Laser plasma formation assisted by ultraviolet pre

Laser plasma formation assisted by ultraviolet pre-ionization Azer P. Yalin, Nick Wilvert, Ciprian Dumitrache, Sachin Joshi, and Mikhail N. Shneider Citation: Physics of Plasmas (1994-present) 21, 103511 (2014); doi: 10.1063/1.4898059 View online: http://dx.doi.org/10.1063/1.4898059 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/21/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A small size 1–3 atm pulsed CO2 laser with series-connected spark gaps ultraviolet preionization Rev. Sci. Instrum. 85, 013109 (2014); 10.1063/1.4861919 Reduction of plasma density in the Helicity Injected Torus with Steady Inductance experiment by using a helicon pre-ionization source Rev. Sci. Instrum. 84, 103506 (2013); 10.1063/1.4824707 Laser induced avalanche ionization in gases or gas mixtures with resonantly enhanced multiphoton ionization or femtosecond laser pulse pre-ionization Phys. Plasmas 19, 083508 (2012); 10.1063/1.4747344 Numerical simulation of the pre-ionization processes during nanosecond-pulse discharge in nitrogen J. Appl. Phys. 111, 013306 (2012); 10.1063/1.3675439 Uniform glowlike plasma source assisted by preionization of spark in ambient air at atmospheric pressure Appl. Phys. Lett. 89, 131503 (2006); 10.1063/1.2356894

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PHYSICS OF PLASMAS 21, 103511 (2014)

Laser plasma formation assisted by ultraviolet pre-ionization Azer P. Yalin,1,a) Nick Wilvert,2,b) Ciprian Dumitrache,1 Sachin Joshi,3,b) and Mikhail N. Shneider4 1

Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, USA Sandia Laboratory, Albuquerque, New Mexico 87123, USA 3 Cummins Inc., Columbus, Indiana 47201, USA 4 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA 2

(Received 19 August 2014; accepted 1 October 2014; published online 16 October 2014) We present experimental and modeling studies of air pre-ionization using ultraviolet (UV) laser pulses and its effect on laser breakdown of an overlapped near-infrared (NIR) pulse. Experimental studies are conducted with a 266 nm beam (fourth harmonic of Nd:YAG) for UV pre-ionization and an overlapped 1064 nm NIR beam (fundamental of Nd:YAG), both having pulse duration of 10 ns. Results show that the UV beam produces a pre-ionized volume which assists in breakdown of the NIR beam, leading to reduction in NIR breakdown threshold by factor of >2. Numerical modeling is performed to examine the ionization and breakdown of both beams. The modeled breakdown threshold of the NIR, including assist by pre-ionization, is in reasonable agreement C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898059] with the experimental results. V

I. INTRODUCTION

Laser breakdown in gases has been extensively studied and finds many potential applications including ignition of combustion mixtures, lightning control, laser triggered spark gaps, and providing a light source for laser induced breakdown spectroscopy (LIBS). Much of the research has focused on breakdown in air and other gases at various pressures using visible to near infrared laser pulses of nanosecond duration. Typically, initial electrons are created by multiphoton ionization (MPI) after which breakdown is primarily by Electron Avalanche Ionization (EAI).1 In the latter process, free electrons gain energy from the radiation field via inverse bremsstrahlung and collide with other gas molecules leading to cascade ionization.1,2 For widely used nanosecond pulses in air at typical conditions, the breakdown process requires a breakdown threshold intensity of 100–300 GW/cm2 (Refs. 3–7) and the plasma may reach a peak temperature of 30 000–100 000 K and peak pressure of 1000 bars.8,9 For combustion ignition applications, laser sparks can provide advantages over traditional electrical spark ignition owing to elevated (“overdriven”) flame speeds,8 increased freedom in positioning the ignition source within the combustor, and lack of quenching from electrodes. There has been specific interest in ignition of reciprocating internal combustion engines in order to avoid problems with spark plugs at high pressure and to extend the lean limit to lower emissions of oxides of nitrogen (NOx),10–12 and ignition of aircraft engines to enable rapid relight and to avoid reliability problems of conventional igniters.13 However, these conventional laser sparks also have limitations including the a)

Author to whom correspondence should be addressed. Electronic mail: [email protected] b) This research was performed while author was at Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, USA. 1070-664X/2014/21(10)/103511/6/$30.00

formation of a blast wave that consumes a significant portion of the energy. Phuoc and White calculated up to 70% of the energy absorbed in a 1064 nm laser spark was used in the propagation of the blast wave, while as little as 7% of the spark energy was actually available for ignition.14 Further, gas dynamic effects from laser sparks also make ignition near the lean limit difficult due to high rates of flame stretch, which is further aggravated at low pressures.8 Given the limitations and somewhat uncontrollable nature of conventional laser sparks, there is emerging interest in methods of controlling and tailoring the laser plasma. Related multi-pulse approaches have been used to enhance LIBS signals,15–17 but in the LIBS case the plasmas are normally formed on solids (with much lower thresholds), and the first pulse already generates a strongly ionized luminous plasma. Past work by some of the present authors has also shown that formation of a strongly ionized NIR laser plasma, in the gas phase, leads to strong absorption of a second NIR laser pulse;18 however, again, the first pulse generated a fully ionized plasma. The present focus is on approaches in which the first pulse pre-ionizes the gas (without full breakdown) and the second pulse sustains or adds energy to the plasma.18,19 Such approaches could allow tailoring of the laser plasma (size, plasma conditions, temperature) for different applications potentially allowing, for example, spatially extended volumes of heated gas (T 1000–2000 K) for thermal ignition,20,21 as well as optimized plasmas for other applications. Shneider et al. have examined the overlap of a nanosecond pulse to sustain a plasma filament from a femtosecond laser showing the possibility of extending the plasma lifetime due to suppressed three-body electron attachment and dissociative recombination processes.19 Scharer and colleagues have examined preionization using ultraviolet (UV) pulses from Argon-Fluoride (193 nm) excimer lasers including their ability to sustain radio-frequency plasmas in air.22,23 Zvorykin et al. have performed studies of pre-ionization and plasma guiding

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with Krypton-Fluoride (248 nm) excimer lasers to guide discharges.24,25 These studies show that pre-ionization can be achieved with relatively low (sub-threshold) UV intensities and that the resulting weakly ionized plasmas may be useful to sustain or control discharges. In this contribution, we study the effects of preionization in air using 266 nm (UV) nanosecond pulses including their effect on overlapped laser plasmas due to 1064 nm (NIR) pulses. In particular, we show that the relatively low density pre-ionization plasmas can substantially reduce the breakdown threshold of the overlapped NIR beams. Such approaches may be useful for practical laser ignition systems, for example, to expand ignition windows, allow thermal ignition, reduce laser power requirements, or enable fiber delivered configurations.26 In Sec. II, we present the experimental setup for pre-ionization, overlap, and breakdown of the UV and NIR beams. In Sec. III, we present numeric modeling approach for breakdown of UV and NIR pulses including effects of pre-ionization and breakdown of the overlapped beams. Results and discussion are provided in Sec. IV with conclusions provided in Sec. V. II. EXPERIMENTAL

The experimental setup shown in Fig. 1 comprises two Nd:YAG lasers to create laser plasma and a third for Schlieren visualization. The fourth harmonic of a first Nd:YAG source (Continuum Powerlite 8010) with pulse duration 10 ns is used to create the UV pre-ionizing laser pulse at 266 nm. A second Nd:YAG source (Spectra Physics PR–II) with pulse duration 13 ns provides a NIR laser pulse at 1064 nm which can be overlapped with the pre-ionizing pulse. The lasers were triggered and synchronized using a pulse delay generator (BNC 555) with the peak of the NIR beam being 10 ns after that of the UV beam for the dual-beam tests. Both the preionizing and energy addition beams were passed through (separate) 300 mm focal length spherical lenses prior to a beam combining optic (labeled as Beam Splitter in the figure). Each beam was focused to a diameter of 150 lm. The beams were made collinear and

meticulously aligned using a CCD profiler (Spiricon SP 503) to ensure overlap (within 5 lm) throughout the focal region. A second beam splitter, positioned after the beams were combined, was used to reflect a fraction of the energy into an energy meter. An additional Nd:YAG laser (New Wave Gemini) was used for Schlieren imaging.27 The beams were focused inside of a vacuum chamber filled with air at P ¼ 0.80 bar, T ¼ 295 K. Air in the chamber was replenished for each measurement to ensure pristine air was used for each measurement point. Experiments were conducted to examine breakdown threshold intensities for the 266 nm and 1064 nm wavelengths, due to the individual beams as well as for overlapped beams to examine possible pre-ionization assist from the UV beam. By operating each beam on its own, we can determine the corresponding breakdown intensity by varying the laser pulse energy and identifying the minimum pulse energy for laser plasma formation. Intensities are found from pulse energy, duration, and area as described below (Sec. IV A). Uncertainty in reported intensity is þ/15% due to measurement repeatability. We identify plasma formation (breakdown) intensity by the presence of visible and audible emission and observe a clear threshold behavior, i.e., abrupt transition from nonbreakdown to breakdown as focused intensity is increased over a certain value.8 All breakdown thresholds are stochastic, and values reported here are the minimum observed thresholds. For the dual beam case, we set the 266 nm beam to a fixed energy (intensity) below its breakdown threshold, and then vary the energy of the 1064 nm beam and identify the threshold for its breakdown. (Note that these double-pulse plasmas are quite different from the double-pulse LIBS plasmas, mentioned earlier, where the first pulse itself is above threshold and already forms a luminous plasma.) III. NUMERIC MODELING

We have developed a model for detailed quantitative analysis of plasma dynamics induced by two overlapped

FIG. 1. Experimental setup for dual pulse pre-ionization experiment.


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laser pulses, one at 266 nm and the other at 1064 nm. Briefly, the model self-consistently integrates plasma-kinetic, NavierStokes, electron heat conduction, and electron vibration energy transfer equations to examine plasma composition, plasma chemistry, and electron photo-detachment processes.19 The model consists of plasma-kinetics rate equations, and the plasma evolution and radial expansion are described by the 1-D time-dependent continuity equations for electrons and ions in the diffusion-drift approximation together with the radial electric field calculated by solving the Poisson equation for the potential19 @ ½ns 1 @ ðr½Cs Þ þ ¼ ½Gs ½Ls ; @t r @r

(1)

þ þ þ þ where ns are the densities of N2þ ; Oþ 2 ; N4 ; O4 ; NO ; O ðnþ Þ; eðne Þ; O2 ; O ðn Þ; O; N; NO; O3 and GS, Ls are their respective generation and loss rates. Cs are fluxes of charged particles. The radial field component E ¼ @/=@r is calculated by solving the Poisson equation for the potential / X 1@ @/ e X r ¼ ni;þ nk; ne ; (2) r @r @r e0 i k

with boundary conditions @/=@rjr¼0 ¼ 0 and /ð1Þ ¼ 0. Summations in Eq. (2) are done over all types of positive and negative ions. All equations, rates and parameters, with the exception of the ionization, are the same as in past work.19 For modeling of laser breakdown (wavelength of 266 nm), multiphoton ionization is taken into account along with avalanche ionization. Multiphoton ionization is computed on the basis of a modified Keldysh theory.28 The contribution of avalanche ionization is calculated independently for UV and NIR laser pulses in the simplest approximation, where the ionization rate is determined by the ratio of the Joule heating power to the ionization potential29 @ne ðr; tÞ @Oþ @N2þ ðr; tÞ 2 ðr; tÞ ¼ þ @t av @t @t av av e2 ne ðm þ c ÞIL ðr; tÞ 0:2 0:8 h i n þ ; (3) me0 c x2L þ ðm þ c Þ2 IO2 IN2 where m and c are the frequencies of transport elastic scattering of electrons in collisions with neutral molecules and Coulomb collisions with ions, respectively; IL and xL are the laser intensity and the angular frequency; IO2 ¼ 12.2 eV and IN2 ¼ 15.6 eV are the ionization potentials of oxygen and nitrogen; and n is an empirical parameter depending on assumed fraction of absorbed energy going into ionization. The results presented below were obtained with n ¼ 0.75. This value, in some sense, is related to the account of the stepwise ionization of the excited states (not considered in this paper), which are accumulated in the process of the avalanche laser breakdown development. Similarly, the multiphoton ionization, based on modified Keldysh theory, results in additional source terms in the continuity equations for electrons and positive ions, respectively:

FIG. 2. Electron number density for the plasma generated by the 1064 nm pulse (alone).

@ne @t

þ @N2þ @O2 ; @t @t mpi mpi mpi þ @N2 ¼ -O2 ðr; tÞO2 ; ¼ -N2 ðr; tÞN2 ; @t mpi ¼

mpi

@Oþ 2 @t

þ

(4)

where -O2 ;N2 ðr; tÞ -O2 ;N2 ðIL ðr; tÞÞ are the (laser intensity dependent) probabilities for multiphoton ionization of O2 and N2 molecules.

IV. RESULTS AND DISCUSSION A. Breakdown thresholds

We first examine the experimental and model results for breakdown at 266 nm and 1064 nm beams acting on their own. Throughout this paper, we define the beam intensity as an average value determined as the laser pulse energy divided by the pulse duration (taken as the temporal fullwidth-at-half-maximum) and divided by the area of the focused beam (taken as a circular beam shape with diameter found from a beam profiler using the D4r method30). For the case of Gaussian spatial and temporal distributions, the peak intensity (in space and time) is lower than the reported average intensity by a factor of 1.536. Experimentally, for our focusing and pulse conditions, we observe breakdown thresholds of 27 GW/cm2 for the 1064 nm beam (on its own), and 15 GW/cm2 for the 266 nm beam (on its own). Figure 2 shows modeled electron number density variation in the plasma formed by a 1064 nm pulse (on its own). We assume that breakdown takes place when ne 1015 cm3, which is a reasonable upper value for streamer breakdown in air at typical conditions31 and consistent with laser plasma measurements by other researchers.24 The modeling results show that the breakdown threshold for the 1064 nm pulse is 40 GW/cm2 which is in reasonable agreement with the experimental value. The relatively small breakdown intensities (measured and computed) in this work versus those often reported are due to the rather large spot sizes of 150 lm for which the electron diffusion loss is reduced.1 Indeed, other researchers have found similar


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trends of reduced breakdown intensities for larger spot sizes4,6,7 that are quite consistent with our results. Figure 3(a) shows the modeled variation of the electron number density due to the 266 nm pulse alone. Probabilities for the multi-photon ionization for O2 and N2 molecules, computed on the basis of generalized Keldysh theory,28 are also shown in Figure 3(b). The calculated intensity for which optical breakdown is achieved at our experimental conditions is 135 GW/cm2. This threshold is an order of magnitude higher than the experimentally observed value of 15 GW/cm2. Scharer et al. have also observed much lower breakdown thresholds for UV radiation, i.e., 5.5 GW/cm2 for 193 nm radiation.23 It is likely that resonant enhanced multi-photon ionization (REMPI) effects (not included in the model) are responsible for the lower thresholds found in experiments. Measurements of electron density versus laser intensity for 248 nm laser pulses, using a conductivity method,24 follow an I2 dependence further suggesting the role of REMPI for UV pulses, i.e., ionization by a 2 þ 1 REMPI process with two photons to reach a resonant state of oxygen or nitrogen followed by ready ionization with an additional photon. The 248 nm measurements show ne 1015 cm3 at breakdown consistent with our model assumptions. Electron measurements in our laboratory for

FIG. 3. (a) Electron number density and intensity for plasma generated by the 266 nm pulse. (b) Probabilities for multi-photon ionization for O2 and N2 molecules (right).

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the 266 nm pulse, using the same method, show very similar results including ne 1015 cm3 at breakdown and I2 dependence.27 Note that similar conductivity measurements of the NIR beam (alone) show no measurable pre-ionization when the NIR beam is below threshold (due to reduced MPI).

B. Pre-ionization breakdown of overlapped beams

We have examined the reduction in breakdown intensity of the 1064 nm beam due to pre-ionization (assist) of the 266 nm beam. The 266 nm beam is operated below its breakdown threshold (i.e., at I266 where I266 < IBD,266 ¼ 15 GW/cm2) and the 1064 nm beam is overlapped with temporal delay of 10 ns. Experimentally, the energy (intensity) of the 1064 nm beam was varied to identify its breakdown condition. Figure 4 shows the threshold breakdown intensity of the 1064 nm beam versus the intensity of the 266 nm beam. With no UV pre-ionization (i.e., I266 ¼ 0), the breakdown requirement for the 1064 nm pulse (on its own) is IBD,1064 ¼ 27 GW/cm2. The results clearly show that the pre-ionization generated by the 266 nm beam reduces the breakdown threshold of the 1064 nm pulse beam, for example, lowering the threshold by factor of 2 for I266 ¼ 7 109 W/cm2. The pre-ionization electrons act as seed electrons so that less NIR intensity is needed to form the cascade and breakdown. The numeric model also captures the reduction in breakdown intensity of the 1064 nm beam due to the preionization of the 266 nm beam. Figure 5 shows the comparison of experimental and modeling results (air, T ¼ 300 K, P ¼ 1.0 bar). Owing to the discrepancy of the modeled and measured 266 nm breakdown threshold, we make the comparison on the basis of the 266 nm intensity normalized by its breakdown threshold value. This approach is also more consistent in matching the corresponding pre-ionization electron densities, e.g., when I266/IBD,266 ¼ 1 both the experiment and model have ne 1015 cm3 (see Sec. IV A). From Figure 5 it can be seen that there is reasonable agreement between

FIG. 4. Variation of 1064 nm breakdown threshold intensity with overlapped 266 nm intensity (air, T ¼ 295 K, P ¼ 0.80 bar). The 266 nm beam is operated below its breakdown threshold. Pre-ionization from the 266 nm beam reduces the breakdown threshold of the 1064 nm beam.


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FIG. 5. Comparison of numeric modeling and experiment: Variation of 1064 nm breakdown threshold intensity versus 266 nm beam intensity.

experiment and model in terms of the 1064 nm breakdown threshold assist by the 266 nm pre-ionization. An example of the two overlapped pulses is given in Fig. 6. In this case, the (modeled) pulse intensity for the pre-ionizing 266 nm pulse is kept constant at 20 GW/cm2, i.e., I266/IBD,266 ¼ 0.15 (based on modeled IBD,266 ¼ 135 GW/ cm2). Several curves are shown corresponding to different intensities of the 1064 nm pulse. Visible breakdown is achieved for 1064 nm pulse intensity of 19 GW/cm2. It is worth mentioning that in this particular example the two pulses are not perfectly overlapped in time. The 266 nm pulse precedes the 1064 nm pulse by 10 ns which is the delay for which the experiments show the largest effect of the pre-ionization assist. Again, breakdown is considered to take place at an electron density of ne 1015 cm3. V. CONCLUSIONS

We have conducted experimental and modeling studies of air pre-ionization using UV laser pulses and its effect on laser breakdown of an overlapped near-infrared NIR pulse.

FIG. 6. Example of electron number density and beam intensity variation of overlapped 1064 nm and 266 nm laser beams. The intensity of the preionizing 266 nm pulse is fixed at 20 GW/cm2 (I266/IBD,266 ¼ 0.15). Different curves show electron densities for different 1064 nm intensities.

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The model accurately determines the breakdown threshold of the NIR pulse. The model finds higher thresholds for the UV breakdown as compared to experiment, likely because REMPI effects (difficult to accurately model) are not included in the model. Model and experiment both show that pre-ionization generated by the UV beam assists in laser plasma formation by the overlapped IR pulse leading to reduced threshold intensity. The results depend closely on the exact density of electrons in the laser plasma which we plan to measure in the future. The ability to use pre-ionization to influence the coupling of overlapped laser beams provides a step towards generating tailored and optimized laser plasma parameters for different applications. For example, though the present work considered cases where the overlapped beam reached full breakdown, it may be possible to find conditions (wavelengths and pulse durations) where the avalanche ionization provides controlled heating to raise the temperature for thermal ignition without full breakdown. ACKNOWLEDGMENTS

The authors acknowledge support from the NSF/DOE Partnership in Basic Plasma Science and Engineering (NSF award PHY-1418845 and DOE award DE-SC0012454). 1

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