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Alpha Particle Confinement Helps Realize Fusion Energy in Helical Fusion Plasmas

For the first time, a group of fusion researchers succeeded in demonstrating that energetic ions containing energy in mega electron volt (MeV) range are highly confined in a plasma in helical systems. This guarantees the alpha particle (helium ion) confinement needed for understanding fusion energy in a helical reactor.

Inside of the Large Helical Device. High-temperature plasma is confined by the two helical superconducting coils. (Image credit: NIFS)

Going forward, the deuterium-tritium reaction in a high-temperature plasma will be employed in fusion reactors. The fusion reaction generates alpha particles with 3.5 MeV energy. These particles transmit their energy to the plasma, and the heating of this alpha particle supports the high-temperature plasma condition needed for the fusion reaction. To realize this plasma, known as a burning plasma, it is essential for the energetic ions in MeV range to be confined in the plasma.

Numerical simulations predicted the positive results of MeV ion confinement in a plasma in helical systems that possess the advantage of steady operation when compared to tokamak systems. A demonstration of MeV ion confinement by experiment is yet to be reported.

The study has been recently improved by MeV ion confinement experiment carried out in the deuterium operation of the Large Helical Device (LHD), which belongs to the National Institute for Fusion Science (NIFS), National Institutes of Natural Sciences (NINS), in Japan.

The 1 MeV tritons (tritium ions) are developed by deuteron-deuteron fusion reactions in deuterium plasmas. The tritons have been studied to have a similar behavior with alpha particles produced in a future burning plasma.

Assistant Professor Kunihiro Ogawa and Professor Mitsutaka Isobe of NIFS together headed this research group, which succeeded in performing MeV triton confinement experiment in LHD. The tritons kept in the plasma go through a secondary reaction and release neutrons with high energy via a fusion reaction with background deuterons (deuterium ions).

The team developed the detector to selectively measure the high-energy neutrons in order to assess the MeV ion confinement performance. The outcome of this study demonstrates the MeV ion confinement for the very first time in helical systems. This indeed guarantees the alpha particle confinement needed for understanding fusion energy in a helical reactor.


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