Study of the generation of the pressure pulse in the laser induced pressure pulse method: Optimization of the process

The laser induced pressure pulse method -LIPP- is commonly used for measurement of distribution of space charge in insulating materials. The aim of this paper is to study the mechanisms giving rise to the generation of the pressure pulse and to optimize the generating process to obtain the better sensitivity of the technique. For transparent liquids at the laser wavelength, the two mechanisms which may generate the pressure pulse, i.e. thermal expansion and thermal vaporization at the liquid/target interface, are clearly identified depending on the laser energy. The surface roughness of the target also played an important role in this generation. For opaque liquids, thermal dilatation and evaporation at the liquid/air interface are involved. With the use of Indian ink, the energetic conversion revealed better for the smaller investigated liquid thickness, i.e. 20 μm. A concentration of 20% of Indian ink in water provides the best sensitivity at 20°C, respectively 25% at 60°C.