The sheath structure under the localized secondary electron emission

The effect of secondary electrons emitted from the solid on the sheath structure is very important, since the heat flux to the solid is limited by the sheath potential. When the secondary electrons emission increases, the sheath potential depth becomes smaller and its isolation effect of primary electron heat flux from the plasma to the solid becomes weaker. Recently, the Nagoya university group derived a new analytic sheath model which includes the secondary electron effects and conformed it experimentally. According to their model, if sufficient secondary electrons with the finite temperature are emitted, the virtual cathode would be formed in front of the target These works, however, have been restricted to one dimension modeling. Locally deformed potential acts as if an electrostatic lens to concentrate or diverse ion heat flux. As the heat flux to the target plate usually has the non-uniform profile, we can expect that some local hot spots are formed on the target surface. If the temperature there is sufficiently high, the thermoelectrons are emitted locally and the sheath structure and the heat flux distribution are affected by them. In order to emulate the local secondary electron emission and study this situation, we install a small electron gun in the target plate and exposed it to low pressure glow plasma. We also made particle simulations and study the condition of the virtual cathode formation and compare it with the one dimensional model. In the no secondary electron case, the potential is almost one dimensional and dependant only on the distance from the target surface. As the secondary electron current increases, localized potential holes ( below than the target surface value) are formed near secondary electron sources. The effect of this deformed potential on plasma flux is under way. In this simulation, when the second electron temperature decreases, the potential holes become shallow and the simulation becomes unstable, which may has relation the - eam-plasma instability.