CO2 Laser Interferometer

Measurement of line electron density at central region of JT-60U plasmas.

[Diagnositic Method]
The interferometer measures the phase shifts of electromagnetic waves of CO2 laser beam. Usually, the phase shift is originated from change in electron density of a plasma and change in the optical path length of the laser beam. In order to know electron density component, two CO2 laser oscillators which wavelength are different as 10.6 and 9.27 m are utilized for a compensation of change in optical path length.

(1) Toroidal tangent line of sight
For the central density measurement, a tangential beam line in the vaccum vessel is introduced.

(2) Two sets of CO2 laser interferometers
The setup of two interferometers are mostly same to minimize their optical path difference.

(3) Common path mode matching
Three different wavelength laser beams (10.6, 9.27, and 0.633 m) apropagated for about 50 m in similar Gaussian shapes in each other.

(4) Frequency shift of different wavelength lasers by a single AOM
A single acoust-optic modulator (AOM) is used in common for lasers of 10.6 and 9.27 m to produce identical IF frequency of heterodyn signal detection.

(5) Development of Very high-Resolution Phase Comparator (VRPC)
The VRPC has been developed for precise density measurement. Phase resolution of the VRPC (2/12800 rad) is more than hundred times better than that of the standard phase comparator (2/100 rad) which is routinely used.

-- IF frequency : 2 MHz
-- Time resolution : 5 s and 1 ms
-- Phase Comparator : 1/100 fringe resolution
-- Density resolution : ~0.5x1019 m~2 (1 ms)
~2x1019 m-2 (5 s)
-- Number of chord : 1 chord (tangential)
-- Port : P8 & P13 section
-- Position : tangent radius =3.11 m

(1) Y. Kawano, et. al., "Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27m for electron density measurement on large tokamaks", Rev. Sci. Instrum. 67, 1520 (1996).