Development of an E-band frequency-hopping microwave reflectometer on the HL-3 tokamak

To locally measure the density fluctuations in the plasma core on the HL-3 tokamak, an E-band (60–90 GHz) frequency-hopping microwave reflectometer was developed. The output frequency of the reflectometer can be tuned arbitrarily between 60 and 90 GHz, though a minimum period of 10 ms is required for each frequency hop. The radial resolution of frequency-hopping microwave reflectometer can be < 1 cm. The reflectometer consists of a frequency-hopping source and intermediate frequency electronics. The frequency-hopping source contains a control voltage module that can linearly output a voltage according to the radio frequency. Laboratory measurements reveal that the relative error of the frequency-hopping source’s output power is below 1.2%, and the phase noise level of the frequency-hopping source meets the measurement requirements. The intermediate frequency electronics are used to launch a high-power microwave into plasma and obtain intermediate frequency signal with low conversion loss. The newly developed reflectometer was successfully applied to high-density discharges on HL-3 tokamak. A sudden increase in the density fluctuations is observed with the microwave reflectometer when the plasma density is close to the Greenward limit. Based on the output frequency of the reflectometer, it can be determined that the sudden increase in density perturbation occurs at least within the normalized minor radius ranging from approximately 0.56 to 0.67.