Design and implementation of a 150 GHz single-channel millimeter wave interferometer on Joint TEXT tokamak

A simple, single-channel millimeter-wave interferometer system has been designed, fabricated, and installed on the J-TEXT tokamak. For the plasma density anticipated on J-TEXT, a solid-state source operating at 150 GHz has been chosen to minimize errors due to both vibration along the beam path and refraction in the plasma. The new aspect of the interferometer design is to use a subharmonic mixer for detection with a frequency doubled 150 GHz source. It employs a single source which is bias-tuned and modulated with a sawtooth wave form up to 100 kHz in order to generate the intermediate frequency. The 12.5 GHz voltage-controlled oscillator is multiplied to 75 GHz before a final doubler raises it to 150 GHz. A portion of the 75 GHZ power is used for the local oscillator (LO) and is directly connected to the LO input of the subharmonic mixer. The phase is evaluated by a digital phase comparator using a software-based algorithm. Detection noise limits the minimum resolvable phase change with the interferometer to ±0.05 fringe, which corresponds to an averaged electron density change along the chord of ±1.1 × 10(17) m(-2). The maximum measurable electron density is expected to be ∼9 × 10(19) m(-3). A comparison of preliminary results from the millimeter wave interferometer with that from the far-infrared hydrogen cyanide laser (wavelength of 337 μm) interferometer shows good agreement during the pulse flat-top period. The millimeter wave interferometer system will be used as a part of the density feedback control system in the future.