Description
A primary objective of the Experimental Advanced Super-conducting Tokamak (EAST) is to demonstrate steady-state long-pulse high-performance plasma operation for future large-scale devices like ITER and CFEDR.
In EAST experiments, the statistical results show that a high chord-averaged density is conducive to the broadening of the current profile as shown in Fig.1, which in turn leads to an improvement in confinement. In experiments with only lower hybrid wave heating, we observed that the LHCD profile is expected to become more off-axis with higher density, because radial penetration of the wave is reduced at higher density, thus the wave is absorbed closer to the plasma edge. Therefore, with only LHCD and bootstrap as sources of plasma current, the total current profile should become broader at higher density. That is, under high-density conditions, achieving a broader current density profile can facilitate weak reversed magnetic shear in the core—characterized by a safety factor (q) profile that decreases modestly toward the center before rising. This (q) profile suppresses turbulent transport, improve confinement, and correlates with a higher bootstrap current fraction, reducing reliance on external drives.
This paper focuses on how density variations in the core and pedestal regions affect the current density profile and plasma confinement. The results show that an increase in core density, by enhancing local pressure gradients, modifying the bootstrap current, and broadening the current profile, is consistent with the off-axis driving effect of lower hybrid waves. In contrast, density variations in the pedestal region but have a minor impact on core kinetics.
These findings reveal a synergistic effect between high density, broad current profile, and enhanced confinement in lower hybrid wave-driven EAST plasmas, providing guidance for tokamaks to achieve efficient current drive and improved confinement performance.
Key Words: Density, Current profile, (q) profile, turbulence