A scanning beam interferometer installed on the Frascati tokamak upgrade (FTU) experiment is presented. The scanning beam scheme combined with the small dimensions of the beams produces a system with very high spatial resolution: more than 30 adjacent
(nonoverlapping) chords sample most of the plasma cross section.
A good time resolution is achieved by the use of a proper scanning device, with a scanning frequency
. Very fast events are measured by three additional fixed lines of sight providing a time resolution
. The instrument is a two-color medium-infrared-compensated-type interferometer; two wavelengths (colors)
are used to measure both the density and the mechanical vibrations of optical components. A
is the main light source,
and a CO laser
is the compensation one. The optical scheme is a double pass Mach–Zehnder type. All the retroreflector mirrors are mounted directly on the FTU mechanical structure thanks to the compensation system that allows for large vibration amplitudes of optical components. Heterodyne detection at 30 and
is obtained by frequency shifting the reference beams with two acousto-optic modulators (Bragg cells). Many features are implemented to achieve high measurement accuracy and reliability.
A real-time system computes the integral density measured on one of the fixed lines of sight and provides an analog signal for density feedback control. The interferometer was used to measure density profiles both in medium-density discharges
and in high-density pellet injected discharges
The measurement error is
under optimal conditions but can be higher in some cases, mainly because of the large tilt of the retroreflector mirrors.
© 2006 Optical Society of America
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(130.6750) Integrated optics : Systems
Original Manuscript: March 24, 2006
Revised Manuscript: August 11, 2006
Manuscript Accepted: August 21, 2006
A. Canton, P. Innocente, and O. Tudisco, "Two-color medium-infrared scanning interferometer for the Frascati tokamak upgrade fusion test device," Appl. Opt. 45, 9105-9114 (2006)