3.2. Measurements of the Ionization Rate of Laser-produced Plasma The conversion efficiency of each part of the energy conversion from laser beam tc electricity is discussed above. The ionization rate of the laser-produced plasma is measured in this chapter to understand the state of the plasma. 3.2.1. Experimental Procedure. The experimental apparatus is shown in Fig. 3. Carbor and the cross-linked polyethylene are used as targets. The ionization rate x is definec by Where Np is the number of ions and Nr is the number of vaporized particles from the target (N„=the number of ions + the number of neutral particles). The number of ions is measured by the Langmuir probe set at 1.5 m from the target surface using equation (2). The number of vaporized particles is measured as follows. About 200-1000 shots of the laser beam are irradiated on the target to measure the vaporized mass, and the number of vaporized particles is calculated by Where NA is the Avogadro number, M is the mean vaporized mass of the target at one shot and m is the atomic weight of the target (in case of the cross-linked polyethylene the ratio of the carbon to the hydrogen is assumed to be 1 to 2, therefore the mean value tm = (12+1 + 1)/3 = 4.67 is used.) 3.2.2. Experimental Results and Discussions. Fig. 11 shows the dependence of the ionization rate on the laser energy. The ionization rate increases with the laser energy using each target. The ionization rate of the cross-linked polyethylene target is about 10 times smaller than that of the carbon. It is supposed that this is due to the difference of the melting point of each target. The melting point of cross-linked
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