Visualising Plasma-Based Acceleration the main pulse o guide it towards interaction along different path o adjust temporal delay ⇒ perfect synchronization ⇒ probe pulse duration similar to main pulse ⇒ record movie from subsequent shots at different delays (requires good shot-to-shotstability!) 11 11 11
Electromagnetic Probe Pulses Measuring B-fields: the Faraday effect • Transverse probing of B-fields in underdense plasma with linearly-polarized probe pulse: Visualising Plasma-Based Acceleration if ⇒ B-field induced difference of η for circularly-polarized probe components ⇒ rotation of probe polarization: ⇒ measure φ rot to get signature of B-fields, measure n e to get amplitude! J. A. Stamper et al., PRL (1975) 12 12 12
Probing Laser-Driven Wakefields Experimental setup I Visualising Plasma-Based Acceleration JETI parameters: E laser = 800 mJ, τ laser = 85 fs, f/6 OAP, I laser ≈ 3x10 18 W/cm 2 probe pulse: τ probe ≈ 100 fs @ 1 ω 13 13 13
Probing Laser-Driven Wakefields Polarimetry results Two polarograms from two (almost) crossed polarizers: Visualising Plasma-Based Acceleration 340 µm polarogram 2 polarogram 1 560 µm Deduce rotation angle φ rot from pixel-by-pixel division of polarogram intensities: 14 14 14
Probing Laser-Driven Wakefields Polarimetry results Visualising Plasma-Based Acceleration simulated feature polarogram 2 polarogram 1 560 µm experimental Faraday feature Experimental evidence for B-fields from MeV electrons and bubble! MCK et al., PRL 105, 115002 (2010) 15 15 15
Probing Laser-Driven Wakefields Experimental setup II Visualising Plasma-Based Acceleration JETI parameters: LWS-20 parameters: E laser = 800 mJ, τ laser = 85 fs, E laser = 80 mJ, τ laser = 8.5 fs, f/6 OAP, I laser ≈ 3x10 18 W/cm 2 f/6 OAP, I laser ≈ 6x10 18 W/cm 2 probe pulse: probe pulse: τ probe = 8.5 fs @ 1 ω τ probe ≈ 100 fs @ 1 ω 16 16 16
Probing Laser-Driven Wakefields Polarimetry results Visualising Plasma-Based Acceleration polarogram 1 polarogram 2 Electron bunch length: Δ z = 4 µm τ FWHM = (6 ± 2) fs, τ RMS = (2.5 ± 0.9) fs A. Buck et al., Nature Physics 7, 543 (2011) 17 17 17
Probing Laser-Driven Wakefields Polarimetry results Visualising Plasma-Based Acceleration • Polarimetry: visualize e-bunch via associated B-fields • change delay between pump and probe ⇒ movie of e-bunch formation • observe e-bunch formation on-line! A. Buck et al., Nature Physics 7, 543 (2011) 18 18 18
Probing Laser-Driven Wakefields Shadowgraphy results Visualising Plasma-Based Acceleration • Shadowgraphy: visualize plasma wave • change electron density ⇒ change plasma wavelength A. Buck et al., Nature Physics 7, 543 (2011) 19 19 19
Probing Laser-Driven Wakefields Experimental setup III • Experiments with 30-TW JETI-laser system Similar resolution, but with 35-fs driver laser: • Visualising Plasma-Based Acceleration frequency-broadening of probe pulse • (in gas-filled hollow fiber) ⇒ shorter τ probe τ probe = (5.9 ± 0.4) fs ⇒ sub-main pulse temporal resolution, 1.1 µm spatial resolution with optimized imaging system M. Schwab et al., Appl. Phys. Lett. 103, 191118 (2013) 20 20 20
Probing Laser-Driven Wakefields Few-Cycle Microscopy • Few-cycle probe pulses LWS 20 Visualising Plasma-Based Acceleration 100 µm 10 µm 10 µm 100 µm JETI 21 21 21 M. Schwab et al., Appl. Phys. Lett. 103, 191118 (2013)
Probing Laser-Driven Wakefields Probing of plasma wakefield acceleration process Measuringthe length of the 2 nd plasma wave period (at Visualising Plasma-Based Acceleration fixed position in the plasma) and the electron charge: critical power for self injection: for our parameters: n e > 1.5x10 19 cm -3 S.P.D. Mangles et al., PRSTAB 15 , 011302 (2012) 22 22 22 A. Sävert et al., Phys. Rev. Lett. 115 , 055002 (2015)
Probing Laser-Driven Wakefields Results from Few-Cycle Microscopy Plasma wave evolution above injection Visualising Plasma-Based Acceleration threshold: n e =1.6x10 19 cm -3 transverse focussing injection acceleration 23 23 23 A. Sävert et al., Phys. Rev. Lett. 115 , 055002 (2015)
Probing Laser-Driven Wakefields Results from Few-Cycle Microscopy Measuringlength of 1 st plasma wave period After injection: strongly (at n e =1.6x10 19 cm -3 ) at different positions: non-linear evolution Visualising Plasma-Based Acceleration „well behaved“ wavebreaking radiation beam-loadingdominated single-bubble regime λ p for n e =1.6x10 19 cm -3 multiple-bubble regime Bubble expansion starts before injection. No beam-loadingbut amplificationofpump pulse: 24 24 24 A. Sävert et al., Phys. Rev. Lett. 115 , 055002 (2015)
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