Producing bright, short-pulse kev radiation with laser-plasma accelerators. Stuart Mangles

Transcription

1 Producing bright, short-pulse kev radiation with laser-plasma accelerators Stuart Mangles

2 Authors S. P. D. Mangles 1, C. Bellei 1, R Bendoyro 3, M. Bloom 1, M. Burza 5, K. Cassou 6, V. Chvykov 2, B. Cros 6, F. Dollar 2, N. P. Dover 1, A Döpp 1, R. Fonseca 3, G. Genoud 5, C. Huntington 2, J Holloway 8, J Jiang 3, C. Kamperidis 5, G. Kalintchenko 2, S. Kneip 1, K. Krushelnick 2, N Lopes 3, J. L. Martins 3, S. F. Martins 3, T. Matsuoka 2, A. Maksimchuk 2, C. McGuffey 2, S. R. Nagel 1, Z. Najmudin 1, H. Nakamura 1, C. A. J. Palmer 1, K. Ta Phuoc 4, A. Persson 5, J. Schreiber 1, L. O. Silva 3, M. J. Streeter 1, D Symes 7, A. G. R. Thomas 2, C.-G. Wahlström 5, F. Wojda 6 and V. Yanovsky 2 1. The Blackett Laboratory, Imperial College London, UK 2. Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, USA 3. GoLP Instituto Superior Técnico, Lisbon, Portugal 4. Laboratoire d Optique Appliquée, ENSTA / Ecole Polytechnique, Palaiseau, France 5. Department of Physics, Lund University, Sweden 6. Laboratoire de Physique des Gaz et des Plasmas, CNRS, Université Paris XI, Orsay, France 7. Central Laser Facility, STFC Rutherford Appleton Laboratory, UK 8. Department of Physics and Astronomy, University College London, UK

3 Outline background: laser-plasma wakefield acceleration how it works current status generating x-rays with laser-plasma wakefield accelerators characterising x-rays from 100 TW laser driven LWFA imaging using LWFA x-rays controlling x-ray spectrum using asymmetric laser pulses to control electron orbits inside the wakefield accelerator

4 Making accelerators smaller ~ 1 m; ~ 40 MV/m ~ 50 µm; ~ 100 GV/m laser driven plasma wave 1 section of Tesla Cavity (DESY) waves in a plasma can travel at close to c and can have very large electric fields ~ 100 GV/m we can use plasma waves to accelerate to high energy in a short distance

5 Driving plasma waves with a laser pulse laser pulse plasma electron density ponderomotive force intense laser pulse expels plasma electrons from regions of highintensity boat Heavier plasma ions do not move - this charge separation leads to very strong electric fields boat

6 Injecting electrons into the wake firing a conventional electron beam into this wake is difficult due to the small size luckily nature is kind to us using appropriately shaped laser pulse the process of wavebreaking can inject electrons into the back of the plasma wave these electrons are injected at the same point so reach the same energy (quasi-monoenergetic beams)

7 laser wakefield acceleration 2D simulation with OSIRIS 1 J, 30 fs laser pulse plasma density of ne = 2 x cm -3

8 LWFA: experimental set-up high intensity laser beam collimator electromagnet supersonic gas jet electron sensitive image plate

9 Current status: LWFA back in 2004 three groups showed that high quality electron beams could be produced 100 MeV in 2 mm with TW lasers b Mangles et al Nature 2004 Geddes et al Nature 2004 Faure et al Nature 2004

10 Current status: LWFA Imperial / Lund: Mangles Phys Plasma 2007 & Lindau IEEE Trans Plasma Sci 2008 Since 2004 various groups have significantly improved stability of selfinjection scheme through improvements to target and lasers stable operation requires working just above the threshold MPQ/ Oxford: Osterhoff Phys Rev Lett 2008

11 Current status: LWFA 250 E (MeV) figure from Malka Phys Plasma 2009 (data originally published in Faure Nature 2006) 30 Number of counts Alternative injection schemes provide an alternative route to stable operation colliding pulse injection sharp density ramp injection Schmid PR-STAB 2010

12 Current status: LWFA State of the art in stability from LBNL group per-cent level stability (charge, energy, energy spread) tuneable electron beam energy up to 400 MeV Gonsalves et al Nature Phys 2011 (doi: /nphys2071)

13 Current status: LWFA Leemans Nature Phys 2006 Leemans Nature Phys 2006 Groups working on scaling to higher energies 1 GeV barrier broken by LBNL in 2006 using a plasma waveguide to guide 40 TW laser over 3 cm Self-guiding reaches 0.8 GeV in 2008 using 150 TW laser in just 0.8 cm Kneip PRL 2009 Kneip Phys Rev Lett 2008 UCLA/LLNL used ionisation injection to reach 1.5 GeV in a broad energy spread Clayton PRL 2010

14 Generating x-rays conventional synchrotrons make bright x-rays using insertion devices called wigglers the electrons are bent by magnets in a periodic structure energy of radiation depends on electron energy (γmc 2 ) and wiggler wavelength λw and strength of magnets

15 Generating x-rays conventional wigglers use permanent magnets (~ 1 T) wiggler periods of a few centimetres - even these are large generating kev requires GeV beams and wigglers a few metres long

16 Plasma accelerators as a photon source focusing force electron wiggling on-axis synchrotron spectrum brightness plasma wave also has very strong focusing forces E / E c electrons self-injected in the wake can oscillate in the strong focusing field and so generate x-rays K = γk for large oscillation amplitudes the spectrum of x-rays is similar β r β to that produced in a wiggler device (first observed Rousse Phys Rev Lett 2004) d 2 I dedω θ=0 ξ 2 K 2 2/3 (ξ/2) ξ = E/E c E c = 3 4 γ2 ω 2 pr β /c

17 X-ray production results Hercules laser at the University of Michigan Ti:Sapphire system (800 nm), 1 beam > 9 J, 30 fs, up to 0.1 Hz

18 X-ray production results This experiment: 3 J, 32 fs, one shot per minute, 100 TW peak power Gas jet target provide helium plasma up to 1 cm long electron spectrum analysed with simple dipole magnet and lanex screen x-ray beam diagnostics knife-edge source size determination filter transmission single shot x-ray spectrum imaging plate measurement of x-ray beam profile

19 x-ray results: profile and source size single shot measurement of the x-ray beam divergence knife-edge measurement of the x-ray source size low divergence (~ 10 mrad) small source size (~ 1-2 µm) assumed pulse duration of 30 fs (in agreement with simulations)

20 x-ray results: spectrum 1.2 d 2I /(d!d") /a.u photon energy /kev 6-fold filter transmission measurement reveals x-ray spectrum is in the multi-kev range (Ec ~ 10 kev) x-rays are brightest when electrons are high-energy and monoenergetic x-ray spectrum is in very good agreement with numerical modelling (JL Martens, GoLP, Lisbon) and predicted scaling (Rousse et at EPJ-D 2010)

21 x-ray results peak brightness photons /s/mm 2 /mrad 2 /0.1%BW Hercules Laser ALS (USA) SPring-8 (Japan) Diamond (UK) photon energy /kev Peak brightness is high (not true for average brightness) ~10 8 photons per shot or ph/kev/srad S Kneip Nature Physics

22 Coherent x-rays for phase contrast imaging Spatial coherence allows phase contrast x-ray imaging phase contrast imaging: allows us to image objects that are transparent to the x-rays S Kneip Applied Physics Letters 99, (2011)

23 Can we do anything to control the x-rays? Experiment at a laser at Lund University smaller laser (1 J, 40 fs ~20 TW) so we expect lower energy electrons and not so hard x-rays Can we make the betatron oscillation amplitude larger? can we make the electron injection happen off-axis using an asymmetric laser spot?

24 promoting off-axis injection asymmetric spot drives an asymmetric wake. trajectory crossing point no longer on the axis (where injection occurs) off axis injection should lead to larger amplitude betatron oscillations and change the x-ray spectrum

25 Generating an asymmetric laser a) b) x /!m x /!m c) x /!m deformable mirror combined with wavefront sensor to tailor laser wavefront and control focal spot shape experiment performed with various degrees of coma in the spot - produces an asymmetric spot ne = 1.5 x cm -3 ; 2 mm helium gas jet

26 evidence for off-axis injection λ coma flat a) Focal spot asymmetry c) number of electrons (/107) per MeV per mrad b) increases divergence of electron beam Also introduces an energy- d) dependent exit angle in the electron spectrometer horizontal exit angle /mrad 20 a signal of large amplitude betatron oscillations (as reported in Glinec EPL 2008)

27 effect of off-axis injection on x-ray spectrum a) b) photon energy /kev Coma increases the critical energy of the synchrotron spectrum electron beam average energy constant indicating increase in Ec is due to increase in oscillation amplitude from rβ = 1.0 ± 0.4 µm to 3 ± 1 µm significant increase in energy range of photons from a betatron source without needing 100 TW laser system Mangles Applied Physics Letters 2009

28 bright x-rays from a laser plasma accelerator ~ 10 kev peak brightness photons /s/mm2/mrad2/0.1%bw Summary 1024 SPring-8 (Japan) 1023 Hercules Laser ALS (USA) Diamond (UK) 1 10 photon energy /kev 100 spatially coherence for imaging a) c) number of electrons (/107) per MeV per mrad b) d) control of the x-ray spectrum by shaping the laser pulse