Laser-plasma accelerators

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When an intense laser pulse propagates through a plasma, the ponderomotive force pushes electrons away from the front and back of the pulse thereby forming a trailing longitudinal density wave. The longitudinal electric field in the plasma wave can be as high as 100 GV m-1, more than three orders of magnitude larger than that found in conventional RF accelerators such as those used at CERN. Particles injected into the correct phase of the plasma wave can be accelerated to energies of order 1 GeV in only a few tens of millimetres. This 'laser wakefield accelerator' is particularly promising for generating beams of short pulse, high-energy electrons for applications in femtosecond electron diffraction, medical imaging, and miniature free-electron X-ray lasers.

One factor which limits the energy to which particles can be accelerated is the distance over which the intensity of the driving laser can be maintained. Over the last few years our group has developed a technique for channelling laser pulses with peak intensities of up to 1018 Wcm-2 over distances which are much longer than the limit set by diffraction. In collaboration with a group at Lawrence Berkeley National Laboratory (LBNL), we have used this technique to extend the length over which acceleration can be maintained by an order of magnitude and thereby generated electrons with energies of 1 GeV. This energy is of the order of that used in many synchrotrons around the world, but the plasma accelerator is only 33 mm long instead of 150 m! For further details, see the further reading suggested below.

Radiation can be generated from laser-accelerated beams in two ways. The beams can be passed through an undulator - a periodic array of dipole magnets with alternating orientation - which forces the electrons to oscillate transversely, and hence to radiate. This technique is already used at synchrotron facilities to generate incoherent radiation from THz to x-ray frequencies. If the electron beams are of sufficiently high quality (i.e. have low energy spread and low emittance etc.) then feedback between the generated radiation and the electron bunch causes "micro-bunching" at the radiation wavelength. Radiation emitted in the forward direction by each micro-bunch is in phase, and hence as the degree of micro-bunching increases so does the intensity of the radiation field; as a consequence the intensity of the radiation grows exponentially with propagation through the undulator.  X-ray "free-electron lasers" (XFELs), based on this principle, have recently been demonstrated for the first time using kilometre-long conventional accelerators. A long-term goal of our work on laser-driven plasma accelerators is to replace the conventional accelerator with a plasma accelerator only a few centimetres long.

A second method for generating x-rays from laser-accelerated electron beams is to use the transverse oscillation caused by the transverse electric fields in the plasma wave. This oscillation leads to the emission of (broad-band) betatron radiation which can extend to photon energies of tens of keV.

The laser wakefield accelerator (LWFA) has many potential applications. However, most of these - including, in the long term, laser-driven particle colliders - will require the accelerator to operate at much higher pulse repetition rates than is possible with the Ti:sapphire lasers used today.

An interesting new approach being developed by a collaboration between groups in the sub-departments of Particle Physics and Atomic & Laser Physics: multi-pulse LWFA, in which a train of low-energy laser pulses drives the plasma wave. If the pulses in the train are spaced by the plasma period then the wakes excited by each pulse interfere coherently to form a large-amplitude wave at the back of the pulse train. The advantage of this approach is that it opens the possibility of using novel laser systems - such as thin-disk and fibre lasers - which can operate at very high pulse repetition rates and with excellent overall efficiency.

For a description of our recent result in this area, see our news article.

  1. J. Cowley, C. Thornton, C. Arran, R. J. Shalloo, L. Corner, G. Cheung, C. D. Gregory, S. P. D. Mangles, N. H. Matlis, D. R. Symes, R. Walczak, and S. M. Hooker, "Excitation and Control of Plasma Wakefields by Multiple Laser Pulses," Phys. Rev. Lett. 119 044802 (2017). DOI: 10.1103/PhysRevLett.119.044802
  2. S. M. Hooker, R. Bartolini, S. P. D. Mangles, A. Tünnermann, L. Corner, J. Limpert, A. Seryi, & R. Walczak, "Multi-Pulse Laser Wakefield Acceleration: A New Route to Efficient, High-Repetition-Rate Plasma Accelerators and High Flux Radiation Sources," J. Phys. B 47 234003 (2014) DOI: 10.1088/0953-4075/47/23/234003
  3. S. M. Hooker, "Developments in laser-driven plasma accelerators," Nature Photonics 775–782 (2013). DOI: 10.1038/nphoton.2013.234
  4. W. P. Leemans, S. M. Hooker et al.[/url], "GeV electron beams from a centimetre-scale accelerator," Nature Physics 696 (2006). DOI: 10.1038/nphys418
    1. J. Osterhoff, S. Karsch, S. M. Hooker et al.[/url], "Generation of Stable, Low-Divergence Electron Beams by Laser-Wakefield Acceleration in a Steady-State-Flow Gas Cell," Phys. Rev. Lett. 101 085002 (2008). DOI: 10.1103/PhysRevLett.101.085002
    2. T.P. Rowlands-Rees, S. M. Hooker et al., "Laser-Driven Acceleration of Electrons in a Partially Ionized Plasma Channel," Phys. Rev. Lett. 100 105005 (2008). DOI:  10.1103/PhysRevLett.100.105005]
    3. M. Fuchs, F. Gruner, S. Karsch, S. M. Hooker et al., "Laser-driven soft-X-ray undulator source," Nature Physics 826 (2009). DOI: 10.1038/NPHYS1404
    4. T. Ibbotson, S. M. Hooker et al., "Laser-wakefield acceleration of electron beams in a low density plasma channel," Phys. Rev. ST Acc. Beams 13031301 (2010). DOI:  10.1103/PhysRevSTAB.13.031301