National Institute for Fusion Science

The 2017 Large Helical Device (LHD) plasma experimental campaign concluded on August 3, 2017. Since the beginning of the experimental campaigns in 1998, this is the nineteenth cycle of plasma experiments. This year’s experiments began on February 8, 2017, and continued for 100 days, over which more than 13,000 plasma shots were conducted. During this cycle, working in collaboration with numerous collaborators from universities and research institutes abroad and in Japan, National Institute for Fusion Science researchers advanced various research projects and continuously gained new insights. Further, we concluded the experimental campaign with no troubles. On June 8 the total number of plasma shots reached 140,000 since the start of LHD experiments in 1998.

In this nineteenth experimental cycle, we generated plasma using deuterium gas, which has two times the mass of hydrogen, and the “Deuterium Experiment,” which aims to improve plasma confinement, was initiated. In this nineteenth cycle, which is the first to use deuterium, during the first month we generated plasma using hydrogen gas to confirm the experimental procedures and apparatus operation. After confirming the above process, we initiated experiments to adjust the conditions, and introduced deuterium gas on March 7, 2017. After confirming safety, we switched to the full-scale deuterium experiment. During the experimental period, together with the increase in heating power, plasma performance favorably improved. After the first week of the deuterium experiment we achieved an ion temperature that exceeded one hundred million degrees, and we extended the temperature to beyond the previous record of 94,000,000 degrees using hydrogen plasma. The deuterium experimental campaign was performed on schedule until July 7, 2017. Experiments that investigate phenomena which express complex phenomena accompanying the enhancement of the high performance of plasma, and the behavior of high-energy particles in a plasma were actively performed. At the final phase of the experimental campaign, we again conducted hydrogen experiments for approximately one month. And we collected reference data for hydrogen plasma in order to perform comparisons with the deuterium experiment data. Thus, in the 19th experimental campaign, together with establishing consistent procedures pertaining to the deuterium experiment, which aims at improving further plasma performance, we also have set the experiment to be well under way. Thus, we were able to achieve numerous important results that could not have been gained through hydrogen plasma experiments.

In the LHD, accompanying the conclusion of the plasma experiments, from August 4 we will return the superconducting coils, whose temperature had been lowered to -270 degrees Celsius, to room temperature. That temperature will be raised incrementally over approximately three weeks. After completing the raising of the coil temperature in late August, maintenance work aiming at next year’s experiment will then be undertaken.