FIDA and other observation of improved fast-ion confinement in the Large Helical Device using shifted magnetic configurations*

Fast-ion confinement in the Large Helical Device (LHD) is investigated at various magnetic configurations to validate the behavior of the synthetic diagnostic code FIDASIM when using a non-axisymmetric distribution from the transport code GNET. Neoclassical confinement in stellarators is improved when the magnetic axis is shifted inward [1]. Improved confinement for fast ions with the inward-shifted configuration has been observed on LHD using neutron measurements [2]. Studies using additional diagnostics have been completed with experiments on MHD-quiescent plasmas with magnetic axes R < 3.6m, R = 3.6m, and R > 3.6m. Fast ions in the experiments are populated by tangential negative-ion neutral beam injection using 150 - 170 keV Deuterium sources. Fast-ion D-alpha, neutral particle analyzer, and vertical neutron camera measurements show improved fast-ion confinement for R < 3.6 m, this trend is compared to GNET-FIDASIM. Successful modeling of neoclassical behavior by GNET-FIDASIM will subsequently be applied to validation of a gyrokinetic code distribution function.

[1] Mynick, H.E., et al. 1982. Phys. Rev. Letters, 48(5), pp. 322–326. doi:10.1103/physrevlett.48.322.

[2] Nuga, H., et al. 2020. J. Plasma Phys., 86(3), 815860306. doi:10.1017/S0022377820000525