Fast Ion Confinement Study by Neutron Emission Rate Measurement after Short Pulse NB Injection in the Large Helical Device

Abstract

The confinement of neutral beam (NB)-injected fast ions has been investigated by the neutron intensity decay time (τ_n) after the short pulse NB injection called “NB-blip.” The Large Helical Device (LHD) has five NB injectors. NBI#1, #2, and #3 are tangential direction injectors with typical energy of 180 keV. NBI#4 and #5 are perpendicular direction injectors with typical energies of 60 keV and of 80 keV, respectively. In this experiment, NBI#1, #2, #3, and #4 each with the pulse width of 40 ms are injected into different configuration plasmas with various electron densities. The τ_n is analyzed with a 0-dimensional fast ion slowing down model. The loss of the fast ion increases significantly with decrease in the electron density. From the comparison of calculated and measured τ_n for different plasma configurations, the fast ion confinement is worst on the outward shifted plasma of R_ax = 3.90 m. On the other hand, it seems that the fast ion confinement is best on the plasma of R_ax = 3.74 m.