EXPERIMENTAL INVESTIGATION OF A 140-GHZ GYROTRON-BACKWARD WAVE OSCILLATOR

We report the experimental operation of a voltage tunable gyrotron bac kward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for applica tion as a fast tuning source for driving high power free electron lase rs or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was ach ieved in the TE1,2 mode with constant magnetic field. However, this tu ning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions fo r a cold beam. The theoretical beam velocity spread was modeled by a 3 D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotr on wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity sprea d would be needed to make the gyro-BWO operate at the desired efficien cy.