Description
Nonlinear power deposition and efficiency loss of lower hybrid current drive
The nonlinear power deposition in the scrape-off layer (SOL) plasma of lower hybrid waves (LHWs) at high plasma density has been one of the major concerns of the studies in lower hybrid current drive (LHCD) since the earliest LHCD experiments, as well as a major challenge for achieving effective non-inductive current drive at future tokamaks.
In this work, we present a theoretical model of the nonlinear power deposition of LHCD in the SOL plasma considering the convective amplification of parametric decay instability (PDI), based on which a self-consistent simulation code characterizing the nonlinear spectrum evolution and power deposition of lower hybrid waves (LHWs) in the SOL plasma is developed [1], and the failure of LHCD at high plasma density on JET, Alcator C-Mod and EAST is reproduced [2]. Such a model predicts that the suppression of PDI can be reached by entering the H-mode to decrease the SOL plasma density (while the line-averaged density holds still), or reducing the toroidal scale of the LHW antenna to shorten the convective length of PDI. Both predictions have been validated in recent experiments on EAST.
Further theoretical analysis reveals that the efficiency loss of LHCD through PDI is also related to the focusing of LHWs caused by the poloidal inhomogeneity of tokamak plasma. As the LHCD systems on conventional tokamaks work under the weak single-pass absorption regime, the focused LHWs can have higher power density during their reflection compared to that during their launch at the antenna mouth, thus leading to stronger PDI during the reflection of LHWs. As a result, mitigating the focusing of LHWs and enhancing the single-pass absorption of LHWs are effective in improving the LHCD performance at high plasma density, which is consistent with recent experimental findings on EAST.
[1] Z. K. Huang et al, Nucl. Fusion 66, 016022 (2025).
[2] K. Chen et al, Nucl. Fusion 65, 094002 (2025).