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Title: Impedance of an intense plasma-cathode electron source for tokamak startup

Abstract

In this study, an impedance model is formulated and tested for the ~1kV, ~1kA/cm 2, arc-plasma cathode electron source used for local helicity injection tokamak startup. A double layer sheath is established between the high-density arc plasma (n arc ≈ 10 21 m -3) within the electron source, and the less dense external tokamak edge plasma (n edge ≈ 10 18 m -3) into which current is injected at the applied injector voltage, V inj. Experiments on the Pegasus spherical tokamak show the injected current, I inj, increases with V inj according to the standard double layer scaling I inj ~ V inj 3/2 at low current and transitions to I inj ~ V inj 1/2 at high currents. In this high current regime, sheath expansion and/or space charge neutralization impose limits on the beam density n b ~ I inj/V inj 1/2. For low tokamak edge density n edge and high I inj, the inferred beam density n b is consistent with the requirement n b ≤ n edge imposed by space-charge neutralization of the beam in the tokamak edge plasma. At sufficient edge density, n b ~ n arc is observed, consistent with a limit to n bmore » imposed by expansion of the double layer sheath. These results suggest that n arc is a viable control actuator for the source impedance.« less

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Univ. of Wisconsin-Madison, Madison, WI (United States)
Publication Date:
Research Org.:
Univ. of Wisconsin-Madison, Madison, WI (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1255444
Alternate Identifier(s):
OSTI ID: 1255165
Grant/Contract Number:  
FG02-96ER54375
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 23; Journal Issue: 5; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; tokamaks; double layer; helicity injection; plasma cathode; electron source; plasma source; plasma sheaths; cathodes; charge injection

Citation Formats

Hinson, Edward Thomas, Barr, Jayson L., Bongard, Michael W., Burke, Marcus Galen, Fonck, Raymond J., and Perry, Justin M.. Impedance of an intense plasma-cathode electron source for tokamak startup. United States: N. p., 2016. Web. doi:10.1063/1.4952628.
Hinson, Edward Thomas, Barr, Jayson L., Bongard, Michael W., Burke, Marcus Galen, Fonck, Raymond J., & Perry, Justin M.. Impedance of an intense plasma-cathode electron source for tokamak startup. United States. doi:10.1063/1.4952628.
Hinson, Edward Thomas, Barr, Jayson L., Bongard, Michael W., Burke, Marcus Galen, Fonck, Raymond J., and Perry, Justin M.. Tue . "Impedance of an intense plasma-cathode electron source for tokamak startup". United States. doi:10.1063/1.4952628. https://www.osti.gov/servlets/purl/1255444.
@article{osti_1255444,
title = {Impedance of an intense plasma-cathode electron source for tokamak startup},
author = {Hinson, Edward Thomas and Barr, Jayson L. and Bongard, Michael W. and Burke, Marcus Galen and Fonck, Raymond J. and Perry, Justin M.},
abstractNote = {In this study, an impedance model is formulated and tested for the ~1kV, ~1kA/cm2, arc-plasma cathode electron source used for local helicity injection tokamak startup. A double layer sheath is established between the high-density arc plasma (narc ≈ 1021 m-3) within the electron source, and the less dense external tokamak edge plasma (nedge ≈ 1018 m-3) into which current is injected at the applied injector voltage, Vinj. Experiments on the Pegasus spherical tokamak show the injected current, Iinj, increases with Vinj according to the standard double layer scaling Iinj ~ Vinj3/2 at low current and transitions to Iinj ~ Vinj1/2 at high currents. In this high current regime, sheath expansion and/or space charge neutralization impose limits on the beam density nb ~ Iinj/Vinj1/2. For low tokamak edge density nedge and high Iinj, the inferred beam density nb is consistent with the requirement nb ≤ nedge imposed by space-charge neutralization of the beam in the tokamak edge plasma. At sufficient edge density, nb ~ narc is observed, consistent with a limit to nb imposed by expansion of the double layer sheath. These results suggest that narc is a viable control actuator for the source impedance.},
doi = {10.1063/1.4952628},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 5,
volume = 23,
place = {United States},
year = {2016},
month = {5}
}

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Works referenced in this record:

    Works referencing / citing this record:

    Public Data Set: H-mode Plasmas at Very Low Aspect Ratio on the Pegasus Toroidal Experiment
    dataset, January 2016


    Public Data Set: On Virial Analysis at Low Aspect Ratio
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    • Bongard, Michael; Barr, Jayson; Fonck, Raymond
    • Pegasus-Supplementary information for journal article at DOI: 10.1063/1.4959808, 1 file (440 KB)
    • DOI: 10.18138/1240359

    Public Data Set: A Novel, Cost-Effective, Multi-Point Thomson Scattering System on the Pegasus Toroidal Experiment
    dataset, September 2016

    • Schlossberg, D. J.; Bodner, G. M.; Reusch, J. A.
    • AIP - Review of Scientific Instruments, Vol. 87, Issue 11, 49.6 MB
    • DOI: 10.18138/1257860

    Public Data Set: Control and Automation of the Pegasus Multi-point Thomson Scattering System
    dataset, January 2016

    • Bodner, Grant; Bongard, Michael; Fonck, Raymond
    • Pegasus, 13 files
    • DOI: 10.18138/1257971

    Public Data Set: A Power-Balance Model for Local Helicity Injection Startup in a Spherical Tokamak
    dataset, May 2018

    • Barr, J. L.; Bongard, M. W.; Burke, M. G.
    • Pegasus Public Data Sets
    • DOI: 10.18138/1345037

    Public Data Set: Non-inductively Driven Tokamak Plasmas at Near-Unity βt in the Pegasus Toroidal Experiment
    dataset, March 2018

    • Reusch, Joshua; Bodner, Grant; Bongard, Michael
    • Pegasus-Supplementary information for journal article at DOI: 10.1063/1.5017966, 1 file (5.1 MB)
    • DOI: 10.18138/1410490

    Public Data Set: Initiation and Sustainment of Tokamak Plasmas with Local Helicity Injection as the Majority Current Drive
    dataset, July 2018

    • Perry, Justin; Bodner, Grant; Bongard, Michael
    • Pegasus-Supplementary information for journal article at DOI: https://doi.org/10.1088/1741-4326/aac72e, 1 file (17.1 MB)
    • DOI: 10.18138/1419641

    Public Data Set: Radially Scanning Magnetic Probes to Study Local Helicity Injection Dynamics
    dataset, July 2018

    • Richner, Nathan; Bongard, Michael; Fonck, Raymond
    • Pegasus-Supplementary information for journal article at DOI: 10.1063/1.5036527, 1 file (9.4 MB)
    • DOI: 10.18138/1432455