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

Journal Article · · Nuclear Fusion
ORCiD logo [1]; ORCiD logo [2];  [1];  [1];  [3]
  1. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
  2. Chalmers University of Technology, Gothenburg (Sweden)
  3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
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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 θpol and the fraction fpol of detected light that was linearly-polarized. Spatial transitions in θpol of 90° —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. Here 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 θpol flips by 90° and fpol is minimal. Using Soft, we determine the dominant RE pitch angle which reproduces measured θpol and fpol values. The spatiotemporal evolutions of θpol and fpol 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.

Research Organization:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
Sponsoring Organization:
USDOE Office of Science (SC), Fusion Energy Sciences (FES); Swedish Research Council (SRC); European Research Council (ERC)
Grant/Contract Number:
FC02-99ER54512; Dnr 2014-5510; ERC-2014-CoG Grant 647121
OSTI ID:
1897057
Journal Information:
Nuclear Fusion, Vol. 59, Issue 9; ISSN 0029-5515
Publisher:
IOP ScienceCopyright Statement
Country of Publication:
United States
Language:
English

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Cited By (1)

Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption journal January 2021

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70 PLASMA PHYSICS AND FUSION TECHNOLOGY
tokamak plasmas
runaway electrons
synchrotron radiation
polarization
synthetic diagnostics