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April 7, 2014
Measuring Electron Temperature above 200,000,000 Degrees: Using the Thompson Scattering Measurement Instrument

In the Large Helical Device (LHD) an electron temperature exceeding 200,000,000 degrees has been measured. Because such a high temperature cannot be measured by a conventional thermometer, we take measurements by utilizing a powerful laser beam. Here we will report on the Thompson scattering measurement instrument that utilizes a laser beam whose improvement has been moving forward at the LHD in order to measure electron temperatures that exceed 200,000,000 degrees.

When we insert a laser beam into a plasma, the light is reflected and scattered by the electrons inside the plasma. At that time, the light is scattered by electrons. Because the wavelengths change in response to the speed of electron movement, when we measure the wavelength of scattered light we can learn the distribution of electron speed, that is, the temperature. We call this temperature measuring device which utilizes a laser beam the Thompson scattering measurement instrument. This principle itself is also being used in capturing speeding vehicles, or “catching mice” (In this instance, it uses radio waves). The Thompson scattering measurement instrument was used for practical applications from the second half of the 1960s into the early 1970s, and at present is being used in research centers around the world. At the LHD, we have developed a system that can take 50 temperature measurements per second of 144 places inside a plasma. And this instrument is providing top-class performance around the world.

The LHD’s Thompson scattering measurement instrument, among the laser beams injected into a plasma, measures the light scattered at a 13 degree angle to the rear, was designed so to measure the electron temperature from zero degrees to 100,000,000 degrees. When this measuring instrument was designed twenty years ago it was thought that measuring a temperature of 100,000,000 degrees would be sufficient. However, LHD experiments have greatly surpassed this expectation, and at present electron temperatures are surpassing 200,000,000 degrees.

For that reason, we dramatically improved the basic structure of the Thompson scattering measurement instrument. We inject the laser beam from the opposite direction, as well, and in addition to light that scatters in the back at a 13 degree angle we also measure light that scatters in the front at a 13 degree angle. When we measure the light that scatters in the front, the range of measurable temperature becomes from 200,000,000 degrees to 1,000,000,000 degrees. Measuring temperatures below 20,000,000 degrees will become difficult, but measuring the scattered light moving to the front becomes particularly beneficial in measuring temperatures that exceed 100,000,000 degrees. Further, combining the measurement of light that scatters to the rear and that of light that scatters to the front we become able to measure temperatures from zero to 1,000,000,000 degrees.

In order to become able to measure light that scatters at an angle to the front using the LHD’s Thompson scattering measurement instrument, it was necessary to extend the transmission range from the laser device to the plasma from fifty meters to eighty meters. Despite that, in order to maintain the laser beam’s position accuracy at less than one millimeter, the condition became 1.6 times more difficult. Adjusting the laser beam’s transmission method and the adjustment method, it became possible to transmit the laser beam eighty meters. As a result, we succeeded in measuring the signal of scattered light slanting to the front. Toward plasma at a temperature as high as 30,000,000 degrees, similar to the measurements to date of scattered light slanting to the rear, we confirmed that we can undertake electron temperature measurements at a measurement error of 10% or less. There remain places where improvement is necessary, but we can see the possibility of accurate measurements even when the electron temperature of an LHD plasma has climbed above 300,000,000 degrees or 400,000,000 degrees.