Imaging of radiation during impurity gas puffing in LHD

In LHD, several methods of detachment have been attempted, including impurity gaspuffing [1], and the application of an m/n=1/1 magnetic perturbation [2]. LHD is equipped with an imaging bolometer (IRVB) [3] that views the plasma from an upper port. Two scenarios are shown and compared, Ne puffing and N2 puffing. In the case of Ne puffing, radiation becomes more intense near the helical divertor X-point as the radiation increases. In the case of N2 puffing, a double stripe pattern evolves around the upper helical divertor X-point, which appears to be localized near the gas puff inlet. In addition, probe data also indicates that the drop in divertor flux with N2 is localized, while uniform with Ne

Experimental observations and modelling of radiation asymmetries during N2 seeding in LHD

N2 gas has been seeded in the Large Helical Device (LHD) to reduce the divertor heat load through enhanced radiation. Radiation is observed by two imaging bolometers, viewing the same poloidal cross-section from top and bottom ports, at a location which is 36° toroidally removed from the N2 gas puff nozzle located at the bottom of the machine. During N2 seeding, these measurements both confirm that additional radiation from the outboard side is coming exclusively from the top of the cross-section, indicating up/down asymmetry, which is also reproduced by modelling with EMC3-EIRENE using a half torus model. In addition, a toroidally localized, magnetic field direction-dependent radiation enhancement is observed with N2 seeding, but is not reproducible by the model

Development of impurity seeding and radiation enhancement in the helical divertor of LHD

Impurity seeding to reduce the divertor heat load was conducted in the large helical device (LHD) using neon (Ne) and krypton (Kr) puffing. Radiation enhancement and reduction of the divertor heat load were observed. In the LHD, the ratio between the total radiated power and the heating power, f rad = Prad/Pheating, is limited up to around 30% in hydrogen plasmas even for high density plasma just below the radiative collapse (ne, bar  >  1   ×   1020 m−3), where ne, bar is the line averaged density. With Ne seeding, the ratio could be raised to 52% at ne, bar ~ 1.3   ×   1019 m−3, albeit with a slight reduction in confinement. f rad ~ 30% could be sustained for 3.4 s using multi-pulse Ne seeding at ne, bar ~ 4   ×   1019 m−3. The localized supplemental radiation was observed along the helical divertor X-points (HDXs) which is similar to the estimated structure by the EMC3-EIRENE code. Kr seeding was also conducted at ne, bar ~ 3.1   ×   1019 m−3. f rad ~ 25% was obtained without a significant change in stored energy. The radiation enhancement had a slower time constant. The supplemental radiation area of the Kr seeded plasma moved from the HDXs to the core plasma. Highly charged states of Kr ions are considered to be the dominant radiators from the plasma core region

Field of View Optimization for IR Imaging Video Bolometers in LHD

An IR imaging video bolometer (IRVB) is a measurement instrument for plasma radiation with the pin-hole projection principle. The IRVB has an advantage of having a large number of detector channels. The advantage is necessary for three dimensional observation of plasma radiation with tomography techniques. The observation also requires the calculation of geometry matrices and optimization of the fields of view for the IRVB to reconstruct accurate plasma radiation distributions. In this study, fields of view for four IRVBs which were installed in LHD have been optimized by changing the aperture positions to minimize the total number of non-visible plasma-voxels in the LHD plasma. The best fields of view were chosen with the geometry matrix which is calculated as a projection matrix of the plasma radiation to the bolometer foil with an assumption of helically symmetry. There were 169 non-visible plasma-voxels which could not be measured by any of the IRVB channels in the setting before optimization. The number could be decreased to 0 by this optimization. By improving the fields of view, the three dimensional plasma radiation distributions will be reconstructed with higher accuracy

Improvement of Infrared Imaging Video Bolometer Systems in LHD

The InfraRed imaging Video Bolometer (IRVB) is a powerful diagnostic to measure a plasma radiation profile especially for three-dimensional measurements. An IRVB mainly consists of a pinhole camera section and an IR camera section. The plasma radiation profile is projected on a thin metal foil through an aperture in the pinhole camera resulting in a two-dimensional temperature distribution. Then, the distribution is observed from the back side by an IR camera as an IR image. Since the image contains the effects of heat diffusion, a calibration of the heat characteristics of the foil is needed to obtain the radiation profile by solving the two-dimensional heat diffusion equation. Some deposition was observed on the foil in the Large Helical Device (LHD) plasma experiment. The effect of this on the heat characteristics of the foil should be studied although it can be compensated for by the calibration. Currently four IRVBs are operating in LHD to investigate the radiation collapse and plasma detachment phenomena. The sensitivities of IRVBs at the 6.5-L and 10-O ports were improved from the experimental campaign in FY 2013 by replacing the IR cameras of these ports. The sensitivity at the 6.5-U port was also improved by applying the periscope system