Spectral investigation of a fireball in low-temperature argon plasma

Summary form only given. By positively biasing an electrode immersed into low-temperature plasma up to a certain threshold value, a complex space charge structure appears in front of it, in form of a luminous quasi-spherical plasma body. This structure was sometimes called anode glow, ball of fire or fireball. Experimental investigation shown that such a structure consists of a positive nucleus (ion-enriched plasma) confined by an electrical double layers. The potential drop across the double layer is almost equal to the ionization potential of the background gas atoms. By accelerating the electrons from the surrounding plasma through this potential drop, the double layer can maintain the equilibrium between the production and the losses of the charged particles (electrons and positive ions), ensuring the existence of the structure. Here we report on the spectral investigation of such a complex space charge structure, in argon plasma. The spectral domain was 550-750 nm, identifying the lines corresponding to different excited states of the argon. The spectral profile of the fireball was obtained by recording the spectra at different positions in front of the electrode. The obtained results shed a new light on the elementary processes at the origin of the appearance and dynamics of the fireball, namely the electron-neutral impact excitations and ionizations.