A technique for the alignment of a ring laser cavity

An alignment technique for a ring laser cavity is presented which is suitable for a resonator into which auxillary laser beams cannot pass.     

Production,installation and test of Nb-sputtered QWRs for ALPI

Eight cryostats, each housing 4 sputtered Nb/Cu, 160 MHz, quarter wave resonators (QWR), are now in operation in ALPI[1]. Two of them house high β cavities; the others are equipped with medium β resonators. Another medium β cryostat is ready and will be installed in the early 2002. Pb/Cu medium β accelerating cavities are now present only in four cryostats and they will have their Pb superconducting (SC) layer replaced by sputtered Nb soon. The substitution of the Pb SC layer in ALPI medium β resonators did not interfere with ALPI operation; the upgrading of resonators went on parallel to the cryostat maintenance programme. The average accelerating field of these resonators, at the design power of 7 W, overcomes 4 MV/m, whereas, when Pb electroplated, their average value was 2.7 MV/m. The sputtered resonator combines the good SC characteristics of Nb with the higher thermal conductivity and better stability to change of He bath pressure, which is typical of copper resonators. This leads to a very high reliability, as routinely experienced during beam acceleration.     

Radiation-induced cavities in aluminium alloy imaged by in line electron holography

In irradiated material, cavities result from the condensation of vacancies induced by collision cascades. The study of their formation is a relevant topic since a high density of cavities may alter significantly the material performance. In this work, a simplified version of in line holography was successfully applied for imaging cavities in ion-irradiated 6061 aluminium alloy. In transmission electron microscopy, the incoming electrons experience a phase shift owing to the potential variation induced by the cavities. The retrieval of this phase shift provides a convenient map to observe and highlight the cavities. Information on density of cavities can be easily obtained. In addition, interstitial clusters may also be detected.     

The role of collision cascades and helium atoms in cavity nucleation

Abstract

The simultaneous production of helium atoms and collision cascades during irradiation may enhance the nucleation of cavities. The influence of parameters such as helium generation rate and recoil energy on the possibility of cavity nucleation is investigated. The three most likely mechanisms by which cascade nucleation might occur are considered. Firstly, helium atoms that are present within the cascade may succeed in preventing the collapse of cascades into dislocation loops. Secondly, helium atoms in the cascades may be swept together during the cooling of the cascade to form cavity embryos. Thirdly, more distant helium atoms may be able to reach an uncollapsed vacancy aggregate in a cascade by diffusion before thermal annealing of the cascade.

Calculations indicate that it is unlikely that a sufficient number of helium atoms would be present in a cascade to prevent its collapse. The probability of several helium atoms being swept together within the volume of a cascade is also found to be rather small. Calculations suggest, however, that the cavity nucleation in cascades might become significant if helium atoms were to diffuse much faster than the substitutional diffusion rate during irradiation. The selfinterstitial replacement mechanism, for instance, would yield such a fast diffusion rate of helium.     

Optical‐fiber microcoil waveguides and resonators and their applications for interferometry and sensing

In this work, recent progress in the theoretical and experimental studies of optical‐fiber microcoil waveguides and resonators, as well as their various applications are reviewed. In particular, the focus is set on sensing and interferometry applications. It is shown that due to its inherently low propagation and fiber‐coupling losses, fiber‐microcoil based sensors and interferometers offer substantial enhancement of sensitivity and compactness compared to other types of devices. Recent progress in the realization and experimental characterization of such structures is presented and the theoretical tools to analyze the impact of real‐world nonuniformities on the characteristics of fiber‐microcoil structures are provided.     

Modelling of a Nd:YAG laser Q-switched by a scanning interferometric mirror

We have modelled a continuously pumped Nd:YAG actively Q-switched by a variable interferometric mirror made up of a scanning Michelson or Fabry-Pérot mirror. We have characterised the three-mirror laser dynamics by using a bifurcation diagram constructed from the plot of peak power-enhancement factor as a function of mirror speed. One observes different chaotic windows separated by period-doubling bifurcations, and stable periodic regime. It is demonstrated that the best performance of the Q-switched laser is obtained rather for low than for high mirror speed (pulse width of 20 ns, and high peak power up to 400 times greater than the continuous emission).     

Implementation of a two-qubit controlled-rotation gate based on unconventional geometric phase with a constant gating time

We propose an alternative scheme to implement a two-qubit controlled-R (rotation) gate in the hybrid atom-CCA (coupled cavities array) system. Our scheme results in a constant gating time and, with an adjustable qubit-bus coupling (atom-resonator), one can specify a particular rotation R on the target qubit. We believe that this proposal may open promising perspectives for networking quantum information processors and implementing distributed and scalable quantum computation.     

Reactive Solid State Dewetting: Interfacial Cavitation in the System Ag-Ni-O

Micron thick silver films, vapour deposited onto high purity polycrystalline nickel substrates, dewet the substrate after high temperature annealing in oxygen rich atmospheres, while the films remain stable after annealing at the same temperature in a nitrogen atmosphere. Dewetting occurs when a nickel oxide layer is formed at the silver-nickel interface as a consequence of oxygen diffusion through the silver film.The sensitivity of the dewetting process on various parameters such as the annealing: temperature, time and oxygen partial pressure has been determined.Scanning Electron Microscopy (SEM) of cross-sections reveal that the main mechanism of dewetting at short annealing time is the nucleation of cavities at the Ag-NiO interface which grow towards the free surface of the Ag film. They are formed not only at Ag grain boundaries and triple junctions but also in the core of Ag grains. Such cavities do not occur when the Ag film is deposited onto a NiO single crystal. We propose a simple model for the cavitation: a vacancy supersaturation is sustained in Ag, at the Ag-NiO interface, as a result of oxygen consumption by the oxidation reaction. In regions of fast oxidation, the vacancy supersaturation is large enough to promote the nucleation and growth of interfacial cavities. The model qualitatively accounts for all the observed trends; quantitatively, on top of the vacancy supersaturation, extra-contributions to the driving force for cavitation must be invoked.     

Building cavities in a fluid of spherical or rod-like particles: a contribution to the solvation free energy in isotropic and anisotropic polarizable continuum model

A general formalism for the calculation of cavitation energies in the framework of the scaled particle theory has been implemented in the Polarizable Continuum Model (PCM), contributing to the nonelectrostatic part of the molecular free energy in solution. The solute cavity and the solvent molecules are described as hard spherocylinders, whose radius and length are related to the actual molecular shape, while the solvent density is estimated from experimental data, or from the solvent molecular volume, suitably scaled. The present model can describe isotropic solutions of spherical and rod-like molecules in spherical or rod-like solvents, and also anisotropic solutions in which the solvent molecules are oriented in space: in this case, the cavitation energy also depends on the relative orientation of solute and solvent molecules. Test calculations have been performed on simple systems to evaluate the accuracy of the present approach, in comparison with other methods and with the available experimental estimates of the cavitation energy, giving encouraging results.