  A high-frequency magnetic probe has been designed and developed on
XuanLong-50 (EXL-50) spherical torus to measure high-frequency magnetic field
fluctuations caused by energetic ions and electrons in the plasma. The magnetic
loop, radio filters, radio-frequency (RF) limiter, and data acquisition system
of the probe are described in detail. The results of the preliminary test show
that the probe can have a frequency response within the 1- 180 MHz range. The
fluctuation data from the EXL-50 plasma were analyzed in the time-frequency
domain using fast Fourier transforms. Using this diagnostic system, distinct
high-frequency instabilities were detected. In particular, significant
frequency chirping was observed, which is consistent with the bump on tail
drive instability predicted by the Berk-Breizman model.
