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January 6, 2016
Building a Wall that Withstands High Temperatures Using Liquid Metal:
The Development of the Tin Shower Divertor

At the National Institute for Fusion Science, we make an extreme high-temperature plasma in a strong magnetic field and prevent the plasma from coming into contact with the vacuum vessel. This is because when a plasma touches the vacuum vessel that high-temperature plasma will momentarily cool. However, no matter the type of the device, there is a local place where a plasma touches the vacuum vessel. That place is called the “divertor.” In the future fusion reactor, hydrogen isotope fuel will cause a fusion reaction and become helium. This helium, too, will become plasma, but in the end it will reach the divertor via magnetic field lines and be evacuated outside the vacuum vessel. The divertor performs the important role of evacuating the ashes of the helium gas. However, when plasma particles including helium touch the divertor, the divertor reaches a high temperature. In the fusion reactor, it is predicted that heat several tens that of the current Large Helical Device (LHD) will be generated at the divertor. Thus, developing materials that can withstand such high temperatures is an urgent task.

In the LHD, the divertor is constructed in a way that graphite tiles are affixed to copper plates which are attached to cooling water pipes. Graphite can withstand high temperatures, and is excellent for good heat removal through heat conduction. However, its weak point is that when plasma particles hit the divertor, the divertor surface suffers scraping damage. The carbon scraped off become impurities and the plasma temperature falls. In order to solve this problem, the idea of using liquid metal on the divertor has gained attention in recent years. Liquid metal is a metal that, similar to mercury, melts. Because the liquid metal is originally liquid, there is no worry of its melting or being scraped away even at high temperatures.

What can be used for liquid metal? Mercury has a strong toxicity and thus cannot be used. Lithium melts at 180 degrees Celsius; tin, which is a principal ingredient of solder, melts at 230 degrees Celsius; and lead melts at 330 degrees Celsius. These metals are rich in resources, and are comparatively inexpensive. Around the world, lithium is frequently used for research, but we have begun research using tin. Because the liquid metal that will be used in the divertor will reach high temperatures, it is best that the vapor pressure be low. Compared to lithium, tin is noted for its low vapor pressure. We are considering making the shower of the melted tin like a wall and using it as a divertor.

In this completely new idea, “the tin shower divertor,” we place in a row many faucets 1 centimeter in diameter. Then, at a speed of several meters per second (about the speed of tap water) we drop the melted tin from a height of one to two meters. Here, we must ensure that the tin falls in a stable manner. If we drop the tin, the globs will scatter widely in the area. One often sees rain gutters linked together with chains for guiding rainwater. We are trying to apply that mechanism for liquid metal. If we can generate a stable wall, then, regarding heat-resistance and heat-removal performance, while performing calculations and comparisons we will investigate issues in detail experimentally. In this way, looking toward the realization of a new divertor, we are planning over the next several years the construction of a wall made from tin showers and are conducting various proof-of-principle experiments. Please look forward to our results.