Beam-assisted low density formation of field reversed configuration (FRC) plasma

A new ionization process of the working gas through the neutral beam particles is utilized in order to assist in the formation of the field reversed configuration plasma. This method improves the lower density limit of the formed plasma. The beam-assisted formation method enables the density of the produced FRC plasma to be lowered from 2.7 x 1021 m-3 to 3.4 x 1020 m-3 and also allows higher temperatures up to 1,300 eV. The separatrix radius and the trapped flux do not depend on the density in the low density regime. The particle confinement time is longer than the empirical scaling by a factor of 2.8. In addition, the attainable minimum density becomes smaller, as the current of the neutral beam is increased. This suggests that more fast neutral atoms produce more electrons due to their collision against molecules of the working gas before pre-ionization. It is inferred that these electrons help the formation. Under a low density regime of less than 1.0 x 1021 m-3, the maximum produced electron number density by the neutral beam injection is estimated to be 2.9 x 1017 m-3 at a filling pressure=130 mPa (the molecule number density=3.6 x 1019 m-3). (author)