The Neptune photoinjector

The RF photoinjector in the Neptune advanced accelerator laboratory, along with associated beam diagnostics, transport and phase-space manipulation techniques are described. This versatile injector has been designed to produce short-pulse electron beams for a variety of uses: ultra-short bunches for injection into a next-generation plasma beatwave acceleration experiment, space-charge dominated beam physics studies, plasma wake-field acceleration driver, plasma lensing, and free-electron laser microbunching techniques. The component parts of the photoinjector, the RF gun, photocathode drive laser systems, booster linac, RF system, chicane compressor, beam diagnostic systems, and control system, are discussed. The present status of photoinjector commissioning at Neptune is reviewed, and proposed experiments are detailed.     

Indentation of an incompressible elastic film

The problem of impressing a rigid flat-ended cylindrical indenter onto an incompressible elastic film is considered. An integral transform solution is developed to reduce the solution of the problem to a Fredholm integral equation of the second kind with a symmetrical kernel depending on the boundary conditions (frictionless/bonded) in the areas of contact. The relationship between the applied load and the indentation depth is derived, which provides a guideline for measuring the elastic constants of thin films and determining the degree of adhesion between a thin film and a stiffer substrate.     

Injection and dynamics of accelerated beams

This paper is a summary of the discussions undertaken by the working group on injection and accelerated beam dynamics at the 1st ICFA Novel and Advanced Accelerator Workshop on Second Generation Plasma Accelerators. The second generation of work on plasma accelerators is aimed to bring the accelerated beams up to the quality needed for applications such as high-energy physics linear colliders. To begin, first generation, or proof-of-principle, experiments and concepts were reviewed. To map the work needed in the second generation of development, the demands of the applications were examined, and an improved framework for discussing the viability of plasma accelerators was constructed. In particular, the issues scaling applications to the short wavelengths characteristic of plasma accelerators was discussed, as was the appropriate characterization of the beam quality in these devices, and the connection between plasma accelerator and conventional accelerator design. Within this framework, the working group discussed electron sources and injectors, the effects of drive beam evolution on accelerated beam dynamics, this effects of nonlinear plasma wave fields on beam phase space, stochastic processes, spatial and temporal beam-plasma wave matching, and future second-generation experimental goals and techniques.     

Electron capture in an electron plasma wave

We study the influence of the radial electric field of a plasma wave on the accelerated electrons.     

Towards a plasma wake-field acceleration-based linear collider

A proposal for a linear collider based on an advanced accelerator scheme, plasma wake-field acceleration in the extremely nonlinear regime, is discussed. In this regime, many of the drawbacks associated with preservation of beam quality during acceleration in plasma are mitigated. The scaling of all beam and wake parameters with respect to plasma wavelength is examined. Experimental progress towards high-gradient acceleration in this scheme is reviewed. We then examine a linear collider based on staging of many modules of plasma wake-field accelerator, all driven by a high average current, pulse compressed, RF photoinjector-fed linac. Issue of beam loading, efficiency, optimized stage length, and power efficiency are discussed. A proof-of-principle experimental test of the staging concept at the Fermilab test facility is discussed.     

Disordering of the Pb (110) surface

Molecular dynamics simulations incorporating a many-body glue model potential (GM) have been used to investigate the atomic structure and dynamics of the Pb (1 1 0) surface in the range from room temperature up to the bulk melting point. The main features of the surface disordering process include: generation of vacancies and the formation of an adlayer, the formation of so-called “local steps” and further their proliferation including wandering, creation and propagation of a quasiliquid surface film. These processes are illustrated by the use of a modern visualization technique.     

One GeV acceleration with laser wakefield in the linear regime

We derive an expression for the maximum energy gain of an accelerated electron, in the limit that the plasma wave created by a laser wake is linear both along the longitudinal direction and in the transverse plane, and with a maximum laser power lower than the critical power for relativistic self-focusing. With an available power of 300 TW, the energy gain is of 1 GeV.     

Survey of seismic fragilities used in PRA studies of nuclear power plants

In recent years, seismic PRA studies have been performed on a large number of nuclear power plants in the USA. This paper presents a summary of a survey on fragility databases and the range of evaluated fragility values of various equipment categories based on past PRAs. The survey includes the use of experience data, the interpretations of available test data, and the quantification of uncertainties. The surveyed fragility databases are limited to data available in the public domain such as NUREG reports, conference proceedings and other publicly available reports. The extent of the availability of data as well as limitations are studied and tabulated for various equipment categories. The survey of the fragility values in past PRA studies includes not only the best estimate values, but also the dominant failure modes and the estimated uncertainty levels for each equipment category. The engineering judgments employed in estimating the uncertainty in the fragility values are also studied.This paper provides a perspective on the seismic fragility evaluation procedures for equipment in order to clearly identify the engineering analysis and judgment used in past seismic PRA studies.     

Critical discussion of the single-fibre pull-out test: does it measure adhesion?

With a comprehensive finite-element model the interface failure process of the single-fibre pull-out test, for the measurement of fibre/matrix adhesion, is investigated on the basis of a fracture-mechanics debonding criterion. Special emphasis is placed on the interface local mixed-mode load, which is shown to have an important influence on the debonding process and is taken into account by a fracture ellipsoid criterion. Additional features investigated are residual thermal stresses, specimen geometrical details (wetting meniscus, drop shape) and a simplistic model of fibre/matrix interfacial friction. For medium debonding lengths the energy release rate runs through a plateau range that can be approximated by a simple analytical approach and can be observed experimentally with a very stiff loading configuration. The mixed-mode state in the plateau range is uniform and dominated by mode 2, but its actual value is quite uncertain. From experimental experience the actual adhesion failure is closely connected with the interface local normal load, while local shear load induces submicroscopic friction and matrix inelasticity which strongly reduce the interface sensitivity, resulting in G_1c<G_2c. G_1c seems to be more significant for adhesion. The interpretation of the plateau range may provide the total critical energy release rate, G_c, for the debonding process, but from a region where mode II prevails. G_c will therefore be far from G_1c, reducing the significance of the tests results for characterization of adhesion.     

A new life extension method for high cycle fatigue using micro-martensitic transformation in an austenitic stainless steel

Most of the conventional strengthening methods for metals and alloys such as work hardening, precipitation hardening, cause a decrease in ductility and are not very effective for cyclic loading. In this study, a new strengthening method, which is effective for high cycle fatigue, has been developed. The intersections of dislocations in a stainless steel are freezed by very fine martensite particles, which are supposed to suppress dislocation motion at low stress amplitudes. Fatigue life in a high cycle regime increased >60 times, and no decrease in ductility was observed in tensile tests, as compared to a work-hardened stainless steel.     

Effect of change in velocity on the development of medical complications in patients with multisystem trauma sustained in vehicular crashes

Medical complications such as sepsis or multiple organ system failure increase the morbidity and mortality associated with injuries sustained in car crashes. This study addresses the question of the association, if any, between one crash characteristic, i.e. change in velocity (Δv), and subsequent medical complications. Data on seventy-six severely injured patients were obtained as part of an in-depth, trauma-center-based study of the biomechanics of vehicular trauma. Factors found to be predictive of the development of complications included patient age≥36 years, Δv≥30 mph, and injury severity score≥25. Vehicle occupants involved in crashes with a Δv≥30 mph had a risk of complications more than five times greater than for those in the lower Δv group. Better knowledge of the dynamics and severity of the crash could help clinical staff anticipate the development of complications and initiate timely prevention strategies.     

The fluidity of presidential policy choice: the space station, the Russian card, and U.S. foreign policy

This paper examines the general executive policy processes that were in place in 1984. These processes, through which the original space station budget proposal was approved, became so uncontrolled that a severe and publicly embarrassing retrenchment became necessary in 1993 to enable the program to survive for another year. The focus here is on how presidential leadership is exercised in the science and technology policy arena which is normally viewed as peripheral to the president's major policy interests. The survival of the current International Space Station now depends upon it remaining central to the president's foreign policy agenda—which prompts recollections of earlier Apollo Program experiences.     

Fatigue performance of 6261-T6 aluminium alloy – constant and variable amplitude loading of parent plate and welded specimens

The work reported in this paper is a summary of the results obtained from several projects run over a five year period. A number of thrusts were encompassed in the project scope, which was to examine the fatigue strength of extruded 6261-T6 I-beams with centrally located, welded cover plates under constant amplitude (CA) and variable (two-level repeated block) amplitude (VA) loading. The CA part of the work examined the effect of cover plate geometry, specimen size, thermal and vibratory stress relief, and differences in fabrication quality on fatigue performance. A number of cases were identified which gave rise to cross-overs in performance in moving from shorter to longer fatigue lives. The aim of the VA part of the work was to identify situations which might give rise to fatigue lives shorter than predicted by linear damage summation models, such as Miner's rule. Two such cases were identified: specimens which had been subjected to vibratory stress relief; and specimens in which weld `quality' (as assessed by profile, heat affected zone hardness, extent and microstructure) varied. Explanations for these observations are proposed and supported by interpretation of a two-level block of small fatigue crack growth.     

Asymptotic failure rate of a continuously monitored system

This paper deals with a perfectly continuously monitored system which gradually and stochastically deteriorates. The system is renewed by a delayed maintenance operation, which is triggered when the measured deterioration level exceeds an alarm threshold. A mathematical model is developed to study the asymptotic behavior of the reliability function. A procedure is proposed which allows us to identify the asymptotic failure rate of the maintained system. Numerical experiments illustrate the efficiency of the proposed procedure and emphasize the relevance of the asymptotic failure rate as an interesting indicator for the evaluation of the control-limit preventive replacement policy.