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December 1, 2014
Maintaining Plasma for Extended Duration: Enhancing the Performance of Steady-state Plasma through Increased Heating

In order to generate energy through nuclear fusion, plasma of a density of 100,000,000,000,000 parts per 1cc must be at a temperature that exceeds 120,000,000 degrees. However, even if such a plasma is achieved, if it can be maintained for only but a short period of time, then the generated energy cannot be used for power generation. In the Large Helical Device (LHD), we are advancing with research that raises the performance of temperature, density, and other aspects of plasma. Together with that research, we are engaged in research on extended duration stable maintenance of high-performance plasma, and we are obtaining important results. Here, we report on the enhancement of the high performance of plasma maintained for an extended duration through increased heating power that is being undertaken in the LHD.

In the LHD, we are undertaking extended duration maintenance of plasmas through this heating device that uses electromagnetic waves. During the 2013 experiment year, at a density of 100,000,000,000,000 parts per 1cc, we succeeded in maintaining plasma at a temperature of 23,000,000 degrees for 48 minutes for both the core electron temperature and the ion temperature. As extended duration maintenance of a high-performance plasma, this result is the highest in the world, and it cannot be matched by other devices.

As heating methods for electromagnetic waves, ECH (electron cyclotron resonance heating), which heats the electrons in the plasma core, and ICH (ion cyclotron range of frequency), which mainly heats ions, are being used in the LHD. The three frequencies for electromagnetic waves being used in ECH are 154,000 megahertz, 84,000 megahertz, and 77,000 megahertz. These frequencies are several tens of times higher than the microwave frequency of 2,450 megahertz used in microwave ovens. One megahertz means that in one second there occur 1,000,000 oscillations. On the other hand, the ICH frequency is approximately 38 megahertz, and is but 1/2000 or 1/4000 of the ECH frequency. This means that the weight of a hydrogen ion is approximately 2,000 times the weight of an electron.

To date in the LHD, through heat energy of 500 kilowatts combined from ICH and ECH, we have already surpassed the world record for extended duration plasma maintenance for 54 minutes of a plasma at the temperature of 10,000,000 degrees at a density of 4,000,000,000,000 parts per 1cc. In response to this, in the 2013 experiment year, we strengthened the heating device by 940 kilowatts of ICH power and 240 kilowatts of ECH power, or a total of approximately 1,200 kilowatts, and greatly increased plasma performance. We were able to significantly improve our record of extended duration plasma maintenance to 48 minutes. Further, we increased to 380 kilowatts the ECH heating power added to the ICH, and we also succeeded in maintaining for 12 minutes a plasma in which the core electron temperature was raised to 46,000,000 degrees.

The LHD experiments this experiment year began on November 6. In preparation for them, we strengthened the ECH system of 154,000 megahertz and, moreover, undertook performance enhancement of the heating device. Based upon this, we are planning further performance enhancement of extended duration maintenance plasmas, and we are strongly moving forward in research relating to extended duration maintenance of steady-state plasmas, which is considered necessary for the future fusion power plant.