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Title: Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics

Abstract

Here we present a method to calculate the ion saturation current, I sat, for Langmuir probes at high frequency (>100 kHz) using the harmonics technique and we compare that to a direct measurement of I sat. It is noted that the Isat estimation can be made directly by the ratio of harmonic amplitudes, without explicitly calculating T e. We also demonstrate that since the probe tips using the harmonic method are oscillating near the floating potential, drawing little power, this method reduces tip heating and arcing and allows plasma density measurements at a plasma power flux that would cause continuously biased tips to arc. A multi-probe array is used, with two spatially separated tips employing the harmonics technique and measuring the amplitude of at least two harmonics per tip. A third tip, located between the other two, measures the ion saturation current directly. We compare the measured and calculated ion saturation currents for a variety of plasma conditions and demonstrate the validity of the technique and it’s use in reducing arcs.

Authors:
ORCiD logo [1];  [1]
  1. Univ. of California, San Diego, CA (United States). Center for Energy Research
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE); USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1374808
Alternate Identifier(s):
OSTI ID: 1373967
Grant/Contract Number:
FC02-04ER54698; FG02-07ER54917
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Review of Scientific Instruments
Additional Journal Information:
Journal Volume: 88; Journal Issue: 3; 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; Electric measurements; Plasma temperature; Probe plasma diagnostics; ECR heating; Tokamaks

Citation Formats

Boedo, J. A., and Rudakov, D. L.. Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics. United States: N. p., 2017. Web. doi:10.1063/1.4978453.
Boedo, J. A., & Rudakov, D. L.. Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics. United States. doi:10.1063/1.4978453.
Boedo, J. A., and Rudakov, D. L.. Mon . "Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics". United States. doi:10.1063/1.4978453. https://www.osti.gov/servlets/purl/1374808.
@article{osti_1374808,
title = {Estimation of plasma ion saturation current and reduced tip arcing using Langmuir probe harmonics},
author = {Boedo, J. A. and Rudakov, D. L.},
abstractNote = {Here we present a method to calculate the ion saturation current, Isat, for Langmuir probes at high frequency (>100 kHz) using the harmonics technique and we compare that to a direct measurement of Isat. It is noted that the Isat estimation can be made directly by the ratio of harmonic amplitudes, without explicitly calculating Te. We also demonstrate that since the probe tips using the harmonic method are oscillating near the floating potential, drawing little power, this method reduces tip heating and arcing and allows plasma density measurements at a plasma power flux that would cause continuously biased tips to arc. A multi-probe array is used, with two spatially separated tips employing the harmonics technique and measuring the amplitude of at least two harmonics per tip. A third tip, located between the other two, measures the ion saturation current directly. We compare the measured and calculated ion saturation currents for a variety of plasma conditions and demonstrate the validity of the technique and it’s use in reducing arcs.},
doi = {10.1063/1.4978453},
journal = {Review of Scientific Instruments},
number = 3,
volume = 88,
place = {United States},
year = {Mon Mar 20 00:00:00 EDT 2017},
month = {Mon Mar 20 00:00:00 EDT 2017}
}

Journal Article:
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  • A high current Electron Cyclotron Resonance (ECR) proton ion source has been developed for low energy high intensity proton accelerator at Bhabha Atomic Research Centre. Langmuir probe diagnostics of the plasma generated in this proton ion source is performed using Langmuir probe. The diagnostics of plasma in the ion source is important as it determines beam parameters of the ion source, i.e., beam current, emittance, and available species. The plasma parameter measurement in the ion source is performed in continuously working and pulsed mode using hydrogen as plasma generation gas. The measurement is performed in the ECR zone for operatingmore » pressure and microwave power range of 10{sup −4}–10{sup −3} mbar and 400–1000 W. An automated Langmuir probe diagnostics unit with data acquisition system is developed to measure these parameters. The diagnostics studies indicate that the plasma density and plasma electron temperature measured are in the range 5.6 × 10{sup 10} cm{sup −3} to 3.8 × 10{sup 11} cm{sup −3} and 4–14 eV, respectively. Using this plasma, ion beam current of tens of mA is extracted. The variations of plasma parameters with microwave power, gas pressure, and radial location of the probe have been studied.« less
  • A Langmuir probe and optical emission spectroscopy were used in a deep reactive ion etch system to correlate plasma parameters (atomic fluorine and argon emission, electron density, ion density, and electron average energy) with the etch rate and via sidewall angle. All data were obtained for coil powers ranging from 200 to 800 W, platen powers ranging from 7 to 16 W, and pressure ranging from 3.8 to 62 mTorr with constant SF{sub 6} and Ar flow rates of 112 and 18 SCCM (SCCM denotes cubic centimeter per minute at STP), respectively. Results indicate that there is a correlation withmore » etch rate for all plasma parameters except for argon emission. For argon emission, the etch rate exhibits a double-valued relation where the etch rate can either increase or decrease with increasing argon emission intensity due to changes in pressure which affect the energy coupling efficiency. As expected, the etch rate increases for measured increases in fluorine emission, electron density, and ion density. The etch rate, however, decreases with increasing average electron energy due to collision processes. In contrast, no correlation is observed between any of the measured plasma parameters with sidewall angle. The last result is consistent with the idea that sidewall angle is primarily controlled by the passivation cycle as opposed to the etching cycle, where all the authors' data were obtained.« less
  • The electron saturation currents measured with the help of fine cylindrical probes at high pressures in the intermediate gas volume of methane-- hydrogen--air flames can be described neither by the Langmuir equations nor the Borgnis probe theory. It was shown that the electrons in the space charge, which at high pressures and low carrier densities completely shield the field of the probe, do not have a constant motion but one dependent on the field and therefore on the position in the space charge layer. It was also shown that in the divergent field of the space charge layer a relaxationmore » of the electron energies occurs which may be neglected at infinitely high pressures. From this viewpoint, a characteristic current-voltage relationship was derived for the electron current at high pressures which permits an exact evaluation of the measurements for determination of the electron densities. The ion saturation currents can be described and evaluated with an equation by Borgnis. (tr-auth)« less