W. A. Gillespie

High quality electron beams from a laser wakefield accelerator

Very stable, high quality electron beams (current ∼ 10 kA, energy spread < 1%, emittance ∼ 1π mm mrad) have been generated in a laser-plasma accelerator driven by 25 TW femtosecond laser pulses.

Temporally resolved electro-optic effect

The electro-optic effect between an ultrafast optical probe pulse and an ultrashort terahertz pulse is shown to depend on the time derivatives of the product of the probe and terahertz electric fields. Application of this theory to temporally resolved single-shot terahertz detection techniques, where the electro-optic effect is temporally localized within an optical probe pulse, shows that the description presented here differs fundamentally and verifiably from that commonly used in literature.

The UK FEL project: Status and measurement of optical gain☆

Abstract The aim of the UK FEL project is to study the characteristics of a single-pass FEL over a wide operating range, using a four-section 5 m wiggler yielding a maximum K value of 2.7. The laser is driven by the 165 MeV Kelvin Laboratory linac. It has a design operating wavelength range of 2–20 μm, although higher harmonics are also under investigation. We report direct measurement of optical gain using a cw CO 2 laser between 9 and 11 μm. Simultaneous spectra are obtained of both gain and spontaneous emission as a function of linac energy or wiggler field as the tuning parameter. The linewidth obtained is close to that expected for homogeneous broadening, and the gain is consistent with that predicted theoretically for the current available. Strong enhancement of both spontaneous emission and measured gain is obtained in the presence of a high- Q FEL cavity, as expected by simple considerations of optical confinement. These results imply that despite the shortness of our macropulse (∼ 2.5 μm), it will be possible to diagnose buildup to oscillation from an injected signal.

1 GHz modulation of a high-current electron gun

Abstract Measurements on a triode electron gun operated with grid modulation at 1 GHz are presented in this article. The gun has a miniaturized cathode—grid s

Upconversion of a relativistic Coulomb field terahertz pulse to the near infrared

We demonstrate the spectral upconversion of a unipolar subpicosecond terahertz (THz) pulse, where the THz pulse is the Coulomb field of a single relativistic electron bunch. The upconversion to the optical allows remotely located detection of long wavelength and nonpropagating components of the THz spectrum, as required for ultrafast electron bunch diagnostics. The upconversion of quasimonochromatic THz radiation has also been demonstrated, allowing the observation of distinct sum- and difference-frequency mixing components in the spectrum. Polarization dependence of first and second order sidebands at ωopt±ωTHz, and ωopt±2ωTHz, respectively, confirms the χ(2) frequency mixing mechanism.

An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator

Narrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs.

Optimization of the FELIX accelerator with respect to laser performance

Abstract In this paper we discuss the performance of the FELIX accelerator in relation to the laser performance. Over the past year, a number of improvements have been made to the accelerator, both to the hardware and to the way in which it was operated, that have resulted in a reduction of the time needed to reach saturation from 9 to 3 μs. Energy spread and stability, both short and long term, and operational flexibility, an important issue for a user facility, are addressed. Surprisingly, “best” FEL performance is not obtained at the same operation point that gives the smallest energy spread, which suggests that the electron bunch length is not fixed. Evidence is presented for the conjecture that the non-isochronicity of the bend plays a major role. Measurements of enhanced spontaneous emission and of coherence between successive optical micropulses, indicating a spatial structure in the electron microbunches on the scale of an optical wavelength, are also discussed.

Role of misalignment-induced angular chirp in the electro-optic detection of THz waves.

A general description of electro-optic detection including non-collinear phase matching and finite transverse beam profiles is presented. It is shown theoretically and experimentally that non-collinear phase matching in ZnTe (and similar materials) produces an angular chirp in the χ(2)-generated optical signal. Due to this, in non-collinear THz and probe arrangements such as single-shot THz measurements or through accidental misalignment, measurement of an undistorted THz signal is critically dependent on having sufficient angular acceptance in the optical probe path. The associated spatial walk-off can also preclude the phase retardation approximation used in THz-TDS. The rate of misalignment-induced chirping in commonly used ZnTe and GaP schemes is tabulated, allowing ready analysis of a detection system.

Bunching of an intense electron beam extracted from a triode gun modulated at 1 GHz

We present measurements of the bunch length and emittance of a high-current electron beam, which is extracted from a triode modulated at 1 GHz and subsequently compressed by means of velocity modulation in a prebuncher cavity. The prebuncher is detuned by about 1 MHz in order to ensure that the total field, which is the sum of the field from an external generator and the beam-induced field, has the correct phase and amplitude. The bunch length is measured by means of a capacitive pick-up probe and a 20 GHz sampling oscilloscope. A bunch length of 40 ps, at a charge of 220 pC, has been obtained at the end of the drift space behind the prebuncher, which has a length of 200 mm. The normalized emittance is 36-pi mm mrad.

A Low-Energy-Spread Rf Accelerator for a Far-Infrared Free-Electron Laser

A high electron current and a small energy spread are essential for the operation of a free electron laser (FEL). In this paper we discuss the design and performance of the accelerator for FELIX, the free electron laser for infrared experiments. The system consists of a thermionic gun, a prebuncher, a buncher and two standard commercial linac sections. The gun is operated with a pulse duration of 280 ps and a bunch charge of 200 pC. After compression to 35 ps by the prebuncher, the bunches are accelerated to 4 MeV in the buncher and simultaneously compressed to 6 ps. The principle of the method is that the order of the electrons is conserved in the buncher, so that the resulting more or less linear energy-phase relationship along each bunch can be compensated effectively against space charge forces and the accelerating field gradient in the linacs, via an appropriate choice of the phase of the rf wave. Behind the linacs an rms energy spread of 0.30% has been measured.