TJ-II:Interferometry

TJ-II has two interferometry systems for the measurement of the line-integrated density.

The Microwave interferometer

 
Diagram showing the geometry of the microwave interferometer at TJ-II

The diagnostic is located in sector B8 (φ = 264.4°). The probing beam has an inclination of 18.7° with respect to the vertical and a frequency of 140 GHz, corresponding to a wavelength of λ = 2.14 mm. The line-integrated electron density is deduced from the cumulative phase change of the probing beam and the theoretical length of the intersection of the probing beam with the plasma (obtained from the magnetic geometry in vacuum).

Data description

The diagnostic delivers a time trace of the line-integrated electron density (65536 points, sampling rate 100 kHz). In the TJ-II database, the raw data are called 'INTERI', 'INTERQ', and the processed data 'Densidad2_' (units: 1019 m-3). The signal is less reliable for high densities (above 2-3 1019 m-3) due to the cumulative effect of spurious phase jumps. This time trace is fundamental for machine operation.

Two-colour interferometer

The double wavelength heterodyne interferometer (CO2: 10.6 μm, NdYAG: 1.064 μm) provides values of the plasma line integral density in scenarios with average densities ranging from a few times 1018 m–3 to more than 5×1019 m–3. The results have been compared with those from the 2 mm (microwave) interferometer, obtaining an excellent agreement. Working with a final bandwidth of 4 kHz, a one pass line integral error level about ±2×1017 m–2 has been achieved. [1] [2] [3] [4] [5] [6] [7] The system is located in sector B1.

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