Plasma diffusion in the atmospheric pressure plasma jets

Summary form only given. The plasma diffusion has been investigated with respect to the atmospheric-pressure plasma jets (APPJ). The ambi-polar diffusion equation is solved in the radial and axial directions. To explain the plasma diffusion process, the plasma diffusion equation is derived with the charged particle motion and the plasma fluid continuity equation. Supposing the initial plasma density n(t=0, r, z), the plasma flux Γ_D=-D_a∇n and the diffusion velocity u_D=-D_a(∇n)/n are calculated. In the diffusion of radial direction, the plasma diffuses and vanishes away at the glass wall by the plasma recombination. The characteristic time of plasma loss is given by τ_r=(r₀/2.4)²/D_a. With the tube radius of r₀~1 mm and the ambi-polar diffusion coefficient of D_a~10^-4 m² /s, the characteristic time is calculated as τ_r~0.5 ms. It corresponds to the operation frequency of plasma jet operation. In the diffusion of axial direction, a high density plasma generated at the area of a high voltage electrode and it diffuses out of the glass tube end with the characteristic time τ_z~0.1 us. The diffusion velocity of high density plasma at the boundary is u_D~10²m/s in the beginning (at the time t~us), and the diffusion velocity becomes slow as u_D~1 m/s after a few ms. Therefore, for the plasma jets whose length is 1 cm, it takes about several milliseconds diffuse from the electrode. These diffusion velocities have a big difference from the velocity of the plasma bullet which is understood as the plasma propagation. These values about the plasma diffusion in APPJ will be basic data to understand plasma density and axial distribution in plasma plume created at the edge of the plasma jets device.