Plasma Bullet as a Plasma Diffusion Wave-Packet in Plasma Jets

The propagation of a plasma bullet in a plasma jet is described under the base of plasma fluid theory in terms of the plasma drift, the gas flow, and the plasma diffusion. The analysis reveals that the plasma bullet originates from the electrostatic diffusion wave in plasma propagating in the plasma column of jets. The plasma diffusion waves, electron waves, and ion waves propagate in the form of a wave-packet modulated by the operation frequency of the voltage pulse. The waves have the dispersion relation of ω ~ ku_αφ, and the wavelength has the order of Debye length λD ~ 10^-4 m as k² λD² ~ 1. The phase velocity is u_αφ ~ (u_αd +ub+un+κ_αun) with the mobility ratio of κe=|μe/μi| for electrons (α = e) and κi=1 for ions (α = i). The terms ued, uid, ub, and un represent the electron drift velocity, the ion drift velocity, the gas blowing velocity, and the plasma diffusion velocity, respectively. The waves are modulated to be the wave-packet of dω ~ ugdk with the group velocity of ug ~ cs²/un ~ (10⁴-10⁵)m/s, where the acoustic velocity is cs ~ 10³ m/s and the diffusion velocity is un ~ (10-10²)m/s estimated in a plasma jet device.