Constant Beam Power Research Articles

MODIFICATION OF CARBON STEEL BY LASER SURFACE MELTING: PART I: EFFECT OF LASER BEAM TRAVELLING SPEED ON MICROSTRUCTURAL FEATURES AND SURFACE HARDNESS

The present study aims to improve the surface hardness of carbon steel by application of laser surface melting of effective conditions. The travelling speed of laser beam during this treatment is one of the important treatment conditions. This study aims to investigate the effect of laser surface melting with different beam speeds on macro and microstructure as well as the hardness distribution through the thickness of carbon steel. To achieve this target, three different travelling speeds (1500, 1000 and 500 mm min-1) at a constant beam power of 800 W were chosen in this study. The resulted laser treated specimens were investigated in macro and microscopically scale using optical and scanning electron microscope. Hardness measurements were also carried out through the thickness of the laser treated specimens. The laser treated areas with all used travelling speeds results in melted and solidified zone on the surface of the steel. In the same time, Plates of acicular martensite structure were observed within the upper part of the melted and solidified zone in almost all experimental conditions, while some bainite structure in ferrite grains are detected in its lower part. By increasing the travelling speed, the depth of the laser treated zone was decreases, while travelling speed has much less significant effect on the laser treated zone width. The size of the formed martensite plates was increased by decreasing the travelling speed from 1500 to 500 mm min-1. On the other hand, the travelling speed has a straight effect on the length of the acicular martensite; as the travelling speed increases, the acicular martensite became longer, while it shows fine acicular martensite at lower travelling speeds. The depth that full martensite structure can be reached is increased by increasing travelling speed. At lower travelling speed (500 mm min-1), large amount of bainite structure is observed at the center of the treated zone up to its lower end. The fast travelling speed (1500 mm min-1) show higher hardness on the free surface than that of slow travelling speed (500 mm min-1). On the other hand, the travelling speed has a reverse effect on the depth of this hardness increment; the slower travelling speed give deeper areas of high hardness than that of fast speed. The Heat Affect Zone (HAZ) areas were increased by decreasing the travelling speed. In all conditions, the heat affected zone areas were composed of partially decomposed pearlite in ferrite grains. Finally, the microstructure of the base metal far from the laser treated areas show normal ferrite-sound pearlite microstructure.

Towards the optimal energy of the proton driver for a neutrino factory and muon collider

Cross section data from the HARP experiment for pion production by protons from a tantalum target have been convoluted with the acceptance of the front-end channel for the proposed neutrino factory or muon collider and integrated over the full phase space measured by HARP, to determine the beam-energy dependence of the muon yield. This permits a determination of the optimal beam energy for the proton driver for these projects. The cross section data are corrected for the beam-energy dependent amplification due to the development of hadronic showers in a thick target. The conclusion is that, for constant beam power, the yield is maximum for a beam energy of about 7 GeV, but it is within 10% of this maximum for $4<{T}_{\mathrm{beam}}<11\text{ }\text{ }\mathrm{GeV}$, and within 20% of the maximum for ${T}_{\mathrm{beam}}$ as low as 2 GeV. This result is insensitive to which of the two HARP groups' results are used, and to which pion generator is used to compute the thick target effects.

Broad beam source of fast atoms produced as a result of charge exchange collisions of ions accelerated between two plasmas

Experimental study of a fast argon atom beam source is carried out and the study results are presented. The source comprises a 90-mm deep and 210-mm in diameter hollow cathode as well as a flat emission grid, both electrodes made of titanium. The study revealed main factors, which influence the zone diameter of homogeneous substrate etching by a broad beam of fast argon atoms, produced as a result of charge exchange collisions of ions, accelerated between a plasma emitter inside the hollow cathode and a secondary plasma in the working vacuum chamber, the plasmas being separated from each other with the grid. It is shown that at a distance from the grid, exceeding the resonant charge exchange length up to 4 times, elastic collisions have no appreciable impact on the spatial distribution of the etching rate in the vacuum chamber. The homogeneous etching zone diameter is mainly influenced by angular characteristics of accelerated particles in the grid plane. At a constant beam power up to 3–5 kW the diameter is rising with a decrease of their energy and with a corresponding increase of the beam current.

Influence of Beam Incidence Angle in Laser Welding of Austenitic Stainless Steel using Finite Element Analysis

This paper presents the influence of beam incidence angle on austenitic stainless steel sheet subjected to a high density laser beam having Gaussian power density distribution. Bead‐on trials are conducted on 3.15 mm thick commercial AISI 304 austenitic stainless steel sheet using a Nd:YAG laser source with a maximum output of 2kW in the continuous wave mode. The effects of beam incident angle on the weld bead geometry are studied using finite element analysis. Experiments are conducted with 600, 1000 and 1400W laser power and 800, 1400 and 2000mm/min welding speed. A three dimensional finite element model is developed for the simulation of non‐linear transient thermal analysis of the weld bead geometry for different beam incident angles using the finite element code ANSYS. The result reveals that by increasing the beam incident angle with constant beam power and welding speed, there is a considerable reduction in the depth of penetration‐to‐width ratio (d/w). Further, it is noticed that the process enters into conduction mode of welding from the keyhole mode of welding as the beam angle is increased beyond 10o. The comparison of the simulation results and the experimental data for weld bead geometry with different beam incident angles show good agreement.

Spatially resolved residual stress assessments of GaN film on sapphire substrate by cathodoluminescence piezospectroscopy

Two cathodoluminescence piezospectroscopic (CL/PS) approaches for measuring the residual stress distribution in thin films are critically examined and compared using an intrinsic GaN film sample (2.5μm in thickness) grown on a (0001)-oriented sapphire substrate. The first approach invokes an analytical model to fit experimental stress distributions as retrieved in both film and substrate at the edge of an artificially created cross section of the sample. Such an edge-stress distribution takes into account both the thermal expansion mismatch between the film and substrate and the mechanistics of film growth process. In the second approach, we directly and nondestructively measure the bulk residual stress field from the sample top surface on the film side using an increase in electron beam voltage (maintaining a constant beam power) as a means for screening the film subsurface. In this latter case, the combined effects of self-absorption and misfit dislocations on the GaN spectrum severely affect the CL/PS assessments; therefore, they need to be analyzed separately from the effect of stress. After spectral deconvolution of the obtained stress profiles, according to either in-plane or in-depth response functions of the electron probe for both film and substrate, cross-section and top-surface stress data were compared and discussed in an effort to substantiate the feasibility of spatially resolved CL/PS approach for the examination of residual stress distributions in film structures.

Radiation-pressure-based cylindrically shaped microactuator capable of smooth, continuous, reversible, and stepped rotation

Long cylindrical objects have been observed to align their central axis with the propagation axis of the illuminating laser beam through the action of radiation-pressure-generated force and torque. A cylindrically shaped microactuator based on this principle and suitable for micromachine applications is examined theoretically. When four in-plane laser beams converging at a common point centered on the cylinder are used, the cylinder can be made to rotate about a pivot point. In one mode, smooth, continuous, and reversible rotation is possible, whereas the other cylinder can be step rotated and locked, similar to the operation of conventional stepping motors. The properties of the device are analyzed based on obtaining either a constant rotation rate with variable beam power levels or a quasi-constant rotation rate with constant beam power levels or on using a fixed beam sequence rate that matches the system parameters and produces smooth or stepped operation.

Measurement of limiter heating due to alpha particle losses during high fusion power deuterium-tritium operation of the TFTR tokamak

Measurements and preliminary analysis have been completed on limiter heating during high fusion power deuterium-tritium (D-T) operation of the Tokamak Fusion Test Reactor (TFTR) in an attempt to identify heating from first orbit and ripple losses of fast alpha particles. Recent operation of TFTR with a 50-50 mix of D-T has resulted in fusion power output (≊6.2 MW) orders of magnitude above what was previously achieved on TFTR. A significantly larger absolute number of particles and energy from fusion products, compared to D-D operation, are expected to be lost to the limiters. Power and energy estimates of total alpha losses were as high as 0.13 MW and 64 kJ. Measurements were made in the vicinity of the outer midplane, where most of the losses are expected, with thermocouples mounted on the tiles of a limiter. With an increasingly more reactive mixture of D and T at constant beam power, there was a measurable increase in the limiter tile temperature as the fusion power and alpha yield increased. The measured temperature increases due to heating from alpha losses corresponded to heat loads of ≤5.9 kJ/m2, which are within a factor of 2 of estimates for alpha heating based on simulations assuming a loss rate of fast alphas of 12%. Estimates are made of heating from the various heating sources using simulations from a number of codes. The observed level of alpha heating indicates that there was probably neither an unexpectedly large fraction of lost alphas nor unexpected localization of the losses. Limits on the stochastic ripple loss contribution from alphas can be deduced.

Estimation of Grown Layer Thickness by Cathodoluminescence Measurement

We propose a new method for measuring the thickness of an epitaxially grown layer with a direct band gap. The beam voltage which gives the maximum cathodoluminescence intensity is related to the layer thickness under a constant beam power. The layer thickness in the range between 0.1 to 1 µm can be estimated without prior knowledge of material parameters such as diffusion length and absorption coefficient.

Intensity dependence of thermal nonlinear optical activity in crystals

The thermal contribution to nonlinear optical activity in a BSO crystal was studied at 514.5 nm. The results show that the effect is only weakly dependent on laser intensity for constant beam power. By varying the laser spot size in the sample the electronic and thermal contributions to nonlinear optical activity may be separated.

Diameter Optimization for Maximum Vertical Penetration of a Beam of Collimated Optical Radiation Normally Incident on the Ocean Surface

The optimum diameter of a circular beam of collimated optical radiation normally incident on the ocean surface, in order to obtain a maximum time-average power density vertically downward, is determined for (1) the case of a given constant beam power density and (2) the case of a given constant total beam power. The resulting optima are functions of the roughness of the ocean surface (wind velocity), the depth of the point of observation, and the index of refraction.