Nr Thompson

Mode Locking in a Free-Electron Laser Amplifier

A technique is proposed to generate attosecond pulse trains of radiation from a free-electron laser amplifier. The optics-free technique synthesizes a comb of longitudinal modes by applying a series of spatiotemporal shifts between the copropagating radiation and electron bunch in the free-electron laser. The modes may be phase locked by modulating the electron beam energy at the mode spacing frequency. Three-dimensional simulations demonstrate the generation of a train of 400 as pulses at gigawatt power levels evenly spaced by 2.5 fs at a wavelength of 124 angstroms. In the x-ray at wavelength 1.5 angstroms, trains of 23 as pulses evenly spaced by 150 as and of peak power up to 6 GW are predicted.

'Near-field optical microscopy with an infra-red free electron laser applied to cancer diagnosis'

We show that the combination of a scanning near field optical microscope and an infra-red free electron laser yields chemical images with sub-cellular spatial resolution that have the potential to provide a diagnostic for oesophageal adenocarcinoma.

4GLS: a new type of fourth generation light source facility

Consideration is now being given in the UK to the provision of an advanced facility at lower energy to complement the DIAMOND x-ray light source. The proposed solution, 4GLS, is a superconducting energy recovery linac (ERL) with an output energy around 600 MeV, delivering both CW beam currents up to 100 mA and alternatively high charge bunches for FEL applications. Production and manipulation of short electron bunches (fs) is a vital part of the source specification. In addition to beam lines from undulator sources in the ERL recovery path there will be three FELs: two will be oscillator types in the infrared and VUV respectively, and the third will be a high gain system for XUV output. The project is outlined, together with its status and the R&D challenges posed. A funded prototype based on a 50 MeV ERL is also described.

Design study of an HHG-seeded harmonic cascade free-electron laser

The UK New Light Source (NLS) project identified a strong scientific case for a next-generation light source to deliver continuous coverage of the photon energy range 50–1000 eV with variable polarisation, 20 fs pulse widths, a high degree of temporal coherence and 1 kHz repetition rate. Three separate seeded free-electron lasers (FELs) were proposed which in combination satisfy these requirements. It was proposed to use a high harmonic generation (HHG) seed source tuneable from 50–100 eV to give direct seeding at the fundamental FEL photon energy up to 100 eV, with one or two stages of harmonic up-conversion within the FEL to reach the higher photon energies. This paper focuses on the optimisation of a two-stage harmonic cascade FEL design operating at the highest (fundamental) photon energy of 1000 eV. Topics investigated include the configuration of the modulator undulators, the required seed power and the effects of the temporal structure of the unfiltered HHG seed on the FEL output. FEL simulations using realistic electron beam distributions tracked from the gun to the FEL are presented, illustrating the predicted coherence properties of the FEL output.

Cavity resonator free electron lasers as a source of stable attosecond pulses

High-gain Free Electron Laser amplifiers are a potential source of single X-ray attosecond pulses and Attosecond Pulse Trains. Single-pulse output from short electron bunches is prone to significant power and arrival-time fluctuations. A Mode Locked Optical Klystron configuration of the FEL amplifier predicts generation of a frequency comb that may be locked to give APT output. In this paper it is shown using numerical simulations that a low feedback (so-called Regenerative Amplifier) FEL cavity resonator configuration can significantly improve output stability for single-pulse operation. The MLOK configuration may also be used in a cavity resonator to generate a frequency comb with spacing much greater than those of the axial cavity modes. As with the MLOK amplifier case, these modes can lock to generate a stable pulse train, each of a few optical cycles.

4GLS: the UK’s fourth generation light source

4GLS is a suite of accelerator-based light sources planned to provide state-of-the-art radiation in the low energy photon regime. Superconducting energy recovery linac (ERL) technology will be utilised in combination with a variety of free electron lasers (IR to XUV), undulators and bending magnets. The 4GLS undulators will generate spontaneous high flux, high brightness radiation, of variable polarisation from 3 - 800 eV, optimised in the lower harmonics up to about 200 eV. Viable radiation at energies up to several keV may be provided from multipole wiggler magnet radiation. The ERL technology of 4GLS will allow shorter bunches and higher peak photon fluxes than possible from storage ring sources. It will also give users the added bonuses of pulse structure flexibility and effectively an infinite beam lifetime. VUV and XUV FELs will be used to generate short pulses (in the fs regime) of extreme ultraviolet light that is broadly tuneable and more than a million times more intense than the equivalent spontaneous undulator radiation. A strong feature of the scientific programme planned for 4GLS is dynamics experiments in a wide range of fields. Pump probe experiments will allow the study of chemical reactions and short-lived intermediates on the timescale of bond breaking and bond making, even for very dilute species. The high intensity of the FEL radiation will allow very high resolution in imaging applications. Funding for the first three years of the 4GLS project was announced by the UK Government in April 2003. This includes the research and development work necessary to produce a design study report, with the construction of an ERL-prototype. Additional funds have recently been awarded that will enable a study of the production of ultra-short pulsed X-rays from the ERL-prototype via Thomson scattering. It is anticipated that the full 4GLS facility will be available to users in 2011.

Isolated Few-Cycle Pulse Generation in X-Ray Free-Electron Lasers

X-ray free-electron lasers (FELs) are promising candidates to deliver high-brightness radiation pulses with duration significantly shorter than the present leading technique, high harmonic generation (HHG). This would extend attosecond science to probe ultrafast dynamics with even finer resolution. To do so requires breaking below a characteristic FEL timescale of typically a few hundred optical cycles, dictated by the relative slippage of the radiation and electrons during amplification. The concept of mode-locking enables this, with the mode-locked afterburner configuration predicted to deliver few-cycle pulses (∼ 1 attosecond at hard X-ray). However such techniques would produce a train of closely separated pulses, while an isolated pulse would be preferable for some types of experiment. Building on previous techniques, a new concept has been developed for isolated few-cycle pulse generation and it is presented alongside simulation studies.