Analyzing Parametric Decay Instabilities due to Helicon Injection at DIII-D

The use of a helicon rf system in the DIII-D tokamak to induce off-axis heating and current drive has presented an opportunity to study parametric decay instabilities (PDI) in tokamak plasmas. These instabilities result in the pump wave driving sidebands which could reduce the total power available for current drive in the mid-radius region. Understanding the extent of these sidebands and which parameters affect pump power loss are important in understanding the physics of helicon heating and current drive. Spectral data taken from 476 MHz helicon experiments at DIII-D demonstrate the existence of daughter waves at frequencies that are excited by the pump wave and carry an unknown fraction of energy away from the pump wave. Detection of these daughter waves is performed using a set of magnetic fluctuation sensing loops at the vessel wall. We examine data from the 2022-2023 DIII-D campaign to identify relationships between various plasma parameters and the strength of the observed PDI in order to determine the optimal parameter range for helicon power deposition in the plasma core.