| AVS 55th International Symposium & Exhibition | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS1-ThA |
| Session: | Plasma Diagnostics, Sensors, and Control II |
| Presenter: | N. Miura, Tufts University |
| Authors: | N. Miura, Tufts University J. Hopwood, Tufts University |
| Correspondent: | Click to Email |
An electrostatic probe for measuring helium metastable density was designed and tested in low-pressure, remote helium plasmas. The measured spatial distribution of helium metastable atoms was then compared with a numerical flow simulation of the plasma. The probe measures secondary electron emission due to helium metastable fluxes at a clean stainless steel surface. The probe consists of a small, planar surface surrounded by an outer guard ring. The outer ring was biased positively to reject plasma ions and the inner part was biased negatively to reject the remaining electrons. In this manner only neutral atoms reach the inner surface, and the inner probe current is due to metastable-induced secondary electrons. Energetic photons generated in the upstream plasma source region were screened from the probe, avoiding photoelectron emission. The experimental gas pressure was 5.5 - 7.5 mTorr inside a 15-cm diameter chamber located downstream from an ICP. Three metastable probes were positioned at different distances downstream from the ICP source and simultaneously swept in the radial direction to obtain spatially-resolved metastable densities. In comparing these measurements to models, the diffusive flow approximation was not completely valid since the mean free path of the metastable atoms was not negligible relative to the chamber dimensions.1 Therefore, the metastable flow was simulated by both the continuous fluid model and the Monte Carlo method. The results are compared and discussed. The conventional method to measure metastable density is optical absorption, which is well-established and non-invasive.2,3 That technique gives the density integrated along the optical path and lacks spatial resolution unless Abel inversion is applied. The probe method described here has good spatial resolution, but it is invasive and the secondary electron emission yield is very sensitive to the probe’s surface cleanliness.4 The probe also relies on electrostatic screening due to the biases applied to its surfaces, so the method is only practical in regions of low electron density such as remote plasmas (~108 cm-3).
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