RF design of Passive Active Multijunction (PAM) launcher for LHCD system of ADITYA-Upgrade tokamak

The LHCD system of the ADITYA-Upgrade tokamak is being upgraded by replacing the existing grill launcher with a Passive Active Multijunction (PAM) launcher. The PAM launcher outperforms the grill launcher with regards to lower reflections, ability to operate in the shadow of the vessel at plasma edge density corresponding to its cutoff density (f = fpe, where, f = source frequency, fpe = plasma frequency) and sustain high temperature due to a provision in its structure for efficient thermal management. This paper reports the RF design of the PAM launcher for the ADITYA-Upgrade tokamak. The launcher is designed at 3.7 GHz to deliver 250 kW of RF power into the tokamak. The plasma parameters, space and mechanical constraints along with the available port size are considered while designing the launcher. The dimensions of the passive active multijunction are optimized using the Advanced LOwer Hybrid Antenna (ALOHA) coupling code to obtain desirable performance when the launcher is loaded with plasma. The PAM launcher along with its transmission line components are analysed in COMSOL Multiphysics to determine its RF performance under matched conditions. Further, the multipaction analysis of the launcher is carried out to determine the possibility of RF breakdown. The evaluated return loss of the designed PAM launcher was ∼33.7 dB at the desired frequency of 3.7 GHz. The multipaction analysis revealed that the designed launcher can handle an input power up to 350 kW.