Kiyofumi Mukai

Calibration of a thin metal foil for infrared imaging video bolometer to estimate the spatial variation of thermal diffusivity using a photo-thermal technique.

A thin metal foil is used as a broad band radiation absorber for the InfraRed imaging Video Bolometer (IRVB), which is a vital diagnostic for studying three-dimensional radiation structures from high temperature plasmas in the Large Helical Device. The two-dimensional (2D) heat diffusion equation of the foil needs to be solved numerically to estimate the radiation falling on the foil through a pinhole geometry. The thermal, physical, and optical properties of the metal foil are among the inputs to the code besides the spatiotemporal variation of temperature, for reliable estimation of the exhaust power from the plasma illuminating the foil. The foil being very thin and of considerable size, non-uniformities in these properties need to be determined by suitable calibration procedures. The graphite spray used for increasing the surface emissivity also contributes to a change in the thermal properties. This paper discusses the application of the thermographic technique for determining the spatial variation of the effective in-plane thermal diffusivity of the thin metal foil and graphite composite. The paper also discusses the advantages of this technique in the light of limitations and drawbacks presented by other calibration techniques being practiced currently. The technique is initially applied to a material of known thickness and thermal properties for validation and finally to thin foils of gold and platinum both with two different thicknesses. It is observed that the effect of the graphite layer on the estimation of the thermal diffusivity becomes more pronounced for thinner foils and the measured values are approximately 2.5-3 times lower than the literature values. It is also observed that the percentage reduction in thermal diffusivity due to the coating is lower for high thermal diffusivity materials such as gold. This fact may also explain, albeit partially, the higher sensitivity of the platinum foil as compared to gold.

Effect of magnetic field ripple on electron cyclotron current drive in Heliotron J

Electron cyclotron current drive (ECCD) experiments have been conducted in the helical heliotron device, Heliotron J. A wide configuration scan shows that the electron cyclotron (EC) driven current is strongly dependent on the magnetic ripple structure where the EC power is deposited. As the EC power is deposited on the deeper ripple bottom, the EC current flowing in the Fisch?Boozer direction decreases, and the reversal of directly measured EC driven current is observed. Measurement results using electron cyclotron emission and soft-x ray spectrum diagnostics imply that high-energy electrons are generated for ripple top heating while they are suppressed for ripple bottom heating, indicating that the generation and confinement of trapped electrons have an important role on ECCD. For ripple top heating, the typical ECCD efficiency is estimated as ? = neIECR/PEC = 0.8 ? 1017?A?W?1?m?2 and , where ne is in 1020?m?3, IEC in A, R in m, PEC in W and Te in keV. The normalized ECCD efficiency is found to be independent of the absorbed EC power for both ripple top and bottom heating cases.

Dynamics of three-dimensional radiative structures during RMP assisted detached plasmas on the large helical device and its comparison with EMC3-EIRENE modeling

The resonant magnetic perturbation (RMP) island introduced in the stochastic edge of the large helical device (LHD) plasma plays an important role in the stabilization of the plasma detachment (Kobayashi et al 2013 Nucl. Fusion 53 093032). The plasma enters in the sustained detachment phase in the presence of an RMP once the line averaged density exceeds a critical value with a given input power. During detachment the enhanced radiation from the stochastic edge of the LHD undergoes several spatiotemporal changes which are studied quantitatively by an infrared imaging video bolometer (IRVB) diagnostic. The experimental results are compared qualitatively and quantitatively with the radiation predicted by the 3D transport simulation with fluid model, EMC3-EIRENE. A fair amount of qualitative agreement, before and after the detachment, is reported. The issue of overestimated radiation from the model is addressed by changing the free parameters in the EMC3-EIRENE code till the total radiation and the radiation profiles match closely, within a factor of two with the experimental observations. A better quantitative match between the model and the experiment is achieved at higher cross-field impurity diffusion coefficient and lower sputtering coefficient after the detachment. In this article a comparison, the first of its kind, is established between the quantified radiation from the experiments and the synthetic image obtained from the simulation code. This exercise is aimed towards validating the model assumptions against the experimentally measured radiation.

Improved signal to noise ratio and sensitivity of an infrared imaging video bolometer on large helical device by using an infrared periscope

An Infrared imaging Video Bolometer (IRVB) diagnostic is currently being used in the Large Helical Device (LHD) for studying the localization of radiation structures near the magnetic island and helical divertor X-points during plasma detachment and for 3D tomography. This research demands high signal to noise ratio (SNR) and sensitivity to improve the temporal resolution for studying the evolution of radiation structures during plasma detachment and a wide IRVB field of view (FoV) for tomography. Introduction of an infrared periscope allows achievement of a higher SNR and higher sensitivity, which in turn, permits a twofold improvement in the temporal resolution of the diagnostic. Higher SNR along with wide FoV is achieved simultaneously by reducing the separation of the IRVB detector (metal foil) from the bolometers aperture and the LHD plasma. Altering the distances to meet the aforesaid requirements results in an increased separation between the foil and the IR camera. This leads to a degradation of the diagnostic performance in terms of its sensitivity by 1.5-fold. Using an infrared periscope to image the IRVB foil results in a 7.5-fold increase in the number of IR camera pixels imaging the foil. This improves the IRVB sensitivity which depends on the square root of the number of IR camera pixels being averaged per bolometer channel. Despite the slower f-number (f/# = 1.35) and reduced transmission (τ0 = 89%, due to an increased number of lens elements) for the periscope, the diagnostic with an infrared periscope operational on LHD has improved in terms of sensitivity and SNR by a factor of 1.4 and 4.5, respectively, as compared to the original diagnostic without a periscope (i.e., IRVB foil being directly imaged by the IR camera through conventional optics). The bolometers field of view has also increased by two times. The paper discusses these improvements in apt details.

Effect of supersonic molecular-beam injection on edge fluctuation and particle transport in Heliotron J

Edge fluctuation in a supersonic molecular-beam injection (SMBI) fueled plasma has been measured using an electrostatic probe array. After SMBI, the plasma stored energy (Wp) temporarily decreased then started to increase. The local plasma fluctuation and fluctuation induced particle transport before and after SMBI have been analyzed. In a short duration (∼4 ms) just after SMBI, the density fluctuation of broad-band low frequency increased, and the probability density function (PDF) changed from a nearly Gaussian to a positively skewed non-Gaussian one. This suggests that intermittent structures were produced due to SMBI. Also the fluctuation induced particle transport was greatly enhanced during this short duration. About 4 ms after SMBI, the low frequency broad-band density fluctuation decreased, and the PDF returned to a nearly Gaussian shape. Also the fluctuation induced particle transport was reduced. Compared with conventional gas puff, Wp degradation window is very short due to the short injection ...

Edge plasma responses to energetic-particle-driven MHD instability in Heliotron J

Two different responses to an energetic-particle-driven magnetohydrodynamic (MHD) instability, modulation of the turbulence amplitude associated with the MHD instability and dynamical changes in the radial electric field (Er) synchronized with bursting MHD activities, are found around the edge plasma in neutral beam injection (NBI) heated plasmas of the Heliotron J device using multiple Langmuir probes. The nonlinear phase relationship between the MHD activity and broadband fluctuation is found from bicoherence and envelope analysis applied to the probe signals. The structural changes of the Er profile appear in perfect synchronization with the periodic MHD activities, and radial transport of fast ions are observed around the last closed flux surface as a radial delay of the ion saturation current signals. Moreover, distortion of the MHD mode structure is clarified in each cycle of the MHD activities using beam emission spectroscopy diagnostics, suggesting that the fast ion distribution in real and/or velocity spaces is distorted in the core plasma, which can modify the radial electric field structure through a redistribution process of the fast ions. These observations suggest that such effects as a nonlinear coupling with turbulence and/or the modification of radial electric field profiles are important and should be incorporated into the study of energetic particle driven instabilities in burning plasma physics.

A reconsideration of the noise equivalent power and the data analysis procedure for the infrared imaging video bolometers

The infrared imaging video bolometer (IRVB) used for measurement of the two-dimensional (2D) radiation profiles from the Large Helical Device has been significantly upgraded recently to improve its signal to noise ratio, sensitivity, and calibration, which ultimately provides quantitative measurements of the radiation from the plasma. The reliability of the quantified data needs to be established by various checks. The noise estimates also need to be revised and more realistic values need to be established. It is shown that the 2D heat diffusion equation can be used for estimating the power falling on the IRVB foil, even with a significant amount of spatial variation in the thermal diffusivity across the area of the platinum foil found experimentally during foil calibration. The equation for the noise equivalent power density (NEPD) is re-derived to include the errors in the measurement of the thermophysical and the optical properties of the IRVB foil. The theoretical value estimated using this newly derived equation matches closely, within 5.5%, with the mean experimental value. The change in the contribution of each error term of the NEPD equation with rising foil temperature is also studied and the blackbody term is found to dominate the other terms at elevated operating temperatures. The IRVB foil is also sensitive to the charge exchange (CX) neutrals escaping from the plasma. The CX neutral contribution is estimated to be marginally higher than the noise equivalent power (NEP) of the IRVB. It is also established that the radiation measured by the IRVB originates from the impurity line radiation from the plasma and not from the heated divertor tiles. The change in the power density due to noise reduction measures such as data smoothing and averaging is found to be comparable to the IRVB NEPD. The precautions that need to be considered during background subtraction are also discussed with experimental illustrations. Finally, the analysis algorithm with all the improvements is validated and found to reproduce the input power well within 10% accuracy. This article answers many fundamental questions relevant to the IRVB and illustrates the care to be exercised while processing the IRVB data.

Radiation Profiles Measured With Imaging Bolometers on LHD During Plasma Detachment and Their Comparison With Synthetic Images

The infrared imaging video bolometer (IRVB) is used for measuring radiation profiles from the large helical device (LHD) helical plasma in 2-D. Currently, the images from the IRVB are used to study the radiation localization during plasma detachment using multiple IRVB modules installed at strategic port locations on LHD. The experimental results from the IRVBs are compared with the modeled radiation from the EMC3-EIRENE impurity transport code, by tracing the sightlines through the code results in 3-D using a synthetic instrument. A correlation can thus be established and measures for convergence between the experiment and model results can then be attempted.

Reconstruction of 3-D Radiation Profile From 2-D Images Viewed by IR Imaging Video Bolometers With SVD

Radiation is one of the major paths of power loss from fusion plasmas. Since the radiation distribution has a complex 3-D structure in helical devices, such as a large helical device (LHD), 3-D radiation measurements, are required for understanding the power balance of fusion plasmas. IR imaging video bolometers (IRVBs) installed in the LHD were examined as one system of 3-D imaging using a projection matrix that is obtained under the assumption of plasma periodicity in the toroidal direction, and using the phantoms obtained numerically with the EMC3-EIRENE code as the impurity distribution. The reconstructed profiles indicate the capability of the current system of IRVBs for imaging the 3-D radiation distribution in LHD.

Observation of Edge Plasma Fluctuations with a Fast Camera in Heliotron J

A fast video camera is verified to be a powerful tool for observation of filaments/blobs near the last closed flux surface (LCFS). In order to extract the fluctuation component from the raw data of the fast camera, a pre-processing technique, sliding time window averaging subtraction (STWAS) has been developed to remove the background of slowly varying emission from the bulk plasma. By using this pre-processing technique, the fast camera data are analyzed. A method to identify the filaments in the pre-processed image is also discussed.