Tests of a two-color interferometer and polarimeter for ITER density measurements

Abstract

A full-scale 120m path length ITER toroidal interferometer and polarimeter (TIP) prototype, including an active feedback alignment system, has been constructed and undergone initial testing at General Atomics. In the TIP prototype, two-color interferometry is carried out at 10.59 μm and 5.22μm using a CO 2 and quantum cascade laser (QCL) respectively while a separate polarimetry measurement of the plasma induced Faraday effect is made at 10.59μm. The polarimeter system uses co-linear right and left-hand circularly polarized beams upshifted by 40 and 44 MHz acoustooptic cells respectively, to generate the necessary beat signal for heterodyne phase detection, while interferometry measurements are carried out at both 40 MHz and 44 MHz for the CO 2 laser and 40 MHz for the QCL. The high-resolution phase information is obtained using an all-digital FPGA based phase demodulation scheme and precision clock source. The TIP prototype is equipped with a piezo tip/tilt stage active feedback alignment system responsible for minimizing noise in the measurement and keeping the TIP diagnostic aligned indefinitely on its 120 m beam path including as the ITER vessel is brought from ambient to operating temperatures. The prototype beam path incorporates translation stages to simulate ITER motion through a bake cycle as well as other sources of motion or misalignment. Even in the presence of significant motion, the TIP prototype is able to meet ITER's density measurement requirements over 1000s shot durations with demonstrated phase resolution of 0.06°and 1.5°for the polarimeter and vibration compensated interferometer respectively. TIP vibration compensated interferometer measurements of a plasma have also been made in a pulsed radio frequency device and show a line-integrated density resolution of d = ńL 3.5 10 17 m −2 .

A heterodyne dispersion interferometer for wide bandwidth density measurements on DIII-D

“…One possible reason for the drift is inhomogeneities of the air temperature or the density around the AO cell. Since the beam paths of the fundamental and the second harmonic components are different around the AO cell, relative phase shifts in this portion (around 60 cmlong) cannot be cancelled without adding a second reference detector as in Reference [1]. As discussed in [1], the other possibility is amount of water vapor, which determines the refractive index and dispersion of the air.…”

“…Since the beam paths of the fundamental and the second harmonic components are different around the AO cell, relative phase shifts in this portion (around 60 cmlong) cannot be cancelled without adding a second reference detector as in Reference [1]. As discussed in [1], the other possibility is amount of water vapor, which determines the refractive index and dispersion of the air. The 5.3 µm second harmonic component is close to a water vapor absorption line and the refractive index of the air at 5.3 µm can be strongly affected by changes of air humidity induced by the air conditioner.…”

“…Unfortunately, as discussed above, the phase shift caused by mechanical vibrations become larger for the shorter wavelengths and can dominate that from the plasma. To overcome this issue, and actually measure both the plasma and vibration induced phase shift, a two-color interferometer which uses two laser sources with different wavelengths sharing the same line of sight in a plasma is usually adopted 1 . Based on the different dependencies on the wavelength, the phase shifts caused by the plasma and that from mechanical vibrations can be separated by resolving separately the phase shifts at each wavelength.…”

“…In this approach, a phase modulation is added to one wavelength component with an electro-optic (EO) cell or a photoelastic modulator (PEM) so that the phase shift can be evaluated independently of the detected intensity. The achieved density and the time resolution with this approach are typically sufficient for density control in fusion devices (~50 kHz) and, in fact, this DI is being proposed to ITER as a backup system 9 to toroidal interferometer polarimeter (TIP) 1 . Although the modulation frequency of EO cells is high, at 250 kHz, thermal instability of EO cells for infrared wavelengths has been reported 4 .…”

Compensation of phase drifts caused by ambient humidity, temperature and pressure changes for continuously operating interferometers

“…To first order it is only sensitive to the dispersion the traversed medium and the wavelength of the employed laser beam. And although vibrations do affect the measured phase, advanced techniques such as dispersion interferometry or the well-established two-color interferometry have successfully eliminated their effect [6][7][8][9][10].…”