Measurements of beam ion loss from the Compact Helical System

Neutral beam ion loss from the Compact Helical System (CHS) has been measured with a scintillator-type probe. The total loss to the probe as well as the pitch angle and gyroradius distributions of that loss have been measured as various plasma parameters were scanned. Three classes of beam ion losses were observed at the probe position: passing ions with pitch angles within 10° of those of transition orbits, ions on transition orbits (as would be expected from these orbits, which have large deviations from their starting flux surfaces) and ions on trapped orbits, typically 15° or more from transition orbits. Some orbit calculations in this magnetic geometry have been performed in order to understand the characteristics of the loss. Simulation of the detector signal based upon the following of orbits from realistic beam deposition profiles is not able to reproduce the pitch angle distribution of the losses measured. Consequently it is conjectured that internal plasma processes induce their loss.