The line integrated density measurement by using a frequency sweep interferometer

Abstract

Frequency modulated continuous wave (FMCW) reflectometer has been widely used to measure the plasma density profile by launching linear frequency modulated microwave into the plasma and measuring the group delay of the microwave reflected at the cutoff density. For a certain frequency range, the microwave just passes through the plasma and reflects back from the vacuum inner wall. By analyzing the phase information of the passing-through microwave, the line integrated density (LID) can be obtained. An FMCW reflectometer operating in this particular regime is called a frequency sweep interferometer (FSI). If the microwave frequency is much higher than the plasma frequency, the group delay of FSI is directly proportional to the LID. This is why a THz interferometer is needed for high density plasma diagnostics in fusion experiments. For a frequency range slightly above the plasma frequency, the FSI measurements depend not only on the LID but also on the details of the plasma density profile. It is analytically shown that the group delay of FSI can be expanded as a Taylor series of inverse frequency squares. The coefficients of the Taylor series correspond to the multiple moments of LID. Therefore LID can be obtained by polynomial fitting the group delay as a function of inverse frequency squares even if the microwave frequency is not much higher than the plasma frequency. This algorithm is tested by simulating the response of the FSI to the given density profile as sweeping the microwave frequency over wide ranges. This algorithm works well even if the microwave frequency is just 3–4 times higher than the plasma frequency and it can be applied for the steady state plasmas.