Periodic forced flow in a nanosecond pulsed cold atmospheric pressure argon plasma jet

Abstract

This paper is devoted to the study of the argon flow modification in a cold atmospheric pressure plasma jet driven by nanosecond high voltage (HV) pulses, from single to multiple HV shots applications. A schlieren optical bench has been designed in order to visualize the argon flow downstream expansion in quiescent air, for moderate flow rates below 1 standard liter per minute. A coupled approach is used between charge coupled device (CCD) schlieren imaging and intensified CCD (ICCD) plasma plume imaging, both time-resolved. It is shown that the application of only one HV pulse (i.e. single HV shot) is enough to disturb the flow. The disturbed flow exhibits ripple propagation, on a timescale similar to the flow velocity. When operating in double HV shots, the second ionization wave can be used as a probe, to instantly visualize the flow structure any time after the first HV pulse application. For some flow rates, the ripple can increase in amplitude up to the point when it strongly deforms, or even stops, the plasma plume expansion, after which it is entrained by the flow and the plasma plume retrieves its full usual expansion. When a series of HV pulses are applied, the maximal disturbance of the flow is achieved for a certain pulse repetition frequency (PRF), specific of each flow rate. It is associated with ripples alternation in the plasma plume, in a 3D helicallike arrangement. For greater PRF, the ripples progressively vanish, and the flow is clearly less disturbed. Once the ripples have vanished, increasing further the PRF does not change the plasma plume and flow structures. We suggest that the repetitive plasma ignition mechanically forces the flow inside the capillary with consequences on the global flow structure, similarly to a forced backward-facing step flow with actuator.

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

“…It should be noted that this resolution can be easily tuned by imaging optics and pixel binning. The proposed system can be helpful to characterize plasmas which have small plasma size of a few mm or steep gradients [4,8,[30][31][32][33][34][35]. Note that the capability to extend 1D measurement is also highly useful to diagnose larger scale test articles as well.…”

Ar(1s5) density in a co-axial argon plasma jet with N2–O2 shielding

“…In this case, the average laminar length is of: The decrease in laminar length points to a more unstable jet for higher oxygen-fraction shieldings. Although we did not perform Schlieren measurements for a 4 kV pulse of 1 µs duration, for a 6 kV pulse of 500 ns of duration at 20 kHz of repetition frequency, the average laminar length, compared with the plasma off case, was reduced by: • ∼8 mm without co-flow shielding; The decrease in laminar length with plasma ON is expected as plasma-flow perturbations have been shown to disrupt the laminar flow [36,37]. Higher oxygen-fraction shieldings also seem to enhance plasma-flow perturbations, although this effect may be due to an already more unstable jet, as observed in the plasma off case.…”

“…Complementary Schlieren imaging was performed to evaluate the laminar length of the argon jet. These experiments were performed in an identical experimental setup to that of the work of Darny et al [36]. Without plasma and in the absence of co-flow (no co-flow), images show a laminar and almost cylindrical argon jet flowing from the nozzle, which eventually transitions into a turbulent flow at about 39.4 ± 2.2 mm from the nozzle.…”

“…A high flow rate might induce turbulence, subsequently influencing the trajectory of the IWs [33] and increasing the quenching of excited species due to an enhanced mixing [34,35]. The geometry of the reactor can also influence the IWs [11,19,32], potentially giving rise to plasma-induced flow perturbations [36,37]. In co-axial reactors, factors like nozzle shape, flow rate, and co-flow rate are crucial in determining turbulence and gas mixing [38].…”

“…In particular, the 811.531 nm line for the radiative transition Ar(2p 9 → 1s 5 ) can be used to probe the density of Ar(1s 5 ). Various studies have examined argon µAPPJs excited by a bell-shaped voltage pulse operating at 20 kHz and maximum amplitude of 6 kV [6,7,23,24,36]. These electric conditions allowed for reproducible IWs with a characteristic diffuse light emission [22].…”