Abstract
When fine particles of several tenth of µm in diameter are injected into a plasma, they are negatively charged and are subjected to an electric force generated by the electric filed in the sheath, and each particle is levitated at the position where gravity and the electric force are valanced. In the present study, we tried to measure the spatial electric filed distribution in the sheath by using spherical particles of synthetic resin containing a fluorescent substance.
Since the shape of each particle is close to a sphere, the mass of the particle is estimated as m = (4/3)πρR3, where ρ is the density of the synthetic resin and R is a radius of the particle. The capacitance of the spherical conductor is C=4πε0R, so that the particle charge q can be estimated accurately as q = CVf = 4πε 0VfR, where Vf is the floating potential on the particle. Therefore, the mass-to-charge ratio, m/q, is proportional to R2 and the electric field at the position where a particle is levitated, E =(m/q) g, is also proportional to R2, where g is the gravitational constant. Large particles are, therefore, expected to be levitated near the lower electrode, while smaller particle float higher away from the electrode.
The fluorescent particles used in the experiments are made of acrylic and have nominal particle diameters of 10 µm (blue emission) and 40 µm (green emission) . Figure 1 shows the size distributions of the two kinds of particles, where the size of the particles was measured by the scanning electron microscope. Figure 2 shows a snapshot of the levitating particles which clearly display the sheath structure. When these particles were injected into the plasma, smaller blue particles were levitated at relatively higher region, while larger green particles were levitated at the lower region as shown in figure 2. If we assume that the particles whose diameters are at the points A, B and C in figure 2 levitate at the height A, B and C in Fig.1, we can estimate the electric field at each height as A: 79.2 V/m, B: 316.8 V/m and C: 1979.8 V/m, respectively.
Our new method has a lot of capabilities for the measurement of the spatial structure of the sheath plasma. When the number of the fluorescent color species increases and the size distribution of the particles is more reduced, the precision of the measurement can be higher. In addition, this method is a real-time observation, and the temporal variation can also be easily measured.
