Escaping fast ion diagnostics in compact helical system heliotron/torsatron

Escaping energetic ions have been measured with a scintillator-based probe which can provide both information of pitch angle and gyroradius of detected ions in compact helical system. The measurement was started in February 1997. It was confirmed that output signals are due to neutral beam-injected fast ions. A Lorentz orbit code was developed and fast ion orbits reaching the probe were computed to understand the loss process of detected ions. Birthplace and initial pitch angle of fast ions were calculated by a Monte Carlo beam deposition code HFREYA. From the comparison between pitch angle variation of computed fast ion orbits and pitch angle distribution of birth fast ions, it turns out that fast ions whose birthplace is in the core region cannot reach the detection point directly. (C) 1999 American Institute of Physics. [S0034-6748(99)64601-9].