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 (<1 ms) and a modified 4CW100000E based driver tetrode, the 120 MHz FMIT has achieved 1.7 MW with reasonable screen current, 4 A, into a matched load. Based on previous reported results, the tube is not near its operating limits [1]. The primary limitation has been the driver and further optimization of the tube-based driver or integration of the SSA offer paths to higher performance. For the SSA, the required performance is dependent upon the gain of the FPA and tolerance to reflected power from the FPA. Based on reported FPA tube operations, a gain factor of 12-14 is expected corresponding to required drive power of 140 - 165 kW to achieve 2 MW output from FPA. We selected a SSA from Cryoelectra that combines ten 20 kW modules for 175 kW output power with VSWR of <1.33. If greater drive power or reflected power tolerance is required, an additional 40 kW can be added without significant design modification. Since the market for tube-based transmitters has been drastically reduced, one of the critical challenges is to secure the tube supply chain. The most critical elements are the pyrolytic screen and control grids. We embarked on a grid deposition, machining and laser cutting development with a goal to create 3 sets of grids for the 4CM2500KG. We obtained acceptable control grids after a few attempts; however, the screen grids required additional adjustments to obtain reliable screen grids. The latest simulations, design and system performance will be presented. Work supported by the MIT Energy Initiative. [ABSTRACT FROM AUTHOR]