The thermal deformation of a metallic mirror used for focusing on or simply directing a laser beam to a workpiece is investigated. The displacement of the front surface is evaluated by using a rapidly converging Dini series. The effects of the distortion on the peak intensity of a focal spot are evaluated by using the normalized intensity function i(P) as a performance figure. Expanding the distorted mirror aberration function in a series of Zernike polynomials, the dependence of i(P) on the mirror illumination and physical parameters is assessed. The results of many calculations illustrate the potential of this analysis.

Mirror Deformations and Wavefront Aberrations Caused by C.w. High Power Laser Beams

Slip bands produced by 100‐nsec 10.6‐μm focused laser pulses have been observed on clean annealed Al single‐crystal targets with (111), (001), and (110) surface‐plane orientations. At energy densities ≳37 J/cm2 the slip banding is not consistent with thermal stress, implying that evaporative compression is the dominant mechanism. This is supported by evidence from neutral particle emission. Possible evidence of thermal stress is present in slip banding observed near and below the threshold for melting. The relationship of observed slip to target orientation can be a valuable source of information on laser‐induced stress fields.

Slip banding in Al single crystals produced by 10.6‐μm laser pulses

The advent of the laser has placed stringent requirements on the fabrication, performance and quality of optical elements employed within systems for most practical applications. Their high power performance is generally governed by three distinct steps, firstly the absorption of incident optical radiation (governed primarily by various absorption mechanisms); secondly, followed by a temperature increase and response governed primarily by thermal properties and finally the elements thermo-optical and thermomechanical response, e.g., distortion, stress birefringenous fracture, etc. All of which needs to be understood in the design as efficient, compact, reliable and useful for many applications high power systems, under a variety of operating conditions, pulsed and continuous wave, rep-rated or burst mode of varying duty cycles.

https://file.scirp.org/pdf/NS_2013052909172563.pdf

High-power optics and its new manifestations

Moving perforation of rocks using long pulse Nd:YAG laser

Metal surface cleaning is highly required in different fields of modern industry. Nuclear industry seeks for new methods for oxidized surface decontamination, and thermonuclear installations require the cleaning of plasma facing components from tritium-containing deposited layer. The laser ablation is proposed as an effective and safe method for metal surface cleaning and decontamination. The important factor influencing the laser heating and ablation is the in-depth distribution of laser radiation. The model of light propagation in a scattering layer on a metal substrate is developed and applied to analyse the features of light distribution. To simulate the contaminated surfaces, the stainless steel AISI 304L was oxidized by laser and in a furnace. Radioactive contamination of the oxide layer was simulated by introducing europium and/or sodium. The decontamination factor of more than 300 was demonstrated with found optimal cleaning regime. The decreasing of the corrosion resistance was found after laser cleaning. The ablation thresholds of ITER-like surfaces were measured. The cleaning productivity of 0.07 m2/hour∙W was found. For mirror surfaces, the damage thresholds were determined to avoid damage during laser cleaning. The possibility to restore reflectivity after thin carbon layer deposition was demonstrated. The perspectives of further development of laser cleaning are discussed.

Laser decontamination and cleaning of metal surfaces : modelling and experimental studies

A theoretical analysis is made of the thermal action of high-power laser radiation on the surface of a solid. Temperature and stress fields generated in an irradiated solid are investigated. Dependences of the maximum laser radiation intensities are calculated as a function of the pulse duration. These calculations are made for three states of a reflecting surface corresponding to maximum permissible elastic stress on the surface, plastic flow producing residual stresses, and melting of the surface layer.

E.M. Shefter

Papers

Thermal action of high-power laser radiation on the surface of a solid