Advances in lower hybrid technology at 4.6 GHz towards long-pulse tokamak operation

To extend the long-pulse operation at higher plasma current and plasma density on Experimental Advanced Superconducting Tokamak (EAST), a new lower hybrid current drive (LHCD) system operating at 4.6 GHz with 4 MW nominal power is being under development. This paper reports the recent advances in lower hybrid technology including the key components of the klystron, launcher, and circulator, which are supported by the Comprehensive Research Facility for Fusion Technology (CRAFT) project. A prototype klystron at 4.6 GHz with one output window has produced 500 kW radio frequency (RF) power with a duration of 1590 s on the test bed with matched load. An active cooling launcher based on the passive-active-multijunction (PAM) concept at 4.6 GHz has been constructed, which is waiting for the plasma experiments on EAST. For the prototype circulator, a full power (namely, forward power = 500 kW) operation over 1135 s with 30% power reflected back to itself has been achieved. These achievements make a large step towards the possible implementation of an LHCD system with the same frequency of 4.6 GHz on China Fusion Engineering Test Reactor (CFETR).