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Title: Improved signal analysis for the calibration of the motional Stark effect diagnostic

Abstract

A Stokes vector and Mueller matrix formalism are adopted to model the optical train of the motional stark effect (MSE) system. The method to find all four elements of Stokes vectors and optical parameters of MSE system is presented in this article. A new fitting model for MSE calibration is discussed. A method for the measurement of offset angle of MSE is also presented. The nonideal effects of the background unpolarized and polarized light of non-Stark effect are analyzed.

Authors:
 [1]
  1. Institute of Plasma Physics, Chinese Academy of Sciences, 230031 Hefei (China)
Publication Date:
OSTI Identifier:
20778738
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 77; Journal Issue: 2; Other Information: DOI: 10.1063/1.2166667; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; CALIBRATION; SIGNALS; STARK EFFECT; VECTORS; VISIBLE RADIATION

Citation Formats

Shi Yuejiang. Improved signal analysis for the calibration of the motional Stark effect diagnostic. United States: N. p., 2006. Web. doi:10.1063/1.2166667.
Shi Yuejiang. Improved signal analysis for the calibration of the motional Stark effect diagnostic. United States. doi:10.1063/1.2166667.
Shi Yuejiang. Wed . "Improved signal analysis for the calibration of the motional Stark effect diagnostic". United States. doi:10.1063/1.2166667.
@article{osti_20778738,
title = {Improved signal analysis for the calibration of the motional Stark effect diagnostic},
author = {Shi Yuejiang},
abstractNote = {A Stokes vector and Mueller matrix formalism are adopted to model the optical train of the motional stark effect (MSE) system. The method to find all four elements of Stokes vectors and optical parameters of MSE system is presented in this article. A new fitting model for MSE calibration is discussed. A method for the measurement of offset angle of MSE is also presented. The nonideal effects of the background unpolarized and polarized light of non-Stark effect are analyzed.},
doi = {10.1063/1.2166667},
journal = {Review of Scientific Instruments},
number = 2,
volume = 77,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}
  • Nonideal effects in the optical train of the motional Stark effect diagnostic have been modeled using the Mueller matrix formalism. The effects examined are birefringence in the vacuum windows, an imperfect reflective mirror, and signal pollution due to the presence of a circularly polarized light component. Relations for the measured intensity ratio are developed for each case. These relations suggest fitting functions to more accurately model the calibration data. One particular function, termed the tangent offset model, is found to fit the data for all channels better than the currently used tangent slope function. Careful analysis of the calibration datamore » with the fitting functions reveals that a nonideal effect is present in the edge array and is attributed to nonideal performance of a mirror in that system. The result of applying the fitting function to the analysis of our data has been to improve the equilibrium reconstruction.« less
  • The magnetic pitch angle and the magnitude from reversed field pinch plasmas in the Madison symmetric torus (MST) have been routinely obtained from fully resolved motional Stark effect (MSE) spectrum analyses. Recently, the spectrum fit procedure has been improved by initializing and constraining the fit parameters based on the MSE model in the atomic data and analysis structure. A collisional-radiative model with level populations nlm-resolved up to n= 4 and a simple Born approximation for ion-impact cross sections is used for this analysis. Measurement uncertainty is quantified by making MSE measurements with multiple views of a single spatial location, rangingmore » 5%-15% for typical MST operation conditions. A multi-view fit improves the goodness of fit of MSE spectral features and background.« less
  • The q profile plays a key role for plasma equilibrium and theories of magnetohydrodynamic instabilities. With the development of the motional Stark effect (MSE) diagnostic, accurate q(r,t) profiles have been measured and utilized for equilibrium and stability analysis. Improvements to the MSE diagnostic on TFTR include a new method for calibrating the system and increasing the number of sightlines from 12 to 21 channels. A technique that uses the known poloidal field at the edge of the plasma is utilized to calibrate each sightline. The major radius is moved over the course of a discharge such that the plasma edgemore » covers each sightline radius. Then with several discharges at various plasma currents each channel is independently calibrated. This method has been compared to our earlier calibration technique using neutral beam injection into a gas filled torus and some differences in the calibration have been found that have been attributed to differences in the excitation and l mixing processes in the Stark spectrum. {copyright} {ital 1997 American Institute of Physics.}« less
  • Motional Stark effect measurements of the magnetic field pitch angle on JET present several difficulties most serious of which is that the injection systems consist of more than one source, each with a different motional stark effect (MSE) angle. Attempts to describe the net polarization angle, within the EFIT equilibrium code, using the weighted sum of Stokes vectors, have proved inaccurate. Instead we rely on spectrally isolating the emission of a single source. Beam power modulation is helpful in differentiating background polarized light, but this technique fails in the presence of strong edge localized mode (ELMs). Calibration is difficult becausemore » of the presence of a mirror in the optical system. The mirror introduces a large optical phase shift which means that the polarimeter has to resolve circular as well as linear polarized components. Basic calibration is carried out off the tokamak using a motorized rotary encoder stage and a six-parameter physical model of the optical and electronic systems. This is readjusted using plasma emission, for mirror operation at 300 deg. C. Some of the operating experience obtained with the JET MSE diagnostic will be relevant to ITER, although the lack of independent power or voltage control of the ITER injectors is seen as the chief challenge facing the ITER MSE design.« less
  • Many motional Stark effect diagnostics around the world make use of a calibration procedure in which the observed neutral beam is injected into a gas-filled torus with known vacuum fields. The instrument is calibrated by reconciling measured angles with vacuum magnetic reconstructions through a range of pitch angles. This in situ gas-filled torus calibration most closely approximates the working conditions of the diagnostic and includes effects such as beam and viewing geometries, beam voltages, Faraday and stress induced birefringence (in most cases) of the transmissive optics, as well as the polarimeter response. However, secondary neutrals, produced after ionization then reneutralizationmore » of a beam neutral, have been found to contaminate measured angles by emitting Balmer alpha with similar Doppler shifts and Stark polarizations as beam neutrals, but with different polarization angles. Simulation results that show spectral and angle behavior versus calibration parameters such as fill gas pressure will be presented. Data from NSTX and C-Mod will be compared to simulations results.« less