One of key physics targets in designing quasi-axisymmetric stellarator CHS-qa (N=2/ Ap=3.2/R=1.5m/Bt=1.5T) [1] is to provide good neoclassical confinement and energetic ion transport compared with those in conventional helical systems like CHS, while realizing a tokamak-like, toroidally-symmetric magnetic field structure in Boozer coordinates. Recent analyses indicate that neoclassical diffusion coefficient in banana regime in CHS-qa is much smaller, by about 2 orders, than that in CHS over the whole plasma region [2]. In this work, we focus on orbit topology and confinement of neutral beam injected energetic ions (38 keV/H+) in CHS-qa. The orbit calculation following the full gyro-motion shows that co-going transit beam ions born on the outboard side of the torus are well confined even at Bt of 0.5 T. On the other hand, counter-going transit beam ions ionized in peripheral region are promptly lost because their orbits largely deviate from flux surfaces toward inboard side and then cross plasma boundary because of relatively low rotational transform of CHS-qa, ranging from 0.36 to 0.38. The interesting observation is that the island structure appears on the Poincar plot of counter-going transit ion in a vacuum magnetic field produced by modular coils under a particular condition, i.e., Bt of 1.0 T, initial pitch angle of 0~40 degrees and launched position of r/a=+0.5-0.6. No island structure appears in Bt of 0.5 T and 1.5 T even if ions are launched at the same r/a and with the same pitch angle. It looks as if appearance of island structure depends on the magnitude of Bt and this may be due to resonance between gyro-motion and variation of magnetic field strength. The mechanism of island formation on energetic ion orbit in CHS-qa is going to be clarified in this paper.

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