A far-infrared ‘polari-interferometer’ for simultaneous electron density and magnetic field measurements in plasmas

A laser-based far-infrared polarimeter-interferometer system utilizing the three-wave technique has been implemented on the J-TEXT tokamak. The polarimeter determines the Faraday effect by measuring the phase difference between two collinear, counter-rotating, circularly polarized laser beams. The first results of the polarimeter-interferometer designed for J-TEXT have been obtained in the most recent J-TEXT experimental campaign. Simultaneous polarimetric and interferometric measurement is achieved, with phase resolution up to 0.1°, at bandwidth of 50 kHz. The temporal resolution, which is dependent on the laser's frequency offset, is ∼1 μs. Continual spatial measurement covering 45 cm (80% of the plasma cross-section) is realized by utilizing 1D parabolic beam expansion optics. Three initial test chords are installed and future plans call for expansion up to 30 chords with 1.5 cm chord spacing, providing high spatial resolution for measurement of electron density and current density profiles. Reliability of both polarimetric and interferometric measurement is confirmed by comparison with computation and data from a hydrocyanic acid (HCN) interferometer. With the high temporal and phase resolution, perturbations associated with the sawtooth cycle and MHD activity have been observed.

Section: J-text Polarismentioning

confidence: 99%

First results from the J-TEXT high-resolution three-wave polarimeter-interferometer

Chen

Zhuang

Wang

et al. 2012

“…15 The method has a high temporal resolution, 3 which can measure magnetic fluctuations. 16 Polarization detection using photoelastic modulators ͑PEMs͒ is one of the alternative methods to measure the Faraday rotation angle.…”

Section: Introductionmentioning

confidence: 99%

Short wavelength far infrared laser polarimeter with silicon photoelastic modulators

Akiyama

Kawahata

Tanaka

et al. 2008

A short wavelength far infrared laser whose wavelength is about 50 m is preferable for a polarimeter and an interferometer for high density operations in the Large Helical Device ͑LHD͒ and on future large fusion devices such as ITER. This is because the beam bending effect ͑ϰ 2 ͒ in a plasma, which causes fringe jump errors, is small and the Faraday and the Cotton-Mouton effects are moderate. We have developed a polarimeter with highly resistive silicon photoelastic modulators ͑PEMs͒ for the CH 3 OD laser ͑ = 57.2 and 47.7 m͒. We performed bench tests of the polarimeter with a dual PEM and demonstrated the feasibility for the polarimeter. Good linearity between actual and evaluated polarization angles is achieved with an angular resolution of 0.05°and a temporal resolution of 1 ms. The baseline drift of the polarization angle is about 0.1°for 1000 s.

“…Since 1970, several different techniques have been proposed for polarimetric measurement and one of those techniques, the three-wave technique, a variation of which was first proposed by Dodel and Kunz in 1976, 3 has been largely developed in recent years. This technique has intrinsic high time response and phase resolution.…”

Section: Introductionmentioning

confidence: 99%

High resolution polarimeter-interferometer system for fast equilibrium dynamics and MHD instability studies on Joint-TEXT tokamak (invited)

Chen

Zhuang

et al. 2014

A high-performance Faraday-effect polarimeter-interferometer system has been developed for the J-TEXT tokamak. This system has time response up to 1 μs, phase resolution < 0.1° and minimum spatial resolution ∼15 mm. High resolution permits investigation of fast equilibrium dynamics as well as magnetic and density perturbations associated with intrinsic Magneto-Hydro-Dynamic (MHD) instabilities and external coil-induced Resonant Magnetic Perturbations (RMP). The 3-wave technique, in which the line-integrated Faraday angle and electron density are measured simultaneously by three laser beams with specific polarizations and frequency offsets, is used. In order to achieve optimum resolution, three frequency-stabilized HCOOH lasers (694 GHz, >35 mW per cavity) and sensitive Planar Schottky Diode mixers are used, providing stable intermediate-frequency signals (0.5-3 MHz) with S/N > 50. The collinear R- and L-wave probe beams, which propagate through the plasma poloidal cross section (a = 0.25-0.27 m) vertically, are expanded using parabolic mirrors to cover the entire plasma column. Sources of systematic errors, e.g., stemming from mechanical vibration, beam non-collinearity, and beam polarization distortion are individually examined and minimized to ensure measurement accuracy. Simultaneous density and Faraday measurements have been successfully achieved for 14 chords. Based on measurements, temporal evolution of safety factor profile, current density profile, and electron density profile are resolved. Core magnetic and density perturbations associated with MHD tearing instabilities are clearly detected. Effects of non-axisymmetric 3D RMP in ohmically heated plasmas are directly observed by polarimetry for the first time.