Collisional ripple transport of neutral beam-injected energetic ions in low aspect ratio helical system CHS

Losses of partially thermalized, pitch angle-scattered NB ions have been experimentally observed by use of a lost fast ion probe (LIP) at the small R side of CHS. The LIP indicates that the energy of escaping fast ions at the probe position ranges from 10 to 20 keV and their pitch angle is around 75 80 degrees although the NB(Eb=38keV) is tangentially coinjected in CHS. Judging from this, collisional ripple transport looks important to understand beam ion behavior and confinement in CHS. Particle simulation in the presence of collisions is performed by the DELTA5D code to verify whether the experimental observation can be explained by classical drift orbit phenomena. The DELTA5D suggests that escaping fast ions can reach the probe position via collisional ripple transport and the their energy is consistent with that observed by the LIP. Comparison will be made between the results from experiment and simulation for ripple transport and resulting losses of beam ions in CHS.