Studies of energetic particle transport due to energetic-particle-driven Alfvénic instability have progressed using neutron and energetic particle diagnostics in Large Helical Device deuterium plasmas. Alfvénic instability excited by injecting an intensive neutral beam was observed by a magnetic probe and a far-infrared laser interferometer. The interferometer showed Alfvénic instability composed of three modes that existed from the core to the edge of the plasma. A comparison between the observed frequency and shear Alfvén spectra suggested that the mode activity was most likely classified as an Alfvénic avalanche. A neutron fluctuation detector and a fast ion loss detector indicated that Alfvénic instability induced transport and loss of co-going transit energetic ions. The dependence of the drop rate of the neutron signal on the Alfvénic instability amplitude showed that significant transport occurred. Significant transport might be induced by the large amplitude and radially extended multiple modes, as well as a large deviation of the energetic ion orbit from the flux surface.

Multi-species impurity seeding is an advanced operation scenario to mitigate the divertor heat load for the realization of future fusion reactors. In the Large Helical Device (LHD), divertor detachment is successfully sustained using higher-Z (krypton, Kr) and lower-Z (neon, Ne) superimposed seeding. Emission from Kr impurities is drastically enhanced if it is followed by Ne seeding. Plasma radiation can be enhanced even at the upstream region in the edge plasma compared with Ne only seeded plasmas with suppression of impurity accumulation toward the central plasma. The characteristics of divertor heat load reduction and energy confinement are comparable between the Kr+Ne seeding and Ne only seeding under the same radiation fraction. However, while the detachment in Ne only seeding is transient, the detachment in Kr+Ne seeding is stable. It indicates that multi-species impurity seeding can be competitive for steady-state operation although further investigation is desired about the balance between divertor heat load reduction, impurity screening, and confinement degradation. The Kr emission enhancement is strongly affected by electron density and temperature at the last closed flux surface resulting in impurity penetration.

A digital phase analysis technique was applied to a 119-µm wavelength far-infrared (FIR) laser interferometer on the Large Helical Device (LHD). High-density plasma measurement without phase jumping was achieved, representing an improvement over measurements made using a conventional analog phase counter. Digital phase analysis became operational at a quarter of the threshold of the analog phase counter. The phase sensitivity was also improved using digital analysis because the noise level was reduced to approximately half that of the conventional analog phase counter. The improved phase sensitivity enabled measurement of small-amplitude fluctuations. c

CO 2 laser interferometers are a promising option for high-density plasma measurements. However, in lowand middle-density measurements, noise due to mechanical vibrations is a serious problem. To remove this noise, we developed a two-color laser imaging interferometer using a CO 2 laser and quantum cascade (QC) laser, called the CO 2 /QC laser imaging interferometer, through benchtop experiments and installed it in the Large Helical Device (LHD). Benchtop experiments provided optical design guidelines for the CO 2 /QC laser imaging interferometer to minimize the influence of the unstable output wavelength of the QC laser. The optical system in LHD was designed according to this guideline, and the vibration noise was successfully reduced to 2.80 × 10 18 m −3 . We also demonstrate measurement examples of hollowed and peaked electron density profiles evaluated using Abel inversion and macro-scale instability. This is the first study to present the measurement results of high-temperature plasma using a CO 2 /QC two-color laser interferometer. The study outcomes provide important insights for the development of two-color laser interferometers in future fusion devices. c