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Title: Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod

Abstract

This paper presents the first experimental analysis of polarized synchrotron emission from relativistic runaway electrons (REs) in a tokamak plasma. Importantly, we show that the polarization information of synchrotron radiation can be used to diagnose spatially-localized RE pitch angle distributions. Synchrotron-producing REs were generated during low density, Ohmic, diverted plasma discharges in the Alcator C-Mod tokamak. The ten-channel Motional Stark Effect diagnostic was used to measure spatial profiles of the polarization angle and the fraction of detected light that was linearly-polarized. Spatial transitions in the polarization angle of 90 degrees---from horizontal to vertical polarization and vice versa---are observed in experimental data and are well-explained by the gyro-motion of REs and high directionality of synchrotron radiation. Polarized synchrotron emission is modeled with the synthetic diagnostic SOFT; its output Green's (or detector response) functions reveal a critical RE pitch angle at which the polarization angle flips by 90 degrees and the polarization fraction is minimal. Using SOFT, we determine the dominant RE pitch angle which reproduces measured polarization angle and fraction values. The spatiotemporal evolutions of the polarization angle and fraction are explored in detail for one C-Mod discharge. For channels viewing REs near the magnetic axis and flux surfaces q =more » 1 and 4/3, disagreements between synthetic and experimental signals suggest that the sawtooth instability may be influencing RE dynamics. Furthermore, other sources of pitch angle scattering, not considered in this analysis, could help explain discrepancies between simulation and experiment.« less

Authors:
; ; ; ;
Publication Date:
DOE Contract Number:  
FC02-99ER54512
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
OSTI Identifier:
1881440
DOI:
https://doi.org/10.7910/DVN/KSXGZH

Citation Formats

Tinguely, R. A., Hoppe, M., Granetz, R. S., Mumgaard, R. T., and Scott, S. Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod. United States: N. p., 2019. Web. doi:10.7910/DVN/KSXGZH.
Tinguely, R. A., Hoppe, M., Granetz, R. S., Mumgaard, R. T., & Scott, S. Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod. United States. doi:https://doi.org/10.7910/DVN/KSXGZH
Tinguely, R. A., Hoppe, M., Granetz, R. S., Mumgaard, R. T., and Scott, S. 2019. "Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod". United States. doi:https://doi.org/10.7910/DVN/KSXGZH. https://www.osti.gov/servlets/purl/1881440. Pub date:Mon Jul 29 00:00:00 EDT 2019
@article{osti_1881440,
title = {Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod},
author = {Tinguely, R. A. and Hoppe, M. and Granetz, R. S. and Mumgaard, R. T. and Scott, S.},
abstractNote = {This paper presents the first experimental analysis of polarized synchrotron emission from relativistic runaway electrons (REs) in a tokamak plasma. Importantly, we show that the polarization information of synchrotron radiation can be used to diagnose spatially-localized RE pitch angle distributions. Synchrotron-producing REs were generated during low density, Ohmic, diverted plasma discharges in the Alcator C-Mod tokamak. The ten-channel Motional Stark Effect diagnostic was used to measure spatial profiles of the polarization angle and the fraction of detected light that was linearly-polarized. Spatial transitions in the polarization angle of 90 degrees---from horizontal to vertical polarization and vice versa---are observed in experimental data and are well-explained by the gyro-motion of REs and high directionality of synchrotron radiation. Polarized synchrotron emission is modeled with the synthetic diagnostic SOFT; its output Green's (or detector response) functions reveal a critical RE pitch angle at which the polarization angle flips by 90 degrees and the polarization fraction is minimal. Using SOFT, we determine the dominant RE pitch angle which reproduces measured polarization angle and fraction values. The spatiotemporal evolutions of the polarization angle and fraction are explored in detail for one C-Mod discharge. For channels viewing REs near the magnetic axis and flux surfaces q = 1 and 4/3, disagreements between synthetic and experimental signals suggest that the sawtooth instability may be influencing RE dynamics. Furthermore, other sources of pitch angle scattering, not considered in this analysis, could help explain discrepancies between simulation and experiment.},
doi = {10.7910/DVN/KSXGZH},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2019},
month = {7}
}

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