Production of Helical Non-neutral Plasmas by Collisionless Electron Penetration

H.Himura,H.Wakabayashi,M.Fukao,K.Ohkuni,Y.Ogawa,Z.Yoshida,
M.Isobe1),C.Suzuki1),S.Nishimura1),T.Akiyama1),S.Okamura1),K.Matsuoka1),H.Yamada1)

University of Tokyo, Graduate School of Frontier Sciences, Tokyo 113-0033, Japan
1)National Institute for Fusion Sciences, Toki, Gifu 509-5292, Japan

For the first time, an experimental study on helical nonneutral plasmas is performed on the Compact Helical System (CHS) device. (1) Remarkably, despite being launched from the outside of closed magnetic surface, the injected electrons travel across the magnetic field and penetrate deeply inside the magnetic surfaces. This penetration of electrons is caused by a collisionless mechanism that has never been observed in past toroidal nonneutral plasmas confined in axisymmetric geometry. (2) The penetrated electrons form a helical non-neutral plasma inside the magnetic surface. The electron density is about 1011-13 m-3 much smaller than the Brillouin density limit. (3) The stable phase of the helical non-neutral plasma continues only for 1-4 ms and then starts to disrupt. About 50 kHz of disruptive instability is observed. This frequency and the other parameters related to the onset time of the instability suggest an ion-related instability as the possible mechanism.