Paper PS-MoA6
Optical Emission Diagnostics for Detection of High Energy Electrons in Argon Plasmas
Monday, October 19, 2015, 4:00 pm, Room 210A
| Session: |
Plasma Diagnostics, Sensors and Control I |
| Presenter: |
Shicong Wang, University of Wisconsin - Madison |
| Authors: |
S. Wang, University of Wisconsin - Madison
J. Boffard, University of Wisconsin - Madison
C.C. Lin, University of Wisconsin - Madison
A.E. Wendt, University of Wisconsin - Madison
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| Correspondent: |
Click to Email |
The electron energy distribution function (EEDF) is one of the most important and fundamental parameters in low temperature plasmas. The high-energy range of the EEDF in particular is responsible for ionization, excitation and gas phase chemistry, which are critical for many industrial applications. Non-invasive OES diagnostics provide an attractive means to measure EEDFs, using emissions from argon states excited by electron collisions [1,2]. Emission lines dominated by excitation of ground state argon atoms are particularly sensitive to the population of high energy (>13 eV) electrons. We report on optical emission diagnostics for detection of energetic electrons in argon plasmas. The OES diagnostic makes use of an emission model based on measured argon excitation cross sections that computes the relative emission intensities at a selected set of wavelengths, with a trial EEDF as input. The trial EEDF that produces the best fit to the spectrum measured in the experiment is determined after a search in which the shape and average energy of the trial EEDF is varied. In both pulsed argon inductively coupled plasmas (ICPs) and capacitively coupled plasmas (CCPs), a high electron temperature has been previously observed at the beginning of the pulse period in time-resolved OES measurements. Meanwhile, the relative argon emission intensities between emissions originating from upper levels populated primarily by high energy and low energy electrons , respectively, (i.e., 420.07/419.83 nm line ratio) imply the existence of a `hot tail' of energetic electrons, as compared to a Maxwellian distribution. In addition, the OES method of detecting high energy electrons is explored in the presence of a supplemental source of energetic electrons. A biased auxiliary set of heated filaments will be used to inject high energy electrons into an argon inductively coupled plasma. The filament bias voltage will be varied to control the energy of emitted electrons, and the filament heater current will be controlled separately to vary the emitted electron flux. The OES diagnostic is used to observe the changes in emission spectra and extracted EEDF caused by the supplemental energetic electrons. The OES method will be examined and compared to Langmuir probe measurements of EEDF as the energy and current of the supplemental electron population are systematically varied.
The authors acknowledge support from NSF grant PHY-1068670.
[1] Wang et al., JVSTA 31, (2013) 021303.
[2] Boffard et al., Plasma Sources Sci. Tech. 19 (2010) 065001.