Research Of The Aperture Effect Of Laser-induced Damage In High-fluence Nanosecond Laser Facilites

Laser-induced damage(LID)in high-power laser facilities is a key bottleneck for their development and application.The aperture effect of laser-induced damage means the difference in LID phenomenon,data and law due to beam aperture.This dissertation studies the pattern of manifestation,cause,mechanism and formation law of LID aperture effect,and puts forwards corresponding solving measures.Besides,the dissertation experimentally investigates the LID performance of large-aperture fused silica and deuterated potassium dihydrogen phosphate(DKDP)under UV laser irradiation respectively.Firstly,we study the transverse stimulated Raman scattering(TSRS)caused by the large-aperture beam which leads to a‘double-lung'damage pattern in a large-aperture crystal.Numerical stimulation results show that the‘double-lung'damage pattern is highly similar to the fluence distribution of TSRS across the crystal.Thus we conclude that the‘double-lung'damage results from TSRS.We propose and experimentally verify a novel TSRS suppression method which uses an array of LID sites to block the propagation the TSRS.The experimental results show that the damage array blocks the transverse propagation of Raman scattering light and halves the TSRS gain length,thus effectively suppressing the TSRS.Secondly,we study the aperture effect of LID with small-aperture laser beam,which shows that the LID threshold decreases with the increase of beam aperture.Results show that this phenomenon is caused by sampling error which is inevitable in LID test with small-aperture laser beam.The proposed solution is to adopt raster scan method which uses small-aperture stitching technology to obtain a large-aperture uniform laser beam.Further analysis shows that from the viewing of maximum the testing area,raster scan method should choose hexagonal stitching pattern and maximize the beam overlap factor.Thirdly,we study the large-aperture beam near-field nonuniformity and the rule of its random change.Results show that beam near-field possesses similarity over the whole beam and dissimilarity at local positions for different laser shots.Due to these two factors,the average fluence of the maximum fluence distribution after N shots increases with laser shots while standard deviation is relatively constant,independent of laser shots.Furthermore,we establish a model to describe the random change of beam near-field,and compare with the experimental result.It shows that prediction of the model is the same as the experimental result.Then,we experimentally investigate LID performance of fused silica on a large-aperture laser facility.Results show that the number of damage sites increases rapidly with laser fluence while increases slowly with laser shots.Results also show that the majority of damage sites grow exponentially with laser shots,and growth rate is influenced not only by laser fluence,but also the sizes of damage sites.And small damage sites grow faster than large damage sites.We aslo experimentally investigate LID performance of DKDP crystal on a large-aperture laser facility.Results show that DKDP appears bulk damage,surface craters and scratches,and the surface and bulk damge threshold are 3J/cm~2?6.7J/cm~2respectively.Bulk damage and surface damage craters keep stable under multiple irradiations with UV fluence of 6J/cm~2,but the length of surface scratches will increase with laser shots while their width remain unchanged.