Femtosecond two-photon laser-induced fluorescence imaging of atomic hydrogen in a laminar methane–air flame assisted by nanosecond repetitively pulsed discharges

The PAC burner consists of a laminar stagnation plate burner fed with a lean methane–air pre-mixture and a nitrogen co-flow, shown schematically in figure 2. The average bulk velocity of the flammable mixture is 1.2 m s⁻¹ and the equivalence ratio is set to 0.76. For these conditions, the thermal power of the flame is about 220 W. The flame stabilizes roughly in the middle of a 10 mm gap between the nozzle and a quartz stagnation plate. Operating conditions are carefully chosen in order to have a stable flame, which is required for phase-locked imaging of the discharges.

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The burner is made out of non-conductive material (polyether ether ketone), allowing an easier integration with high voltage components. Additional details of the burner can be found in [31]. Two pin electrodes are used: one placed in the quartz plate and the other located inside the nozzle along the symmetry axis of the flame. The cathode is made out of a 1 mm diameter pure tungsten welding electrode and features a conical tip. The anode instead consists of a thin electrochemically etched tungsten wire. Even though this wire is only 0.1 mm in diameter, it still affects the velocity profile of the flow at the outlet, preventing the flame surface from being flat (see figure 3(a)). This electrode arrangement allows the discharge to cross the flame perpendicularly and to develop in both cold and hot gases. A high voltage nanosecond discharge generator (FPG Series, FID GmbH) is connected to the electrodes for the creation of the discharge. A BNC 575 delay generator (not shown in figure 1) is employed to generate the desired pulse repetition frequency and to synchronize the discharges with the laser system and camera. Tuning the delay between the discharge pulse and the camera acquisition allows to perform TALIF imaging at an arbitrary time after the discharge event. We observed an overall jitter of about ±13 ns that limited our ability to investigate the hydrogen fluorescence very close to the onset of the discharge.

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NRP discharges can appear and behave quite differently in different experimental condition where parameters such as voltage, pulse repetition frequency, voltage rise-time, electrodes geometry, gap spacing, gas composition in the gap, pressure, etc are known to affect the discharge [32]. Discharges in the glow regime [33, 34] are considered in the experiments discussed in this paper. This particular non-thermal discharge regime allows low power deposition, high chemical reactivity, and low gas temperature, making it interesting for applications [33].

A high voltage probe (Tektronix P6015A) is used to measure the voltage applied to the electrodes and a Pearson current monitor 6585 for measuring the current. Typical results are shown in figure 4. The purpose of these electrical measurements is to provide estimations of the plasma power. The instantaneous discharge power can be computed by multiplying voltage and current traces, and the energy is obtained by integrating the power over the duration of a discharge event. In the voltage trace shown in figure 4, one can recognize the 10 ns long high voltage pulse, and observe some ringing and a significant overshoot. Interestingly, the energy transfer seems to occur during the first 140 ns after the high voltage pulse before settling to its final value.

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In this study, the chosen applied voltage is 8.4 kV and the pulse repetition frequency is set to 10 kHz. Under these conditions, typical values of plasma power are 1–2 W, corresponding to less than 1% of the thermal power of the flame.

The PAC burner is mounted on top of a translation stage, which moves vertically (perpendicularly to the page plane in figure 1) and it is synchronized with the camera, allowing fast multi-frame acquisitions of the same object at different heights for an automatic flame scanning. Alternatively, the PAC burner can be replaced by a McKenna burner with a central nozzle capable of generating a narrow cone flame, suitable for calibration measurements.

A Ti:sapphire chirped pulse amplification (CPA) femtosecond laser system (coherent, Hidra-50) delivers 800 nm laser pulses with a duration of 125 fs, at 10 Hz repetition rate. The laser beam then pumps a travelling wave optical parametric amplifier (Light Conversion, HE-TOPAS-PRIME) followed by a frequency mixing apparatus (NirUVis unit), which is finally capable of providing the required 205 nm laser pulses with pulse energy ∼35  μJ/pulse for exciting atomic hydrogen, from n = 1 to n = 3, while detection occurs at 656 nm [15]. The 205 nm laser beam, roughly 5 mm in diameter, is then focused with a cylindrical lens to form a vertical laser sheet, with the same height of 5 mm and an estimated thickness of about 200 μm, right across the flame in the PAC burner.

An intensified charge-coupled device (ICCD) camera (Princeton instruments, PI-MAX4 1024f), fitted with a Nikon Nikkor 135 mm f/2.8 lens is used to capture the 656 nm H-atom fluorescence. The camera is synchronized with the laser and is setup in a 90°-side configuration perpendicular to the laser beam propagation direction. Typical camera acquisition settings are 3 ns gate width and 150 on-chip accumulations. Suppression of the background radiation is achieved with a narrowband band-pass interference filter with a center wavelength at 655 nm (Semrock, FF01-655/15-25).

Simultaneously with fluorescence measurements, an energy meter (Gentec, SOLO 2) placed right after the burner, is used to continuously log the laser pulse energy. A spectrometer (Princeton instruments, Acton SP2500, spectral resolution ∼0.018 nm) is used to facilitate spectral analysis of the emission signals.

Figure 5(a) shows the measured fluorescence spectrum. The spectral peak is centered at 656 nm and the peak intensity is sensitive to the detuning of the excitation wavelength, confirming that the fluorescence signal comes from H atoms. Figure 5(b) shows the excitation spectrum, from which we can see that the optimal excitation wavelength for detecting H atoms is about 204.67 nm. The rather broad excitation peak is a result of the large linewidth, about 0.5 nm, of the fs excitation laser.

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In ideal conditions the TALIF process would result in a squared dependence of the fluorescence yield on the pump laser energy. However, the laser pulse could photolytically generate additional H [35] that is then detected, leading to a higher than quadratic energy dependence. Two main precursors, H₂O [28] and CH₃ [36, 37], are both abundant in the flame. At the same time, stimulated emission could lead to a dependence index lower than quadratic [38, 39]. This latter possibility can be ruled out since we did not observe any signature of stimulated emission of H atoms in the forward or backward direction. Additionally, interference from O₂ photolysis, leading to O-atom production, that can deplete the H-atom population is also ruled out, given that the laser fluence used in this experiment (6 mJ cm⁻²) is more than 60 times lower than reported fluence thresholds for similar conditions [35].

In order to estimate the possible impact of photolytic interference, we measured the laser pulse energy dependence of the H fluorescence signal, as shown in figure 6. For this characterization, the PAC burner is operated without applying any discharge. Each experimental data point accumulates the fluorescence signal over 100 laser shots, while the error bar shows the laser pulse energy standard deviation over the same laser shots. A second order power function is also shown together with experimental data in figure 6, showing a good agreement. It suggests that in the range of the pump pulse energies we employed in the experiments, photolytic interference is not a significant issue.

A more robust way to evaluate effect of photolytic interference would be to analyze the spatial shape of the fluorescence signal for different laser pulse energies [35, 40], since even in case of severe photolytic interference sometimes the power-law may still not significantly differ from 2 [35]. This more advanced validation procedure will be performed in further investigations dedicated to quantitative measurements of atomic species densities.

In a real laser sheet, the laser intensity is not uniform. It has a 2D distribution, depending on the actual beam profile and how the beam is focused into a sheet. Since the fluorescence response depends on the laser intensity squared, spatial non-uniformity of laser intensity has a direct and significant influence on the fluorescence distribution, and must be accounted for.

The laser intensity profile can be characterized by measuring the TALIF signal from a known spatial distribution of atomic H. Unfortunately, it is not possible to fill a volume with a uniform concentration of atomic H, and other techniques must be used. For example, in [41] a homogeneous concentration of krypton was employed, since krypton features similar excitation and detection scheme. The resulting TALIF map was used to normalize the measurements, correcting for the non-uniform laser intensity distribution.

In the present study, a different approach is used to obtain the correcting map. The atomic H naturally present in a ∼25 mm tall CH₄–air conical flame is used to probe the laser sheet at different locations. This flame has been chosen because the H radial profiles at slightly different heights are very similar (this assumption is acceptable if the height of the laser sheet is small compared to the height of the cone flame [18]). The laser beam crosses the flame around its middle part, generating a TALIF signal as shown in figure 8(a). Around 130 TALIF images are collected while moving the flame along the laser beam. By properly averaging all those images, an intensity map can be obtained, as in figure 8(b), that approximates the TALIF response to an uniform atomic H field. Figure 8(b) confirms that the laser intensity in the sheet is far from uniform.

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In the following, this correction map will be referred as laser beam profile. The characterization just described is conducted twice, before and after any set of measurements, and the average map is used for correction. It is worth noting that the laser beam profile obtained with this method is also affected by the non-uniform pixel responsivity and intensifier gain. These effects are corrected as well when a TALIF raw image is divided by the correction map.

Several measurements are performed at different heights (z) by moving the burner vertically in order to cover the full region of interest. In these measurements, 7 different positions are considered with a ∼0.9 mm displacement between two adjacent positions. Recall that typical camera acquisition settings for each frame at a certain position are 3 ns gate width and 150 on-chip accumulations. Given that the laser repetition frequency is 10 Hz, the time required for a complete measurement is roughly 2 min. Examples of these frames at different z can be seen in figure 9(a).

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In this paper we present TALIF images that are collected at different times after the discharge event. We define t = 0 as the time when we observe the maximum emission from the discharge. At around 656 nm there is a broadband plasma emission [3] whose intensity, integrated over a 3 ns gate width, appears to be a couple of order of magnitude larger than the H fluorescence signal. This fact, together with the ∼±13 ns jitter discussed in section 2.1, prevents acquisition of meaningful data close to t = 0. The strong plasma emission quickly decays via fast collisional quenching, which takes ∼30 ns [42]. After about 50 ns the background plasma emission becomes negligible and reliable data of the H fluorescence can be acquired. Measurements are repeated twice to evaluate repeatability.

Several corrections are applied to the raw TALIF images (see figure 9(a)), as discussed in the following:

• Background correction: the background images are captured with the laser excitation wavelength tuned 3 nm off-resonance, and subtracted from the TALIF images.
• Beam non-uniformity and camera correction: each raw image is corrected by using the laser beam profile obtained as described in section 3.1. Thanks to that, corrections for non-uniformity in the laser intensity distribution, in pixel responsivity and intensifier gain can be accomplished.
• Collection efficiency correction: the flame stabilizes few mm away from a large quartz plate. For this reason, part of the fluorescence emitted close to the plate will not be able to reach the camera, and because of the occlusion by the plate a lower signal intensity will be recorded. The collection efficiency is calculated on geometrical grounds. The occlusion scheme will be different for each burner position, so each raw image frame in figure 9(a) has to be multiplied by its own corresponding collection efficiency.
• Energy correction: each image is re-scaled using the average squared pulse energy during the collection of each image itself, to correct for any laser energy fluctuation. We noticed that this procedure generally leads to a slight over-correction, however it still helps reducing data spread between repeated measurements.

After corrections, individual frames are merged by means of a weighted average, using the laser intensity profile described in section 3.1 as weight. A similar approach for images stacking can be found in [21] (page 72). Finally, the processed TALIF image, after all corrections and stacking of all frames (figure 9(a)), is achieved as shown in figure 9(b).

Once non-uniformity effects in the laser beam are taken into account and provided that photolytic effects are negligible, collisional quenching of excited atoms is the main source for the difference between the fluorescence map and the H concentration map. The quenching rate affects the fluorescence yield [22], and it depends on the nature and the number density of the collisional partners. According to [19] the quenching rate can be up to 7 times larger in the cold gases compared to the hot flame front region, therefore different locations having the same TALIF signal level might have quite different H number density. Following [43], the H-TALIF signal can be roughly expressed as S_TALIF ∝ N₀/(Q₂ + A₂) where N₀ is the concentration of H atoms in the ground state, Q₂ the quenching rate and A₂ the Einstein coefficient for spontaneous emission. The quenching rate, in principle, could be calculated from the position-dependent temperature and flame composition and the species and temperature dependent quenching cross sections [18]. Besides the difficulties of mapping the 2D temperature and composition, a comprehensive set of the quenching cross sections of atomic hydrogen is also not available for the current experimental conditions. Most of the available quenching rate constants [29] are provided at room temperature and it is not clear how to extrapolate them to high temperature [18, 21]. Therefore, quantitative analysis of H-TALIF signal is challenging and direct point measurements of quenching rates are preferred for quantitative TALIF [21]. Two-dimensional quantitative measurements is in principle also possible for TALIF of H by using fluorescence lifetime imaging [44].

Nevertheless, if local conditions (such as composition and temperature) are not very different, then the fluorescence intensity can be used to infer trends of actual atomic H concentration. This allows us to estimate the relative change in H concentration in specific points. As an example, at the flame tip, we can assume that collisional quenching does not vary dramatically when we apply discharges. This is supported by the fact that NRP glow discharges do not change the flame temperature much (see for example [45]), neither the concentration of major species (see for example [42]). Then the relative change in the TALIF signal can be assumed to represent the change in H number density.

For a methane–air flame of 0.76-equivalence ratio at ambient conditions, 1D simulations of a wall stabilized flame (using Cantera and GRI-Mech 3.0) predict a peak H number density at the flame front, close to 8 × 10¹⁵ cm⁻³, in the case without plasma. Based on the results presented in figure 11, a rough estimation of the H density in the discharge channel can be carried out. Using the same procedure as described in [19], taking the quenching constants and H natural fluorescence lifetime from [21, 29] and the estimated species concentrations form the 1D simulation, we found the fluorescence yield A₂/(A₂ + Q₂) to be about 5.6 times lower in the cold gases than in the flame front. At about 150 ns after the discharge, the average fluorescence signal in the part of the discharge occurring in the fresh gases (dark blue curve, z = 1 mm) is about 40% of the peak intensity in the flame front (black curve, z = 4 mm). From those numbers, we obtain that the NRP discharges produce about 1.8 × 10¹⁶ cm⁻³ of atomic H, upstream of the flame front. This local production of atomic H, comparable with the atomic H density naturally present in the flame, could explain the strong effect of the NRP discharges on the stabilization height of the flame. Assuming that the electron impact reaction (CH₄ + e → CH₃ + H + e) is the main source of atomic H production, this atomic H density would correspond to about 1% of dissociation of CH₄. Quantitative measurements are necessary to validate this simplified analysis.

A recent study on a CH₄–air Bunsen flame [18] showed that the peak of fluorescence emission was located at the flame tip. That fluorescence peak was attributed to the diffusional focusing of H radicals. In our case, even without plasma actuation, we observe a significant fluorescence response in the flame tip region (figure 10(a)). Flame curvature and fast H diffusion in the hot gases could play a role there. A local increase of atomic H fluorescence at the flame tip could also be induced by a higher local flame temperature, increasing the local fluorescence signal. Higher temperatures can lead to (i) a higher atomic H production, (ii) a decrease in the overall gas density and (iii) a change in the quenching rates. By changing the unburned temperature in the 1D simulation described in the previous section, and estimating the fluorescence response as $\propto \left[\mathrm{H}\right] {A}_{2}{\left({A}_{2}+{Q}_{2}\right)}^{-1}$, we verified that the overall effect of a temperature increase would be an increase in both the atomic H fluorescence and number density.

Plasma forcing causes an increase in the atomic H fluorescence and thus an increase of concentration as well, if assuming similar quenching environment. This can be observed in the 2D fluorescence images in figure 10 as well as in figures 11 and 13. In the flame tip region, crossed by the discharge, a local increase of the H concentration up to 50% is observed. This production of atomic H is usually referred to during analysis of the chemical impact of nonequilibrium discharges on combustion [2, 9]. However, only a few studies presented some observations of atomic H production by nanosecond discharges in combustion environment (see for example [21]), and only for point measurements. In the present study, the 2D imaging of atomic H fluorescence allows a discussion on the local effect of NRP glow discharges in the fresh gases, in the flame front and in the burnt gases.

Atomic H fluorescence can be detected in the discharge channel developing in the fresh gases between the flame tip and the bottom electrode, but not on the burnt-gases side of the discharge. There might be several reasons for this. The first fact to consider is that the discharge channel in the burnt gases appears wider (see figure 3(b)) compared to the one developing in the fresh gases, because of the lower density in the hot gases and possibly because of the fact that the discharge does not occur always exactly in same location. In this situation any production of atomic H as the result of the electron impact reaction would be spread in a wider volume, making the detection of any extra H more difficult. Another factor may be the difference in the H precursors on the two sides of the discharges. On the fresh-gases side H stems from CH₄, while in the burnt gases its originates from H₂O. In [21], H concentration is reported to increase by about one order of magnitude after ns-discharges applied in the burnt gases. In that case, the discharge was probably in the spark regime, causing a significant increase in temperature and about 90% of the increase in H was attributed to thermochemistry, while the reminder 10% to plasma-enhanced kinetics.

The lifetime of H in the fresh-gases discharge region appears to be around 2.5 μs. This was estimated directly by the decline of the fluorescence signal in the discharge channel in figure 12. We estimate that in 100 μs H molecular diffusion could account for the displacement of a mere 40% of the H atoms from the laser sheet. Since the observed decay time is on the order of few μs the molecular diffusion process can be regarded as not dominant and therefore neglected in this analysis. Effects related to thermal expansions have been neglected as well.

Most of the H generated in the fresh gases is not reaching the flame because it is consumed earlier, within few μs. Atomic hydrogen in post-discharge chemistry, besides recombining, may as well be consumed in the production of radicals such as HO₂ and OH [46, 47]. This will need to be further studied.