[1] Natl Inst Lasers Plasma & Radiat Phys, Electron Accelerators Lab, POB MG-36, RO-071125 Bucharest, Romania
[2] Bulgarian Acad Sci, Inst Elect, 72 Tzarigradsko Shossee Blvd, Sofia 1784, Bulgaria
[3] Univ Chem Technol & Met, 8 Kl Ohridski Blvd, Sofia 1756, Bulgaria