Description
Accurate measurements of plasma electron density can be challenging for long running fusion reactions as complete temporal information from startup is necessary. Furthermore traditional techniques can introduce errors due to fringe jumping. Here we present a prototype interferometer that can measure relative phase. It is based on dual frequency comb spectroscopy with quantum cascade lasers at the wavelength of ~8 um. This method relies on phase differences between neighbouring comb lines eliminating the need for pervious plasma electron density knowledge and minimizing fringe jumping errors. The instrument performance was verified: 1) by measuring the phase induced by a material with known absorption characteristics (polypropylene). The frequency comb phase shift after propagating through a sample with both negative and positive phase gradients was measured and showed good correlation with the expected one calculated using the known refractive index and absorption characteristics of this material. 2) by measuring the relative phase change induced by two counter rotating plane-parallel plates. The measurements were taken by adjusting the tilting angle of plane-parallel plates. Good correlation between the measured slope of the phase shift w.rt. the wavenumber and that simulated with the known dispersion parameters of the plates was demonstrated. The noise of this system was evaluated. The phase error occurs due to comb stability, strength and the phase drift between the two frequency comb sources. This phase error combined with the frequency of each comb line was used to determine the plasma electron density error. With this suggested instrument the error associated with the line integrated plasma electron density is in the order of $10^{17} m^{-3}$, with an upper limit measured as $1.3×10^{27} m^{-3}$. The cut-off plasma density is $4.4×10^{23} m^{-3}$. This instrument was tested in the presence of vibrations and show no significant change to either the transmission or phase measurements. The results of measurement and simulation demonstrate the validity of the built instrument for reliable phase shift measurements applicability for use in long running fusion reactions necessary for powerplant operations.