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

Abstract

In order to improve both the density and particularly the temporal resolution beyond previous dispersion interferometers (DI), a heterodyne technique based on an acousto-optic (AO) cell has been added to the DI. A 40 MHz drive frequency for the AO cell allows density fluctuation measurements into the MHz range. A CO2 laser based heterodyne DI (HDI) installed on DIII-D has demonstrated that the HDI is capable of tracking the density evolution throughout DIII-D discharges, including disruption events and other rapid transient phenomena. The data also show good agreement with independent density measurements obtained with the existing DIII-D two-color interferometer. The HDI line-integrated density resolution sampled over a 1s interval is ~9×10 17 m-2. Density fluctuations induced by MHD instabilities are also successfully measured by the HDI.

Recent Progress on dispersion interferometers for nuclear fusion and low-temperature plasmas

“…Regardless of such a long beam transmission, alignment with the IR camera enables sufficiently good overlapping. As a result, vibration phase shifts were well cancelled even at the startup of the ohmic solenoid coil and when large vibrations occur [22]. The reported line-integrated density resolution is 9 × 10 17 m -2 , defined as the standard deviation over a 1 s interval.…”

“…For the phase matching, one uses conventional birefringence of a nonlinear material, the other uses the periodic domain inversion structure. For 10 mm wavelength, the former used for the DIs are ZnGeP2 [11], AgGaSe2 [13][14][15][16][17][18], and AgGaS the latter is OP-GaAs [21][22][27][28]. One of the advantages of former is that they are easy to use and suitable for use at room temperature.…”

“…3.3. Considering power loss along the long beam path such as 100 m on DIII-D [22] and ITER [28], the higher efficiency is preferable for the heterodyne DI. Therefore, OP-GaAs is presently the best candidate for the phase-modulated DI on ITER.…”

“…For the purpose of demonstration of wide bandwidth density measurement with the DI, a heterodyne DI was developed on DIII-D [22]. As discussed in Sec.…”

Study for a tangential dispersion interferometer/polarimeter for DTT

“…The solution proposed for this diagnostic is the second harmonic dispersion interferometer (DI) [2][3][4][5] which has the advantage to be quite insensitive to vibrations. This is a common-path interferometer based on the generation of an higher harmonic, in this case the second harmonic λ/2, which crosses the plasma collinearly with the beam at its fundamental λ.…”