MTX/ELF II microwave power measurements and calibration for the 2-GW, 140-GHZ, ELF II free-electron laser

Techniques for measuring the power and frequency of the Electron Laser Facility (ELF)-II free-electron laser (FEL) used for plasma heating experiments on the Microwave Tokamak Experiment (MTX) have been developed. A multichannel 140-GHz receiver capable of measuring FEL power levels from 10 mW to 0.1 μW within an accuracy of ±1 dB and with a 50-dB dynamic range and a 2-ns response time has been designed. By using calibrated attenuators, it is possible to measure power levels from 10 GW to 0.1 μW. The microwave output of the FEL in a microwave load tank is sampled by using WR-8 or WR-28 stub waveguide antennas. Microwave turning mirrors are used to guide the microwave beam down an evacuated beam tube to the MTX. Stub, WR-8, fundamental-mode waveguide antennas are used for beam detection on the microwave turning mirrors. Orthogonal, WR-8, stub waveguides are machined into the surfaces of the mirrors and used as directional couplers to measure forward and reflected power from the FEL. The microwave power is then transported to the microwave receiver by means of a low-loss, over-moded, WR-28 waveguide. A movable modes probe in the microwave load tank is used to scan across the microwave beam to determine the modes content of the beam. Frequency stability of the FEL is measured with a fast, frequency-modulation detector (FFMD) capable of measuring frequency shifts and modulation on a 2- to 4-ns time frame.