a betatron , adapted , e . g ., for use as a high - energy electromagnetic radiation source in a borehole well logging tool , includes modulator circuitry for actively controlling the electron beam radius during acceleration and for extracting the electron beam at or near maximum magnetic field strength . such control may be effected by suitable separate pulsing of field - coil and core - coil magnets , and results in enhanced efficiency of betatron magnet - excitation power conversion and beam end - point energy stability and intensity over a range of operating temperatures .