The laser deposition of shaped metallic wire using holographic optical elements

This work covers an investigation of the use of customised laser beam profiles to improve the characteristics of wire-based laser deposition, by using the modified beam to control the thermal cycle of the deposition process. Inherent disadvantages of wire fed deposition vs. powder have been previously identified; namely that it is highly sensitive to changes in processing parameters, a tendency to low surface quality and a high incidence of porosity and cracking. Two potential avenues are identified and explored for the purpose of improving these disadvantages: Laser beam shaping and wire cross-sectional modification. The investigation is presented in both theoretical modelling and physical experiments. Surface reflectivity calculations are predicted that relate the incidence angle profile across the wire width with it absorption profile. Further heat transfer simulations were used to compare the heat conduction within the wire for the various wire cross-sections and experimental beam profiles. From this, melting simulations are presented with a variety of wire shapes and beam profiles. A series of experimental studies are presented comparing the use of different beam profiles on a single wire geometry, and then a single type of beam profile on different wire geometries. These are analysed principally with optical microscopy, with selected samples also studied via EBSD analysis. These corroborated the simulation results, where the use of altered beam profiles was found to give improved results in regard to the ability to form a melt pool, reduced power requirements and improved dilution characteristics. Combining a shaped beam with shaped wire gave further improvements. Subsequent to this, experimental clad tracks are presented that show the ability to create multiple layers in different directions, as well as the ability to use shaped wire with a wire feeder. This work shows that through the ability to modify the laser beam and wire cross-sectional profiles together with each other, the deposition properties can be improved; with a reduction in required power, an improvement in clad track quality and a reduction in process sensitivity

Optical guiding in meter-scale plasma waveguides

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_0 beam, generates the core of the waveguide, while the delayed second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on axis plasma densities as low as N_e0=5x10^16 cm^-3

Solving nonlinear multicommodity flow problems by the analytic center cutting plane method

The paper deals with nonlinear multicommodity flow problems with convex costs. A decomposition method is proposed to solve them. The approach applies a potential reduction algorithm to solve the master problem approximately and a column generation technique to define a sequence of primal linear programming problems. Each subproblem consists of finding a minimum cost flow between an origin and a destination node in an uncapacited network. It is thus formulated as a shortest path problem and solved with Dijkstra's d-heap algorithm. An implementation is described that takes full advantage of the supersparsity of the network in the linear algebra operations. Computational results show the efficiency of this approach on well-known nondifferentiable problems and also large scale randomly generated problems (up to 1000 arcs and 5000 commodities

Solving variational inequalities defined on a domain with infinitely many linear constraints

We study a variational inequality problem whose domain is defined by infinitely many linear inequalities. A discretization method and an analytic center based inexact cutting plane method are proposed. Under proper assumptions, the convergence results for both methods are given. We also provide numerical examples to illustrate the proposed method

Air hydrodynamics of the ultrafast laser-triggered spark gap

We present space and time resolved measurements of the air hydrodynamics induced by ultrafast laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel initiated by the laser plays the dominant role in the gap evolution leading to breakdown