Mode transitions and spoke structures in E×B Penning discharge.*

Two-dimensional (radial-azimuthal) particle-in-cell simulations of the cylindrical ExB Penning discharge are presented. The low-pressure discharge is self-consistently supported by plasma ionization from the axial electron bean, along the direction of the external magnetic field. It is shown that with increasing the strength of the external magnetic field the discharge undergoes a sequence of transitions between several azimuthal modes. Azimuthal m>1 spiral arm structures are excited at low values of the magnetic field as plasma confinement is improved by the magnetic field and the radial density profile becomes peaked. With a larger field the spiral arms with m>1 are replaced with the m=1 spoke mode presented in the plasma density. A transition from the spiral arms to the spoke regime occurs at the point where the plasma potential in the center changes from weakly positive (or zero) to the negative and the radial electric field becomes inward. Further increase of the magnetic field results in the well-developed m=1 spoke mode with additional small scale higher frequency m>1 structures inside the spoke. MUSIC spectra analysis further confirms the coexistence of the low-frequency modes related to m=1 spoke and higher frequency m>1 modes.