Measurement of ion average velocity in Riemann´s transition layer in front of sheath

The potential and ion average velocity distribution in weakly-ionized, weakly-collisional plasmas were measured. The plasma sheath geometry in weakly-ionized, collisionless plasmas was generally known to have two distinctive layers which are the Bohm´s presheath, and the Child-Langmuir (C-L) sheath. When the collisional effects like ionization collision exist, however, Riemann theoretically suggested that there must be the `transition layer´ between the presheath and the C-L sheath to couple the numerical solutions of each distinctive layer. Oksuz and Hershkowitz first defined the experimental existence of the Riemann´s transition layer in a multi-dipole plasma source by measuring potential variation with emissive probes (EPs). They also measured the ion average velocity variation inside the layer with Mach probes (MPs) and showed that the ion velocity reached the Bohm velocity inside the transition region. It can be convincing that the observation of the point where ions reach the Bohm velocity is the key to understand the physical role of the transition Bohm´s layer, because the formation of sheaths in weakly-collisional plasmas is an electric-field dominant phenomenon in which the Bohm´s criterion should be satisfied well. Especially, the clarification of the potential at which the ions reach the Bohm velocity is very important to estimate an ion incident dose to a target like Langmuir Probes (LPs), since the plasma in transition region is known to follow the Boltzmann´s relation. Experiments similar to the Oksuz´s were done in our multi-dipole plasma source. The measurement of the ion average velocity and potential variation in the transition layer with various collisional conditions were focused. The bulk plasma parameters were measured with LPs. Potential profile formed in front of the biased plate was measured with EPs in the limit of zero emission technique. The average ion velocity variation was measured with non-invaded high power laser induced flu- rescence (LIF; Nd-YAG/Dye laser) system instead of metallic MPs to avoid the perturbation on plasma sheath properties. From these phenomenological results we could understand the role of the transition layer (including the preshealth), on the formation of the plasma shealth.