Date of registration 2020-11-24
In 1995, South Korea announced it will build a medium size tokamak equipped with full superconducting magnets. Because no one has tried such concept before, it would be the first full superconducting tokamak in the world. 12 years later in 2007, South Korea reported the completion of the tokamak and started to operate the device. The world was excited to witness its first operation. Although they have no previous experiences on tokamak operation, Korean scientists ignited the sparks of plasmas successfully without failure. The temperature of KSTAR’s first plasma was 2 million °C and the duration was 248 milliseconds. In 2019, after 12 years of operation, KSTAR achieved the driving record of 100 million °C plasma for eight consecutive seconds. The plasma temperature is 50 times higher and driving time is 30 times stronger.
To counter much stronger plasma operation, KFE plans to upgrade the plasma-facing materials inside of KSTAR from carbon to tungsten.
KSTAR has been using a high-purity carbon tile attached to the inside of the vacuum vessel to protect the device from events of sudden collapse of the high temperature plasma and from very high heat flux at the divertor. During the plasma operation, the temperature tiles rises to more than 1000°C due to plasma radiation and particle flux. Carbon tiles are frequently used in tokamaks due to their superior thermal properties and costs. Carbon is a low Z material which brings another advantage as a radiator that reduces the particle flux onto the wall.
However, carbon is chemically very active species with hydrogen forming hydrocarbon molecules such as methane (CD4) when plasma particles bombard the surface of carbon tiles. Methane formation in a fusion device causes problems because it contains four hydrogen per carbon atom resulting in the co-deposition of fusion fuel (D or T) as hydrogenated amorphous carbon film on the surface of remote areas inside the fusion device. Furthermore, methane entering into the plasma core degrades the performance of plasma, thus fusion reaction. The situation will be getting worse and worse as the plasma performance dramatically increases.
Inside the ‘KSTAR’ vacuum vessel of the superconducting nuclear fusion research device
In this context, the fusion communities are paying attention to tungsten as a new first wall material to replace carbon. With atomic number 74, melting point 3,422°C, boiling point 5,930°C, density 19.25g/cm3, tungsten, which means ‘heavy(tung) stone(sten)’ in Swedish, has the best melting point among existing metals. Therefore, it has excellent heat and chemical resistance, and there is little thermal expansion at high temperatures. Mass also reaches about 20g/cm3, which is 10 times heavier than carbon, making plasma-induced erosion very small.
Not only the ITER, but also Germany's ASDEX-U and China's EAST are replacing plasma materials with tungsten. KSTAR has launched a project to replace carbon a diverter, which plays a critical role in removing the heat and particle from plasma in the tokamak. Its goal is to complete the installation of new tungsten diverter by July 2022 in order to carry out high-performance plasma experiments and research related to DEMO reactor. The life time of tungsten divertor in DEMO is approximately 2.5 years in which much stronger plasma experiments take place., it can be used without any replacements for two and a half years.
The development of tungsten bonding technology is another big challenge. Two different materials, tungsten and copper, have to be bonded in order to sustain erosion by plasma (tungsten) while the heat load on the surface of the tungsten has to be removed by copper with flowing water. Since the properties of the two materials are so different and the thermal expansion rate varies by four times, high-level bonding processes are needed. Since 2012, KFE has promoted tungsten research and developed its own technology for bonding tungsten and copper alloys (CuCrZr).