Dear Jian,

Yes, It is due to the outside pressure. As the pressure is decreased, the plasma plume expands both in length and width. These expansions are stretched and built up farther from the nozzle exit. The primary reason for this expansion is the decrease in absolute pressure. As the number of surrounding molecules decreases, their collision with the species exiting the nozzle also decreases. As a result, the jet requires more distance to equilibrate pressure. When both the exit and outside pressures are equal, the jet's static pressure is in equilibrium with the surrounding pressure, and no expansions occur.