Laser shock cleaning (LSC) has been proved an effective method to clean sub-micron and micron particles from solid surface during last five years. In this report, dynamics of the interaction between plasma shock wave and adhered spherical particles is analyzed in theory, considering the change of particle contact radius induced by the load of the shock wave. Analysis of the rolling mechanism at the initial contact of the shock wave with particles shows working gap has a serious influence to the cleaning and smaller diameter particles are more difficult to be removed with smaller cleaned area. Moreover, particle energy obtained from the shock wave is analyzed through which particle removal trace and cleaned area are studied combined reflection shock wave and irregular turnover of the particle into account. Removal of micron copper particles on a silica surface in air is experimented at different working gap. Results show that particles can be effectively removed within the suitable working gap, i.e., 0.8 mm for 150 mJ explosion energy, and higher working gap represents poorer cleaning efficiency. Moreover, the cleaning situation of the heavy contamination shows out an interesting phenomenon of the cleaned area (0.4cm2) profile that is an ellipse caused by the non-uniform pressure distribution of plasma shock wave.
A novel method, namely, fiber-coupling zig-zag beam deflection method has been proposed to investigate the attenuation
of plasma shock waves in air in this paper. The main innovation of this method is to use a zig-zag laser beam as the
probing beam, instead of a straight beam in traditional beam deflection method. The zig-zag beam is formed by eight
times successive reflections on a pair of parallel mirrors. Shock waves propagate through the space between two mirrors
which contains nine zigzags in the horizontal plane. This space can be designated as the testing field. After the probing
beam leaves the testing field, it is coupled into a single mode optical fiber which guides the beam into a photomultiplier
to complete the process of photoelectrical conversion.
Plasma shock waves are generated during laser ablation of Fe target in air. The laser used in our experiment is a Qswitched
Nd: YAG laser operating at wavelength of 1064nm and pulse width (FWHM) of 7ns. The output of
160mJ/pulse of this laser is focused on the surface of the target which is far exceeds the ablation threshold of Fe. When a
shock wave propagates in the testing field, it will meet the zig-zag probing beam nine times one after another.
Correspondingly, nine deflection signals will be induced by the perturbations of the shock wave, which can be utilized to
illuminate the propagating behavior of the shock wave. The whole attenuation process of the shock wave can be
demonstrated intuitively only through one experimental curve with the nine deflection signals. From the curve, the
average velocity of the shock wave can be calculated out with high and reliable precision. It is found that shock waves
attenuate into acoustic waves within 10mm in air because of the inhalement of environmental media. The development
of this novel optical technology provides a powerful tool for the detection of shock wave propagation and riches
diagnostic methods of shock waves.
A new feature of relationship between macro-bending losses and bending length of mono-mode fibers is found experimentally: when bending radius is 4mm nearly no loss is detected on the condition that bending angle is less than 20 degrees. The similar phenomena happen when bending radius is 3mm or 2mm. The turning bending degree that bending losses will increase rapidly is called critical angle by us. The result will give us some new clues that how the bending losses change with bending length on earth. And some theoretical work should be done to explain the phenomenon.