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, HeNe: 0.63 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.

References

[1] H. Lamela et al, Laser interferometric experiments for the TJ-II stellarator electron-density measurements, Rev. Sci. Instrum. 72, 96 (2001)

[2] M. Sánchez at al, High resolution CO₂ interferometry on the TJ-II stellarator by using an ADC-based phase meter, Rev. Sci. Instrum. 75, 3414 (2004)

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