Velocity Distribution of Fast Ions Generated by ICRF Heating in Heliotron J

"Fast ion velocity distribution is investigated using ICRF minority heating in Heliotron J with special emphasis on the effect of the toroidal ripple of magnetic field strength. The effect of the magnetic configuration on the fast ion confinement is one of the most important issues in helical devices. Here, the pitch angle dependences of energy spectra for three bumpy cases are measured for the first time, then, the fast ions up to 34 keV are observed in the high bumpy case during ICRF heating in Heliotron J. The configurations used in this study are as follows; the bumpiness (B_04/B_00, where B_04 is the bumpy component and B_00 is the averaged magnetic field strength) are 0.15 (high), 0.06 (medium) and 0.01 (low) at the normalized radius of 0.67. The configuration of B_04/B_00= 0.06 corresponds to the standard configuration in Heliotron J. In high bumpy cases, the higher energy flux is measured near 120 deg in pitch angle although the ions are considered to be accelerated in the perpendicular direction by ICRF heating. To understand experimental results, Monte Carlo calculation is performed. The numerical model consists of orbit tracing, Coulomb collisions and acceleration by the ICRF heating. Minority protons are regarded as test particles and the heating is simulated by the velocity kick in the perpendicular direction in velocity space when ions cross the cyclotron layer. The calculation results using Monte Carlo method represents that the accelerated ion distribution has its peak in the range between 20 deg and 30 deg from the perpendicular direction. This result is considered to be caused mainly due to the existence of the loss region around the perpendicular direction.