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

Tsui, C. K.; Boedo, J. A.; Stangeby, P. C.

doi:10.1063/1.4995353

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

Journal Article · Wed Jan 17 00:00:00 EST 2018 · Review of Scientific Instruments

DOI:https://doi.org/10.1063/1.4995353· OSTI ID:1526018

Tsui, C. K. ^[1]; Boedo, J. A. ^[2]; Stangeby, P. C. ^[3]

Univ. of California, San Diego, CA (United States); Swiss Federal Inst. of Technology in Lausanne (EPFL) (Switzerland). Swiss Plasma Center (SPC)
Univ. of California, San Diego, CA (United States)
Univ. of Toronto, ON (Canada). Inst. for Aerospace Studies

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 l_p, and the model is thus referred to as the “perimeter sheath expansion method.” l_p 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 = V_f.

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Research Organization:: Univ. of California, San Diego, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: SC0010529

OSTI ID:: 1526018

Alternate ID(s):: OSTI ID: 1417232

Journal Information:: Review of Scientific Instruments, Vol. 89, Issue 1; ISSN 0034-6748

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 17 works

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