National Institute for Fusion Science
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Research

Degradation of the energetic-ion confinement independent of MHD instabilities is observed

We observed that the degradation phenomenon of energetic-ion confinement was independent of MHD instabilities in an LHD deuterium experiment. The degradation depends on neutral beam power and is localized on the plasma core region. Because the cause of the degradation phenomenon is still unclear, further investigation is required.

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The vertical axis indicates the neutron emission rate per beam power normalized to that at t = 4.2 s and the horizontal axis indicates the time from the start of the discharge. The beam power increases by two times after t = 4.3 s. Curves in different colors indicate the discharges with different plasma densities. In most discharges, the neutron emission rate per beam power is reduced by up to 20 % after the beam power increases. This reduction reflects the degradation of the energetic-ion confinement.

Neutral beam injection (NBI) is often used for plasma heating in magnetic confined fusion devices. Although the output power of NBI can be measured, the actual power absorbed in the plasma (in other words, heating efficiency) is not measurable. Therefore, in general, the heating efficiency is estimated by simulations. If there are some unknown phenomena, the estimation of the heating efficiency is not accurate because simulation tools calculate according to theory.

Before our research, the confinement of energetic ions generated by NBI was considered able to estimate sufficiently when magnetic hydrodynamics (MHD) activities are negligible. For this reason, though there are a lot of investigations into the interaction between energetic ions and MHD activities, there are few investigations into the energetic-ion confinement without MHD instabilities. Although the confinement degradation observed in this paper depends on the NB power, it does not depend on MHD activities.

Although the conventional estimation of the heating efficiency was approximately 70 %, our estimation considering the confinement degradation decreased to approximately 40 %.

This confinement degradation was clarified through the neutron measurement, impurity measurement, and integrated simulation. The neutron emission due to the fusion reaction between deuterons is the most important criterion of the energetic-ion confinement. For this reason, if the LHD deuterium experiment had not taken place, this phenomenon could not be discovered.

Although this research discovered the confinement degradation, the mechanism of this degradation is still unclear. To develop the heating efficiency, clarification of the degradation mechanism is necessary. The reduction of the heating efficiency decreases the economic efficiency of a power plant. Therefore, clarification of the degradation mechanism is important for fusion science. Investigation of the mechanism is for future works.

This research result was published on April 18, 2024 in Nuclear Fusion, a journal on controlled thermonuclear fusion by the International Atomic Energy Agency.

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