Development of filamentary structure of atmospheric breakdown on 170GHz Gaussian beam

This paper investigates the development of the atmospheric breakdown on the high power Gaussian beam observed using a high speed video camera. The movie with 5-1 mus frame interval time was taken. A high power millimeter wave beam generated by 170 GHz gyrotron was used. First the breakdown at the focal point was observed. The beam was focused using an offset parabolic mirror. A separated ionization spot was found in front of the ionization front of the previous frame. At the next frame, the next new spot is produced and the old ionization spot decays. The array of separated spots is formed on the beam. Next, the ionization propagation on the beam with 20 mm radius was observed. The small luminescence plasma spot formed a thin outline of the ionization front. Each plasma spot propagated towards the outside from the beam center. The trajectory of spots shows the similar structure to the filament structure of the integrated picture.