A Fixed Frequency Reflectometer to Measure Density Fluctuations at Aditya Tokamak

Abstract

Amongst modern diagnostics of fusion plasmas, microwave methods, both passive and active, play an important role. Microwave Reflectometer is used to measure the plasma density and its fluctuations in fusion research device like tokamak. A fixed frequency (O – mode) microwave reflectometer at 22 GHz (cut – off density nc = 6 × 1012 cm-3) has been designed, developed and used to measure the critical density layer and its fluctuations in Aditya tokamak. It can measure the plasma density fluctuations from r = 11 to 22 cm for central electron density 7.5 × 1012 cm-3 and more, respectively. The output signal of reflectometer is analyzed and compared with the density measurement from the microwave interferometer. When the density measured by interferometer is constant, then the fluctuations of local density are seen from the reflectometer signal. Analysis of initial results show that density fluctuation at r = 21 cm in the main plasma has correlation time of about 8 μsec and frequency spectrum is broad. Use of 22 GHz incident wave allows the observation of density fluctuation with wave number in the range of 0 – 9.2 cm-1 from the reflecting region at the receiving horn. Radial variation of the fluctuation level is observed from 5% to 22% for minor radius 11 to 22 cm, respectively.