Influence of refraction on plasma density measurements

Abstract

The plasma density profile in the T-11M tokamak is measured with a multichannel Cotton-Mouton polarimeter and also with a time-of-flight refractometer. Both instruments operate at frequencies in the range of 140 GHz. Since, in this case, the trajectory of the probing microwave beam deviates from a straight line, the maximum refraction angle being as large as ∼0.25 rad, the measurement results should be corrected for refraction. In order to solve this problem, a computer code has been developed for calculating the trajectories of both O-and X-mode probing beams. In calculations, it is assumed that the plasma is cold and that the inequality ω ≫ 2πνp (where ω is the probing frequency and νp is the collision frequency) is satisfied. The wavelength λ is assumed to be much less than the plasma inhomogeneity scale length, so the calculations are performed in the geometric optics approximation. The dependences of the phase difference ΔΦ (for the Cotton-Mouton polarimeter) and the propagation time of a microwave pulse (for the time-of-flight refractometer) on the linear plasma density 〈nl〉 are calculated. For comparison, similar dependences are calculated for a conventional interferometer operating at the same frequency. It is shown that, in the latter case, the influence of refraction on the measurement results is about three times less. The dependences obtained are presently being used in measurements of 〈nl〉 by means of the Cotton-Mouton polarimeter and the time-of-flight refractometer with allowance for the density profiles recovered by abelianizing the data from the multichannel polarimeter (a series of iterative Abel inversion procedures are performed for each instant of time). The code can also be used to find corrections for refraction when performing interferometric measurements in other devices.