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Title: Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas

Abstract

A Child-Langmuir law-based method for accounting for Debye sheath expansion while fitting the current-voltage I-V characteristic of proud Langmuir probes (electrodes that extend into the volume of the plasma) is described. For Langmuir probes of a typical size used in tokamak plasmas, these new estimates of electron temperature and ion saturation current density values decreased by up to 60% compared to methods that did not account for sheath expansion. Changes to the collection area are modeled using the Child-Langmuir law and effective expansion perimeter lp, and the model is thus referred to as the “perimeter sheath expansion method.” lp is determined solely from electrode geometry, so the method may be employed without prior measurement of the magnitude of the sheath expansion effects for a given Langmuir probe and can be used for electrodes of different geometries. This method correctly predicts the non-saturating ΔI/ΔV slope for cold, low-density plasmas where sheath-expansion effects are strong, as well as for hot plasmas where ΔI/ΔV ~ 0, though it is shown that the sheath can still significantly affect the collection area in these hot conditions. The perimeter sheath expansion method has several advantages compared to methods where the non-saturating current is fitted: (1) Itmore » is more resilient to scatter in the I-V characteristics observed in turbulent plasmas. (2) It is able to separate the contributions to the ΔI/ΔV slope from sheath expansion to that of the high energy electron tail in high Te conditions. (3) It calculates the change in the collection area due to the Debye sheath for conditions where ΔI/ΔV ~ 0 and for V = Vf.« less

Authors:
 [1];  [2];  [3]
+ Show Author Affiliations
  1. Univ. of California, San Diego, CA (United States); Swiss Federal Inst. of Technology in Lausanne (EPFL) (Switzerland). Swiss Plasma Center (SPC)
  2. Univ. of California, San Diego, CA (United States)
  3. Univ. of Toronto, ON (Canada). Inst. for Aerospace Studies
Publication Date:
Research Org.:
Univ. of California, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES)
OSTI Identifier:
1526018
Alternate Identifier(s):
OSTI ID: 1417232
Grant/Contract Number:  
SC0010529
Resource Type:
Accepted Manuscript
Journal Name:
Review of Scientific Instruments
Additional Journal Information:
Journal Volume: 89; Journal Issue: 1; Journal ID: ISSN 0034-6748
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY

Citation Formats

Tsui, C. K., Boedo, J. A., and Stangeby, P. C. Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas. United States: N. p., 2018. Web. doi:10.1063/1.4995353.
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Tsui, C. K., Boedo, J. A., & Stangeby, P. C. Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas. United States. https://doi.org/10.1063/1.4995353
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Tsui, C. K., Boedo, J. A., and Stangeby, P. C. Wed . "Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas". United States. https://doi.org/10.1063/1.4995353. https://www.osti.gov/servlets/purl/1526018.
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@article{osti_1526018,
title = {Accounting for Debye sheath expansion for proud Langmuir probes in magnetic confinement fusion plasmas},
author = {Tsui, C. K. and Boedo, J. A. and Stangeby, P. C.},
abstractNote = {A Child-Langmuir law-based method for accounting for Debye sheath expansion while fitting the current-voltage I-V characteristic of proud Langmuir probes (electrodes that extend into the volume of the plasma) is described. For Langmuir probes of a typical size used in tokamak plasmas, these new estimates of electron temperature and ion saturation current density values decreased by up to 60% compared to methods that did not account for sheath expansion. Changes to the collection area are modeled using the Child-Langmuir law and effective expansion perimeter lp, and the model is thus referred to as the “perimeter sheath expansion method.” lp is determined solely from electrode geometry, so the method may be employed without prior measurement of the magnitude of the sheath expansion effects for a given Langmuir probe and can be used for electrodes of different geometries. This method correctly predicts the non-saturating ΔI/ΔV slope for cold, low-density plasmas where sheath-expansion effects are strong, as well as for hot plasmas where ΔI/ΔV ~ 0, though it is shown that the sheath can still significantly affect the collection area in these hot conditions. The perimeter sheath expansion method has several advantages compared to methods where the non-saturating current is fitted: (1) It is more resilient to scatter in the I-V characteristics observed in turbulent plasmas. (2) It is able to separate the contributions to the ΔI/ΔV slope from sheath expansion to that of the high energy electron tail in high Te conditions. (3) It calculates the change in the collection area due to the Debye sheath for conditions where ΔI/ΔV ~ 0 and for V = Vf.},
doi = {10.1063/1.4995353},
journal = {Review of Scientific Instruments},
number = 1,
volume = 89,
place = {United States},
year = {Wed Jan 17 00:00:00 EST 2018},
month = {Wed Jan 17 00:00:00 EST 2018}
}
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Journal Article:
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Publisher's Version of Record
https://doi.org/10.1063/1.4995353
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Cited by: 17 works
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Figures / Tables:

FIG. 1 FIG. 1: A poloidal cross section of the TCV tokamak (left) showing the magnetic geometry and the location of the reciprocating probe. The insets (right) show the reciprocating probe head geometry. The graphite electrodes extend out of a boron nitride insulator (in white) which is held in place and protectedmore » by a graphite shroud. The Mach probe array is shaded red, and the double probe array is shaded teal.« less
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Works referenced in this record:

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    Works referencing / citing this record:

    The flush-mounted rail Langmuir probe array designed for the Alcator C-Mod vertical target plate divertor
    journal, April 2018

    • Kuang, A. Q.; Brunner, D.; LaBombard, B.
    • Review of Scientific Instruments, Vol. 89, Issue 4
    • DOI: 10.1063/1.5023905

    Analysis of wall-embedded Langmuir probe signals in different conditions on the Tokamak à Configuration Variable
    journal, May 2018

    • Février, O.; Theiler, C.; De Oliveira, H.
    • Review of Scientific Instruments, Vol. 89, Issue 5
    • DOI: 10.1063/1.5022459

    Shaping effects on scrape-off layer plasma turbulence: A rigorous validation of three-dimensional simulations against TCV measurements
    journal, January 2020

    • Riva, F.; Tsui, C. K.; Boedo, J. A.
    • Physics of Plasmas, Vol. 27, Issue 1
    • DOI: 10.1063/1.5123451

    Langmuir probe electronics upgrade on the tokamak à configuration variable
    journal, August 2019

    • De Oliveira, H.; Marmillod, P.; Theiler, C.
    • Review of Scientific Instruments, Vol. 90, Issue 8
    • DOI: 10.1063/1.5108876

    3D particle-in-cell modeling of Langmuir probe effective collecting area in magnetized plasma
    journal, June 2018

    • Podolník, A.; Komm, M.; Adámek, J.
    • Plasma Physics and Controlled Fusion, Vol. 60, Issue 8
    • DOI: 10.1088/1361-6587/aac701

    The effect of the secondary x-point on the scrape-off layer transport in the TCV snowflake minus divertor
    journal, December 2018

    Filamentary velocity scaling validation in the TCV tokamak
    journal, July 2018

    • Tsui, C. K.; Boedo, J. A.; Myra, J. R.
    • Physics of Plasmas, Vol. 25, Issue 7
    • DOI: 10.1063/1.5038019
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      Figures / Tables found in this record:

      FIG. 1 (p. 4)figure
      FIG. 2 (p. 4)figure
      FIG. 3 (p. 5)figure
      Table I (p. 5)table
      FIG. 4 (p. 6)figure
      FIG. 5 (p. 6)figure
      Table II (p. 6)table
      FIG. 6 (p. 7)figure
      FIG. 7 (p. 8)figure
      Table III (p. 8)table
      FIG. 8 (p. 9)figure
      Table IV (p. 9)table
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