Fig. 2-6 Compensation for optical path length changes using close-wavelength dual laser interferometry, and time variation of electron density during a major disruption in JT-60 (a) The traces show phase signals obtained by close-wavelength dual laser interferometry, for a case where the mechanical vibration signal is of the order of ten times larger than the density signal. The line electron density is obtained after correction (the bottom trace) and the phase shift here is very small, corresponding to less than a half a wavelength at its maximum. Closed circles show electron densities obtained by Thomson scattering measurements. The average electron density in m-3 is obtained by dividing one line density by the laser path length in the plasma. (b) We now have a means of measuring a very fast change in the electron density, which will facilitate detailed studies of disruption phenomena.

The electron density of a plasma can be measured by laser interferometry, but large tokamak experiments often suffer from the superposition of a large, spurious vibration upon the phase signal of the density changes. The spurious signal is due to mechanical vibrations of the optical system caused by tokamak operation, and it may be up to 100 times larger than the phase signal that we wish to measure. Simultaneous measurement using a combination of different laser wavelengths is therefore required to measure fast density variations correctly (Fig. 2-6 (a)), and for this purpose we have recently developed a dual carbon dioxide (CO2) laser interferometry system for JT-60, that uses two close wavelengths of 10.6 micrometer and 9.27 micrometer. This close-wavelength combination can be readily realized using common optical components and a simplified optical layout, but unfortunately the close wavelength combination causes a reduction in the density resolution. In order to overcome this problem, we have developed a very high resolution phase comparator that has a phase resolution corresponding to 10-4 of a wavelength. Because of these technical developments, the electron density behavior can now be measured with high accuracy in JT-60 even during a fast major disruption (Fig. 2-6 (b)). On the basis of the feasibility obtained in JT-60, we are confident that the developed system can be used for future large devices such as ITER.

Reference Y. Kawamoto et al., Dual CO2 Laser Interferometer with a Wavelength Combination of 10.6 and 9.27 micrometer for Electron Density Measurement on Large Tokamaks, Rev. Sci. Instrum., 67, 1520 (1996).

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