W.A. Gillespie

Feasibility of a far-infrared free-electron laser as voltage-controlled optical oscillator

Abstract The use of a far-infrared free-electron laser as voltage-controlled optical oscillator is reported. The optical spectrum is able to follow a set of pre-programmed waveforms applied to the electron energy. The time-resolved spectral output has been measured at several wavelengths and cavity desynchronisms. Maximum sweep rates of 1.4%/μs were measured at 20μm wavelength and total sweeps of 2.0% in wavelength have been produced.

Real-time single-shot electron bunch length measurements

Linear accelerators employed as drivers for X-ray free electron lasers (FELs) require relativistic electron bunch with sub-picosecond bunch length. Precise bunch length measurements are important for the tuning and operation of the FELs. Previously, we have demonstrated that electro-optic detection is a powerful technique for sub-picosecond electron bunch length measurements. In those experiments, the measured bunch length was the average of all electron bunches within a macropulse. Here, for the first time, we present the measurement of the length of individual electron bunches using a development of our previous technique. In this experiment, the longitudinal electron bunch shape is encoded electro-optically on to the frequency spectrum of a chirped laser pulse. Subsequently, the laser pulse is dispersed by a grating and the spectrum is imaged with a CCD camera. Single bunch measurements are achieved by using a nanosecond gated camera, and synchronizing the gate with both the electron bunch and the laser pulse repetition rates. The electron bunch length is determined by measuring the laser pulse spectra with and without the presence of an electron bunch. We demonstrate that this method enables a real-time diagnostic for the bunch length of single electron bunches with a time resolution of 370 femtoseconds and a high signal-noise-ratio.

HIGH TEMPORAL RESOLUTION, SINGLE-SHOT ELECTRON BUNCH-LENGTH MEASUREMENTS

Publisher Summary A new technique combining the electro-optic detection method of the Coulomb field of an electron bunch with the single-shot cross-correlation of optical pulses is used to provide the single-shot measurements of the shape and length of sub-picosecond electron bunches. That technique has been applied at the Free Electron Laser for Infrared eXperiments (FELIX) facility for free-electron laser (FEL) showing bunches of around 600 fs, where the resolution is limited primarily by the electro-optic (EO) crystal thickness and the relatively low γ of the electrons. X-ray FELs require dense, relativistic electron bunches with bunch lengths significantly shorter than a picosecond. For operating and tuning those lasers, advanced electron bunch length monitors with sub-picosecond temporal resolution are essential. Ideally, non-destructive and non-intrusive monitoring of a single electron bunch should be available in real-time. A promising candidate for such monitors, and the subject of an on-going research and development project at the FELIX facility, is the determination of the electron bunch longitudinal-profile via electro-optic detection of the co-propagating Coulomb field. This chapter presents the results of the first application of the cross-correlator technique to longitudinal-profile electron bunch characterization.

Application of electro-optic sampling in FEL diagnostics

Abstract The electro-optic sampling technique has been used for the full characterization (both amplitude and phase) of freely propagating pulsed electromagnetic radiation (such as FEL pulses, transition radiation) and for the quasistatic electric field of relativistic electron bunches. Measurements of the electron bunch length and shape have been performed with both non-intercepting and intercepting methods. Sub-picosecond time resolution has been obtained with this electro-optic detection technique.

Time‐resolved electron spectrum diagnostics for a free‐electron laser

Time‐resolved electron‐beam diagnostics have been developed for use with free‐electron lasers (FELs) and associated electron sources, based on the techniques of secondary electron emission and optical transition radiation (OTR). The 32‐channel OTR detector forms part of a high‐resolution (0.18%) electron spectrometer with a time resolution of 50 ns. Variable‐magnification optics allow the spectrometer to view single‐macropulse spectra with widths in the range of 0.2%–7%; wider spectra are taken with several momentum settings. Design criteria for the spectrometer are presented, and experience of operating with the diagnostics over a range of FEL physics experiments is summarized. The spectrometer is used, in conjunction with optical diagnostics, in studies at FELIX of efficiency enhancement, pulse chirping, and stepped‐undulator operation.

Development of a real-time, 20 mega-samples per second, 32 channel data acquisition system

Abstract A 32 channel data acquisition system has been developed to monitor, at intervals of 50 ns, the variation in the electron energy spectrum of a free electron laser during the 10 μs duration of the electron beam pulses which repeat at a frequency of approximately 10 Hz. The data are made available to the user in a variety of ways: as graphics displays on a VDU and as an archive that allows the latest data to be accessible to users at workstations elsewhere on the laboratory network. A low-cost, simple and robust system is described that will find application in other areas where real-time, rapid, multi-channel sampling is required, and an insight is provided into the design of both hardware and software aspects of a multitasking multiprocessor system.

Parametric amplification of picosecond mid-infrared radiation

The aim of this work is to produce mid-infrared picosecond pulses with energies of the order of 1 mJ, tunable in the range of 4-10 /spl mu/m, at a repetition rate of 10 Hz. An optical parametric chirped-pulse amplification scheme and a longer crystal are used to increase the amplified pulse energy while maintaining a high peak-power. Parametric amplification (type-I difference-frequency mixing) in a AgGaS/sub 2/ crystal is used to increase the energy of micropulses provided by Free Electron Laser for Infrared Experiments (FELIX), while preserving tunability over the 4-10 /spl mu/m wavelength range.

Cavity losses and extraction efficiencies in a short-pulse infrared free-electron laser

Recent data taken at the FELIX facilit y are presented. Losses in the optical cavity have been artificiall y increased by the introduction of fine wires, and measurements of the resulting optical and electron energy spectra have been made over a range of cavity losses. The experimental data show that the extraction eff iciency varies as the inverse square root of the cavity losses, consistent with the observation of superradiance in the free-electron laser oscillator. * Corresponding author. Tel. +44 1382 308242, Fax +44 1382 308261, email a.m.macleod@tay.ac.uk. The electron-to-photon conversion eff iciency, η, is an important parameter in a free-electron laser (FEL). Measurements of the intrinsicall y low eff iciency of Compton FELs have been performed at a number of laboratories and have been reported previously. [1,2]. Eff iciency measurements performed at the FELIX facilit y [3] used an electron energy spectrometer equipped with a transition radiation detector [4] and demonstrated eff iciencies as high as 2.3%. During these measurements electron energy spreads regularly exceeded the acceptance of the electron spectrometer and caused background problems, and therefore a technique was developed to calculate the eff iciency from a combination of the electron and optical data. The optical power envelope may be used to determine the extraction eff iciency as a function of time within the macropulse, up to an arbitrary scaling factor [3]. During the onset of lasing, the energy spreads observed fall easil y within the 8 % energy bite of the detector. Fitti ng this part of the data to the eff iciency derived from the optical power allows the scaling factor to be determined. In this way the optical data may be used to determine the eff iciency even in those cases where the energy spread is too large for the spectrometer to handle. It should be noted that when the energy spread fall s within the acceptance of the spectrometer, consistent results are obtained for the eff iciency values calculated from the optical data and those measured directly from the electron energy. In this paper we discuss the effects of cavity losses on the extraction eff iciency in a FEL operating in the strong slippage regime with short electron bunches, short optical pulse lengths and relatively long radiation wavelengths. The experimental data were taken on FEL–2 at the FELIX facilit y [5] operating at a wavelength of 15.2 μm and with round-trip cavity losses which were varied from 4 % to 16 %. The results obtained are consistent with the recent work of Jaroszynski et al. [6,7] which describes superradiance in short-pulse free-electron lasers and is relevant to situations where the optical pulse length is considerably shorter that the electron bunch length. In our case the optical pulse length is of the order of 600 fs [8] and the electron bunch length is of the order of 3 ps. Eff iciencies considerably in excess of the conventional 1 2Nu estimate are produced at small cavity detunings and arise because the ultra-short optical pulses interact with electrons at a given position in a bunch for only part of the journey down the undulator before contact is lost due to slippage. Our measurements show that the eff iciency η depends on the cavity losses α as η α ∝ − 1 2 and that there is also good numerical agreement between our results and the effective and measurable eff iciencies as defined in [6] The electron diagnostics comprise a high-resolution time-resolved electron spectrometer employing a curved OTR radiator, and variable imaging optics set to give a dispersion of 0.22 % per channel [4]. The time resolution of the spectrometer is 50 ns. The optical power envelope is measured by a pyroelectric detector with a 100 ns response time. Cavity losses are increased by introducing fine wires into the optical cavity and measured using the ring-down time of the cavity at the end of the macropulse. Following [6] we use the expression η ρ α = l l b

Nonlinear evolution of the equilibrium electron orbits in a helical wiggler with a positive axial guide magnetic field at magnetoresonance

Abstract A model of the coupled nonlinear evolution equations is presented for the slow-varying amplitude and phase of the electron perpendicular velocities in a helical wiggler with positive axial guide magnetic field at magnetoresonance. The transverse motion is dominated by the nonlinear effect, which arises from the correlation between the longitudinal and perpendicular velocities. The analytical results are in very good agreement with numerical calculations.