A large diameter thin annular intense relativistic electron beam was modulated at a frequency of 10 GHz. The electron beam propagated inside a narrow annular drift tube in which gaps feeding radial cavities were inserted. An external source injected microwave power of 180 kW into a single input cavity followed by a simple idler structure. A stable 10 kA rf current modulation was induced on the 400 keV electron beam. ©1997 American Institute of Physics.