Comparative measurements of plasma potential with ball-pen and Langmuir probe in low-temperature magnetized plasma

Abstract

The ball-pen probe (BPP) is used for direct plasma potential measurements in magnetized plasma. The probe can adjust the ratio of the electron and ion saturation currents Isat−/Isat+ to be close to one and therefore its I-V characteristic becomes nearly symmetric. If this is achieved, the floating potential of the BPP is close to the plasma potential. Because of its rather simple construction, it offers an attractive probe for measurements in magnetized plasma. Comparative measurements of plasma potential by BPPs of different dimensions as well as one Langmuir probe (LP) in an argon discharge plasma of a cylindrical magnetron were performed at various experimental conditions. An additional comparison by an emissive probe was also performed. All these types of probes provide similar values of plasma potential in a wide range of plasma parameters. Our results for three different BPP dimensions indicate that the BPP can be operated in a cylindrical magnetron DC argon discharge if the value of the ratio of the magnetic field and neutral gas pressure, B/p, is greater than approximately 10 mT/Pa.

Ball pen probe in strongly magnetised RF plasmas

“…The dip width is larger than the conical height of 1.5 mm and is more pronounced for the flat collector. A similar feature was also observed by Adámek et al (2013) and Bousselin et al (2013) but not by Zanáška et al (2015). By observing the dip width to increase with λ De (as the magnetic field is increased) Bousselin et al (2013) attributed the dip to the expanding sheath surrounding the BPP.…”

“…The saturation currents at the floating potential can then be obtained by a linear fit to the data. The highly nonlinear variation of collector current with applied voltage, as shown in figure 6(b), has also been observed in magnetised low temperature plasmas by Adámek et al (2013) and Zanáška et al (2015). This method of estimating α is not applicable for these plasmas.…”

“…Inside the chamber the ceramic tube extended so that its open end was positioned in the centre of the chamber halfway between the electrodes. The 2 mm BPP inner diameter (table 1) is similar to that used by Zanáška et al (2015) in their experiments. The importance of a high impedance buffer to measure the collector voltage has been pointed out by several authors (Adámek et al 2013, Bousselin et al 2013and Zanáška et al 2015.…”

“…The 2 mm BPP inner diameter (table 1) is similar to that used by Zanáška et al (2015) in their experiments. The importance of a high impedance buffer to measure the collector voltage has been pointed out by several authors (Adámek et al 2013, Bousselin et al 2013and Zanáška et al 2015. Following Zanáška et al (2015) a voltage follower using a Texas Instruments op-amp (OPA452) with input impedance of 10 13 Ω was used.…”

“…The second conclusion is inconsistent with the simple ball pen model which requires D>r Le . A further study was performed by Zanáška et al (2015) in a cylindrical magnetron with field strengths up to 40 mT and pressures from 1 to 20 Pa in argon. In their experiments the BPP agreed with a cylindrical Langmuir probe to within 1.5 V. The main conclusion of this work is that the BPP becomes operational when the field to pressure ratio, B/P, exceeded 10 mT Pa −1 .…”