1. 연사: 백승규 박사 (MIT Plasma Science and Fusion Center)

2. 일시: 2018. 9. 12(수), 오후 2시

3. 장소: 공학3동 201호 (세미나실)

4. 제목: High Density Lower Hybrid Current Drive Experiment on Alcator C-Mod

Lower hybrid current drive (LHCD) is expected to play a vital role in realizing steady-state tokamak operation, yet it has been suffered from the so-called lower hybrid density limit problem, a sudden loss of current drive efficiency at high density. In this talk, we will be focusing on the experimental study conducted on the Alcator C-Mod tokamak, which operates at the highest density and magnetic field to date. While the anomalous loss of efficiency has been reported previously, C-Mod has identified that this could worsen in a diverted tokamak. This talk will begin with reviewing tokamak basics to understand why tokamak is inherently a pulsed device. We will also briefly discuss the basic ideas behind current drive, which is to produce the plasma toroidal current relying on wave-particle interaction without the application of the electric field. Early Alcator C-Mod experimental results will be then presented to characterize the observed LH density limit, suggesting that the injected LH wave is damped near the plasma boundary. The discrepancy observed between the experiment and modeling has motivated to look for experimental evidence of non-linear wave interactions, such as parametric decay instabilities that can result in edge damping. The instability onset density is found to be correlated with the observed loss of efficiency. In the 2016 campaign, efficient LHCD is demonstrated up to near the accessibility limit beyond the density limit previously identified at the line-averaged density of 10^{20} m^{-3}. It is achieved by increasing plasma current from 0.5 MA  to 1.4 MA. Not only the global density but also the plasma current is found to determine the scrape-off-layer characteristics and its interaction with the LH wave. The new results implicate that a control in the edge/scrape-off-layer plasma is crucial in attaining LHCD at high density, and, in this regard, the placement of the LH antenna at the high-field-side of the tokamak may be an optimal approach in implementing LHCD in a reactor.