In order to realize the future’s fusion energy through basic research it is necessary to achieve plasma at temperatures exceeding 120,000,000 degrees and to investigate through academic study the qualities of that high-temperature plasma. For that reason, using the Large Helical Device (LHD) we are focusing on the important research topic of “How to achieve high-temperature plasma?” Here we will report on advances achieved last year in high-temperature plasma performance.
Plasma is an ionized gas in which electrons separate from atoms, and ions and electrons move about separately, and it is in a high-temperature state. In fluorescent light, too, it is in a plasma state, and its temperature exceeds 10,000 degrees. Because it is at a low density of four parts per thousand (4/1000) of atmosphere, it is cooled by the wall of the glass container and the temperature reaches but 10,000 degrees. Conversely, the LHD, through the superconducting magnet, plasma is confined with magnetic field and maintains it there so that it does not touch the wall of the vacuum vessel. Through this, in order for the plasma to not be cooled by the wall the density is approximately 1/5,000 of fluorescent light plasma, but it has achieved a temperature that exceeds tens of millions of degrees.
In order to raise the plasma temperature, the plasma must be heated. In the LHD, in order to raise the ion temperature, a high-energy hydrogen particle beam is injected into the plasma, and the plasma is heated through that energy. Using five particle beam injection devices, in 2011 we achieved an ion temperature of 80,000,000 degrees. In experiments conducted in 2012, prior to the generation of high ion temperature plasma, we succeeded in making conditions of a vacuum vessel wall appropriate for the conditions required to generate high ion temperature plasma by repeatedly generating plasma using the electro-magnetic waves of FM radio frequency bands. The plasma does not directly touch the wall, but because it receives influence from the wall, it is important to establish a method for adjusting for wall conditions. Based upon the optimization of this last year we achieved an ion temperature of 85,000,000 degrees.
Conversely, in order to raise the temperature of electrons, we heat plasma using high frequency microwaves. In 2012, we introduced a microwave generation device, a gyrotron jointly developed with the University of Tsukuba. This gyrotron produced a heretofore unreached 154 gigahertz (154 billion hertz) and oscillator power at 1,000 kilowatts, and together with three gyrotrons developed and introduced by 2011, electric power of microwaves reached a 4,600 kilowatts, though only for the short period of two seconds. From high power plasma heating experiments that used this power in plasma of a density of 10 trillion/1 cubic centimeter (approximately 1/2,700,000 of atmosphere), we achieved an electron temperature of 150,000,000 degrees. This greatly enhanced the results that had been achieved prior to 2011 in which a temperature of 100,000,000 degrees was achieved from the same density.
Plasmas of a high ion temperature or of a high electron temperature achieved to date were gained through separate experiments conditions, and for that reason the electron temperature and the ion temperature are high. In the future fusion power plant it will be necessary for the ions and the electrons in the plasma to be at the same temperature. To investigate such conditions, alongside research seeking to raise the ion temperature further we are moving forward with research that will realize plasma with the same temperature of ion and electron even when those temperatures are low.