Coupling optimization experiment on HL-2A based on Passive-Active Multijunction antenna for 3.7GHz Lower Hybrid system

System Introduction. A new Lower Hybrid Wave (LHW) system (3.7GHz/2MW/2s) has been built on HL-2A tokamak. A Passive-Active-Multijunction (PAM) concept antenna [1], [2] was designed and built, fed by 500kW× 4 TH2103A Klystrons, delivering ITER-relevant power density, i.e. 25MW/m2 at f = 3.7GHz [3],[4]. The antenna is designed to launch a peak parallel refractive index of N||=2.75 with a low theoretical Reflection Coefficient (RC), i.e. less than 1% [5], [6]. A specific gas puffing system for improving the antenna coupling and a set of Langmuir probes for antenna mouth density measurements are separately located on one side of the antenna each. An auxiliary vacuum system is installed at the rear of the antenna to improve the pumping efficiency. Vacuum leak tests and low power microwave scattering parameter measurements were done before the PAM antenna was installed on HL-2A, allowing the antenna to be conditioned at RF power of 100kW/100ms before starting plasma operation.