Alternating current probe system allows access to a wide electron energy range in plasmas

MAR 20, 2020

Obtaining the electron energy distribution function in the low energy region of low pressure plasmas is a difficult problem. A Langmuir probe with alternating current may be the solution.

Anashe Bandari

DOI: 10.1063/10.0000989

Alternating current probe system allows access to a wide electron energy range in plasmas internal name — Alternating current probe system allows access to a wide electron energy range in plasmas lead image

Determining the electron energy distribution function (EEDF) in low pressure plasmas is important for identifying the parameters and understanding the rates of processes for plasmas. However, traditional methods of obtaining the EEDF are difficult, requiring complicated probe systems and mathematical techniques. Adrian Heiler, Roland Friedl and Ursel Fantz demonstrated a Langmuir probe that uses an alternating current (AC) system to measure the EEDF down to low energy regions.

“A trustworthy determination of the EEDF is highly desirable, since the electrons are generally not in thermal equilibrium in weakly ionized, low pressure plasmas,” Heiler said.

Typically, the EEDF is calculated using the second derivative of the current-voltage curve. By applying a sinusoidal voltage to the probe, the AC modulation technique can measure this second derivative directly.

“The AC method can complement conventional measurements,” said Heiler. “This technique does not rely on numerical differentiation of noisy data and has hence proven to be less prone to error, especially in the EEDF low energy region.”

In a proof-of-concept test, the group compared how well EEDFs determined by the AC approach and by numerical differentiation agree with simulations. They found the AC technique can reliably access electrons in a wide energy range and is more effective at accessing low energy ranges than differentiation, which displays a nonphysical drop in this region.

The next step of the work is to reduce the AC system’s signal-to-noise ratio to access high energy tails of the EEDF. This can be done using noise reduction techniques or by the application of a higher modulation amplitude.

Source: “Application of a Langmuir probe AC technique for reliable access to the low energy range of electron energy distribution functions in low pressure plasmas,” by A. Heiler, R. Friedl, and U. Fantz, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5139601 .