A parasitic effect in neutral particle diagnostic using a helium probing beam

This paper describes the characteristic and the physical picture of a parasitic disturbance, which can occur because of the drift motion of probing beam ions trapped at the plasma edge, in active neutral particle measurements for magnetically confined fusion devices. In the JT-60 experiments, the disturbance is observed under the condition in which a neutral particle analyzer views the high recycling region, i.e., the divertor, and the occurrence of the parasitic effect is substantially dependent on the safety factor at the plasma boundary. Also discussed are the validity of our interpretation and some measures for avoiding the disturbance.