Energetic electrons moving along peripheral magnetic field lines in magnetically expanding plasmas

Two-dimensional structures of a floating potential of a Langmuir probe are measured in a magnetically expanding plasma using permanent magnets for two choices of the argon gas pressures of 1 and 3 mtorr, where an electric double layer (DL) forms for 1 mtorr while it disappears for 3 mtorr. The narrow region of the negative floating potential, which indicates the presence of energetic electrons, is observed to spread out along the outermost magnetic field line not intersecting the source wall for both pressures. It is found that the energetic electrons are transported from the source along the field lines, irrespective of the presence of the DL. Here, the typical structure observed for 1 mtorr is presented.