Laser Plasma Acceleration: Observing Mono-energetic Electrons

1. Acceleration of particles with lasers at RAL Peter A Norreys Physics Group Leader Central Laser Facility CCLRC Rutherford Appleton Laboratory visiting Professor of Physics, Imperial College London.

2. Electron Energy World's First Observation of Mono-energetic Electrons from a Laser Plasma Accelerator • Laser Plasma Accelerators have 1,000 higher electric field than conventional accelerators • Implies kilometer’s to centimeters reduction in size for same electron energy - attractive • To date have always produced broad range of energies which severely limited application • Quasi Mono-energetic electrons up to 100 MeV produced for the first time at RAL • Astra “Gemini” may increase this to the GeV level IC / RAL / Strathclyde / UCLA collaboration Mangales et al Nature, 431 , 535 (2004) Astra Target Area 2 • Experiments performed on the Astra ultra-high power laser system in the CLF • Major success for the RCUK Basic Technology Programme

3. Outline • Introduction to the ASTRA laser facility • Basic concepts for electron acceleration • Laser wakefield acceleration • Beam pointing • Photon acceleration in laser wakefield accelerators • Scaling to multi-GeV energies - Astra Gemini laser at RAL • Physics group modelling team in the CLF

4. diffraction gratings Laser characteristics • 15 TW = 1.5 1013 W • Power = Energy = 0.6 J = 1.51013 W • pulse duration 40 fs • To maximise the intensity on target, the beam must be focused to a small spot. • The focal spot diameter is 20 mm and is focused with an f/17 off-axis parabolic mirror • Intensity = Power = 1.5 x 1018 W cm-2 • Focused area Astra

5. Astra laser • Single Beam Titanium Sapphire laser system • 10 TW optical pulse at 10Hz / 25TW at 1Hz • Operates to 2 target areas • Experiments in Laser-Plasma Physics

6. Electron motion in an intense laser field • A single electron in an intense infinite plane polarised laser field exhibits a figure of eight motion due to the vxB term in the Lorentz force F = -e(E+vxB) • At relativistic intensities, electrons are accelerated in the direction of the propagation direction k twice every laser cycle. • The kinetic energy the electron acquires is roughly proportional to the ponderomotive potential energy Up E B k Intensity on target 1016 Wcm-2 1018 Wcm-2 1021 Wcm-2 Up 1 keV 0.4 MeV 14 MeV

7. Laser wakefield acceleration • Perturbing ‘object’ passes through a medium which is displaced from equilibrium • The medium then returns creating oscillations • Areas of high and low electron density create extreme electric fields High intensity laser pulse Electron density Gas Jet

8. OSIRIS simulations Electron density plot at 3.1ps

9. Electron acceleration Plasma electrons are trapped and accelerated by the laser’s wakefield Laser pulse Wakefield Electron injection Projection of electron density Collaboration between CCLRC, IST Lisbon, University of Strathclyde Glasgow, UCLA, and Imperial College London

10. Mono-energetic spikes in the spectra observed Measured electron spectra a) ne= 1.6 x 1019 cm-3 b) ne= 1.8 x 1019 cm-3 c) ne= 3 x 1019 cm-3 d) ne = 5 x 1019 cm-3 at E = 350 mJ, t = 40 fsec.

11. Analysis ~600μm ~600μm Two independent measurements of the plasma length When the density is such that the dephasing length, is shorter than the plasma, the monoenergetic features are lost

12. Mono-energetic electron beams observed Measured electron spectrum using a 500 mJ laser pulse at a density of 2  1019 cm-3. The energy spread is ± 3%. Plasma density: 2.1 x 1019 cm-3 (1) Self-focusing of laser: electrons first appear (2) Wavebreaking first occurs (3) More breaking occurs - multiple bunches (4) Dephasing causes smoothing of the spectrum

13. Good beam properties measured at LOA in France • Fritzler et al., measured the emittance of an electron beam from a laser wakefield accelerator (with a ‘thermal’ distribution) using the pepperpot technique and radiochromic film as the diagnostic. • The normalised emittance is defined as the rms correlation between the space (x) and reduced momentum (x’) coordinates of all beam electrons in the (x,x’) 2D phase space • x’ = px/pz is the electron angle wrt laser axis. • (x,x’) phase space plot is a measure of the divergence of the electron beam as a function of position across the beam. • At 55 MeV electron energy, measurements of the normalised emittance were 2.7 (0.9)  mm mrad. S.Fritzler et al., Phys. Rev. Lett. 92 165006 (2004)

14. 3D simulations of laser wakefield accelerators According to Wei Lu and Warren Mori (UCLA) the theoretical energy gain E is provided that the laser focal width is matched to the bubble size. This indicates that the maximum energy gain is simply proportional to the laser power (within the dephasing limit) Should scale to multi-GeV energies Courtesy of Prof L.O.Silva, IST, Lisbon 3D explicit PIC simulations using OSIRIS typically take 1 week to run on a 256 node cluster with 2GB memory per node. 2D simulations are needed for parameter scans. They take 12 - 24 hours to run on 32-node clusters

15. Proof of principle: towards a super compact proton accelerator Basic set-up (top view) Target: 3-25 µm Al CPA Laser Proton/ion beam Proton/ion beam A.P.Fews, P.A.Norreys et al., Phys. Rev. Lett. 73,1801 (1994) E.L.Clark, K.Krushelnick et al., Phys. Rev. Lett. 84,670 (2000) • Large accelerating fields exist throughout the target • Hydrocarbon surface contaminants provide the protons for acceleration on both front and rear surface. • Acceleration takes place over 10’s of µm (E~1012 V/m)(Standard accelerators E~106 V/m, typical scale 10’s of metres)

16. The Physics Group, Central Laser Facility Raoul Trines Alex Robinson Peter Hakel James Green Kate Lancaster Christopher Murphy took the photo! Dr Mark Sherlock has also joined us and Prof Roger Evans has arrived as a consultant

17. Summary • The first observations of mono-energetic electron beams from laser wakefield accelerators has been made using the ASTRA laser facilities at RAL. • These beams have excited wide interest because of the huge accelerating electric fields generated (> GeV m-1). • There is much to do - pointing stability of the beam, shot to shot energy fluctuation, scaling with laser power. • Theory indicates that the energy gain is proportional to the laser power - multi-GeV energies may be possible on ASTRA-GEMINI. Scaling to 10 PW, it is possible that energies of interest to HEP science can be generated - needs experimental validation on the Vulcan 10 PW capability. • There also have been a number of proposed applications for these beams such as for injectors into subsequent conventional acceleration stages, new light sources, probing of dense plasmas and for inertial fusion energy.

18. ACKNOWLEDGEMENTS The mono energetic electron acceleration work described here was performed as part of the RC UK Basic Technology alpha-X grant • S. P. D. Mangles, C.D.Murphy, Z. Najmudin, A. G. R. Thomas, J. L. Collier, A. E. Dangor, E. J. Divall, P.S. Foster, J.G. Gallacher, C. J. Hooker, D.A. Jaroszynski, A. J. Langley, W. B. Mori, R. Viskup, B. R. Walton, and K. Krushelnick • CLF laser, target area and engineering staff. • RCUK, EPSRC & CCLRC

19. ACKNOWLEDGEMENTS Long-Wavelength Hosing Instability in a Self-Injected Laser-Wakefield Accelerator M. C. Kaluza, S. P. D. Mangles, A. G. R. Thomas, C. D. Murphy, Z. Najmudin, A. E. Dangor, K. M. Krushelnick Plasma Physics Group, Imperial College London J. L. Collier , E. J. Divall, K. Ertel, P. S. Foster, C. Hooker, A. J. Langley, D. Neely, J. Smith CCLRC Rutherford Appleton Laboratory

20. Photon acceleration The laser’s photons are accelerated by the laser’s own wakefield! Final photon distribution Initial photon distribution Wakefield Photon frequency (rad/s) Scaled electron density x – ct (m) Image taken from simulations using a dedicated wave-kinetic code

21. Photon spectra • Asymmetric redshift/blueshift qualitatively reproduced • Numerical shifts too big; caused by use of 1-D plasma model • Large blueshift because scaled wakefield amplitude exceeds 1 • No blueshift of the spectrum as a whole • Rise and fall of blueshift with increasing density explained from wakefield behaviour. C.D.Murphy, R.Trines et al., Phys. Plasmas, March 2006