Propagation of ion plasma wave-packet as the plasma bullet in a plasma jet-plume

Summary form only given. The velocity of plasma bullet is known to be (10⁴-10⁵) m/s in the plasma jet experiments. To explain the high propagation velocity of the plasma bullet under a very weak field condition, Lu and Laroussi have explained a photo-ionization based model. However, the mechanism of bullet propagation has not been understood clearly. Similar to the propagation of plasma bullet, the observation of wave-packet propagation in a long positive column discharge tube has been reported where the wave-packet speed is the same as that of plasma bullet in the pencil-type plasma jet devices. In this study the plasma bullets are analyzed to be an ion plasma wave-packet propagating by the plasma density gradient along a plasma plume. When a high density plasma is generated at the high field region of plasma jet device, the plasma diffuses longitudinally to be formed the plasma plume column of low electric field. At the steady state the background plasma column has a diffusion velocity u_n due to a plasma density gradient. The ion plasma wave is generated by the pulses of the operating frequency having a several 10s kHz and it propagates along the plasma plume column without damping due to the plasma density gradient. The wave packet of ion plasma wave has the group velocity u_g~c_s²/u_n~(10⁴-10⁵) m/s where the ion acoustic velocity is c_s~10³ m/s and the plasma diffusion velocity u_n~(10¹-10²) m/s in the plasma jet column of low electric field. In the conclusion, the plasma bullet is the ion wave-packet propagating along the plasma column having a plasma density gradient.