High frequency, high power ICRF source for fusion plasmas

Ion Cyclotron Range of Frequency (ICRF) heating systems are anticipated to be a primary auxiliary heating source in next step fusion tokamak experiments. For high field devices, multi-megawatt ICRF sources above 100 MHz will be required since the ion resonance frequency scales with magnetic field. In this project, we seek to develop a 2 MW, 120 MHz RF source that incorporates a single solid-state amplifier stage (SSA) with a final power amplifier (FPA) utilizing the 4CM2500KG tetrode since it previously achieved 1.7 MW for 5.4 s at 131 MHz [1]. The plan is to modify a fixed 80 MHz Fusion Material Irradiation Test (FMIT) [2] system which is a three-stage amplifier with a 10 kW solid-state initial power stage (IPA) and two consecutive tetrode-based amplifier stages. Based upon C-Mod experience, a single SSA driver offered increased modularity, decreased operational cost, and higher reliability. From the output power perspective, the most restrictive limitation is the FPA tube screen power dissipation thus we sought a plate impedance between 50-70 Ω to minimize the screen current. With an input impedance of 2.7-4 Ω, a full surrogate dummy tube was manufactured to allow exploration of the existing cavity matching range and guide modifications. As expected, the cavities required significant reduction. Using short pulses (