A High-efficiency 33 GHz Free Electron Laser With A Reversed Axial Magnetic Field

Summary form only given, as follows. We report on a new regime of five electron laser operation using a helical wiggler and a reversed axial guide magnetic field. The orientation of the axial field is such that the helical wiggler and the cyclotron motion of the electrons in the guide field have opposite handedness. The mildly relativistic electron beam (750 keV) is generated by a Marx capacitor bank (Physics Intemational Pulserad 110A). The field-emission cathode yields 300 A, with a beam radius of 2.54 mm. The bifilar helical wiggler has a period of 3.18 cm and provides a magnetic field of 1.5 kG on axis. The FEL is driven by a n-mgnetron operating at 33.39 GHz. About 20 kW of linearly polarized radiation is injected into the TE/sub 11/ mode of the circular drift tube. The output power of the device is transmitted by a conical hom into a microwave anechoic chamber and then received by a small horn. By moving the receiving horn we determine the angular pattern of the emitted radiation. We have measured over 60 MW of output power, corresponding to an efficiency of 27%. The results are compared with those obtained when the axial guide field is in its conventional orientation, where considerable loss of power and efficiency is observed. The frequency spectrum has been determined by mixing the emitted power with radiation from a K-band local oscillator and then by feeding the beat wave into an X-band filter bank. In the conventional geometry (unlike in the Reversed Field configuration), an unexpected frequency upshift from the magnetron input frequency of the order of 100 MHz has been observed, and will be discussed.