The hydrogen/deuterium (H/D) isotope effect on a zonal flow and its configuration dependence are experimentally studied in a helical device, Heliotron J. The isotope dependence of a toroidally symmetric potential fluctuation in the frequency range of <∼4 kHz, a zonal flow, is observed in low-density electron cyclotron resonance heated plasmas in Heliotron J. In the standard configuration, the zonal flow interacting with turbulence is enhanced as the D gas fraction increases. The zonal flow is more dominated against turbulent fluctuation in D plasmas, which should have a favourable impact on turbulence transport. Interestingly, however, the opposite dependence of the zonal flow activity on the isotope ratio is also observed in the different magnetic configurations. The configuration dependence observed in this experiment could be one of the factors to explain the difference in the isotope effect of confinement improvement between tokamaks and helical devices. Impurities and radial electric field, which can also be influenced by isotope mass through the difference in neoclassical transport and plasma-wall interaction, have insignificant influences on the zonal flow behaviour in this experiment.

A new 320 GHz solid-state source interferometer is installed in the Heliotron J helical device to explore the physics of high-density plasmas (ne > 2–3 × 1019 m−3, typically) realized with advanced fueling techniques. This interferometry system is of the Michelson type and is based on the heterodyne principle, with two independent solid-state sources that can deliver an output power of up to 50 mW. A high time resolution measurement of <1 µs can be derived by tuning the frequency of one source in the frequency range of 312–324 GHz on the new system, which can realize the fluctuation measurement. We successfully measured the line-averaged electron density in high-density plasma experiments. The measured density agreed well with a microwave interferometer measurement using a different viewing chord, demonstrating that the new system can be used for routine diagnostics of electron density in Heliotron J.

The spatial radiation distribution has been measured with an infrared imaging video bolometer (IRVB) in a neutral beam injected (NBI) plasma from the Heliotron J device. The temperature distribution on the IRVB foil is consistent with radiation simulated by the three-dimensional transport code, EMC3-EIRENE. The foil temperature increase is proportional to the radiation intensity measured with an Absolute eXtreme UltraViolet (AXUV) diode diagnostic. These results suggest that the IRVB can be used for plasma radiation measurements in small and medium size devices.

The sensitivity of an infrared imaging video bolometer (IRVB) was improved for the measurement of relatively low energy plasma radiation from the viewpoint of the metal foil absorber material. The photon energy of the radiation was considered up to 1 keV for the divertor plasma measurement. The thickness of the foil absorber was evaluated not only for conventional heavy elements, e.g., platinum, but also for light elements by the relation between the photon energy and attenuation length and by mechanical strength. A heat-transfer calculation using ANSYS suggested that light elements with practical foil thickness provide a higher temperature rise of the foil absorber compared with heavier elements with practical foil thickness. The maximum of the temperature rise was evaluated using He–Ne laser irradiation onto absorber samples. The material dependence of the temperature rise has a similar tendency between calculations and experiments. Experimentally, the sensitivity of the IRVB improved from 280 to 110 µW/cm2 using titanium with 1 µm thickness compared with conventional platinum with 2.5 µm thickness. Consequently, the signal-to-noise ratio of the IRVB could be improved from 2.8 to 9.1.

We report the development of a new interferometer with two stable, high-power, 320 GHz solid-state sources in Heliotron J. A heterodyne Michelson interferometer optical scheme is employed. Two solid-state oscillators are utilized as sources with a fixed frequency at 320 GHz and frequency tunable of 312–324 GHz. Quasi-optical techniques are used for beam transmission. The beam is elongated in the vertical direction with two off-axis parabolic mirrors and injected into the plasma as a sheet beam for the multi-channel measurement (&gt;5 ch.). Passing through the plasma, the beam is reflected at a retroreflector-array installed at the vacuum chamber wall. The retroreflector-array is a bunch of retroreflector structures, which can suppress the beam refraction caused by plasma without much space inside a vacuum chamber unlike a single retroreflector and can facilitate the system design. The source, detectors, and the retroreflector-array are tested to evaluate their basic performance on a tabletop experiment.