Colorimetry in Nuclear Fusion Research

*Gen Motojima*

#### **Abstract**

Colorimetry is a unique technique among research fields. The technique is also utilized in nuclear fusion research. The motivation is to evaluate the wide range of distribution of the deposition layer on the surface of the vacuum vessel. The deposition layer affects the control of fuel particles. Therefore, the result from colorimetry can contribute to the study of particle control in fusion plasma. In a particle control study, global particle balance analysis is usually conducted. Also, long-term samples irradiated by plasma have been analyzed. Colorimetry has the role of a bridge between these analyses. In this chapter, a demonstration of colorimetry in fusion devices is introduced.

**Keywords:** nuclear fusion, colorimetry, color analyzer, reflection rate, deposition layer, wall retention, Large Helical Device, Wendelstein 7-X

#### **1. Introduction**

We are now facing challenging times in the global environment. While people's social lives are becoming more affluent, global changes (a global crisis, if you will) such as global warming must be solved on a global scale. The world is aiming to become carbon neutral by 2050. Here, carbon neutrality, which is often used in conjunction with the term "a decarbonised society", is a concept that aims to limit carbon dioxide emissions, which are the main cause of global warming. To achieve carbon neutrality, hydrogen is attracting attention as the energy of the new age. Hydrogen is considered to be the most abundant material in the universe and can be used as heat energy through combustion. The main advantage of the use of hydrogen is that hydrogen does not produce carbon dioxide when it is used as energy. The energy efficiency of hydrogen is so high that it is even used as fuel for rockets. Nuclear fusion energy is the key energy source, using hydrogen as a fuel.

Nuclear fusion is a reaction in which two or more nuclei are transformed into nuclei of higher atomic numbers. The easiest fusion reaction to initiate on the earth is a nuclear reaction in which helium (He) and neutrons (n) are produced from deuterium (D) and tritium (T). 17.6 MeV (=2.82 × 10−12 J) of energy is released in a single nuclear reaction. The reaction equation can be expressed as an Eq. (1).

$$\mathrm{H\_{1}^{2}D} + \mathrm{\,}\_{1}^{3}\mathrm{T} \rightarrow \mathrm{\,}\_{2}^{4}\mathrm{He} \,(\ $.52\,\mathrm{MeV}) + \mathrm{\,}\_{0}^{1}\mathrm{n} \,(\$ 14.06\,\mathrm{MeV}) \tag{1}$$

The nuclear reaction of 1 g of D-T fuel corresponds to the thermal energy of the combustion of about 8 tons of fossil fuel oil. For a reaction as shown in Eq. (1) to

occur, it is necessary to increase the relative velocity of the nuclei so that the kinetic energy is greater than the electric potential energy at the position where the nuclear force is effective. The relative velocity of hydrogen can be increased by heating it to a high temperature and by the increase of the thermal motion of the nuclei. For a fusion reaction to occur on earth, the hydrogen fuel particles would have to be heated to several hundred million degrees. In such a high-temperature state, the constraint of electrical attraction between nuclei and electrons is broken and they become discrete. This state is called "plasma". There are several ways to confine the plasma, but here we introduce research on fusion plasmas with the "magnetic confinement method". Few readers may be able to imagine the relevance of colorimetry to fusion research. The author hopes that this chapter will make the reader aware that colorimetry is a method that can make a significant contribution to fusion research. Especially, colorimetry helps the understanding of particle control in nuclear fusion research.

#### **2. Contribution of colorimetry to nuclear fusion research**

To gain stable fusion energy, a stable fuel supply control is necessary. For performing fuel particle control, understanding of the particle absorption process at the plasma-facing wall is an important issue, as well as the establishment of a fuel supply method. For example, in the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) in Japan, which is one of the largest superconducting machines among helical plasma experimental devices [1], a global particle balance analysis was carried out in a 48-minute-long helium discharge with a total heating power of 1.2 MW using ion cyclotron heating and electron cyclotron heating, and dynamic wall retention of fuel particles was found [2]. It has been found that the dynamic wall retention can be explained by the temperature dependence of the particle retention of the plasma-facing walls composed of the divertor plate (carbon) and the first wall (stainless steel) [3–5]. In this discharge, 60% of the fuel particles were absorbed by the wall, and long-term samples irradiated by plasmas showed that the absorbed amount increased linearly with the thickness of the carbon-based deposition layer [5]. To understand the particle retention in the wall, it is important to identify "where" and "how much" the deposition layer is distributed in the vacuum vessel over a wide area. However, it is not practical to evaluate the thickness of the deposition layer on all the plasma-facing walls of the vacuum vessel using irradiated samples, because it takes time to analyze the samples and only a limited number of samples can be installed in the vacuum vessel. Therefore, a new method has been devised: color analysis. So far, color analysis has been carried out on plasma experimental devices such as the TEXTOR-94 and ASDEX-U [6, 7]. In these machines, the hue of the color was measured to assess the thickness of the deposition layer. For this purpose, a CCD camera was used in the TEXTOR-94 and an imaging camera was used in the ASDEX-U. Although, there have been reports of using cameras to evaluate the thickness of deposition layers in the past, the measurement area is still limited, and it is difficult to extend the measurement to a wide area in a vacuum vessel. In addition, when analyzing a shiny object such as metal, the reflected light from the specular reflection is strong. Therefore, the position of the sensor is affected by this effect. Thus, the color analysis of the object may not be accurate due to the strong influence of the specular reflection. The earlier study of color analysis indicates that the measurement area was not sufficient, and the accurate reflection rate was difficult to be evaluated in the metal object.

In this chapter, we introduce an example of the application of the color analysis method to the LHD using a color analyzer, which can be utilized on the metal

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

**Figure 1.**

*The process from colour analysis to deposition layer thickness evaluation.*

surface and evaluate the thickness of the deposition layer over a wide area. To evaluate the thickness of the deposition layer by color analysis, four processes are carried out, as shown in **Figure 1**. The evaluation of the thickness of the deposition layer from the color analysis is a four-step process, as shown in **Figure 1**. Process 1 is a color analysis using a color analyzer. Process 2 shows that the color analysis measurement is equivalent to the reflection measurement from a physical point of view, and Process 3 shows that the relationship between the reflection and the thickness of the deposition layer can be explained by the single-layer model. Finally, in Process 4, the thickness of the deposition layer is evaluated from the results obtained from color analysis. In this chapter, each process is explained in detail.

### **3. Compact color analyzer**

A compact color analyzer (model: DM-1) manufactured by Hitachi Metals in Japan was used to evaluate the thickness of a wide range of deposition layers on the first wall of the LHD vacuum vessel [8]. A photograph of the color analyzer is shown in **Figure 2(a)**. The main feature of this analyzer is that it has an LED integrating sphere inside the analyzer. The internal structure of the integrating sphere is shown in **Figure 2(b)**. The light emitted from the LED is injected into the object as a homogeneous standard light by a diffuser. After that, the photodiode sensor captures the light reflected from the object, which includes both positive reflection and diffusive light. Then, numerical values output as the intensity of three specific types of light, so-called R, G, B (Red, Green, Blue), with central peaks at red (615 nm), green (540 nm) and blue (465 nm). This color analyzer can measure not only R, G and B but also hue, saturation and brightness. The specifications of the color analyzer are given in **Table 1**.

The color analyzer is lightweight, small in size, and user-friendly, so that it can be easily carried into the vacuum vessel opened to the atmosphere after the plasma experiment. In addition, the measurement time is only about 3 seconds, which makes it possible to measure many points in a short time. Furthermore, the internal memory enables continuous data storage, and the rechargeable battery eliminates the need for a continuous AC power supply. Here, to evaluate the accuracy of the color analyzer, we calibrated it using about 400 color sample books (DIC Color Guide, 19th Edition, PART1, 3) whose R, G and B values were known in advance [9]. A summary of the calibration results is shown below; the R, G and B values have an offset, and their values are all similar. The R, G and B values have similar characteristics of sensitivity with high sensitivity in large values and low sensitivity in small values. A high RGB value indicates that the image is close to white, and

#### **Figure 2.**

*(a) Photograph of the colour analyzer (model: DM-1) and (b) Operation principle of colorimetry method. Formation of incident light from the integrating sphere and capture of light reflected from the target [10].*


#### **Table 1.**

*Specifications of the colour analyzer.*

a low RGB value indicates that the image is close to black, i.e., the image is more sensitive when the color is white and less sensitive when the color is black. This is similar to the human eye in general, which finds it more difficult to distinguish the difference in color between black than white. From the viewpoint of the evaluation of the deposition layer, it is desirable to have sensitivity up to the region of low RGB value, and further development of the color analyzer is expected in the future.
