The Tungsten Guard Limiter of 4.6 GHz Lower Hybrid Current Drive Under EAST Operations

Abstract

The 4.6 GHz lower hybrid current drive (LHCD) waveguide is a dominative auxiliary heating and current drive facility in Experimental Advanced Superconducting Tokamak (EAST), but a large number of hot spots were observed on the previous guard limiter (GL) of graphite monoblocks plated with SiC during 2016&#x2013;2017 campaigns, which limited to the plasma density, the superconductor currents, and the heating power of LHCD in EAST operations and also led to the plasma disruptions due to the carbon impurity. Therefore, we developed a tungsten GL for 4.6 GHz LHCD facility employed in the operations. The thermal transfer issues are resolved with a numerical simulation method. The fluid circuit is designed symmetrically considering the interchangeability. The joint water box is schemed based on the physical requirements with explosive welding technology. The new GL experienced the plasma discharges in the LHCD operations under the conditions of <inline-formula> <tex-math notation="LaTeX">$I_{p} \sim$ </tex-math></inline-formula> 400&#x2013;600 kA, <inline-formula> <tex-math notation="LaTeX">$n_{e}~\sim ~2$ </tex-math></inline-formula>&#x2013;<inline-formula> <tex-math notation="LaTeX">$4\times 10^{19}\,\,\text{m}^{-3}$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$I_{t}\,\,\sim$ </tex-math></inline-formula> &#x2212;10 000/&#x2212;11 000 A in the plasma discharges. The maximum operated power of LHCD reached 3.7 MW and a short pulse discharge lasted 20 s at operated power of 2.9 MW. In consequence, the plasmas discharges were controlled well without any disruptions due to the hot spot reasons and ended smoothly. It indicates that the hot spots of LHCD GL ever considered as an inevitable event in the LHCD operations could be reduced to minimum via the new engineering design.