Impact of electrical grounding conditions on plasma–liquid interactions using Thomson scattering on a pulsed argon jet

Abstract

The interaction between an argon plasma jet excited using microsecond duration voltage pulses and a liquid target was examined using Thomson scattering to quantify the temporal evolution of the electron density and temperature. The electrical resistance between a liquid target and the electrical ground was varied from 1 to $$680\, \text {k}\Omega $$ 680 k Ω to mimic different conductivity liquids while the influence of the varying electrical properties on the electron dynamics within the plasma were examined. It was demonstrated that the interaction between the plasma jet and a liquid target grounded via a high resistance resulted in typical dielectric barrier discharge behaviour, with two discharge events per applied voltage pulse. Under such conditions, the electron density and temperature reached a peak of $$1\cdot 10^{15}\, \text {cm}^{-3}$$ 1 · 10 15 cm - 3 and 3.4 eV, respectively; with both rapidly decaying over several hundreds of nanoseconds. For liquid targets grounded via a low resistance, the jet behaviour transitioned to a DC-like discharge, with a single breakdown event being observed and sustained throughout the duration of each applied voltage pulse. Under such conditions, electron densities of $$2{-}3 \cdot 10^{15}\, \text {cm}^{-3}$$ 2 - 3 · 10 15 cm - 3 were detected for several microseconds. The results demonstrate that the electron dynamics in a pulsed argon plasma jet are extremely sensitive to the electrical characteristics of the target, which in the case of water, can evolve during exposure to the plasma.

Towards plasma jet controlled charging of a dielectric target at grounded, biased, and floating potential

“…The influence of the grounding conditions of a target under plasma exposure has been examined before regarding the electronic properties of the discharge. For a different coaxial-plasma jet operated using argon gas instead of helium and 5 s pulses (16 kHz), it was shown that the resistance of a conductive liquid target towards the ground determines whether it behaves as a DBD-type discharge (for a high impedance path to ground) or a DC-type (low impedance path to ground) 20 , 28 . This had significant consequences for the temporal dynamics of electron density and temperature in the center of the plasma plume.…”

“…When the material is in direct contact with a plasma, the electric fields are induced by surface charge deposition and therefore this approach gives a way to examine the influence of the plasma on the target. This is complementary to other diagnostic techniques for the study of electrical properties of a plasma which focus on the characterization of the plasma plume, such as Thomson scattering, Stark broadening, Stark shift, or coherent anti-Stokes Raman scattering 11 , 16 – 20 .…”

Low-temperature plasmas in contact with liquids—a review of recent progress and challenges

“…The calibration may be performed by Raman scattering on a known amount of the molecular gas [160] or by Rayleigh scattering on a known amount of neutral gas particles [188]. In practice, the Thomson scattering method requires the use of high-power lasers for a sufficient signal and triple grating spectrometers or Bragg grating for the removal of Rayleigh scattering from atoms and molecules and a stray light [137,159,189]. Besides the stray light, the nonintrusive influence of a laser on the observed plasma should be verified, so laser heating, optical pumping, detachment of the negative ions etc, should be excluded [161].…”

“…The Thomson scattering technique is successfully applied for the diagnostics of a bulk plasma in contact with liquid in several configurations [137,189,191,192]. Thomson scattering measurements in near-liquid surface plasma are extremely challenging due to the elastic laser scattering on the surface and due to a Raman scattering on liquid water [193].…”

High resolution spatially extended 1D laser scattering diagnostics using volume Bragg grating notch filters

“…Thus, to detect Raman and Thomson scattering signals, the light near the laser line (Rayleigh scattering/stray light) should be effectively removed to prevent saturation of the detector. Such laser line rejection can be performed with multiple techniques such as a triple grating spectrograph [4][5][6][7][8][9][10][11][12], a vapor cell [13][14][15], a glass/interference filter [16], a physical mask [17,18], and a volume Bragg grating (VBG) filter [19][20][21][22][23][24][25][26].…”