TFTR 1mm Density Interferometer

A primary control diagnostic instrument on the Princeton Tokamak Fusion Test Reactor (TFTR) will be a 1mm double-pass interferometer. The interferometer will provide: (a) line average electron density measurements for densities up to 2 × 1014 cm−3, (b) a permissive signal for control of neutral beam firing, and (c) an input signal for the gas feedback controlling system that regulates the electron density level. A 1mm wavelength was selected because of the following considerations: 1. A reliable 1 watt carcinotron source is available. 2. The cut-off density for the probing frequency, 1015 cm−3, is a factor of 3 above the highest average density to be measured. 3. At 1mm wavelength refraction problems are not severe along the available horizontal mid-plane transmission path (i.e., along the density gradient). 4. Low-loss in over-sized waveguide allows the interferometer to be located remotely from the tokamak and thus minimize environmental problems. 5. The interferometer is relatively insensitive to vibration.