Matthew Harvey

Shear-banding in polyacrylamide solutions revealed via optical coherence tomography velocimetry

We used optical coherence tomography velocimetry inside a fluids rheometer to study the rheology of a family of polyacrylamide (PAM) solutions that contain different polymer molecular weights and concentrations, with picolitre probing volumes. The linear velocity profiles obtained from low molecular weight samples, characteristic of Newtonian fluids under shear, become shear-banded when longer polymer chains (molecular weights 5 000 000 and above) are used at sufficiently high concentrations. Upon increasing the concentration further, shear-banding becomes less dominant and significant wall-slip takes place on the two plates of the rheometer. We describe the shear-banding and wall-slip phenomena in our samples in terms of parameters calculated from the velocity profiles, and use our data to suggest a dynamic phase diagram indicating the linear, shear-banding, and wall-slip regimes as functions of PAM molecular weight and concentration.

An ultracold low emittance electron source

Ultracold atom-based electron sources have recently been proposed as an alternative to the conventional photo-injectors or thermionic electron guns widely used in modern particle accelerators. The advantages of ultracold atom-based electron sources lie in the fact that the electrons extracted from the plasma (created from near threshold photo-ionization of ultracold atoms) have a very low temperature, i.e. down to tens of Kelvin. Extraction of these electrons has the potential for producing very low emittance electron bunches. These features are crucial for the next generation of particle accelerators, including free electron lasers, plasma-based accelerators and future linear colliders. The source also has many potential direct applications, including ultrafast electron diffraction (UED) and electron microscopy, due to its intrinsically high coherence. In this paper, the basic mechanism of ultracold electron beam production is discussed and our new research facility for an ultracold, low emittance electron source is introduced. This source is based on a novel alternating current Magneto-Optical Trap (the AC-MOT). Detailed simulations for a proposed extraction system have shown that for a 1 pC bunch charge, a beam emittance of 0.35 mm mrad is obtainable, with a bunch length of 3 mm and energy spread 1%.

Low-Cost Computer-Controlled Power Supplies for Optimization and Control of Electron Spectrometers

Synopsis New Arduino-based power supply designs are presented that have direct applicability to electron spectrometers such as the (e,2e) spectrometer that is used in Manchester. Voltages from these supplies can be manually selected using digital potentiometers, or can be remotely controlled using LabVIEW. These new control routines allow the apparatus to explore both ground state and laser-excited targets.

The AC-MOT Cold Atom Electron Source (CAES)

Progress towards observing ultra-fast dynamics on atomic length scales is limited by the achievable brightness, or current per unit solid angle, of available electron beams. The brightness of an electron beam is related to its characteristic temperature [1], and hence by reducing the temperature of the electron beam, higher brightness beams can be produced. In a cold atom electron source (CAES), cold atoms held in a magneto-optical trap (MOT) are photo-ionized near threshold by a laser. This process yields electrons with an energy spread on the order of 1 meV, corresponding to a characteristic temperature of ~10 K [2]. Electrostatic fields are then used to extract the electrons into a bunched beam for delivery to experiments. The magnetic B-fields found in a conventional MOT perturb the electron beam trajectory and its quality. The new source described here uses an AC-MOT [3] to ensure the trapping fields are zero at the time of electron extraction. This allows field-free electron extraction whilst retaining a high source density. In the ACMOT, the trapping laser polarization is switched synchronously with an alternating Bfield. Once the B-fields are zero, the atoms are ionized using pulsed, two colour photoionisation, and an electron beam is extracted. To determine the brightness B of the CAES electron beam, its beam current J and characteristic temperature T are measured, since

(e,2e) Ionization Studies of N2 at Low to Intermediate Energies from a Coplanar Geometry to the Perpendicular Plane

Ionization triple differential cross sections (TDCS) have been determined experimentally and theoretically for neutral N2 over a range of geometries and energies, from a coplanar geometry through to the perpendicular plane. Data were obtained at incident electron energies from ~10 eV to ~100 eV above the ionization potential (IP) of the 3σg, 1πu and 2σg states, using equal and non-equal outgoing electron energies, and using symmetric and asymmetric geometries. Data were taken with the incident electron beam in the scattering plane (ψ = 0°), as well as at angles ψ = 45° and ψ = 90° (see figure 1).

Electron-Impact Ionization of Laser-Aligned Atoms – Contributions from both Natural and Unnatural-Parity States.

In this work experimental and theoretical results will be presented for (e,2e) ionization measurements from laser-excited and aligned atoms. The experimental data are taken in Manchester, whereas theoretical results are from the groups of Don Madison in Missouri, and James Colgan at Los Alamos Labs in the USA. The motivation for these studies arises since time-independent distorted wave (DWBA, 3DW) models predict zero flux for atoms aligned orthogonal to the scattering plane (as in fig 1,2), in disagreement with experimental data. By contrast, time-dependent close coupling (TDCC) models predict a non-zero crosssection under these conditions, and conclude it is the unnatural parity contributions to the cross section that produce this flux [1-4]. An unnatural parity state has parity (−1), compared to a natural parity state that has parity (−1).

Natural & unnatural-parity contributions in electron-impact ionization of laser-aligned atoms

Differential cross section measurements from laser-aligned Mg atoms are compared to theoretical calculations using both time dependent and time-independent formalisms. It is found that both natural and unnatural parity contributions to the calculated cross sections are required to emulate the data when the state is aligned out of the scattering plane.

Studying Cold Potassium Rydberg Atoms with an AC-MOT

A new method is described for the detection and study of cold Rydberg atoms. Cold atoms held in an AC-driven magneto optical trap (AC-MOT) are selectively laser excited to a Rydberg state in a stepwise process. Ions or electrons from interactions between Rydberg atoms are then electrostatically extracted and detected via a channel electron multiplier. The apparatus is capable of studying Rydberg atoms from n = 15 to n ~ 220.

Electron impact ionization and excitation studies of laser prepared atomic targets

A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.

Electron scattering from cold atoms - Design and operation of the AC driven Magneto Optical Trap (AC-MOT)

Electron scattering from cold atoms provides significant advantages compared to collisions with atoms emanating from a hot effusive source. Here we describe a new type of cold atom trap (the AC-MOT) that allows measurements from cold and trapped atoms at low impact energies, by eliminating the MOT magnetic fields very rapidly. The design and operation of the AC-MOT will be described, together with results from experiments that have been performed.