Analysis of runaway electron driven whistler wave instability experiments

Whistler waves are a fundamental phenomenon in terrestrial and planetary ionospheres. They are destabilized by energetic electron populations which can arise from lightning strikes and solar mass ejections leading to solar sub-storms. In tokamak plasmas, energetic runaway electrons can be created by toroidal electric fields arising either from the external Ohmic fields used to drive current, or by sudden drops in the plasma current caused spontaneously by instabilities, or deliberately produced by pellet/gas injection. With the start of the frontier science initiative on DIII-D tokamak, repeatable experimental scenarios have been developed for producing runaway electron beams at various intensity levels. This capability along with enhanced high frequency (~ 100s MHz) electromagnetic wave diagnostics were used in 2017 to demonstrate that whistler waves can be excited by runaways. Subsequent experiments done at higher runaway electron intensity levels indicated that other waves in the compressional and shear Alfvén frequency ranges (~ MHz) could also be excited. While simple models have verified the basic features of these waves, much remains to be understood about the observations. Topics such as the runway-wave coupling mechanisms, the observed multiple discrete frequency bands, the nonlinear saturation, and the induced scattering of the runaways by the waves remain topics of ongoing investigation. An improved understanding of the runaway electron – wave interaction physics is important not only to deepen the connections with ionospheric physics, but also offers opportunities to control and better predict runaway electron behavior in tokamaks. The additional manpower provided by this grant will allow the team to develop analysis and modeling tools to understand these observations in a more methodical way than has been possible previously. In addition to the earlier experiments, the DIII-D Frontier Science initiative has granted the team new experiments in the FY20-21 time frame. The new data will be analyzed as well.