composed of two mesh electrodes and one ion collector. The earthed grid is used so that the electric field of the electrostatic separation does not influence the plasma expansion, and the suppressor grid with negative potential repels the electrons. The ions which pass through the suppressor grid are collected by the ion collector and the collector output voltage is nearly equal to the voltage which corresponds to the expansion energy Tu (eV). The collector current is given by the value of tiieuS taken out as an electrical energy, where nb e and 5 are the ion density arriving at the collector, charge and the collector area, respectively. The conversion efficiency of the PDC is given by the following equation. where The value of depends on the geometrical transmittance of the suppressor grid, the reflected ions by the potential Vrand the secondary electrons from the mesh grid. On the other hand, the value of lc depends on the geometrical transmittance and the secondary electrons from the suppressor grid. It is necessary that the width of the mesh window is nearly equal to the Debye length kD( = 740 V 7(eV)//?(cm 3) (cm), 7 and n are the electron temperature and the electron density, respectively) to separate the electrons and ions effectively. Although the Debye length becomes short in the high density plasma and the mesh interval becomes small, the geometrical transmittance of the mesh cannot be increased due to the heatresisting problem. The electron density n should be decreased by making the expansion area of the plasma large, but in this case, the converter tends to be large in size. For the above reason, the mesh which is strong against the sputtering, heating and high geometrical transmittance with short interval should be developed in future. 4. EXPERIMENTAL APPARATUS The experimental apparatus for generating the electricity using the produced plasma confined by the surface magnetic field is shown in Fig. 4. The output energy of the TEA CO2 laser (10) is 2-4 J/pulse and the full width at half maximum (FWHM) of it is 100-200 ns. A germanium lens with the focal length of 40 cm is used to focus the laser beam. The pressure of the vacuum chamber is about IO-6 torr and the cross-linked polyethylene is used as a target. The magnetic coil shown in Fig. 5 is installed inside along the vacuum chamber and it is divided by twenty-four pairs with two layers and distributed cylindrically. The inner and outer diameters of the coil are 12 cm and 20 cm respectively, and the magnetic wall of 4 cm width is made when the current flows in the coil as shown in Fig. 6. The length of the coil for the axial direction (Z-direction) is 56 cm. The gradient dR/dZ and the decreasing inner diameter of the coil are 0.25 and 6 cm at both ends respectively, where, R is the radial
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