Tungsten-Based Materials Applied in French West Tokamak Project

Recently, a team of scientists at the US Department of Energy's Princeton Plasma Physics Laboratory won the US Department of Energy Science Award, which resulted in the development of two new X-ray diagnostic instruments for the French West Tokamak project. This instrument can be used to diagnose the possible risk of melting of the first wall material of the plasma in the ITER artificial solar project.

WEST is the European ITER artificial solar project, which is mainly used to carry out prototype component tests to promote the construction of the International Thermonuclear Experimental Reactor (ITER). WEST was upgraded from the Tore Supra installation and echoes China's EAST Oriental Super Ring project. The early Tore Supra device was a large device that used carbon plasma components. Now, WEST researchers have replaced carbon components with tungsten components, the most important of which is the divertor.

The divertor is the most important key part of ITER. Its main function is to effectively shield the impurities from the wall, reduce the pollution of the central plasma, and realize the power evacuation of the central plasma particle flow and heat flow. The appearance of the divertor makes The acquisition of clean core plasma is made possible. In the current ITER study worldwide, the divertor is mainly made of tungsten alloy material.

It is well known that tungsten is one of the metals with the highest melting point.  Tungsten and tungsten alloys have excellent properties such as high melting point, high thermal conductivity, high density, low coefficient of thermal expansion, low vapor pressure, low enthalpy retention, low sputtering yield and high self-sputtering threshold. In the early ITER study, although carbon can withstand high temperatures, carbon materials react with hydrogen and then release radioactive contamination, which is very dangerous. Tungsten has an advantage in this respect. The tungsten-based material can withstand the superheating temperature of the molten plasma without absorbing the gas in the plasma. Therefore, not only the divertor is made of tungsten material, but also after ITER enters the third stage of research. The carbon and niobium-based materials on the inner wall were also eliminated, and all the wall materials were changed to tungsten, which mainly considered the low physical sputtering rate of tungsten alloy materials, high sputtering threshold energy, and low impurity generation.

However, tungsten-based materials also have short plates. Its biggest drawback is that if there is an unexpected explosion inside the ITER to generate heat, tungsten may melt. Tungsten does not immediately destroy the plasma like carbon, and stops melting, which easily causes unpredictable risks and losses. At this time, two new devices studied by American scientists will come in handy.

It is understood that the first device newly researched by scientists is called "Multi-Energy Hard X-ray (ME-HXR) Camera Diagnostic System", which can not only measure X-ray emission in a wide range of energy from plasma that promotes fusion reaction. It is also possible to detect X-ray radiation that will cover the tungsten metal tiles inside the Tokamak. This information will reveal whether the extreme heat of the machine has removed tungsten atoms from the tiles and pushed them into the plasma if the tungsten atoms are in the plasma.  The presence of a large amount in the body may indicate that the tungsten divertor has begun to melt. This monitoring of tungsten content is essential to prevent machine damage.

Another device, called the “Compact X-Ray Imaging Crystal Spectrometer”, is more like the plasma positioning system in ITER, which creates a low-resolution two-dimensional cross-sectional image of the plasma to show impurities. The overall position (including argon, molybdenum, niobium and tungsten) can more specifically monitor the operating state of the plasma in the ITER.

Using these two different but complementary tools for plasma diagnostics at WEST can provide important information for future fusion device research and play an important role in advancing the progress of the all-human "artificial sun" ITER project.

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