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Abstract
In this article we present burst-mode nitric oxide laser-induced fluorescence (LIF) measurements at 250 kHz probing the first 500 µs evolution of a nanosecond repetitively pulsed discharge in the spark regime. The discharge energy was varied from 140 µJ to 180 µJ by adjusting the excitation voltage. The probe laser beam at 226 nm was produced by doubling the 452 nm output of an optical parametric oscillator (OPO) pumped by 355 nm output of a burst-mode laser. It was observed that the nitric oxide produced by the discharge developed a toroidal distribution around the electrode central axis over the first 200 µs after the last spark discharge with the maximum local nitric oxide concentration of 1.8 × 1024 m−3. The expansion of the toroidal distribution was tracked over time and space by the pulse-burst laser pumped OPO system for the first time.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Laboratory testing of fast dynamic phenomena in fields of plasmas, hypersonics, combustion, etc., benefit from non-intrusive and fast diagnostic techniques. Laser diagnostics in particular provide non-intrusive and short exposure measurements, among other electromagnetic methods [1,2]. Recent advancements in optical materials and multilayer dielectric coatings enabled development of high energy, high repetition rate systems [3]. Commercialization of pulse-burst laser technology has provided new capabilities for measuring high speed dynamic and transient phenomena with the increased repetition rate and user-friendly operation [4].
Laser diagnostic techniques that rely on a specific wavelength of the laser output are molecular tagging, laser-induced fluorescence (PLIF/LIF), and those based on atomic filtering. Typically, the desired result is achieved either by carefully selecting atomic/molecular transitions to match a commercially available laser, such as Filtered Thomson Scattering [5], or use of frequency tuning devices – optical parametric systems or dye lasers [6,7]. The significant advantage of OPOs over more conventional dye lasers is the solid-state gain media that allows integration with high repetition rate pump systems: laser clusters or pulse-burst lasers (PBLs) [8–10]. This brings tunable laser radiation to 250 kHz and even MHz repetition rates with bursts of 1 – 10 ms duration. Current work on design and implementation of OPOs includes: efforts to bring OPOs to chip-scale light sources [11]; improved tunability range of uninterrupted narrow linewidth operation [12]; integration of OPOs with pulse-burst lasers – previous works [4,13,14] as well as the study presented here.
In this work we present an implementation of an unseeded OPO pumped by a pulse-burst laser at 250 kHz to produce long bursts of 226 nm laser radiation for nitric oxide formation in nanosecond repetitive pulsed (NRP) spark discharges. Repetition rate was chosen based on the previous studies that suggested a minimum repetition rate of 50 kHz for an efficient capture of flow dynamics, and pump laser radiation available [15]. Spark discharge regime was chosen over both glow and corona regimes because of higher temperature and electron number density produced [16–18]. A series (3 to 25 pulses) of 10 ns FWHM spark discharges were spaced by 10 $\mu s$ time intervals. Formation of nitric oxide in the afterglow was observed using a 250 kHz imaging system synchronized with the PBL laser.
2. Experimental methods
Optical parametric oscillators utilize a solid-state gain media with a nonzero second-order susceptibility $\chi ^{(2)}$, placed inside a singly or doubly resonant cavity [19]. A smooth wavelength tuning is achieved by varying crystal angle with respect to the pump beam. Such continuous tuning is possible because of the birefringent properties of the crystal [20]. Continuous tuning range for a seeded OPO in visible spectrum of up to 200 nm (450-650 nm) was demonstrated previously [12]. In this experimental study, a singly resonant cavity design was used in order to minimize the total optical exposure on reflective coatings of the cavity and minimize the chance of laser-induced damage (LID). Two Type I $\beta - Ba(BO_2)_2$ crystals of 10x8x7 (HxWxL) $mm^3$ dimension pre-cut at $\theta = 31^{\circ }$ with respect to the optical axis were chosen for the OPO’s gain media. The counter-rotating design of two BBO crystals reduced total costs, allowed replacement of only half the gain media in case of LID, and eliminated beam walk-off as the second crystal doubled as a compensator. A custom lever-based crystal holder was designed and built in-house allowing precise independent angle tuning within a minimal cavity length.
A pump beam was supplied by a custom Continuum pulse-burst laser (PBL) based on Nd:YAG gain media. A conventional master oscillator power amplifier (MOPA) configuration provides stable operation at 1 MHz pulse repetition rate with burst duration of up to 10 ms. Each subsequent burst is delayed by 10 seconds allowing heat dissipation and cooling. Fundamental 1064 nm pulses had 11 ns FWHM duration and were doubled and mixed upstream to produce 4.3 ns FWHM 355 nm third harmonic. For this study, the OPO was pumped by 355 nm laser output at 250 kHz with an average energy per pulse of 55 mJ.
Figure 1 shows a complete optical setup of the OPO with wavelength separation components. The cavity is constructed by two flat mirrors (Lattice Electro Optics, Inc.): a high reflectivity back mirror and 80% reflectivity output coupler designed for a signal range of 450-650 nm. Both mirrors are designed for transparency to the idler beam (780-1680 nm). Two pump beam dichroics are installed inside the cavity to couple the pump 355 nm beam in and out. This significantly elongated the cavity, however it allowed manufacturing of better performing coatings for the 80% reflective output coupler. Several key cavity parameters are: finesse $\mathcal {F} = 25$, mode spacing $\Delta \nu _c = 1.5 \space$ GHz, and total linewidth $\Delta \nu _{broad} = 500$ GHz. Signal conversion efficiency of 1 % results in 452 nm beam energy of 0.5 mJ per pulse. A relatively low conversion efficiency was dictated by a significantly lowered laser-induced damage threshold (LIDT) of the crystal protective coatings at 250 kHz and low energy density of the pump beam. Signal beam is then doubled by a BBO doubling crystal placed outside the cavity to produce a 226 nm UV beam. A residual 452 nm beam is used to monitor the OPO output by routing it into Princeton Instruments IsoPlane 160 spectrometer equipped with a PI-MAX4 ICCD camera. An efficient separation of UV beam from the rest of wavelengths is achieved with a Pellin-Broca prism shown in yellow in Fig. 1. The UV beam was subsequently routed to a reference cell or an experimental setup.
Fig. 1. Optical setup schematic with ray-tracing of the primary beams; 452 nm OPO signal beam profile.
In this experiment, the focus was on nitric oxide laser-induced fluorescence as means to study production and spatial evolution of a NO plume. A-X(0,0) band of nitric oxide, that corresponds to 226 nm absorption line, was chosen based on previous studies that showed the highest fluorescence level in this band within our accessible range of 225 - 325 nm [21]. Close spacing between absorption spectra of A-X(0,0) also allowed for an optimal coupling of energy from a broadband energy source such as an unseeded OPO. Based on the spectrometer measurements of the unseeded OPO 452 nm output and wavelength optimization done using a 5 torr nitric oxide reference cell, broadband UV output was estimated to have a central wavelength located at 226.076 nm with FWHM linewidth of 0.187 nm (1.1 THz). Energy per individual 226 nm pulse was around 10 $\mu J$. Plasma in ambient air was generated by a FID FPG 30-100NM high frequency nanosecond voltage pulser. The electrical circuit and overall experimental setup is shown in Fig. 2. A potential difference was applied to a 1.8 mm air gap between two conical electrodes. Maximum voltage drop across the gap was 700 V with maximum current flowing at 35.5 A. Current was monitored by a Pearson current monitor, while voltage was measured by a 200 MHz bandwidth Cal Test CT4028 High Voltage Oscilloscope Probe. The dissipated energy ranging from 140 $\mu J$ to 180 $\mu J$ was calculated by time integration of the voltage-current product. Plasma discharges were created at a 100 kHz repetition rate with 2-25 pulses in a single burst.
Fig. 2. Set-up of the laser system and discharge circuit; experimental timeline and camera synchronization.
The UV laser beam was focused by a 125 mm focal length lens and had a diameter of approximately 1 mm between the electrodes. A photodiode was placed behind the experiment to monitor pulse-to-pulse variations of the laser beam intensity. This allowed effective noise reduction in LIF data during post-processing.
Timing of the system was controlled by two Berkeley Nucleonics (BNC 577) delay generators as shown in Fig. 2. The master trigger pulse, corresponding to the first laser pulse, was sent from the internal laser triggering system to the first BNC delay generator set to 100 kHz. It triggered a set number (from 2 to 25) of plasma discharges and, after the last plasma discharge, sent a signal to the second BNC delay generator set to 250 kHz. The second delay generator, delayed by 18 $\mu s$ after the last plasma discharge, triggered both a Shimadzu HPV-X2 camera and LaVision IRO X S20 (Intensified Relay Optics) equipped with a CERCO UV 100 F/2.8 by SODERN lens. IRO was used for providing gain to the weak NO fluorescence as well as accurate exposure gating. A total of 125 frames with 200 ns exposure spanning half a millisecond were recorded for each laser burst.
For a quantitative number density estimate the working equation for laser-induced fluorescence is needed. In the weak excitation limit, the fluorescence signal becomes [22]
The reference cell used was a sealed 5 torr nitric oxide cell without any buffer gas (25 mm in diameter and 200 mm of length). The value for $\sum _{\nu "}A_{\nu '\nu "} = 4.95\times 10^6$ $s^{-1}$ was found at the experimental study of Piper and Cowles [24]. The assumed value of quenching cross-section in air ($O_2 - 30$ Å$^2$, $N_2 - 0.014$ Å$^2$) and self-quenching in nitric oxide (40.1 Å$^2$) were obtained from the study by Drake [25]. A temperature of 1000 K was assumed for quenching rate calculation in the vicinity of plasma, based on a heat transfer model and previous works [15,26]. Measurement in the reference cell yielded a fluorescence signal relative standard deviation (RSD) of $45\%$ which is attributed to dynamic variation in the spectral mode structure of the OPO. The NO concentration presented hereafter should be interpreted within the context of the RSD and quenching assumptions above.
3. Results and discussion
The aim of the experiment was on spatial evolution of NO produced by NRP discharges. Due to high repetition rate of both laser and data acquisition, we were able to acquire the first, to the best of our knowledge, nitric oxide number density spatial distribution after a single production event with a 4 $\mu s$ resolution.
Figure 3 shows a data post-processing method (Figs. 3(a)–3(d)) as well as a graphical 3-D illustration (Figs. 3(e), 3(f)) of the nitric oxide, shown in orange, distribution shift over time from cylindrical to the suggested toroidal shape, which does not, however, possess a perfect symmetry. The position of electrodes is shown in grey and the UV laser beam is shown in purple. Unprocessed LIF images (red line indicates the electrodes’ central axis) at 18 $\mu s$ and 250 $\mu s$ with the quenching rate corrected reference image are shown in Figs. 3(a)–3(c). Integration of the vertical spatial axis, as shown in Fig. 3(d), represents the horizontal distribution of nitric oxide. Such horizontal distributions at varying time delay are later stacked and presented in Fig. 4(c)–4(d), after appropriate background and laser scattering subtractions.
Fig. 4. Spatial evolution of nitric oxide plume for 10 and 25 discharge pulses: raw LIF images (a)-(b); temporal summary (c)-(d).
Figure 4 shows an ensemble of single-shot raw LIF images (Figs. 4(a) and 4(b)) to illustrate nitric oxide 2-D behaviour as well as a temporal summary of horizontal variation of NO concentration in the first 500 $\mu s$ after 10 and 25 plasma discharges, Figs. 4(c) and 4(d), respectively, for 180 $\mu J$ deposited per discharge. Signal to noise ratio (SNR) of the LIF images has values ranging from 9 to 11 for higher fluorescence measurements during the first 200 $\mu s$ and slowly decreases for longer time delays. Since NO is produced under ambient conditions of NRP spark discharge, an accumulation of nitric oxide over longer plasma bursts is observed with the highest local concentration of up to $1.8\times 10^{24}$ $m^{-3}$, assigned to 1 on the relative scale of Figs. 4(c) and 4(d). The streaking in the figure is attributed to the mode competition within the OPO output resulting in varying energy coupling into absorption bands of NO with RSD of $45\%$. Initial distributions of nitric oxide were observed to be peaked in close proximity to the electrode centerline. After several hundred microseconds, the spatial distribution evolves away from the centerline and acquires a more bimodal form, consistent with vortex ring formation from baroclinic torque. The evolution of the OH radical distribution into a toroidal shape under similar conditions was previously reported by Stepanyan, et al. [15]. A study by Singh and Bane also supports the proposed shape formation by showing significant vortex rings induced by a spark discharge creating flows directed outward pushing the hot gas kernel [27]. Dumitrache, in his study of laser-induced breakdowns, showed formation of similar asymmetric torus shapes as part of the kernel evolution [28]. It was observed, the higher the energy deposition, the higher the nitric oxide concentration will be accumulated. That is attributed to higher heating of the air and consequently more favorable conditions for NO production. Another notable trend is the nitric oxide "torus" radius increases with both time and number of pulses. We hypothesize that the total number of pulses affects the magnitude of torque applied to the flow as well as gasdynamic expansion of the heated region. Consequently, the maximum number of plasma pulses (25) results in the widest distribution of NO.
Figure 5 shows the total LIF signal collected, integrated over both space (entire region of interest) and time (all 125 shots spanning 500 $\mu s$) for varying energies deposited per discharge and number of plasma discharges. For the first 10 plasma discharges we see a close to linear growth of LIF signal that steadily drops for the higher number of pulses. The drop in LIF signal is attributed to both diffusive effects and baroclinic torque present over a longer time after the first plasma pulse, as well as slow dissociative effects. Both plasma energies follow similar trends with, in general, the highest energy deposited resulting in the highest LIF signal collected. LIF signal fluctuations are primarily caused by plasma instability as well as variations in the flow dynamic resulting in different overlap of the NO plume with the UV laser beam. Data for one or two plasma discharges was not represented in this study as plasma was unstable and the definite transition from streamer to spark happened for the second or third discharge within plasma burst.
Fig. 5. Average LIF signal over 5 runs as a function of number of pulses and voltage, with indicated min/max values.
4. Summary and conclusions
Recent advancements in pulse-burst laser technology allow for the production of stable high energy bursts of third harmonic Nd:YAG radiation for pumping optical parametric oscillators at rates exceeding 100 kHz. In this article, we present the first, to the best of our knowledge, nitric oxide laser-induced fluorescence study of the spatial evolution of a NRP spark discharge using an OPO pumped at 250 kHz by a pulse-burst laser. A direct relationship between the number of plasma discharges per burst, individual energy deposited per discharge, and amount of NO generated is clearly established. A maximum local NO concentration as high as $1.8 \times 10^{24}$ $m^{-3}$ was measured during first 50 $\mu s$ of the plume development. A maximum in the integrated LIF signal was observed with 10 discharges within the plasma burst, with signal gradually decreasing with more discharges. This behavior was attributed to high temperature and dissociation of NO with increasing energy deposition.
Funding
Army Research Office (W911NF-19-2-0243).
Acknowledgments
The authors thank Dr. Albina Tropina for a valuable discussion and Mr. John Kochan for technical support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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