For suppressing the energy of runaway electrons in tokamak plasma

we analyzed the X-ray energy spectra by runaway electrons in different discharges of the HT-7 tokamak experiment performed in the autumn of 2009. The resonant phenomenon between runaway electrons and magnetic ripple was found. Although

the energy of runaway electrons in the plasma core can be as high as several tens of MeV

but when they are transported to the edge

the electron energy are limited to a certain range by resonance with the magnetic ripple of different harmonic numbers. The runaway electrons under high loop voltage resonate with low step magnetic perturbations

with high energy gain; whereas the runaway electrons under low loop voltage resonate with high level magnetic perturbations

with low energy gain. Using this mechanism

the energy of runaway electrons can be restricted to a low level

and this will significantly mitigate the damage effect on the equipment caused by runaway electrons.