Abstract
The isotope effect, which has been a long-standing mystery in the turbulent magnetically confined plasmas, is the phenomena that the plasma generated with heavier hydrogen isotope show a mitigated transport. This is on the contrary to what is predicted with the simple scaling theory, in which the heavier ions easily diffuse because of its larger gyro-radius. Thanks to the newly developed analysis method and a comprehensive parameter scan experiment in the steady-state plasmas in the Large Helical Device (LHD), the isotope effect was clearly observed in the self-organized internal transport barrier (ITB) structure for the first time. Comparing the ITB intensity in deuterium (D) and hydrogen (H) plasmas, two distinct hydrogen isotope effects are found: stronger ITB is formed in D plasmas and a significant edge confinement degradation accompanied by the ITB formation emerges in H plasmas. This observation sheds light on a new aspect of the turbulent plasmas regarding how the basic properties of the fluid material affect the turbulent structure formation in the open-system.
Highlights
new aspects of the isotope effect in the ITB formation were found in LHD
the ion temperature profile was characterized by the dome-shaped profile
in which the diffusion coefficient linearly scales with the local ion temperature
Summary
LHD is a magnetically confined fusion plasma device of the heliotron magnetic configuration. The confinement magnetic field is mainly produced by the external helical coils, the plasma current need not be maintained. In the vacuum magnetic field configuration, the rotational transform, ι/2π = 1/q, where q is the safety factor, monotonically increases with the radius. The plasma is heated by five neutral beams (NBs); three are tangentially injected and two are perpendicularly injected. Heating power ratio of the tangentially injected NBs to the perpendicularly injected NBs are 6 MW/14 MW and 11 MW/9 MW in D and H plasmas, respectively. Because of this difference, the absorption power by ions is slightly higher in D plasmas
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