**4.4 Evaluation of reflection rate distribution and thickness distribution of deposition layer on the first wall of helical coils**

The reflection rate of the first wall on the helical coil in the same toroidal section, where the long-term irradiated samples described in 3.2 were installed, was measured using a color analyzer after the opening to the atmosphere in the vacuum vessel. The measured number of stainless steel protection plates was 530. The reproducibility of the measurements was confirmed by performing the measurements twice. **Figure 6(a)** shows the results of the reflection rate

**Figure 6.**

*(a) CAD showing the measured reflection rate, (b) developed view, and (c) deposition layer thickness distribution evaluated from reflection measurements [11].*

#### *Colorimetry in Nuclear Fusion Research DOI: http://dx.doi.org/10.5772/intechopen.101634*

measurements [11]. On the outside of the torus, the RGB values of most of the protection plates are low and the reflection rate is low. On the other hand, inside the torus, the RGB values are high, and the reflection rate is high except near the divertor plate. For clarity, **Figure 6(b)** shows the development figure of the measured stainless steel plates. These results suggest that deposition is dominant outside the torus, while erosion is dominant inside the torus. The reflection rate of the deposition layer is determined by the balance between erosion and deposition processes, which depends on the distance from the plasma and the view from the divertor plate. The RGB values are higher when the stainless steel plate is closer to the plasma and lower when it is farther from the plasma [9]. This relationship of the distance between the protection plate and the plasma with the reflection rate is interesting (for details, refer to [9]). Using the reflection rate measured by the color analyzer, and assuming that the thickness of the deposition layer follows the single-layer model, the thickness distribution of the deposition layer on the helical coil was evaluated. **Figure 6(c)** shows the thickness distribution of the deposition layer evaluated from the reflection rate measurement. As predicted from the reflection rate measurements, deposition is dominant on the outside of the torus, while erosion is dominant on the inside of the torus except near the divertor plate. In terms of the distribution of the deposition layers, 37% of the layers were 10 nm thick, 44% were 10–100 nm thick, and 19% were thicker than 100 nm. 60% of the first wall of the helical coil in the toroidal section was covered by the deposition layer. This result suggests that the region covered by the deposition layer shows an important role in the wall absorption of fuel particles. Quantitative comparison with global particle balance analysis is also performed using this experimental result, please refer to Ref [3].

Colorimetry has been applied to plasma experimental devices other than the LHD. Here we introduce an example of the application of colorimetry on the Wendelstein 7-X (W7-X) device at the Max-Planck-Institute for Plasma Physics in Germany [10]. Similar to the LHD, the W7-X is a large superconducting stellarator device [13]. **Figure 7** shows the results of colorimetry measurements in the W7-X on the surface of the panels covering the vacuum vessel and the evaluation of the deposition layers in different experimental campaigns. **Figure 7(a)** shows

**Figure 7.** *Colour pattern of first wall panels after (a) OP1.2a and (b) OP1.2b in W7-X [10].*

the estimated deposition layer after the experimental campaign of OP1.2a, where the average thickness of the deposition layer is predicted to be 10 ca. On the other hand, the deposition layer after the experimental campaign of OP1.2b is shown in **Figure 7(b)**. The thickness of the deposition layer is evaluated to be 25 ca. Thus, colorimetry provides useful information for discussing strategies for removing the deposition layer.

#### **5. Summary**

In this chapter, a color analysis method using a color analyzer is introduced, and it is shown that it is possible to evaluate the deposition layer formed on the plasmafacing wall by color analysis. As an example, the color analysis of the first wall on the helical coil of the LHD shows that the thickness distribution of the deposition layer on the inner and outer sides of the torus is different. Also, the different deposition layer thickness has been obtained in the different experimental campaigns in the W7-X. Although, color analysis is useful for surface analysis of plasma-facing walls, it may not be an in-situ diagnosis, because the current method requires a color analyzer to be brought into the vacuum vessel for measurement. However, using the principle of evaluating the thickness of the deposition layer from the color analysis, it could be applied as an in-situ diagnostic measurement. For example, if the color analyzer is held by a robot arm and operated remotely, in-situ diagnosis is possible. The color analysis method using the color analyzer has already been applied not only to the LHD and W7-X but also to the QUEST at Kyushu University and the GAMMA10 at Tsukuba University in Japan. In the future, color analysis will facilitate the understanding of the plasma-wall interaction in fusion plasma.

### **Acknowledgements**

The author G.M. would like to express his gratitude to Professor Emeritus Naoaki Yoshida of Kyushu University, who allowed him to discuss color analysis methods and Dr. Kenji Matsumoto of Honda R&D Co., Ltd., who developed the color analyzer. G.M. also thanks Dr. Suguru Masuzaki of National Institute for Fusion Science, Dr. Chandra Prakash Dhard and Dr. habil. Dirk Naujoks of Max-Planck-Institute for Plasma Physics for their strong support of this study. This work was supported in part by the National Institute for Fusion Science under Grants (NIFSUMPP003-1 and NIFSULPP801) and by the NIFS Stellarator-Heliotron Association Committee (URSX209) and by JSPS KAKENHI grant (18H01203).

#### **Abbreviations**


*Colorimetry in Nuclear Fusion Research DOI: http://dx.doi.org/10.5772/intechopen.101634*

