Experiment on a cold cathode gyrotron

Summary form only given. A gyrotron oscillator operating at frequencies less than 20 GHz and driven by an electron beam from a cold cathode has been designed, fabricated and tested. The cavity with length 168 mm was installed in a solenoid coil which produced a magnetic field up to 1.2 T for 1 sec duration. Three aluminum cavities with various radii (viz., 10, 15, and 19 mm) were prepared. The beam source was placed 270 mm from the upstream end of the cavity, where the magnetic field strength was 33 % of that at the cavity. The anode was a stainless-steel disk with a circular hole of radius greater than the radius of the edge of the aluminum annular cathode. The cathode edge was wrapped with thin velvet to improve electron emission. The anode was at ground potential and a -80 kV high voltage pulse with duration of 100 nsec was applied to the cathode. The output microwave pulse was observed using an X-band horn antenna located 230 mm from the output glass disk window. A waveguide directional coupler divided the signal into two parts. One was fed to a crystal detector to observe the prompt signal through semi-rigid cable and coaxial attenuators. The other was fed to a 18.75 m waveguide delay line to form a delayed signal. Both prompt and delayed signals were displayed on a digital oscilloscope to measure the frequency and power. Output pulses corresponding to TE[111], TE[211] and TE[011] modes were observed near the expected values of magnetic field for the three cavities. The frequency measurements were restricted to a range near 10 GHz, because we had available only X-band waveguide delay line. The radiation patterns indicated multi-mode oscillations which might have been expected since the values of beam current were large. For cavity radii 15 and 10 mm, the outputs reached 5 MW. The frequency range of the measurement system was limited to be less than 20 GHz. We are improving the system by extending its frequency range up to 30 GHz.