1. Mechanical Engineering and Aeronautics Department, University of Patras, 26 504 Rion, Patras, Greece
2. High Voltage Laboratory, Electrical and Computer Engineering Department, University of Patras, 26504 Rion, Patras, Greece
3. School of Science and Technology, Hellenic Open University, 26 222 Patras, Greece
4. Physics Department, University of Patras, 26 504 Rion, Patras, Greece