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September 8, 2014
Measuring Ion Temperature from the Movement of Electrons: The Microwave Collective Thomson Scattering Method

In the future, in order to produce energy through fusion, at a density of 100 trillion parts per one cubic centimeter the temperature of hydrogen ions must exceed 120,000,000 degrees. In the Large Helical Device (LHD), we have achieved a temperature of 94,000,000 degrees. In this temperature measurement, we are using the light that ion impurities in the plasma emit. However, because impurities lessen when the performance of plasma increases, this hinders the measurement of the ion temperature. Here, we will introduce the collective Thomson scattering method, which uses microwaves that are being developed in the LHD as a diagnostic for measuring the ion temperature, which despite being the most important temperature in fusion research is also the most difficult.

A fusion plasma is formed from hydrogen ions and electrons. Compared to electrons, the measurement of hydrogen ions is not simple. Because the ion impurities that have not been fully stripped emit light even in a high-temperature plasma, we can use ion impurities for measuring temperature. However, because hydrogen ions that do not bear electrons do not emit light, we cannot perform a similar measurement. Further, because the electrons in a plasma are light and move about easily, responding to microwaves and laser light, we can accurately measure density and temperature. Because a hydrogen ion is approximately 1,800 times heavier than an electron, it is not easily affected by microwaves and laser light, and direct measurements based upon hydrogen ions are extremely difficult.

Thus in order to measure the temperature and density of hydrogen ions, we are working at a frequency of 77 gigahertz to develop a new diagnostics method that uses a microwave at the 3.8mm wavelength (one gigahertz is 1,000,000,000 hertz, and 2.45 gigahertz of microwaves are used by an electric range). However, using microwaves that respond only to electrons, how are we to measure the ions? Ions that carry positive charge and electrons that carry negative charge move about together in a plasma as if they are protecting electrical neutrality. Thus, focusing on the movement of electrons that move about tracking the movement of ions, when we inject microwaves into the plasma and then measure microwaves that were scattered by such movements of electrons, we can learn about the movement of ions. Because ions and electrons cooperate and move together, this movement is called “collective” movement. And we call the method for measuring the scattering measurement of this type of collective movement the collective Thomson scattering diagnostic. Although we cannot directly measure ion movement, by using microwaves to measure the electrons that move together with the ions we can indirectly measure ion movement. This is, so to speak, similar to knowing the movement of swans by watching the changes that swans make in the water’s surface, and not by watching swans swimming on a lake’s surface.

Microwaves utilize a powerful microwave generation source that uses plasma heating, but the signal by which collective Thomson scattering is measured is extremely weak. That signal is 10 billion parts to one the strength of an injected microwave. Because a small amount of noise greatly affects the measurement, the development of special diagnostics devices and their careful adjustment are necessary. By microwave collective Thomson scattering we can not only measure the temperature and the density of ions in the plasma, but also the high energy ions that heat the plasma.

The microwave collective Thomson scattering method is considered an essential measuring method for the future fusion power plant, and, at present, it is scheduled to be one of the diagnostics devices affixed to the ITER (International Thermonuclear Experimental Reactor). The device to be affixed to ITER is being developed by a group in Denmark, and while exchanging information with the group we are planning to enhance measuring performance in the LHD.