Description
To advance the steady-state operation of ITER and achieve high fusion gain (high-$Q$) in tokamak devices, experimental research on high poloidal beta ($\beta_p$) plasma scenarios was conducted on EAST, focusing on ITER-relevant compatibility, fully non-inductive operation, and tungsten (W) divertor adaptation. High-performance high-$\beta_p$ plasma was successfully demonstrated via radio frequency only heating (ECRH, LHCD, ICRF), with key parameters including excellent confinement $H_{98y2}=1.5$, bootstrap current fraction $f_{\mathrm{BS}}=50\%$, Greenwald fraction $n_e/n_G=0.82$, and $\beta_p \sim 3.0$ / $\beta_N \sim 1.8$ maintained for 100 seconds at $B_t=2.5\,\mathrm{T}$ and $q_{95}\sim7.0$.
For steady-state operation of W-divertors towards high-$Q$, fully non-inductive high-$\beta_p$ plasmas with a boron-coated wall were developed at $q_{95}=6.0$, $n_e/n_G=0.65$, $B_t=2.5\,\mathrm{T}$. Driven by dominant electron heating, this plasma achieved $H_{98y2}=1.35$ and $\beta_p=2.1$ / $\beta_N=1.85$, and exhibited a series of superior physical characteristics: an internal transport barrier (ITB) that effectively suppresses plasma turbulence, small edge-localized modes (ELMs) with a frequency $f_{\mathrm{ELM}}>2.5\,\mathrm{kHz}$, effective control of tungsten impurity accumulation, a flat core current density profile with $q_{\min}>1$, and suppression of $3/2$ tearing modes via ICRF heating. Transport simulation results showed that $\alpha$-stabilization induced by Shafranov shift is the dominant factor governing turbulent transport. The electron thermal ITB is mainly controlled by trapped electron mode (TEM) turbulence, and zonal flow shearing in the ITB region exerts a regulatory effect on TEM-induced energy transport. Notably, increased plasma density and $\beta_p$ were found to elevate the bootstrap current fraction $f_{\mathrm{BS}}$, broaden the current density profile, and further improve plasma energy confinement.
Subsequent experiments were performed with ITER-like heating and ITER-shaped (LSN) combinations (ECRH, ICRH, NBI), alongside plasma current scanning in the range $q_{95}=5.0$–$6.0$. The results indicated that increased plasma current caused $H_{98y2}$ to decrease from 1.35 to 1.1 and reduced the SOL density. Additionally, edge plasma parameters were found to exert a significant influence on the coupling and absorption efficiency of RF heating.
For further optimization of ITER-configuration plasma scenarios with enhanced capability on EAST, future research will focus on adopting a moderate safety factor $q_{95}=5.0$ to extend fusion performance, employing ITER-like heating configurations, adjusting the ion-to-electron temperature ratio $T_i/T_e$ via ion heating, and broadening the current density profile $j(r)$ to form a full-channel ITB with $q_{\min}>2.0$, which can effectively prevent the onset of low-order tearing modes.
The experimental results obtained on EAST provide important technical support for ITER’s new research plan, including the material switch of the first wall to tungsten. Furthermore, these findings offer critical physical insights for the development of high-performance plasma scenarios in future fusion power plants.