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물리학과/첨단원자력공학부 특별세미나 개최 안내
물리학과2018-09-04462
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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.
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