W. Allan Gillespie

Low secondary electron yield engineered surface for electron cloud mitigation

Secondary electron yield (SEY or δ) limits the performance of a number of devices. Particularly, in high-energy charged particle accelerators, the beam-induced electron multipacting is one of the main sources of electron cloud (e-cloud) build up on the beam path; in radio frequency wave guides, the electron multipacting limits their lifetime and causes power loss; and in detectors, the secondary electrons define the signal background and reduce the sensitivity. The best solution would be a material with a low SEY coating and for many applications δ < 1 would be sufficient. We report on an alternative surface preparation to the ones that are currently advocated. Three commonly used materials in accelerator vacuum chambers (stainless steel, copper, and aluminium) were laser processed to create a highly regular surface topography. It is shown that this treatment reduces the SEY of the copper, aluminium, and stainless steel from δmax of 1.90, 2.55, and 2.25 to 1.12, 1.45, and 1.12, respectively. The δmax furt...

Diffractive optical element embedded in silver-doped nanocomposite glass

A diffractive optical element is fabricated with relative ease in a glass containing spherical silver nanoparticles 30 to 40 nm in diameter and embedded in a surface layer of thickness ~10 μm. The nanocomposite was sandwiched between a mesh metallic electrode with a lattice constant 2 μm, facing the nanoparticle containing layer and acting as an anode, and a flat metal electrode as cathode. Applying moderate direct current electric potentials of 0.4 kV and 0.6 kV at an elevated temperature of 200 °C for 30 minutes across the nanocomposites led to the formation of a periodic array of embedded structures of metallic nanoparticles. The current-time dynamics of the structuring processes, optical analyses of the structured nanocomposites and diffraction pattern of one such fabricated element are presented.

Picosecond pulsed laser induced optical dichroism in glass with embedded metallic nanoparticles

Picosecond (~10 ps) pulsed laser irradiation at 532 nm led to the efficient and scalable fabrication of dichroic areas in glass with spherical silver nanoparticles of ~30 - 40 nm in diameter embedded in a surface layer of thickness ~20 μm. The observed dichroism is due to the uniform and permanent shape transformation of the nanoparticles - from spherical to spheroidal shapes - throughout the irradiated areas and along the laser polarization direction, paving the way for affordable manufacture of polarization-selective diffractive optical elements. The shape modification threshold and the dichroism as a result of Surface Plasmon Resonance band separation were identified. The process was then studied as a function of the laser polarization, repetition rate and the number of pulses fired per spot.

Azimuthally and radially polarized light in conical diffraction

Azimuthal and radial polarization states of light are used to produce conical diffraction (CD) from a KGd(WO4)2 crystal. The patterns produced in the ring plane in each case display marked differences than those seen when linearly polarized incident light is used, with the production of a splitting of the CD ring into two concentric rings of equal intensity. The free space evolution for each type of polarization state is also experimentally recorded and investigated. Comparison with theory shows agreement with the experimentally observed results.

Status of the UK free electron laser project

The aim of the UK 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 of 2.7. The laser is driven by the 165 MeV Kelvin Laboratory linac fitted with a modified electron gun incorporating a subharmonic buncher and an energy compression system to reduce the energy spread to 0.1%. The design wavelength operating range is 2–20 μm, although higher harmonics are also under investigation. Recent progress is reported, with emphasis on electron beam transport and diagnostics. Measurements have been made with the high-resolution electron spectrometer, and emittance measurements have been performed under different linac operating conditions. Details are given of the multichannel spectrum/profile monitors common to these measurements.

An electron spectrum and profile monitor for the UK free electron laser

Abstract A versatile electron spectrum monitor has been developed for the UK free electron laser project. The monitor uses secondary emission from 96 tungsten foils and microprocessor-controlled sampling electronics to produce a high-resolution electron energy spectrum. A 30-channel prototype is currently operating as a beam profile monitor and has been extensively used in emittance measurements at the Kelvin Laboratory linac.

Radially and azimuthally polarized laser induced shape transformation of embedded metallic nanoparticles in glass

Radially and azimuthally polarized picosecond (~10 ps) pulsed laser irradiation at 532 nm wavelength led to the permanent reshaping of spherical silver nanoparticles (~30 - 40 nm in diameter) embedded in a thin layer of soda-lime glass. The observed peculiar shape modifications consist of a number of different orientations of nano-ellipsoids in the cross-section of each written line by laser. A Second Harmonic Generation cross-sectional scan method from silver nanoparticles in transmission geometry was adopted for characterization of the samples after laser modification. The presented approach may lead to sophisticated marking of information in metal-glass nanocomposites.

Laser nano-processing of sliver-doped nanocomposite glass for tailored nonlinear response

In recent years, the fabrication, structural modification and characterisation of nanostructured materials have gained a great deal of attention due to their distinctive properties and potential use in technological applications. Glass containing silver (Ag) nanoparticles (NP), or Metal-Glass Nanocomposite (MGN), is routinely used as high-contrast optical polarisers [1], and has been recently shown as a promising medium for high-volume optical storage of information [2]. Strong second and third harmonic generation has been demonstrated when the MGN containing mechanically reshaped NPs was subjected to fs pulse excitation [3], thus showing potential as an effective nonlinear optical medium for photonics nanodevice applications.