7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Plasma potential fluctuations in cold micro-plasma jets : interactive surface
feedback effects on reactive species generation
Sudeep Bhattacharjee1, Michael Keidar2, and Deepika Behmani1
1

Department of Physics, Indian Institute of Technology Kanpur, 2 Mechanical and Aerospace
Engineering, George Washington University
e-mail (speaker): sudeepb@iitk.ac.in

Cold atmospheric pressure micro-plasmas, are nonequilibrium plasmas with widely different electron (Te ∼6000 K) and ion temperatures (Ti ∼ 300 K). Unlike
conventional low-pressure plasmas, these plasmas do not
require expensive vacuum and magnetic systems forgeneration and confinement, and can be produced rather
inexpensively in the ambient environment. The directinteraction of the charged particles with the ambient air
gives rise to a rich gaseous chemistry leading to theformation of reactive oxygen and nitrogen species(RONS) such as OH, HO2, H2O2, NO, N2O5 and radiationsuch as UV photons [1]. The RONS play a critical role inbiomedical applications such as sterilization, blood
coagulation, wound healing, and cancer therapy [2].
Moreover, the lower gas temperatures of these plasmas
provide a conducive environment for the treatment of
biological cells. In our recent work, it has been found thatthe fluctuations in plasma floating potential (FP) lie in thefrequency range from 0.5 to 9 kHz, and depend upon
experimental operating parameters [3]. The time scale ofthe observed fluctuation frequencies (~ 0.1 – 1 ms)
corresponds to the typical time scales of RONS evolution:
0.5 – 1 ms in RF [4] and ~1 ns – 100 s in pulsed jets [5].
Hence, fluctuations can influence RONS development,and affect particle and energy transport.
A schematic of the experimental setup is shown infigure 1. A ring-to-ring electrode configuration isemployed for generating the helium plasma jet by
applying a sinusoidal waveform of amplitude 11 kV (pp)and frequency of 10 kHz [3]. After ignition, the plasma
emerges out of the orifice of the capillary tube as a fine jetof diameter ~ 0.9 mm. The plasma plume is diagnosed bya single-pin probe, whose signal (voltage or current) is fed
to the input of a lock in amplifier for FP and current
measurements, using a Keithley multimeter. A TTL outputof the applied voltage signal and the signal of the internal
oscillator of the lock in amplifier have been used as

Figure 1 Schematic diagram of experimental setup tomeasure fluctuations in the jet interacting with the substrates

reference signals.
In this work, the fluctuations are investigated for a jetinteracting with substrates of varying permittivity such ascopper, p-type silicon, quartz, Teflon, and goat skin.
Beside FFT analysis, the amplitude of FP throughfrequency locked measurements has been estimated in thefrequency range from 0.5 to 9 kHz. Figure 2 shows theamplitude of FP as a function of frequency. The inset
shows the potential captured by the probe pin without any

Figure 2 Floating potential as function of frequency in caseof different samples, near the sample’s surface. The inset
shows the potential captured by pin probe without plasma

plasma which is considerably lower (approx. 5 order),implying that the lower frequencies arise after plasma
ignition. The peak amplitude of FP lies at around 1 kHz
and comparatively higher for low permittivity samples
such as goatskin, quartz, and teflon. The intensity oftypical RONS (OH and N2+) lines, especially OH,
drastically drops to about 98 % for quartz and teflon fromits maximum value (in case of copper), possibly due tohigher level of fluctuations. The results of the frequency
locked measurements are compared with the peak valueof the fluctuation frequencies determined from the FFT.
The plasma potential and plasma current locked at driving
frequency (10 kHz) have also been looked into, as afunction of applied voltage, to understand the effect ofsample’s permittivity on the ignition voltage of plasma.
References[1] Li Lin et al., Appl. Phys. Rev. 8 011306 (2021)[2] M. Keidar, Plasma Sources Sci. Technol. 24 033001(2015)[3] D. Behmani et al., AIP Adv. 11 085128 (2021)[4] W.V. Gaens et al., J. Phys. D: Appl. Phys. 46 275201(2013)[5] R. Barni et al., J. Appl. Phys. 97 073301 (2005)