Publications by Simon Hooker


Laser-driven soft-X-ray undulator source

Nature Physics 5 (2009) 826-829

M Fuchs, R Weingartner, A Popp, Z Major, S Becker, J Osterhoff, I Cortrie, B Zeitler, R Hörlein, GD Tsakiris, U Schramm, TP Rowlands-Rees, SM Hooker, D Habs, F Krausz, S Karsch, F Grüner

Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research1; however, their dependence on large-scale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasma-wave ccelerators2-10provide 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.


Chirped pulse trains for quasi-phase-matching high harmonic generation

Optics InfoBase Conference Papers (2009)

T Robinson, K O'Keeffe, SM Hooker

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.


Simulating sub-wavelength temporal effects in a seeded FEL driven by laser-accelerated electrons

FEL 2009 - 31st International Free Electron Laser Conference (2009) 119-122

SI Bajlekov, SM Hooker, R Bartolini

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.


Comparison of Parallel and Perpendicular Polarized Counterpropagating Light for Quasi-Phase-Matching High Harmonic Generation

ULTRAFAST PHENOMENA XVI 92 (2009) 15-+

T Robinson, K O'Keeffe, M Landreman, B Dromey, M Zepf, SM Hooker, RW Schoenlein


Stable laser-driven electron beams from a steady-state-flow gas cell

AIP Conference Proceedings 1086 (2009) 125-130

J Osterhoff, A Popp, Z Major, B Marx, TP Rowlands-Rees, M Fuchs, R Hörlein, F Grüner, D Habs, F Krausz, SM Hooker, S Karsch

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.


Generation of stable, low-divergence electron beams by laser-wakefield acceleration in a steady-state-flow gas cell

Physical Review Letters 101 (2008)

Laser-driven, quasimonoenergetic electron beams of up to ∼200MeV in energy have been observed from steady-state-flow gas cells. These beams emitted within a low-divergence cone of 2.1±0.5mrad FWHM display unprecedented shot-to-shot stability in energy (2.5% rms), pointing (1.4 mrad rms), and charge (16% rms) owing to a highly reproducible gas-density profile within the interaction volume. Laser-wakefield acceleration in gas cells of this type provides a simple and reliable source of relativistic electrons suitable for applications such as the production of extreme-ultraviolet undulator radiation. © 2008 The American Physical Society.


OPALS: The Oxford plasma accelerator light source project

30th International Free Electron Laser Conference, FEL 2008 (2008) 163-166

S Bajlekov, R Bartolini, N Delerue, G Doucas, SM Hooker, K Peach, D Urner, JS Wark

Recent progress in Laser Plasma Accelerators has demonstrated the possibility of generating GeV electron bunches with very interesting beam qualities. It is now conceivable that the further development of such devices could generate beams with emittance, energy spread and peak current suitable for FEL operation in the XUV range with relatively short undulator trains. In this context the OPALS project aims at the construction of a XUV radiation source, driven by a Laser Plasma Accelerator, capable of generating ultrashort fs XUV pulses. Such a source is small enough to be hosted in an academic or industrial institution and could therefore have a major impact on time-resolved science.


Laser-driven acceleration of electrons in a partially ionized plasma channel.

Phys Rev Lett 100 (2008) 105005-

TP Rowlands-Rees, C Kamperidis, S Kneip, AJ Gonsalves, SPD Mangles, JG Gallacher, E Brunetti, T Ibbotson, CD Murphy, PS Foster, MJV Streeter, F Budde, PA Norreys, DA Jaroszynski, K Krushelnick, Z Najmudin, SM Hooker

The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel.


Bright quasi-phase-matched soft-X-ray harmonic radiation from argon ions

Physical Review Letters 99 (2007)

M Zepf, B Dromey, M Landreman, P Foster, SM Hooker

Selective enhancement (>103) of harmonics extending to the water window (∼4nm) generated in an argon gas filled straight bore capillary waveguide is demonstrated. This enhancement is in good agreement with modeling which indicates that multimode quasi-phase-matching is achieved by rapid axial intensity modulations caused by beating between the fundamental and higher-order capillary modes. Substantial pulse energies (>10nJ per pulse per harmonic order) at wavelengths beyond the carbon K edge (∼4.37nm, ∼284eV) up to ∼360eV are observed from argon ions for the first time. © 2007 The American Physical Society.


Modeling of a square pulsed capillary discharge waveguide for interferometry measurements

Physics of Plasmas 14 (2007)

BHP Broks, W Van Dijk, JJAW Van Der Mullen, AJ Gonsalves, TP Rowlands-Rees, SM Hooker

Slow pulsed capillary discharges in round capillaries are currently under investigation for use as plasma channel laser waveguides in laser-wakefield acceleration, x-ray lasers, and higher-harmonic generation. In this study, a capillary discharge with a square cross section is presented. The electron density, which determines the laser guiding properties, can be measured by means of transverse interferometry in this device. Using a numerical model of the plasma and the capillary wall, an analysis of the discharge is made. The results predict that the square channel is capable of guiding circular laser pulses. The guiding properties are quite similar to those of a round channel with nearly the same diameter as the channel width. This suggests the results obtained by measuring the square capillary discharge are applicable for round channels as well. It was found that the wall heating was inhomogeneous, which makes the wall more susceptible to ablation. The heating of the wall changes the transverse optical pathlength in the interferometry experiments. © 2007 American Institute of Physics.


Transverse interferometry of a hydrogen-filled capillary discharge waveguide.

Phys Rev Lett 98 (2007) 025002-

AJ Gonsalves, TP Rowlands-Rees, BHP Broks, JJAM van der Mullen, SM Hooker

Transverse interferometric measurements are presented of the plasma channel formed in a hydrogen-filled capillary discharge waveguide recently used to generate 1 GeV electrons in a laser-driven plasma accelerator for the first time. The measurements were found to be in good agreement with nonlocal thermal equilibrium simulations, but showed significant differences with the results of a quasistatic model developed by Bobrova et al. [Phys. Rev. E. 65, 016407 (2001)]. The measurements are used to determine scaling laws for the axial electron density and matched spot size of the plasma channel, enabling optimization of the channel to specific applications.


Electron acceleration in a gas-discharge capillary

34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts 31 (2007) 57-60

A Popp, J Osterhoff, TP Rowlands-Rees, Z Major, M Fuchs, B Marx, R Hörlein, K Schmid, B Hidding, L Veisz, F Grüner, U Schramm, F Krausz, SM Hooker, S Karsch


Quasi-phasematching of harmonic generation via multimode beating in waveguides

Optics Express 15 (2007) 7894-7900

B Dromey, M Zepf, M Landreman, SM Hooker

A new scheme for quasi-phasematching high harmonic generation (HHG) in gases is proposed. In this, the rapid variation of the axial intensity resulting from excitation of more than one mode of a waveguide is used to achieve quasi phasematching. Numerical modeling demonstrates enhancement of the harmonic signal over that achieved for a single coherence length by factors >10 4. © 2007 Optical Society of America.


GeV electron beams from a centimeter-scale channel guided laser wakefield accelerator - art. no. 056708

PHYS PLASMAS 14 (2007) 56708-56708

K Nakamura, B Nagler, C Toth, CGR Geddes, CB Schroeder, E Esarey, WP Leemans, AJ Gonsalves, SM Hooker

Laser wakefield accelerators can produce electric fields of order 10-100 GV/m, suitable for acceleration of electrons to relativistic energies. The wakefields are excited by a relativistically intense laser pulse propagating through a plasma and have a phase velocity determined by the group velocity of the light pulse. Two important effects that can limit the acceleration distance and hence the net energy gain obtained by an electron are diffraction of the drive laser pulse and particle-wake dephasing. Diffraction of a focused ultrashort laser pulse can be overcome by using preformed plasma channels. The dephasing limit can be increased by operating at a lower plasma density, since this results in an increase in the laser group velocity. Here we present detailed results on the generation of GeV-class electron beams using an intense femtosecond laser beam and a 3.3 cm long preformed discharge-based plasma channel [W. P. Leemans et al., Nature Physics 2, 696 (2006)]. The use of a discharge-based waveguide permitted operation at an order of magnitude lower density and 15 times longer distance than in previous experiments that relied on laser preformed plasma channels. Laser pulses with peak power ranging from 10-40 TW were guided over more than 20 Rayleigh ranges and high quality electron beams with energy up to 1 GeV were obtained by channeling a 40 TW peak power laser pulse. The dependence of the electron beam characteristics on capillary properties, plasma density, and laser parameters are discussed. (C) 2007 American Institute of Physics.


Generation of a train of ultrashort pulses from a compact birefringent crystal array.

Appl Opt 46 (2007) 5142-5146

B Dromey, M Zepf, M Landreman, K O'keeffe, T Robinson, SM Hooker

A linear array of n calcite crystals is shown to allow the generation of a high contrast (>10:1) train of 2(n) high energy (>100 microJ) pulses from a single ultrafast laser pulse. Advantage is taken of the pulse-splitting properties of a single birefringent crystal, where an incident laser pulse can be split into two pulses with orthogonal polarizations and equal intensity, separated temporally in proportion to the thickness of the crystal traversed and the difference in refractive indices of the two optic axes. In the work presented here an array of seven calcite crystals of sequentially doubled thickness is used to produce a train of 128 pulses, each of femtosecond duration. Readily versatile properties such as the number of pulses in the train and variable mark-space ratio are realized from such a setup.


GeV plasma accelerators driven in waveguides

PLASMA PHYS CONTR F 49 (2007) B403-B410

SM Hooker, E Brunetti, E Esarey, JG Gallacher, CGR Geddes, AJ Gonsalves, DA Jaroszynski, C Kamperidis, S Kneip, K Krushelnick, WP Leemans, SPD Mangles, CD Murphy, B Nagler, Z Najmudin, K Nakamura, PA Norreys, D Panasenko, TP Rowlands-Rees, CB Schroeder, CS Toth, R Trines

During the last few years laser-driven plasma accelerators have been shown to generate quasi-monoenergetic electron beams with energies up to several hundred MeV. Extending the output energy of laser-driven plasma accelerators to the GeV range requires operation at plasma densities an order of magnitude lower, i.e. 10(18) cm(-3), and increasing the distance over which acceleration is maintained from a few millimetres to a few tens of millimetres. One approach for achieving this is to guide the driving laser pulse in the plasma channel formed in a gas-filled capillary discharge waveguide. We present transverse interferometric measurements of the evolution of the plasma channel formed and compare these measurements with models of the capillary discharge. We describe in detail experiments performed at Lawrence Berkeley National Laboratory and at Rutherford Appleton Laboratory in which plasma accelerators were driven within this type of waveguide to generate quasi-monoenergetic electron beams with energies up to I GeV.


Simple technique for generating trains of ultrashort pulses.

Opt Lett 32 (2007) 2203-2205

T Robinson, K O'Keeffe, M Landreman, SM Hooker, M Zepf, B Dromey

A simple method for generating trains of high-contrast femtosecond pulses is proposed and demonstrated: a linearly polarized, frequency-chirped laser pulse is passed through a multiple-order wave plate and a linear polarizer. It is shown theoretically that this arrangement forms a train of laser pulses, and in experiments the production of a train of approximately 100 pulses, each of 200 fs duration, is demonstrated. In combination with an acousto-optic programmable dispersive filter this technique could be used to generate and control pulse trains with chirped spacing. Pulse trains of this type have widespread applications in ultrafast optics.


GeV electron beams from a centimeter-scale laser-driven plasma accelerator

Proceedings of the IEEE Particle Accelerator Conference (2007) 1911-1915

AJ Gonsalves, K Nakamura, C Tóth, CGR Geddes, CB Schroeder, E Esarey, E Cormier-Michel, WP Leemans, D Bruhwiler, JR Cary, SM Hooker

Results are presented on the generation of quasimonoenergetic electron beams with energy up to 1GeV using a 40TW laser and a 3.3 cm-long hydrogen-filled capillary discharge waveguide [1, 2]. Electron beams were not observed without a plasma channel, indicating that self-focusing alone could not be relied upon for effective guiding of the laser pulse. Results are presented of the electron beam spectra, and the dependence of the reliability of producing electron beams as a function of laser and plasma parameters. ©2007 IEEE.


Performance of capillary discharge guided laser plasma wakefield accelerator

2007 IEEE PARTICLE ACCELERATOR CONFERENCE, VOLS 1-11 (2007) 2222-+

K Nakamura, E Esarey, CGR Geddes, AJ Gonsalves, WP Leernans, D Panasenko, CB Schroeder, C Toth, SM Hooker, IEEE


Comparison of parallel and perpendicular polarized counterpropagating light for suppressing high harmonic generation

Journal of the Optical Society of America B: Optical Physics 24 (2007) 2421-2427

M Landreman, K O'Keeffe, T Robinson, M Zepf, B Dromey, SM Hooker

The use of counterpropagating laser pulses to suppress high harmonic generation (HHG) is investigated experimentally for pulses polarized parallel or perpendicular to the driving laser pulse. It is shown for the first time that perpendicularly polarized pulses can suppress HHG. The intensity of the counterpropagating pulse required for harmonic suppression is found to be much larger for perpendicular polarization than for parallel polarization, in good agreement with simple models of the harmonic suppression. These results have applications to quasi-phase-matching of HHG with trains of counterpropagating pulses. © 2007 Optical Society of America.

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