Motivated by the experiment [1], two-dimensional particle-in-cell simulations of a magnetized plasma have been carried out and show that there is an autonomous oscillation if a positively charged electrostatic plate with a finite size is immersed in the plasma. The amplitude of the potential oscillation is comparable to the potential of the plate. The region in which plasma potential oscillates periodically is limited to the zone in which magnetic field lines are connected with the plate. The particles hitting the plate are absorbed there. This is a kind of bifurcation phenomenon and appears in the parameter range in which the potential of the plate is less than a critical value. If the potential is larger than that value, this oscillation disappears. The size of the plate must be finite in the direction perpendicular to the magnetic field, because the negative sheath facing the plate is influenced by the electron and ion motions parallel and perpendicular to the magnetic field. The oscillation of the sheath width parallel to the magnetic field is synchronized with that of the amplitude of the potential. The relevance to the positively charged electrostatic plate immersed in a tokamak will be discussed.

References