The Shoelace antenna: A Device for inductively coupling to low frequency, short wavelength fluctuations in the plasma boundary

Short-wavelength, low-frequency, frequently coherent fluctuations regulate transport across the plasma boundary in all steady-state tokamak confinement regimes of interest for a reactor. In the ideal case, these fluctuations expel particles, especially impurities, without significantly degrading energy confinement. The Shoelace antenna is a novel tool employed on the Alcator C-Mod tokamak designed to couple to such fluctuations inductively. The antenna is a ladder-like structure, constructed of lanthanum-doped molybdenum wire rungs wound around alumina tension wheels. There are 19 rungs approximately 15 cm long and 2 cm apart. Adjacent rungs carry equal and opposite currents over a range of frequencies from 50-300 kHz. These parameters are selected to match two frequently-occurring fluctuations: the Quasi-Coherent Mode (QCM) and the Weakly Coherent Mode (WCM). The antenna must withstand high heat flux and large JxB forces due to its proximity to the plasma. The outputs of two commercial broadband RF amplifiers are combined with transformers to drive the antenna. An agile tuning and matching network, consisting of 80 series and 80 parallel capacitors with fast solid-state RF switches, can couple up to 8 kW of power to the antenna over the entire operational band. The frequency input to the amplifiers can be swept to hunt for an appropriate mode, and then a phase lock loop can take over to track the plasma fluctuation signal in real-time. Initial operation of the antenna indicated a weakly-damped resonant plasma response centered at the QCM frequency, a result which suggests that the antenna-induced fluctuation may drive beneficial transport across the plasma edge.