Lower Hybrid Antenna Design for MST

Inter‐digital line antennas are being used to test the feasibility of lower hybrid current drive in MST. The antennas use λ/4 resonators and launch slow waves at 800 MHz with n∥ ∼ 7.5. Routine operation has been achieved with a good impedance match between antenna and plasma. High power antenna design improvements include larger vacuum feed‐throughs, better impedance matching, and rf instrumentation on all resonators. The antenna and feed‐through modeling was performed with CST Microwave Studio™. The pulse‐forming network that powers the klystron is being upgraded to a 50 kV – 30 ms pulse. The goal for the LHCD system on MST is a modular design that can handle 300 kW per antenna.