In order to realize fusion energy in the future it is necessary to confine high-temperature high-density plasma for a long period of time. For that reason, first, it is important to know the characteristics of plasma. In the Large Helical Device (LHD), in order to obtain information regarding phenomena that greatly affect plasma confinement, in particular, detailed information regarding the flow and turbulence of plasma that recently has come to be considered an important topic, we are developing new methods of measurement. In this Research Update we will introduce a new measuring method, the Doppler Reflectometry.
The hybrid cars that we now see frequently on the street combine the good features of a gasoline engine and of an electric motor. This is splendid technology realized in a car with low fuel consumption and high performance. Similarly, in the measurement of high-temperature plasma we are developing greatly improved measuring instruments by combining multiple technologies.
Because fusion plasma is at a very high temperature, using electromagnetic waves and other tools it is necessary to examine the plasma’s properties without touching directly the plasma. Recently, the flow and turbulence of plasma have come to be focused upon as features that greatly affect high-temperature plasma confinement performance. Thus, methods to measure these information are being sought. When we inject an electromagnetic wave into the plasma, by precisely measuring the scattered wave we can learn the speed and directions of plasma flows because the electromagnetic wave will be scattered by interacting with the collected electrons in the plasma. Until now, in order to precisely measure various places in a plasma, a method using two antennas had been adopted; two antennas emits electromagnetic waves and receives scattered electromagnetic waves, respectively. And the directions of antennas are set to intersect. However, as in the LHD, in seeking to measure a large plasma of more than one meter in diameter, the structure of these antennas became very large and conducting the measurement was difficult.
In order to solve this problem, we sought to hybridize a measuring instrument. As a method to measure a scattered wave, we combined reflection radar technology used for a plane’s operation and weather observation. Reflection radar technology is one that when focused on a target measures the reflected wave. This is used in identifying automobiles that are exceeding the speed limit. In the LHD, if we select an electromagnetic wave of the appropriate wavelength of several millimeters, in the plasma a reflected wave will always appear. For that reason, through the combination of these technologies with one small-sized antenna together with the reflection of the electromagnetic wave it became possible to measure the reflected wave of a broad area of plasma to a very high spatial accuracy. This radar technology uses the Doppler Shift phenomenon, which shows changes in the reflected wave’s wavelength through the speed of the object that caused the electromagnetic wave to reflect. This measuring instrument is called the Doppler Reflectometry. By precisely measuring the small differences in the wavelengths of the injected electromagnetic wave and the reflected wave, we are now able to know in detail the speed and the directions of the plasma flow, the intensity of turbulence, and other quantities. We are installing high-precision instruments in the LHD so that we may be able to simultaneously measure many places.
Using the Doppler Reflectometry we were for the first time able to learn in detail that when we apply voltage from outside to the plasma edge and force changes in the condition of the plasma, the plasma’s flow gradually changes in response to the applied voltage, and under certain conditions the turbulences are suddenly controlled. In the future, too, we will enhance the performance of measuring instruments, clarify unknown characteristics of plasma, and seek to raise the level of research on plasma confinement.