1 - Introduction

A far infrared (FIR) interferometer is used on the FTU tokamak for measuring the electron plasma density. The density is one of the fundamental parameters of the plasma and must be measured reliably for the experiments on FTU. The line averanged density is measured by detecting the phase shift of an electromagnetic wave crossing the plasma.

FIR radiation must be used for this purpose because only at these wavelengths can the refractive index of the plasma be measured; at longer wavelengths strong refraction would take place.

The system also provides a polarimetric measurement of the toroidal plasma current profile.

System setup
The system consist of two lasers, the interferometer structure and the detectors. The molecular discharge lasers have been developed by C.E.A. (France) and built under license by Bertin (France). The lasers can operate at 337 um (HCN), 194 um (DCN) or at 119 um (H2O), depending on the gas used in the discharge. Of the two lasers, one is used operationally, while the other serves as backup for the density measurements, during maintenance or repair of the first.

The optical elements of the interferometer are supported by a large frame surrounding the tokamak. Since the measurements are affected by mechanical vibrations, the frame must be very stable during a tokamak discharge. The magnitude of the required stability can be estimated by noting that a path variation equal to one wavelength of 337 um (HCN), corresponds to a line density of 1.1 exp 19 e/m, while the minimum line density, obtained on FTU is about 2x1019 m-3. One general design consideration was the limitation on the size of the interferometer imposed by the extreme compactness of the FTU machine. Every effort has been made to ensure maximum rigidity. The frame design was optimized by use of a computer finite element analysis and the structure itself was subjected to static and dynamic tests to verify the design data. Stainless steel tubing has been used for linear elements and honeycomb fiberglass for flat elements. Special attention was devoted to avoiding electrical loops or the use of magnetic materials.

Optical system
The output radiation from the laser is directed to the top of the supporting frame by a complex mirror system that prevents diffraction losses from the beam in the tokamak volume. Gaussian beam optics have been used in the optical design. A set of quartz beam splitters forms the five signal and reference channels. The number of channels is limited to five in order to avoid cross talk due to the bending of the beams by plasma refraction. After passing through the plasma, the beams are mixed with reference beams and carried to the detector by a glass wave guide. The total path length from laser to detector is about 30 m. The reference beam is frequency shifted 10 kHz by a rotating grating. This phase modulation scheme makes it possible to convert the phase shift to a time delay so that the measurement is not affected by amplitude fluctuations. The optical path must be enclosed in a moisture free atmosphere, because radiation at 194 um is strongly absorbed by ambient water vapor. The optical system consists of about 200 remotely controlled elements, including mirrors, quartz beam splitters, and wire grid polarizers. Where the magnetic field is strong, pneumatic step motors are used (C.E.A. design). Remote control is necessary because the mirrors cannot be reached manually. Furthermore, it facilitates alignment.

The detector is a fast electron bolometer cooled to liquid helium temperature (4.2 K). The sensitive element is high purity Indium Antimonide (InSb) whose resistivity decreases when it is exposed to FIR radiation. Each element is mounted at the bottom of a 9 mm gold plated stainless steel waveguide about 1m in length. The six detector element are assembled together in a conventional 60 liter storage vessel, so as to form a six channel unit. The helium evaporation rate is about one liter/day and the sensitivity at 337 micron is about 2 V/W with the detector noise negligible compared to that of the preamplifiers (10 nV/ÌHz)

The six channel detector was completely developed and built in our laboratory, beginning from InSb monocrystalline material.


A view of large frame that supports the optical elements of the interferometer.

The mirror and beam splitter assembly of the interferometer.

The mirror assembly of the glass waveguide.

The part of the supporting frame located in the tokamak hall.

The six channel In-Sb detector.

A typical density measurement in two channels of the interferometer.

FTU Diagnostics