Two-wavelength millimeter wave “unambiguous” heterodyne interferometer

The paper presents the results of upgrading and description a two-wavelength “unambiguous” microwave interferometer for line integrated electron density measurements at the “COMPASS” tokamak. The probing signals in the frequency range of 131 and 133 GHz are generated by IMPATT diode oscillators. Double conversion heterodyne receivers are used for detection. Use of each oscillator is twofold: as a source for the probing signals and as a local oscillator for heterodyne detection. The plasma is probed with two separate waves of different wavelengths traveling along identical paths to meet each other. Each probing beam experiences a different phase shift φ₁ and φ₂ and the “unambiguous” interferometer determines the difference φ₁-φ₂. The phase measurement is realized by using the 3 pieces of phase detectors based on pairs of phase/gain evaluation boards (AD8302). Application of the phase detectors provides information on the average plasma concentration in real time and restore the values of plasma density in the case of phase jumps by an amount > 2π.