Study of magnetic axis effect on fast-ion confinement using FICXS and neutron camera profile measurements in the Large Helical Device*

Fast ions in the core region of hydrogenic plasmas with a range of magnetic axes positions are measured in the Large Helical Device (LHD). Thermal plasma confinement is known to improve in LHD-type stellarators when the magnetic axis is shifted inward [1]. One of the reasons for this improvement is believed to be the achievement of an omnigenous condition, similar to the one described in [2]. Passive measurements have shown improved confinement  of fast ions with an inward-shifted axis on LHD for limited regions of phase space [3, 4]. Experiments have been conducted with MHD-quiescent hydrogenic plasmas over a range of magnetic axis positions: Rax < 3.6m, =3.6m and > 3.6m. The plasmas are heated by a tangential negative-ion neutral beam with injection energies around 160 keV. A radial positive-ion neutral beam is used for active measurements of the fast-ion profile using a fast-ion charge exchange spectroscopy (FICXS) system. Neutron emission profiles are also provided by a vertical neutron camera (VNC) system for deuterium cases. Validation of the measurements is conducted using simulated profiles provided by the transport code GNET and the synthetic diagnostic code FIDASIM. Preliminary analysis shows that any effect on fast-ion confinement will be primarily dependent on the plasma parameters of the core region.

[1] Komori, A., et al. 2001. Phys. Plasmas 8(5). doi:10.1063/1.1344561

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

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

[4] Murakami, S., et al. 2004. Fusion Sci. Technol. 46(2), pp. 241-247. doi:10.13182/FST04-A561.