TJ-II:Interferometry
TJ-II has two interferometry systems for the measurement of the line-integrated density.
The Microwave interferometer
The diagnostic is located in sector B8. The probing beam has an inclination of 18.7º with respect to the vertical and a frequency of 140 GHz, corresponding to a wavelength of λ = 2.14 mm. The diagnostic provides a time trace of the line-integrated electron density (65536 points, sampling rate 100 kHz). The time trace is called 'Densidad2_' in the TJ-II database. The signal is less reliable for high densities (above 2-3 1019 m-3). 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] The system is located in sector B1.