Zhonghua Yan

Ultraviolet Laser-induced ignition of RDX single crystal

The RDX single crystals are ignited by ultraviolet laser (355 nm, 6.4 ns) pulses. The laser-induced damage morphology consisted of two distinct regions: a core region of layered fracture and a peripheral region of stripped material surrounding the core. As laser fluence increases, the area of the whole crack region increases all the way, while both the area and depth of the core region increase firstly, and then stay stable over the laser fluence of 12 J/cm2. The experimental details indicate the dynamics during laser ignition process. Plasma fireball of high temperature and pressure occurs firstly, followed by the micro-explosions on the (210) surface, and finally shock waves propagate through the materials to further strip materials outside and yield in-depth cracks in larger surrounding region. The plasma fireball evolves from isotropic to anisotropic under higher laser fluence resulting in the damage expansion only in lateral direction while maintaining the fixed depth. The primary insights into the interaction dynamics between laser and energetic materials can help developing the superior laser ignition technique.

Bulk damage and stress behavior of fused silica irradiated by nanosecond laser

Abstract. The laser-induced bulk damage and stress behaviors of fused silica are studied by using a neodymium-doped yttrium aluminum garnet laser operated at 1064 nm with pulse width of 11.7 ns. Three zones of bulk damage are defined: columned cavity zone, compacted zone, and crack zone. The damage morphology and stress distribution are characterized by a three-dimensional digital microscope and a polarizer stress analyzer. The results show that the stress in the columned cavity zone and compacted zone is approximately zero. From the laser beam center to fringe, both tensile and compressive stresses in the crack zone increase abruptly and linearly and then decrease exponentially. Thermal annealing is used to prove the phase retardation caused by the residual stress. The formation mechanism of bulk damage is also discussed.

Scratch defects modulated hot spots generation in laser irradiated RDX crystals: a 3D FDTD simulation

Insights into the interaction between laser and energetic materials offer possibility of manipulating and initiating explosives in a more precise, controllable way. Herein, we investigate the interaction between laser and cyclotrimethylene trinitramine (RDX) energetic crystals with scratch defects in the framework of three-dimensional finite difference time domain. Modulation effects on the hot spot formations have been explored for single scratch of parabolic/triangular geometries as well as two-scratch combination in parallel and intersecting configurations. The calculation results disclose that for single scratch, the width of scratch affects more on the light intensification than its depth. Meanwhile, scratch defects of triangular geometry exhibit stronger modulation effect than the parabolic ones. For two-scratch combinations, including both parallel and intersecting configurations, a coupling effect is found to promote generation of hot spots under laser irradiation. In particular, the triangular & triangular interaction presents largest enhancement on light intensification among diverse configurations. It also reveals that RDX crystals are more easily initiated when the intervals between two parallel scratches are appropriate (λ–4λ), or the intersecting scratches are perpendicular with each other. This work offers a primary inspiration for better understanding the formation of hot spots under laser irradiation and provides practical guideline for the surface quality managements of explosive materials.

Quantitative correlation between facets defects of RDX crystals and their laser sensitivity

In this work, the {210} facets of cyclotrimethylenetrinitramine (RDX) single crystals with different quality were studied by scanning electron microscopy and atomic force microscopy. Their laser sensitivity was then assessed using a direct laser ignition test irradiated with ultraviolet laser (wavelength: 355nm, pulse width: 6.4ns). Quantitative relationships between laser sensitivity and surface defects of RDX (210) and (2¯1¯0) facets were investigated. It is determined that the laser sensitivity exhibits significant correlation with the surface roughness, size of which is comparable with scales of laser wavelength. 3D FDTD simulations disclose that this relationship can be well explained with light intensity modulation effects induced by micro-defects on the initial plane wave.

Influence of outgassing organic contamination on the transmittance and laser‐induced damage of SiO2 sol‐gel antireflection film

Abstract. The influence of organic contamination (rubber outgassing) on the transmittance of the SiO2 sol‐gel antireflection (AR) film and laser‐induced damage threshold (LIDT) at 355 nm for 3ω AR film and at 1064 nm for 1ω AR film is studied. The correlation between the contamination time and the transmittance loss/LIDT of 1ω/3ω AR film is also investigated both in atmospheric and vacuum environments. The results show that the transmittance loss increases with increasing contamination time, and the LIDT decreases with increasing contamination time for both in atmospheric and vacuum environments. In addition, the resistance against contamination of the 1ω film is stronger than 3ω film, and the contamination is more serious in vacuum than in an atmosphere environment for the same contamination time. Meanwhile, the damage mechanism is also discussed. It indicated that both the porous structure and photo‐thermal absorption contribute to the decreasing LIDT of the sol‐gel AR film.

Defects-Induced Hot Spots in TATB

We investigate the interaction between the laser and energetic materials with different defects. The three-dimensional models of triaminotrinitrobenzene (TATB) explosives containing spherical pores, craters, and cracks are established, respectively. The laser ignition process of TATB is simulated with three-dimensional finite difference time domain (3D-FDTD) method to study the electromagnetic field distribution surrounding these defects with 355 nm laser incidence. It indicates that the larger defects in the TATB energetic materials have the stronger electric field modulations to initial incident laser for all the three defects, which is easier to lead to the generation of hot spots. Furthermore, TATB materials with spherical pore defects and crater defects are easier to form hot spots than those with narrow crack defects.

Numerical simulation of modulation to incident laser by submicron to micron surface contaminants on fused silica

Modulation caused by surface/subsurface contaminants is one of the important factors for laser-induced damage of fused silica. In this work, a three-dimensional finite-difference time-domain (3D-FDTD) method is employed to simulate the electric field intensity distribution in the vicinity of particulate contaminants on fused silica surface. The simulated results reveal that the contaminant on both the input and output surfaces plays an important role in the electric field modulation of the incident laser. The influences of the shape, size, embedded depth, dielectric constant (er), and the number of contaminant particles on the electric field distribution are discussed in detail. Meanwhile, the corresponding physical mechanism is analyzed theoretically.