[IEEE Proceedings of 9th International Seminar/Workshop on Direct and Inverse Problems of Electromagnetic and Acoustic Wave Theory - Tbilisi, Georgia (2004.10.11-2004.10.14)] Proceedings of the 9th International Seminar/Workshop on Direct and Inverse Problems of Electromagnetic and Acoustic Wave Theory, 2004. DIPED 2004. - Electron beam alternating rotation in periodic permanent magnet focusing field has an effect on parasitic backward-wave oscillation in helix traveling wave tube