Optimization of plasma current profile by lower hybrid current drive on EAST for steady-state operation with high internal inductance*

A H-mode discharge with a relatively high value of the internal inductance $l_{\mathrm{i}}$ that may improve confinement and raise the stability limit to high $\beta_{\mbox{N}} $ has attracted considerable attentions in recent years[1], and it is proposed as one of promising scenarios to achieve steady-state (SS) operations with RF heating only on EAST. To assess how $l_{\mathrm{i}}$ varies with plasma parameters, predictive modeling and experiments are conducted for different operation parameters. It is shown by phase space analysis and GENRAY/CQL3D simulations that the variation of the parallel wave refractive index $N_{\mathrm{//}}$ which determines the power deposition profile of lower hybrid (LH) waves, depends mainly on the wave frequency, $q$ profile, and density profile, and so does the current profile[2]. The modeling indicates that a discharge with a lower density and plasma current is favorable to obtain a higher $l_{\mathrm{i}}$. A series of experiments are carried out in which only LH current drive and electron cyclotron heating are applied to sustain the SS H-mode discharges. The experimental results confirm that $l_{\mathrm{i}}$ decreases with the plasma density and current. The performance of plasma will be evaluated with more experiments and integrated modeling simulations over a broader parametric regime. [1] Ferron, et al., 2015 Nucl. Fusion 55, 073030. [2] Zhai, et al., 2019, Plasma Phys. Control. Fusion 61, 045002.