Development and initial results of 320 GHz interferometer system in Heliotron J.

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.

Three-dimensional dynamics of fluctuations appearing during pellet ablation process around a pellet in a fusion plasma experiment.

Understanding pellet ablation physics is crucial to realizing efficient fueling into a high temperature plasma for the steady state operation of ITER and future fusion reactors. Here we report the first observation of the formation of fluctuation structures in the pellet plasmoid during the pellet ablation process by a fast camera in a medium-sized fusion device, Heliotron J. The fluctuation has a normalized fluctuation level of ~ 15% and propagates around the moving pellet across the magnetic field. By comparing the fluctuation structures with the shape of magnetic field lines calculated with the field line tracing code, we successfully reconstruct the spatio-temporal structure of the fluctuations during the pellet ablation process. The fluctuations are located at the locations displaced toroidally from the pellet and propagate in the cross-field direction around the pellet axis along the field line, indicating a three-dimensional behavior and structure of fluctuations. The fluctuation would be driven by a strong inhomogeneity formed around the pellet and invoke the relaxation of the gradient through a cross-field transport induced by the fluctuations, which could affect the pellet ablation and pellet fueling processes. Such fluctuations can be ubiquitously present at the inhomogeneity formed around a pellet in the pellet ablation process in fusion devices.

Development of a multi-channel 320 GHz interferometer for high density plasma measurement in Heliotron J.

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 (>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.

Sensitivity improvement of infrared imaging video bolometer for divertor plasma measurement.

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.