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

Abstract

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.

Single-shot calibration of soft x-ray mirrors using a sinusoidal transmission grating

“…The small phase noise results from the high gain FIR laser cavity design. 7 It is also consistent with measured very narrow laser line width of 0.33 ± 0.18 kHz. The Gaussian phase noise is intrinsic to the lasers.…”

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

“…4,5 To explore the equilibrium and perturbation of magnetic field and electron density, especially the fine structure of perturbation during MHD activity, a high resolution polarimeterinterferometer system (POLARIS) based on three-wave technique has been developed on J-TEXT tokamak. [6][7][8] To achieve this goal, J-TEXT POLARIS is intentionally developed with high phase resolution (less than 0.1 • for Faraday angle and less than 1 • for line-integrated density at 50 kHz bandwidth), high temporal resolution (<1 μs), high spatial resolution (15 mm minimum) and high spatial coverage (multiple chords covering the entire plasma cross section). In this paper the diagnostic and its application will be presented.…”

Design and development of a synchronized operation control system for Thomson scattering diagnostic on J-TEXT

“…A laser TS system has been designed or constructed for many tokamaks such as DIII-D, JT60-U, KSTAR, EAST, HL-2A, and ITER, of which the last one is considered the largest tokamak in the world [3][4][5][6][7][8]. Currently, at Joint Texas Experimental Tokamak (J-TEXT), T e and n e are obtained using an electron cyclotron emission (ECE) diagnostic and polarimeter-interferometer diagnostic, respectively [9,10]. However, ECE can be used to measure electron temperature over a small range of plasma parameters [11].…”