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

The confinement of neutral beam (NB)-injected fast ions has been investigated by the neutron intensity decay time (τn) 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 τn 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 τn for different plasma configurations, the fast ion confinement is worst on the outward shifted plasma of Rax = 3.90 m. On the other hand, it seems that the fast ion confinement is best on the plasma of Rax = 3.74 m.