1. Institute of Physics, State University St. Petersburg, 198504 Russia.
2. Boeing Space and Intelligence Systems, Boeing Electron Dynamic Devices, 3100 West Lomita Boulevard, Post Office Box 2999, Torrance, CA 90509–1999, USA.
3. International Center for Relativistic Astrophysics (ICRA), University of Rome La Sapienza, I-00185 Roma, Italy.