Antenna-driven Compressional Alfven Eigenmodes in Relativistic Electron Beam Plasmas

Compressional Alfven Eigenmodes (CAEs) are a kinetic instability observed in tokamak plasmas with significant ion heating power. While CAEs are typically excited by a fast ion population, recent work has reported that they can also be driven by a runaway electron (RE) population following a plasma disruption (Chang Liu et al 2021 Nucl. Fusion 61 036011). Whether naturally occurring or driven by an antenna these modes have the potential to deconfine the RE population. During recent DIII-D discharges, CAEs were driven using a low-power ICRF antenna, located on the low-field side, in both pre- and post-disruption plasmas. Waves were launched for a range of frequencies from 0.1 MHz to 10 MHz and were observed to excite multiple wave harmonics, as shown in frequency spectrograms of magnetic fluctuations, in both the pre-disruption and RE plasma. The dependence of the low frequency cut-off on several plasma parameters, such as toroidal field and plasma density, will be presented; and a comparison will be made between those conditions that support the propagation of the driven mode post-disruption and those conditions in which no modes propagate. The results of this study will improve the fundamental understanding of CAEs and how they may deconfine the RE population.