Emittance preservation in a plasma-wakefield accelerator.

Radio-frequency particle accelerators are engines of discovery, powering high-energy physics and photon science, but are also large and expensive due to their limited accelerating fields. Plasma-wakefield accelerators (PWFAs) provide orders-of-magnitude stronger fields in the charge-density wave behind a particle bunch travelling in a plasma, promising particle accelerators of greatly reduced size and cost. However, PWFAs can easily degrade the beam quality of the bunches they accelerate. Emittance, which determines how tightly beams can be focused, is a critical beam quality in for instance colliders and free-electron lasers, but is particularly prone to degradation. We demonstrate, for the first time, emittance preservation in a high-gradient and high-efficiency PWFA while simultaneously preserving charge and energy spread. This establishes that PWFAs can accelerate without degradation-an essential step toward energy boosters in photon science and multistage facilities for compact high-energy particle colliders.

Energy-Spread Preservation and High Efficiency in a Plasma-Wakefield Accelerator.

Energy-efficient plasma-wakefield acceleration of particle bunches with low energy spread is a promising path to realizing compact free-electron lasers and particle colliders. High efficiency and low energy spread can be achieved simultaneously by strong beam loading of plasma wakefields when accelerating bunches with carefully tailored current profiles [M. Tzoufras et al., Phys. Rev. Lett. 101, 145002 (2008)PRLTAO0031-900710.1103/PhysRevLett.101.145002]. We experimentally demonstrate such optimal beam loading in a nonlinear electron-driven plasma accelerator. Bunches with an initial energy of 1 GeV were accelerated by 45 MeV with an energy-transfer efficiency of (42±4)% at a gradient of 1.3 GV/m while preserving per-mille energy spreads with full charge coupling, demonstrating wakefield flattening at the few-percent level.

Local two-sided bounds for eigenvalues of self-adjoint operators.

We examine the equivalence between an extension of the Lehmann-Maehly-Goerisch method developed a few years ago by Zimmermann and Mertins, and a geometrically motivated method developed more recently by Davies and Plum. We establish a general framework which allows sharpening various previously known results in these two settings and determine explicit convergence estimates for both methods. We demonstrate the applicability of the method of Zimmermann and Mertins by means of numerical tests on the resonant cavity problem.

Computer hardware and software to assist the visually impaired and blind.

Modern developments in computer hardware and software for the business world have been adapted to allow the partially sighted and blind to participate successfully in computer usage in education, employment and recreation. Adaptations such as the enlargement of the whole screen display, production of enlarged print and voice output of screen contents can assist the partially sighted. Conversion of computer screen information into speech or braille can be used in conjunction with sound boards and touch pads to allow the totally blind access to the exact spatial layout of screen contents. Optical character recognition devices whereby the printed page can be accessed by computer, and recent voice recognition techniques which allow a computer to interpret voiced commands, can also be used to assist the totally blind. Developments in these areas are discussed together with specific examples of equipment now available which make use of the various methods described.

A clinical evaluation of equipment which allows the partially sighted and blind to access computer-based information.

Twenty-five visually handicapped people, including two who were completely blind, were trialled on recently developed equipment which enables such severely visually impaired people access to the modern computerised world. Twenty-one of the people found that such equipment would enhance their study, work and recreational capabilities and allow them to utilise computerised technology alongside their normally sighted peers. Two already had equipment sufficient for their needs, and two children were too young to require it at this stage.

Ten years' experience in low vision practice.

The Low Vision Clinic at the Palmerston North Hospital has now been operating for 10 years. Over the course of these ten years a number of factors have emerged which can be as readily applied to general ophthalmological practice as to low vision practice. The philosophy of low vision care is one of which all ophthalmologists should be aware and includes factors to be taken into account when dealing with children, people in the workplace, and everyday factors involved in daily living activities, all of which are equally relevant in routine ophthalmological practice. This paper endeavours to share some thoughts on these factors and also discusses means by which the visually handicapped can be helped in areas where specialist low vision services are not readily available.