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December 7, 2015
Transmitting Electromagnetic Waves for Plasma Heating with High Efficiency and High Quality: Numerical Simulation of Electromagnetic Wave Propagation in Waveguides

In the Large Helical Device (LHD) we heat electrons in plasma by using high frequency electromagnetic waves. In the method called electron cyclotron resonance heating (ECH) we inject powerful electromagnetic waves generated by a device called a gyrotron. Energy from those waves is absorbed by electrons and heats the plasma. Between the gyrotron and the LHD is the “waveguide,” which leads the electromagnetic wave toward the LHD plasma. In order to heat electrons to high temperatures using ECH, it is necessary to increase the power of the gyrotron and to make electrons absorb the electromagnetic wave’s energy with less waste. In addition, transmission of the electromagnetic wave through the waveguide must be optimal. Here, we introduce computer simulation research being undertaken for designing such a waveguide.

The waveguide, which connects the gyrotron and the LHD, reduces the loss of energy in the electromagnetic wave, and makes transmission of the electromagnetic wave highly efficient. Further, because the electromagnetic wave phase, the oscillation direction, and the wave energy profile all have an important influence upon electron heating, maintaining transmission in an appropriate condition, that is, maintaining a high quality of transmission, is necessary. In order to achieve the high efficiency and the high quality, the internal surface of the waveguide has been micromachined to resemble the teeth of a comb.

The transmission of electromagnetic waves inside the waveguide receives strong influence from the electrical current that flows along the waveguide’s wall surface. For this reason, the configuration of the internal surface has been improved through repeated experiments and measurements of prototypes that are manufactured based upon approximate calculations for surface resistivity, which expresses the quantity of flow difficulties of the electrical current on the waveguide’s wall surface. However, in approximations of the surface resistivity that will be the basis of the prototype there has occurred the problem of not being able to incorporate the precise surface features of the internal surface of the waveguide and the effects of the eddy current generated on the internal surface. For this reason, there is a limit to enhancing the high performance of electromagnetic wave transmission and enhancing the high quality that this method can realize. More precise calculations were desired.

Thus, in order to perform more precise calculations we have developed a calculation program for simulating an electromagnetic wave transmitted in a waveguide with a complicated wall configuration. The waveguide path that connects the gyrotron and the LHD is composed of a combination of a “corrugated waveguide” in which the internal surface is designed to resemble the teeth of a comb and a waveguide for a 90 degree bend of the electromagnetic wave path called a “miterbend.” Based upon the program developed for this project, we have taken account of the internal surface configuration far more precisely than in the past, and are now able to perform calculations of the transmission of electromagnetic waves along the waveguide. As a result, after the electromagnetic wave had passed through the waveguide, we became able to evaluate at a much higher level to what extent the composition of the electromagnetic wave changed. Further, we became able to determine the distribution of the eddy current on the waveguide’s wall surface. From this, we reduced the generation of the miterbend’s eddy current and suggested a new method for raising the transmission efficiency of electromagnetic waves. In the miterbend, a “polarizer,” which can change the oscillation direction of electromagnetic waves has been attached. From the program used this time, it also has become possible to calculate more precisely the changes in the oscillation direction of the electromagnetic wave through the polarizer, which until now had been problematic. In this way, optimal use of the waveguide has become possible by utilizing this new program, and we anticipate further enhancement of the efficiency and the quality of electromagnetic wave transmission for heating plasma through use of this new program.