Design and Development of Microwave Interferometer and Reflectometer Systems for Plasma Diagnostics in Tokamak

Abstract

The microwave interferometry and reflectometry have been extensively used to measure electron density and its fluctuations in various tokamaks. The tokamak has become the leading research tool in the world-wide effort to develop valuable electrical power based on controlled thermonuclear fusion. This is the most successful scheme for confining plasma magnetically. The present work aims to design and development of microwave interferometer and microwave reflectometer systems for plasma diagnostics in tokamak. A 100 GHz microwave interferometer with seven-channel is designed, established and used for the measurement of the radial profile of the electron density (𝑛𝑒 ) of plasma in Aditya Tokamak. The resolution of measurement of the density is 𝑛𝑒 = 2.0 × 1011𝑐𝑚−3. The time resolutions and spatial are 10 μs and 7 cm and, respectively. The lineaveraged measurements of plasma density in Aditya Tokamak from seven channels are carried out. The radial profile of the plasma density is obtained by using Abel inversion. The density measured by this interferometer is well matched with the measurement of the Thomson scattering diagnostics. A 3mm wave interferometer was designed and developed to measure the plasma density online with the central chord of the Aditya Tokamak. A real-time processing method for fringe signals with a hardware logic up/down-counting circuit that converts fringes to phase shift in units of 𝜋 radians was developed. The microwave circuit of the central chord of a homodyne interferometer system is modified to make a quadrature circuit by using phase shifters and magic tees. This is used to produce the sine/cosine fringe signals. These outputs are converted into pulses and passed to wired logic up/down fringe counter. Digital synchronous logic circuit is implemented in a CPLD, followed by DAC and scaler, which produce a voltage proportional to increase or decrease in plasma density in real time. Initially, fringe counter is tested with function generator and then with a prototype quadrature interferometer in laboratory. Finally, fringe counter is used in interferometer at Aditya tokamak. The line-averaged plasma density 𝑛𝑒 = 0.9 × 1013𝑐𝑚−3 is measured online in the Aditya Tokamak. Magnetohydrodynamics (MHD) has been studied using microwave interferometer system. Sawtooth and MHD activities are observed in many discharges in the signal of 100 GHz interferometer in Aditya Tokamak. Two types of discharges have been found, one with prominent mode of a single frequency and the other with modes of multiple frequencies during the current flat top of discharge. Time frequency and the bispectral analysis of the interferometer signals in these discharges allow mode coupling studies and its role in sustenance of plasma discharges and its disruption. Role of mode coupling in studying the disruption phenomena in the discharges at Aditya Tokamak is presented using microwave interferometer diagnostics. A microwave reflectometer with fixed-frequency (O-mode) at 22 GHz (nc = 6 10 12 cm -3 ) is planned and developed. It is used in the measurement of plasma density fluctuations. The measurement of the fluctuations have been done from r=11 to 22 cm for different central densities of plasma by this reflectometer. The signal measured by the reflectometer is studied and compared with the microwave interferometer signal. The measured autocorrelation time of the signal of reflectometer is ~8 μs. The power-spectrum analysis demonstrates that the frequency spectrum is broadband. The incident wave of 22 GHz allows studies of density fluctuations with wave number up to 9.2 cm-1 from the reflecting region. The variation of density fluctuation level from 5 to 22% is found for the minor radius 11 to 22 cm.