Time-resolved measurements of electron density in nanosecond pulsed plasmas using microwave scattering

Journal of Applied Physics

Bane

et al. 2021

Self Cite

We present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in single-pulse mode (no memory effects). Discharge parameters were measured using microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, and fast photography. Spark and corona discharge regimes were studied for electrode gap sizes 2-10 mm and discharge pulse duration of 90 ns. The spark regime was observed for gaps < 6 mm using discharge pulse energies of 0.6-1 mJ per mm of the gap length. Higher electron number densities, total electron number per gap length, discharge currents, and gas temperatures were observed for smaller electrode gaps and larger pulse energies, reaching maximal values of about 7.510 15 cm -3 , 3.510 11 electrons per mm, 22 A, and 4,000 K (at 10 s after the discharge), respectively, for a 2 mm gap and 1 mJ/mm discharge pulse energy. Initial breakdown was followed by a secondary breakdown occurring about 30-70 ns later and was associated with ignition of a cathode spot and transition to cathodic arc. A majority of the discharge pulse energy was deposited into the gas before the secondary breakdown (85-89%). The electron number density after the ns-discharge pulse decayed with a characteristic time scale of 150 ns governed by dissociative recombination and electron attachment to oxygen mechanisms. For the corona regime, substantially lower pulse energies (~0.1 mJ/mm), peak conduction current (1-2 A), and electron numbers (3-510 10 electrons per mm), and gas temperatures (360 K) were observed.

Section: Introductionmentioning

confidence: 99%

Experimental study of atmospheric pressure single-pulse nanosecond discharge in pin-to-pin configuration

Journal of Applied Physics

Bane

et al. 2021

Self Cite

“…The individual electron cross-section correspondingly scales as . Previous experiments on microwave scattering off microplasmas have been conducted in this regime as discharges were operated at elevated pressures 3 – 5 , 9 , 10 , 35 . A detailed knowledge of is consequently required for an accurate characterization of scattering.…”

Section: Resultsmentioning

confidence: 99%

“…The Thomson regime is particularly advantageous for the evaluation of photoionization rates and cross-sections in comparison with collisional scattering at 1 atm used previously 4 , 5 , 9 , 10 . In addition to -independent measurement results in the Thomson regime (as discussed in previous paragraph), it also enables measurements for an extended range of laser intensities by delaying the onset of nonlinear optical phenomena to even higher laser intensities.…”

Section: Resultsmentioning

confidence: 99%

“…Emitted radiation can then be measured/isolated using temporally resolved frequency mixer technologies and attributed to an absolute electron count after system calibration 3 . Both calibrated and uncalibrated variants of in-phase coherent microwave scattering (CMS) have become valuable non-intrusive diagnostic techniques in applications ranging from photoionization and electron-loss rate measurements 3 – 10 to trace species detection 11 , gaseous mixture and reaction characterization 12 – 14 , molecular spectroscopy 15 , and standoff measurement of local vector magnetic fields in gases through magnetically-induced depolarization 16 . Notable advantages of the technique include a high sensitivity, good temporal resolution, low shot noise 11 , non-intrusive probing, and the capability of time gating due to continuous scanning.…”

Section: Introductionmentioning

confidence: 99%

“…A careful understanding of electromagnetic-plasma and scattering interactions is essential for the proper extraction of information on total electron numbers and subsequent derivable quantities (e.g., the electron number density ) from CMS. Vast historical art exists for weakly-ionized, strongly collisional microplasmas in which electron motion is restricted by electron-neutral collisions 4 , 5 , 9 , 10 . However, application of the diagnostic to collisionless plasmas has remained relatively unexplored (no N e correspondence is given by 11 , 13 , 18 )—despite garnering some attention in recent years 19 due to a prevalence in studies on electric propulsion devices 20 , photoionization in low-pressure conditions 21 , 22 , etc.…”

Section: Introductionmentioning

confidence: 99%

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Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

Ranjan

et al. 2021

Sci Rep

Self Cite

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5–12 GHz) microwave scattering off laser induced 1–760 Torr air-based microplasmas (287.5 nm O2 resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons $$N_{e}$$ N e in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson “free-electron” regime—where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract $$N_{e}$$ N e from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Combined microwave and laser Rayleigh scattering diagnostics for pin-to-pin nanosecond discharges

Journal of Applied Physics

Shashurin

2021

In this work, the temporal decay of electrons produced by an atmospheric pin-to-pin nanosecond discharge operating in the spark regime was measured via a combination of microwave Rayleigh scattering (MRS) and laser Rayleigh scattering (LRS). Due to the initial energy deposition of the nanosecond pulse, a variation in the local gas density occurs on the timescale of electron decay. Thus, the assumption of a constant collisional frequency is no longer applicable when electron number data are extracted from MRS measurements. To recalibrate MRS measurements throughout the electron decay period, temporally resolved LRS measurements of the local gas density were performed over the event duration. The local gas density was calculated to be 30% of the ambient level during the later stages of electron decay, and it recovers at about 1 ms after discharge. A shock front traveling approximately 500 m/s was additionally observed. Coupled with plasma volume calibration via temporally resolved intensified charge-coupled device imaging, the corrected decay curves of the electron number and electron number density are presented with a measured peak electron number density of 4.5 × 1015 cm−3 and a decay rate of ∼(0.1–0.35) × 107 s−1. A hybrid MRS and LRS diagnostic technique can be applied for a broad spectrum of atmospheric-pressure microplasmas where a variation in the gas number density is expected due to energy deposition in the discharge.