Radiation Effects On Optical Properties And Laser Damage Performance Of Sapphire And Fused Silica

Optical materials, such as sapphire and fused silica, have been widely used as electrical insulators and optical components/windows due to their excellent optical properties, which are under consideration for a number of critical applications in magnetic confinement fusion reactors and high-power solid state laser-driven inertial confinement fusion. Therefore, the optical materials will be inevitably damaged and their microstructures and optical properties as well as laser damage resistance will change. Based on the wide applications of optical components in the field of controlled nuclear fusion, it is significantly meaningful to investigate the radiation damage and microstructure evolution of optical components for their safe use and life-time under the special operation environment.In this dessertation, sapphire and fused silica have been irradiated by means of helium ions, electron beam and γ-ray. The radiation effects on their surface morphologies, microstructures, optical properties and laser damage performances are investigated. The specific research contents are listed as follows:(1) A series of sapphire samples are irradiated by helium ions at different nominal fluences at room temperature. The effects of radiation on their surface morphologies, optical properties and laser damage performances are investigated. After He ions irradiation, the density and amount of defects increase with increasing irradiation fluence. The surface morphologies have changed distinctly when the fluence exceeds 1×1017 ions/cm2. In addition, two new broad absorption bands and an obvious photoluminescence band at 330 nm are observed, respectively. With the increase of irradiation fluence, the intensities of the absorption bands increase greatly, whereas the intensity of the photoluminescence band decreases and tends to saturation. The original strong infrared band at 790 cm-1 shifts and broadens with increasing irradiation fluence. After laser irradiation, it is found that the irradiation fluence has great effect on the laser-induced damage threshold(LIDT), which decreases monotonously with increasing irradiation fluence. In the light of the experimental results, a mechanism for the degradation of laser damage resistance is presented.(2) The microstructures of sapphire irradiated by helium ions at the fluence of 1×1017 ions/cm2 have been studied in detail. The(006) and(0012) diffraction peaks of sapphire decrease and broaden. The irradiated area and matrix of sapphire has been detailedly investigated by transmission electron microscope(TEM). The high resolution TEM images indicate that sapphire partially appears amorphous structure after helium ion irradiation. There is a lattice expansion up to ~4.5% in the irradiated area which can be measured from two sets of diffraction spots of(003) and(110) and the lattice distortion measured from dispersion of(110) diffraction is ~4.6°. In addition, a large number of nano-scaled helium bubbles and crystallites with different orientations are observed. Furthermore, due to the formation of helium bubbles, the laser induced damages of sapphire are totally different before and after helium irradiation.(3) Sapphire samples have been implanted successively by helium ions with a series of energies to obtain a uniform layer of impurities. The effects of helium doping on the surface morphologies, optical properties and laser-induced damage resistance are investigated. After helium ion doping, the surface morphology has been greatly changed. The intensity of optical absorption significantly increases. In addition, a phenomenon of the original infrared band obviously shift and broaden has been observed. After laser irradiation, on account of helium doping, an obvious degradation of LIDT is up to 40% and the laser damage has changed from burst damage induced by stress effect to melting damage induced by thermal effect.(4) A 60 kW high-power pulsed electron beam is used to study the surface morphologies, optical property and laser damage performance of high purity fused silica at the different fluences. The cracks appear at the surface of fused silica after electron beam irradiation, owing to the thermal effect, and that the crack density and size increase with increasing radiation fluence. There are a large number of debris particles on the surface. The absorbance increases at the beginning then decreases with increasing electron radiation fluence and a weak absorption peak at 394 nm is observed. The LIDT of fused silica decreases with increasing fluence. Based on the experimental results, a mechanism for the degradation of laser damage resistance is discussed.(5) High purity fused silica samples have been irradiated by 60 Co γ-ray at the different doses, the effect of γ-ray irradiation on the surface morphologies, optical properties and laser damage performances of fused silica have been investigated. After γ-ray irradiation, there is no obvious change on the surface of fused silica observed. However, the surface roughness increases slightly with increasing γ-ray irradiation dose. When the irradiation dose reaches 5×105 Gy and above, a broad absorption band at 215 nm is observed and its intensity increases greatly with increasing γ-ray dose. After laser irradiation, the distributions of LIDT are related to γ-ray doses, and the LIDT decreases monotonously with increasing γ-ray dose.In the work, helium ions, electron beam and γ-ray are employed to simulate the irradiation environments of the controlled nuclear fusion. Radiation effects on their surface morphologies, optical properties and laser damage performances of sapphire and fused silica irradiated by different fluences/doses are investigated. It is concluded that helium ion irradiation has great effect on the laser damage performance of sapphire; high dose of electron beam irradiation will lead to disaster damage on the surface of fused silica optical components; γ-ray irradiation has little effect on the laser induced damage morphology of fused silica. Therefore, radiation effect of sapphire and fused silica produced by high fluence/dose irradiation will significantly shorten their life-time, which will greatly limit the engineering application of optical components.