The ac-coupling between a Langmuir probe and a plasma and its effect on the plasma density measurement in the afterglow

• Published in: Journal of physics. D, Applied physics Applied physics, 2014-02, Vol.47 (7), p.75201-9
• Main Authors: Liu, Fei-Xiang, Wang, Zhen-Bin, Pu, Yi-Kang
• Format: Article
• Language: English
• Subjects: ac-coupling; Afterglows; Argon plasma; compensation electrode; diagnostics; Distortion; Electrodes; langmuir probe; low temperature plasmas; Plasma (physics); Plasma density; Plasma potentials; Volt-ampere characteristics
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/0022-3727/47/7/075201

The ac-coupling between a Langmuir probe and a plasma is commonly enhanced by using a large-area compensation electrode (CE) in rf discharges in order to reduce the rf distortion on the probe I-V curves. However, in the early afterglow of pulsed discharges the ac-coupling induces a spurious electron current which prevents obtaining the correct values of the plasma parameters if conventional methods are used. In this work, the ac-coupling effect is investigated experimentally in a pulsed rf capacitively coupled argon plasma at 20 mTorr. An equivalent circuit model is also developed to simulate this effect. Both results show that the strong ac-coupling can lead to a significant misestimate of the plasma density in the early afterglow. However, the determination of the effective electron temperature and the plasma potential is not significantly affected by the ac-coupling if the Druyvesteyn method is used. In the last part of this work, two methods are proposed to estimate the correct value of the plasma density, using the data collected from a probe with a CE.