Experimental investigation of a fire-rod in weakly magnetized diffusion plasma

When a positively biased electrode is immersed in plasma, up to a certain value of the applied potential a complex space charge structure develops in front of it. This structure looks like a quasi-spherical intense luminous plasma blob, attached to the surface of the electrode, known as ball of fire or fireball. Emissive probe measurements proved that the structure consists of a positive ion enriched plasma region, confined by an electrical double layer. When a magnetic field is applied perpendicular to the electrode surface, the spherical structure will along in the direction of the field, taking a rod-like form. Thus, the magnetic field introduces anisotropy in the electron velocity field, the electrons moving towards electrode surface in the perpendicular direction on it. By using different Langmuir probes inside and outside the fire-rod, we extracted the electron energy distribution function, which emphasize the existence of different groups of electrons. In this way, we analyzed the important role of the electron-neutral impact excitations and ionizations in the appearance and dynamics of the fire-rod in magnetized plasma. (authors)