Observation of Ion Tail in ECH/ECCD Plasmas in Helical Devices

The high energy ion tail, having the energy more than five times higher than the bulk temperature, has been observed in the electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) plasmas of Heliotron J and CHS. The tail temperature of ion increases with decreasing the density and increasing the ECH power. The ion tails appear under the condition of high tail temperature of electrons measured by the soft X-ray charge-coupled-device system. The comparison of the fraction of the ion tail density between CHS and Heliotron J shows that the tail ions are accelerated selectively in the parallel direction to the magnetic field. The high energy electron is a key factor of the formation of the ion tail