Efficiency improvement of an E x B Penning discharge source by enhanced cross-field transport of electrons

Abstract

Precise control of the particle motion in externally applied electric and magnetic fields is of great significance in the development of the E x B source to generate high-density plasma and deliver a stable ion beam current. Especially, in the E x B Penning discharge source, the heating and energy relaxation of the beam electrons is concentrated in the plasma column along the magnetic field line. Plasma researchers have thus far focused on the relevant physical phenomena of the partially magnetized plasma that arises from the gradient of the plasma properties in the E x B Penning source. Here, we point out that current methods of radially centered electron confinement do not guarantee efficient ion beam extraction, and newly introduce the improvement of the efficiency of a cylindrical E x B Penning source targeting radial extraction of ion beam. We concentrate on the method to enhance the cross-field transport of electrons toward the extraction region. The generation of a spatially asymmetric sheath structure allows the beam and energetic electrons to be transported to the extraction region via the E x B drift of the electrons. The transported electrons contribute to expansion of the electron heating and ionization regions to the extraction region by breaking of axial symmetry of the sheath, thereby increasing the temperature and density of the electrons in the extraction region as the magnetic field strength increases. The enhanced discharge efficiency defined as the ratio of the electron density to the discharge current is noticeable, recording approximately twice the improved efficiency compared to the conventional mode with symmetric sheath structure.