Experimental investigation of the ripple induced losses of perpendicularly injected beam ions in the low aspect ratio helical system CHS

Abstract

Confinement of perpendicularly injected beam ions has been investigated experimentallyboth in the standard configuration and in the drift optimized configuration of theCHS heliotron/torsatron by means of the neutron technique. The experimental resultsindicate that the confinement of trapped beam ions is poor in both configurations. Theobserved loss rate of trapped beam ions was very high and this high rate is not explainedby losses due to charge exchange or collisional scattering from the timescale perspective.Full gyro-motion following orbit calculations showed that in the peripheralregion of CHS plasmas, trapped beam ions are promptly lost because they immediatelyintersect the vacuum chamber wall due to their large Larmor radii. It also suggested thatin the core domain the collisionless transition behaviour is the key to understanding the rapid lossof beam ions. It was shown that the magnetic moment of perpendicularly injected beamions is not well conserved in CHS. The rapid losses of perpendicularly injected beamions are probably due to non-adiabatic effects resulting in the transition behaviour of energeticions.