A relativistic electron beam (500 keV, 200 A, 10 ns) generated magnetically
tunable microwave radiation in a frequency range of 9-13 GHz when it is in
jected into an X-band rectangular waveguide immersed in a uniform axial mag
netic field (4-10 kG). The mechanism of the microwave radiation was identif
ied as the gyrotron backward wave interaction. The output power of the radi
ated microwave increased exponentially with the increase of the cavity leng
th.