CO2 Laser Polarimeter

Measurement of line averaged electron density along a tangential central chord in JT-60U plasmas.

[Diagnostic Method]
The polarimeter measures the Faraday rotation angles of a linearly polarized CO2 laser wave. Along a toroidally tangential chord, the Faraday rotation angle is approximately proportional to neBt//dl, where ne is the electron density and Bt// is the parallel component of the toroidal magnetic field to the diagnostic chord. Since the Bt// is easily known, ne can be obtained. In principle, polarimetry is more reliable than conventional interferometry because polarimetry does not suffer from fringe jump.

(1) Toroidal tangent line of sight
For the central density measurement, a tangential beam line in the vacuum vessel is selected.
(2) Detection of polarization angle
A couple of photoelastic modulators (PEMs) and a polarizer plate are utilized to detect a polarization angle of a CO2 laser wave.
(3) Simultaneous measurement with CO2 laser interferometry
An identical probing laser beam is utilized for both polarimetry and interferometry. Cross calibration between two methods is easily available.
(4) Two-color polarimetry concept
In order to eliminate the Faraday rotation component at vacuum windows from measured signal, two-color polarimetry concept has been developed. Two polarimeters with different probing laser wavelength of 10.6 and 9.27 m are installed.
(5) Dimond window
In order to reduce the Faraday rotation component at vacuum windows, CVD diamond windows are installed as vacuum windows. Their low Verdet constant and thin thickness results in that the Faraday rotation component at vacuum windows is negligible.

-- PEM modulation frequencies : 37 kHz and 50 kHz
-- Temporal resolution : 4 ms (Standard)
-- Density resolution : ~3x1018 m-3
-- Number of chord : 1 chord (tangential), tangent radius =3.11 m
-- Port : P8 & P13 section

(1) Y. Kawano, et al., "Infrared laser polarimetry for electron density measurement in tokamak plasmas", Rev. Sci. Instrum. 72, 1068 (2001).
(2) Y. Kawano et al., "Application of diamond window for infrared laser diagnostics in a tokamak device", Rev. Sci. Instrum. 75, 279(2004).