  In this study, the first observation of high-frequency instabilities driven
by runaway electrons has been reported in the EXL-50 spherical torus using a
high-frequency magnetic pickup coil. The central frequency of these
instabilities is found to be exponentially dependent on the plasma density,
similar to the dispersion relation of the whistler wave. The instability
frequency displays chirping characteristics consistent with the Berk-Breizman
model of beam instability. Theoretically, the excitation threshold of the
instability driven by runaway electrons is related to the ratio of the runaway
electron density to the background plasma density, and such a relationship is
first demonstrated experimentally in this study. The instability can be
stabilized by increasing the plasma density, consistent with the wave-particle
resonance mechanism. This investigation demonstrates the controlled excitation
of chirping instabilities in a tokamak plasma and reveals new features of these
instabilities, thereby advancing the understanding of the mechanisms for
controlling and mitigating runaway electrons.
