Professor Simon Hooker

Abstract:

Ultrashort electron bunches from laser-driven plasma accelerators hold promise as drivers for short-wavelength free electron lasers. While FEL simulation techniques have been successful in simulating lasing at present-day facilities, the novel sources investigated here are likely to violate a number of widely-held assumptions. For instance the HHG seed radiation, as well as the radiation generated by the bunch, may not conform to the slowly-varying envelope approximation (SVEA) on which the majority of codes rely. Additionally, the longitudinal macroparticle binning precludes the modeling of the full physics of the system. In order to more completely simulate the sub-wavelength effects which arise, we have developed an unaveraged 1-D time-dependent code without the SVEA. We use this to perform numerical analyses and highlight some of the additional features that these new systems present. We conclude that while coherent spontaneous emission from ultra-short bunches may significantly affect start-up, the sub-wavelength structure of HHG seeds has little effect.

Abstract:

Quasi-monoenergetic, laser-driven electron beams of up to ∼ 200 MeV in energy have been generated from steady-state-flow gas cells [1], These beams are emitted within a low-divergence cone of 2.1 ± 0.5 mrad FWHM and feature unparalleled shot-to-shot stability in energy (2.5% rms), pointing direction (1.4 mrad rms) and charge (16% rms) owing to a highly reproducible plasma-density profile within the laser-plasma-interaction volume. Laser-wakefield acceleration (LWFA) in gas cells of this type constitutes a simple and reliable source of relativistic electrons with well defined properties, which should allow for applications such as the production of extreme-ultraviolet undulator radiation in the near future. © 2009 American Institute of Physics.

Abstract:

A method for producing non-uniformly spaced (chirped) trains of ultrafast pulses is demonstrated, using an acousto-optic programmable dispersive filter (AOPDF). Programmable pulse trains of this type may find applications in quasi-phase matching of high-harmonic generation. © 2009 Optical Society of America.

Abstract:

Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research 1 ; however, their dependence on large-scale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasma-wave ccelerators 2-10 provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray sources to the niversity-laboratory scale. Here, we demonstrate the generation of soft-X-ray undulator radiation with laser-plasma-accelerated electron beams. The well-collimated beams deliver soft-X-ray pulses with an expected pulse duration of ∼ 10 fs (inferred from plasma-accelerator physics). Our source draws on a 30-cm-long undulator and a 1.5-cm-long accelerator delivering stable electron beams with energies of ∼ 210 MeV. The spectrum of the generated undulator radiation typically consists of a main peak centred at a wavelength of ∼ 18 nm (fundamental), a second peak near ∼ 9 nm (second harmonic) and a high-energy cutoff at ∼ 7 nm. Magnetic quadrupole lenses ensure efficient electron-beam transport and demonstrate an enabling technology for reproducible generation of tunable undulator radiation. The source is scalable to shorter wavelengths by increasing the electron energy. Our results open the prospect of tunable, brilliant, ultrashort-pulsed X-ray sources for small-scale laboratories. © 2009 Macmillan Publishers Limited. All rights reserved.