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Nitrogen Laser Emissions of Short and Long Durations Generated in Air Mladen M

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Nitrogen Laser Emissions of Short and Long Durations Generated in Air Mladen M IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 48, NO. 3, MARCH 2020 647 Nitrogen Laser Emissions of Short and Long Durations Generated in Air Mladen M. Kekez and David M. Villeneuve Abstract— This article describes two sets of experiments. pressure, the excitation of the N2 molecules, and consequently An eight-stage, pulse-forming network (PFN) Marx generator the laser emission would yield only short duration light pulses, of 50- internal impedance produces a “square”-shaped voltage from one nanosecond down to some tens of picoseconds. waveform and induces multiphoton ionization, excitation of molecules and ions to generate laser pulses of long (<3 μs) and From the research and development on CO2 lasers, it was short (∼10 ns) durations in air, when the principal laser line of learned that the delay in the transition from the glow-like the N laser at 337.1 nm was observed. The visible blue laser discharge to a fully developed spark channel is possible, when 2 + lines coming from the first negative band of N2 ions due to the the characteristic impedance of the driver is of relatively large 2 + → 2 + B u X g transition at 391.4 and 427.8 nm were studied. value (>10 ). The wideband (300–800 nm) Schott BG 39 interference filter was To explore the importance, the nine-stage Marx generator also employed to examine whether some other lines participate in the emission being generated. The scaling law was derived of 18- internal impedance was used in [5]. The electro- describing the attenuation of theemissionversusthedistance magnetic radiation pulses were observed both at the single for long-duration pulses (<3 μs) at 337.1 nm. The second set microwave frequency close to 1 GHz and the pulses of long of experiments was introduced to support the data obtained. duration (<220 ns) at 337.1 nm (= prominent N2 laser line) It offers information pertinent to the visible blue line emissions in the pressure range from 100 to 200 torr in nitrogen, as well as to the photoionization processes, lighting phenomena, and processes occurring at high altitudes. and also in air, without the use of the laser resonator and mirrors. These radiation pulses can be considered as the Index Terms— Corona, lighting, nitrogen laser, photoioniza- coherent radiation due to the maser action and the laser action, tion, spark channels. respectively [5]. I. INTRODUCTION To explore further the importance of impedance, an eight- LASER is a useful source of pulsed ultraviolet (UV) stage, pulse-forming network (PFN) Marx generator with laser radiation. With the low (<1 )-internal impedance 50- internal impedance was used in [6]. The pulses of N2 < μ Blumlein driver, the commercial N laser operates usually nitrogen emission at 337.1 nm of long duration ( 3 s) and 2 ∼ with voltages ranging from 8 to 14 kV and pressures between of short duration ( 10 ns) were obtained. This article is organized as follows. For both the short- 70 and 250 torr of N2. Under optimum conditions, the laser provides pulses with energies of <30 mJ and the width of the and long-duration pulses, attention is given to the electronic– 3 → 3 ) pulse not exceeding 19 ns [1]. vibrational–rotational transitions (C u B g of the second positive system of a nitrogen molecule with a charac- The classical N2 laser is pumped by an electrical discharge teristic peak at 337.1 nm. The visible blue laser lines coming to create numerous filamentary streamer/corona channels in + + from the first negative band of N ions due to the (B2 → the glow-like discharge formed between the two parallel elec- + 2 u X2 ) transition at 391.4 nm and 427.8 nm were studied. trodes. The gas discharges formed in N2 and/or air provides g the N laser gain medium. The preionization had to be applied Emphasis is given toward understanding the attenuation of the 2 = to produce a semi-uniform glow-like column [1]–[4]. 337.1 nm line ( principal laser line of N2 laser) of long- < μ The duration of the laser pulse as a function of pressure is duration pulses ( 3 s) as it propagates in air proportional to the time interval of the voltage fall from the To support the data obtained, the second set of experiments full value of the voltage supplied by the Blumlein driver to was introduced to provide additional information regarding the almost zero value, due to the formation of the spark channel. visible blue line emissions, as well as regarding the processes of photoionization, ionization by the electric field, lighting When the N2 laser is made to operate at a high (<760 torr) phenomena, and processes occurring at high altitudes. Manuscript received September 14, 2019; revised December 9, 2019; accepted January 20, 2020. Date of publication February 19, 2020; date of current version March 10, 2020. The review of this article was arranged by II. EXPERIMENTAL SETUP Senior Editor S. J. Gitomer. (Corresponding author: Mladen M. Kekez.) Mladen M. Kekez, deceased, was with High-Energy Frequency Tesla Inc., In this article, an eight-stage PFN Marx generator described Ottawa, ON K1H 7L8, Canada (e-mail: [email protected]). in [6] was used. David M. Villeneuve is with the National Research Council of Canada, Ottawa, ON K1A 0R6, Canada, and also with the Department of Physics, When the nipple 4 in the experimental setup (see Fig. 1) Faculty of Science, University of Ottawa, Ottawa, K1N 6N5, Canada (e-mail: is removed and the voltage impulse of negative polarity from [email protected]). an eight-stage PFN Marx generator is applied, it creates the Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org. spark channel breakdown between the copper wire 6 and the Digital Object Identifier 10.1109/TPS.2020.2969420 metallic electrode 5. The gap between the tip of the wire 6 and 0093-3813 © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: National Research Council Canada. Downloaded on March 10,2020 at 20:14:58 UTC from IEEE Xplore. Restrictions apply. 648 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 48, NO. 3, MARCH 2020 To avoid the interference problems mentioned in [5], the laser signals recorded by the photodiode have been viewed with the aid of a low (150 MHz)-pass filter. IV. EXPERIMENTAL RESULTS The emissions recorded with four narrow bandpass interference filters and a wideband BG 39 filter are shown in Figs. 2–4. The analysis of the data obtained is given in Figs. 5 and 6. It may be useful to understand the mechanism of the laser beam generation of short duration, as registered with the aid of a 337.1-nm filter. When the Plexiglas tube was replaced with the nipple 4 shown in Fig. 1, the data were obtained and are Fig. 1. Schematics of the experimental setup. 1: Hamamatsu R 727 pho- shown in Fig. 2. When an eight-stage PFN Marx generator todiode, 2: optical filter, 3: Plexiglas flange with an aperture (i.e., orifice) is energized, the spark breakdown takes place between the of 6.25 mm and the additional recess to accommodate the filter, 4: 2¾-in (Var- ian) nipple, 5: metallic post, 6: copper wire attached to the central electrode, 7: metallic post 5 and the copper wire attached to the central central electrode, 8: upper edge of the conical structure, 9: Plexiglas thimble electrode 6. The spontaneous emission coming from the spark with a hole in the center machined to fit well the polyethylene insulation of channel causes the photoionization of air. the coaxial cable, 10: conical structure connected to the ground of the Marx generator, and 11: inner conductor of the coaxial cable with connection to the In general, photoionization from excited states is the mech- generator. anism of gas breakdown by the visible and UV spectrum electromagnetic waves of high intensity, without the benefit the metallic electrode 5 is ∼3 mm. The separation between the of ionization by the external electric field [7]. As a function metallic electrode 5 and the metallic post 7 is varied between of gas density/pressure, the electron densities produced by 4 and 7 mm until the reproducible data for the emission at the UV source have been derived for several gases and gas 337.1 nm of short duration is obtained. The diameter of the mixtures using, for example, the microwave interferometry central electrode 7 is 15.9 mm with the height set initially at method [8]. The microsecond time scale was applied in these 55 mm and later made to be 80 mm in length. The metallic measurements. post is of 2.5-mm diameter. The detailed studies of plasma density created by photoion- The thimble 9 machined out of Plexiglas was placed over ization versus distance from the spark channel d reported in the polyethylene surface of the RG 217/U coaxial cable to [8] and [9] show that the experimental data can be summarized minimize the harmful effect of corona tracking along the by the following expression: surface of the cable. The height of the thimble above the upper ( −3) =[ .(− / )]/ 2. edge of the conical structure 8 was 32 mm. Plasma density cm A exp d k d (1) The precise alignment in the vertical direction is required, At 6 cm away from the spark channel, the plasma density so that the photodiode 1 indicated in Fig. 1 can view the glow- reaches the level of 1011cm−3, and at 30 cm away, the density like discharges formed between the central electrode 7 and the falls to 2 × 108cm−3.
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