Multichannel far-infrared polarimeter-interferometer system on the MST reversed field pinch

Brower, D. L.; Jiang, Yong; Ding, W. X.; Terry, S. D.; Lanier, N. E.; Anderson, J. K.; Forest, C.B.; Holly, D. J.

doi:10.1063/1.1321744

Cited by 43 publications

(27 citation statements)

References 10 publications

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“…In order to obtain the correct Faraday rotation angle, this response must be measured and used to calibrate the measured data. 24 The measured Faraday rotation angle is equal to the line integral of the product of the electron density and the poloidal magnetic field. Therefore, changes or fluctuations in the Faraday rotation can be caused by either density or magnetic field fluctuations.…”

Section: Methodsmentioning

confidence: 99%

Measurement of current profile dynamics in the Madison Symmetric Torus

et al. 2004

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The current profile and core magnetic field fluctuation amplitudes in a reversed-field pinch are measured by using a high-resolution polarimetry–interferometry system. This paper presents data showing a redistribution of the current during a sawtooth crash. Also, the core magnetic field fluctuation amplitude is observed to increase at a sawtooth crash consistent with the idea of nonlinearly driven dynamo current. In addition, the parallel current density increases in the outer region of the plasma during auxiliary pulsed parallel current drive. This was expected, as the external application of an edge parallel electric field is designed to flatten the current profile providing an equilibrium closer to the minimum energy Taylor state. However, the current density also increases in the core, relative to standard plasmas. This increase can be explained by a reduction of the dynamo (anti)current drive in the core that should accompany the measured reduction of magnetic fluctuations and by a drop in resistivity caused by the increased confinement of fast electrons.

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Section: Methodsmentioning

confidence: 99%

Measurement of current profile dynamics in the Madison Symmetric Torus

et al. 2004

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“…Details of this procedure have been described elsewhere. 17 Typical polarimeter system rms noise levels are 1 mrad ͑Ϸ0.1°͒ at 100 kHz bandwidth ͓0.4 mrad ͑0.04°͒ at 20 kHz bandwidth͔. The data can be ensemble averaged over similar time windows ͑e.g., sawtooth cycle͒ for further noise reduction.…”

Section: Polarimeter-interferometer Systemmentioning

confidence: 99%

Laser polarimetric measurement of equilibrium and fluctuating magnetic fields in a reversed field pinch (invited)

Brower

Ding

Terry

et al. 2003

Review of Scientific Instruments

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Articles you may be interested inFar-infrared polarimetry diagnostic for measurement of internal magnetic field dynamics and fluctuations in the C-MOD Tokamak (invited)a) Rev. Sci. Instrum. 83, 10E316 (2012); 10.1063/1.4731757 Density fluctuation measurements by far-forward collective scattering in the MST reversed-field pincha) Rev. Sci. Instrum. 83, 10E302 (2012); 10.1063/1.4728098 Electron thermal transport within magnetic islands in the reversed-field pincha) Phys. Plasmas 17, 056115 (2010); 10.1063/1.3388374 Measurement of electron transport in the Madison Symmetric Torus reversed-field pinch (invited) Rev. Sci. Instrum. 72, 1039 (2001); 10.1063/1.1319613First results from the far-infrared polarimeter system on the Madison Symmetric Torus reversed field pinch Rev.New developments in Faraday rotation polarimetry have provided the first measurements of current density profile and core magnetic fluctuations in the core of a high-temperature reversed field pinch. This has been achieved by a fast-polarimeter system with time response up to 1 s and phase resolution Ͻ1 mrad. Recent experiments on Madison Symmetric Torus have directly measured radial magnetic field fluctuations in the plasma interior with amplitude 33 G, ϳ1%. A broad spectrum of magnetic fluctuations is observed up to 100 kHz. Relaxation of the current density profile at the sawtooth crash occurs on the timescale of 100 s. Reversed-field pinch behavior is determined in large part by magnetic fluctuations driven by the radial gradient in the parallel current density. Hence, measurement of magnetic fluctuations and the current density profile is essential to understand the link between the current density profile, fluctuations, and transport.

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“…A practical method to examine the polarization of counter-rotating circularly polarized probe beams is polarization modulation. 13 To accomplish this, we add a rotating (few Hz) half-wave plate in the optical path to replace the plasma and modulate the polarization. If the probe beams' polarizations are counter-rotating circularly polarized, phase difference between the R beam and L beam will change linearly and equal four times the rotation angle of the half wave plate; any distortion of polarization will lead to a nonlinear response.…”

Section: Systematic Errorsmentioning

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

Review of Scientific Instruments

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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.

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Multichannel far-infrared polarimeter-interferometer system on the MST reversed field pinch

Cited by 43 publications

References 10 publications

Measurement of current profile dynamics in the Madison Symmetric Torus

Measurement of current profile dynamics in the Madison Symmetric Torus

Laser polarimetric measurement of equilibrium and fluctuating magnetic fields in a reversed field pinch (invited)

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

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