Design of a dispersion interferometer combined with a polarimeter to increase the electron density measurement reliability on ITER

Design of a dispersion interferometer on a field-reversed configuration device

Dispersion interferometry (DI) is a promising method for density measurement. Compared with the traditional interferometer, the DI is immune to mechanical vibration and can avoid the fringe jump error. In addition, a simple optical configuration is also one of the advantages of the DI. The electron density of the Huazhong University of Science and Technology field-reversed configuration (HFRC) device can reach 10²⁰ m^-3 with a pulse length of 50 µs. In this case, the DI based on the CO₂ laser on the HFRC device adopts the heterodyne technique based on the acousto-optic modulator, which can increase the temporal resolution to 40 MHz. It can realize density fluctuation measurements in the MHz range. The test of each optical element, especially the nonlinear crystal, has been completed. The AgGaSe₂ crystal can produce a second harmonic wave of about 52.5 µW when the incident CO₂ power is 10 W. Based on these designs and tests, a DI system can be expected on the HFRC device.

Bench test of phase measurement on dispersion interferometer for EAST

In recent years, different traditional interferometers have been the necessary diagnostic of electronic density measurement on fusion devices. Until now, two main problems always influence the density measurement: the mechanical vibration and fringe jump in the calculation. The dispersion interferometer (DI) with a long-wavelength infrared wavelength is a good choice because mechanical vibrations can be canceled and the fringe jump can be inhibited. This paper describes the bench test of phase measurement using a wedge instead of plasma on the DI. The results show good agreement with the theoretical calculations. In the background measurement, this DI without a vibration isolation system has good performance, and the drift of the baseline is less than 2 × 10¹⁷ m^-2 in 3 s and less than 5 × 10¹⁷ m^-2 in 400 s. Plasma data will be obtained during the next campaign on EAST (Experimental and Advanced Superconducting Tokamak).

Advanced Helical Plasma Research towards a Steady-State Fusion Reactor by Deuterium Experiments in Large Helical Device

The Large Helical Device (LHD) is one of the world’s largest superconducting helical system fusion-experiment devices. Since the start of experiments in 1998, it has expanded its parameter regime. It has also demonstrated world-leading steady-state operation. Based on this progress, the LHD has moved on to the advanced research phase, that is, deuterium experiment, which started in March 2017. During the first deuterium experiment campaign, an ion temperature of 10 keV was achieved. This was a milestone in helical systems research: demonstrating one of the conditions for fusion. All of this progress and increased understanding have provided the basis for designing an LHD-type steady-state helical fusion reactor. Moreover, LHD plasmas have been utilized not only for fusion research, but also for diagnostics development and applications in wide-ranging plasma research. A few examples of such contributions of LHD plasmas (spectroscopic study and the development of a new type of interferometer) are introduced in this paper.

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

In order to improve both the density and particularly the temporal resolution beyond previous dispersion interferometers (DIs), 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 1 s interval is ∼9 × 10¹⁷ m^-2. Density fluctuations induced by MHD instabilities are also successfully measured by the HDI.