ISSN 1063-780X, Plasma Physics Reports, 2015, Vol. 41, No. 10, pp. 858–861. © Pleiades Publishing, Ltd., 2015.
`Original Russian Text © I.G. Rudoy, N.G. Solovyov, A.M. Soroka, A.O. Shilov, M.Yu. Yakimov, 2015, published in Fizika Plazmy, 2015, Vol. 41, No. 10, pp. 929–932.
`
`LOW-TEMPERATURE
`PLASMA
`
`Xenon Plasma Sustained by Pulse-Periodic Laser Radiation
`I. G. Rudoy, N. G. Solovyov, A. M. Soroka, A. O. Shilov, and M. Yu. Yakimov
`A. Ishlinsky Institute for Problems in Mechanics, Russian Academy of Sciences,
`pr. Vernadskogo 101-1, Moscow, 119526 Russia
`e-mail: yakimov@lantanlaser.ru
`Received March 16, 2015
`
`Abstract―The possibility of sustaining a quasi-stationary pulse-periodic optical discharge (POD) in xenon
`at a pressure of p = 10–20 bar in a focused 1.07-μm Yb3+ laser beam with a pulse repetition rate of frep ≥ 2 kHz,
`pulse duration of τ ≥ 200 μs, and power of P = 200–300 W has been demonstrated. In the plasma develop-
`ment phase, the POD pulse brightness is generally several times higher than the stationary brightness of a
`continuous optical discharge at the same laser power, which indicates a higher plasma temperature in the
`POD regime. Upon termination of the laser pulse, plasma recombines and is then reinitiated in the next
`pulse. The initial absorption of laser radiation in successive POD pulses is provided by 5p56s excited states of
`xenon atoms. This kind of discharge can be applied in plasma-based high-brightness broadband light sources.
`
`DOI: 10.1134/S1063780X15100086
`
`1. INTRODUCTION
`The development of the physics of a continuous
`optical discharge (COD) [1], as well as progress in
`technology of powerful lasers used for their sustaining,
`has made it possible to create efficient light sources
`with a spectral brightness substantially exceeding that
`of traditional plasma-based sources. An important
`achievement of the recent years was application of
`near-IR solid-state lasers to sustain COD [2–4],
`because the radiation of near-IR lasers may be effi-
`ciently absorbed by high-pressure noble-gas plasmas
`due to the broadening of spectral transitions between
`excited atomic levels [5]. Several light sources on this
`basis were developed and patented [2, 3, 6].
`COD provides a high plasma spectral brightness,
`reaching several units of W cm–2 nm–1 sr–1 in the visi-
`ble region for high-pressure xenon, which is several
`times greater than the maximum brightness of arc-dis-
`charge plasma under similar conditions. In this case,
`due to high temperature, the COD brightness in the
`UV region is one order of magnitude higher than that
`of arc-discharge plasma [4, 7].
`The laser energy deposited in COD plasma is lim-
`ited by refraction on the density gradients of the neu-
`tral gas and free electrons [5]. Refraction manifests
`itself in defocusing of the laser beam as it propagates
`through regions with heated ionized gas and, along
`with absorption of laser radiation in plasma, deterio-
`rates conditions for COD sustaining. Similar to
`absorption, the influence of refraction increases with
`increasing pressure of the plasma-forming gas. To a
`certain extent, refractive defocusing can be compen-
`sated for by a tighter focusing of the laser beam. Nev-
`
`ertheless, as the pressure is increased further (e.g., to
`raise the brightness of the plasma-based source),
`refraction leads to a decrease in the plasma tempera-
`ture and saturation of the growth of the electron den-
`sity with increasing pressure.
`When the refraction effects are dominating (i.e., at
`weak focusing and/or high pressure), the electron
`temperature and density in an optical discharge can be
`substantially increased by applying pulse-periodic
`excitation. Such a mode of an optical discharge is
`called the pulse-periodic optical discharge (POD).
`This possibility was demonstrated in experiments on
`sustaining COD and POD in high-pressure hydrogen
`by means of continuous and pulse-periodic CO2
`lasers, respectively [8].
`So far, PODs with repetition rates of a few kHz to
`several hundred kHz and higher were implemented
`using CO2 lasers [8–10]. It is important that, in those
`experiments, the laser intensity in the focal spot corre-
`sponded to the threshold for optical breakdown (with
`allowance for the high repetition rate) and the plasma
`in its characteristics was closer to the plasma of optical
`breakdown than to COD plasma.
`In this work, the conditions are determined for the
`first time for sustaining a pulse-periodic plasma that is
`closer to COD plasma in the threshold sustaining
`power, stability, and reproducibility as compared to
`the optical breakdown plasma, but surpasses COD
`plasma in the temperature and brightness due to spe-
`cific features of pulse-periodic excitation.
`
`858
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`XENON PLASMA SUSTAINED BY PULSE-PERIODIC LASER RADIATION
`
`859
`
`Pin(t)
`
`Pout(t)
`
`300
`
`200
`
`100
`
`Pin, Pout, W
`
`0
`
`123
`
`Ip, arb. units
`
`0
`
`τd
`
`200
`
`400
`
`t, μs
`
`600
`
`Fig. 1. Typical waveforms of the incident laser power Pin
`and the power Pout transmitted through the plasma, as well
`as of the intensity of thermal radiation Ip of POD plasma.
`
`and 5p56s' metastable states, as well as in the resonant
`states the relaxation of which at a high pressure and a
`temperature of a few thousands of kelvins is hampered
`due to the small free path length (capture) of resonant
`radiation. Fast gas heating is also assisted by the high
`rate of collisional relaxation of excited Xe atoms pro-
`duced due to the absorption of laser photons in the 6s–
`6p transitions.
`Figure 2 shows time-averaged images of COD and
`POD plasmas (obtained using a mirror optical system)
`depicted by contour lines of the brightness with a step
`of 10% of its maximum value. The images were
`obtained under similar conditions. The laser peak
`pulse power in the case of POD was nearly equal to the
`continuous laser power in the case of COD. It can be
`seen that both images are similar in appearance; the
`difference is that the COD image represents a station-
`ary process, while the POD image is a result of the
`time averaging of the plasma propagation process in
`the beam channel, which is repeated in each pulse.
`The straight lines show a conditional boundary of the
`laser beam in the focal region in the absence of plasma.
`To measure the pulsed spectral brightness of
`plasma, the point of the maximum brightness was
`determined on a 40-fold magnified POD image and
`the 0.45-mm-diameter entrance aperture of the fiber
`guide was placed there. The point of the instantaneous
`maximum brightness nearly coincided with the point
`
`2. EXPERIMENT ON SUSTAINING POD
`The experiments on sustaining COD and POD
`were performed in quartz bulbs filled with xenon at a
`pressure of p0 = 8–14 bar. Optical discharges in a
`focused laser beam were initiated by a short-time arc
`discharge excited between auxiliary electrodes. In this
`case, the pressure increased to p = 13–21 bar due to
`gas heating.
`A YLR-150/1500-QCW fiber ytterbium laser man-
`ufactured by IPG Photonics was used as a source of
`pulse-periodic radiation with a wavelength of 1.07 μm
`and a bandwidth of 3–5 nm (depending on the
`power). The peak pulse power was up to P = 1.5 kW,
`the average power was up to Pa = 160 W, the pulse
`duration was τ = 0.2–10 ms, the pulse repetition rate
`was frеp ≤ 2.7 kHz, and the beam diameter at the colli-
`mator exit was d0 = 9 mm. The laser bean radiation
`pattern is described by the so-called beam parameter
`product (the product of the beam radius at the colli-
`mator exit and the half-angle of the beam divergence
`in the far field), BPP = 1.2 mm mrad, or by the laser
`beam propagation factor, K = 1/M2 = 1/3.5.
`The laser beam with a diameter of d ≥ d0 was
`focused by a quartz lens with a focal length f and focus-
`ing parameter of F = f/d = 3.3–8, which for P = 260–
`300 W provided a focal intensity at a level of If ≈ 107–
`108 W cm–2. To compare POD with COD, the same
`laser operating in a continuous mode with a maximum
`radiation power of up to 260 W was employed to sus-
`tain COD.
`It was found that, after initiation by the arc dis-
`charge and termination of it, POD in xenon could be
`steadily sustained similarly to COD plasma by a
`focused laser beam at a pulse repetition rate of frep ≥
`2 kHz, pulse duration τ ≥ 200 μs, and minimum peak
`pulse power of P = 200–300 W (depending on the
`pressure, which was varied in the range of p = 10–
`21 bar).
`Figure 1 shows typical waveforms of the pulse-peri-
`odic laser power incident onto the plasma and passed
`through it and the integral plasma radiation intensity,
`which characterizes the plasma formation time after
`discharge initiation in each pulse, as well as the plasma
`decay time upon termination of the laser pulse. The
`processes of plasma formation and plasma decay were
`almost completed in a time τd ≤ 100 μs.
`Although plasma almost fully recombines upon
`termination of the laser pulse over a time of about
`50 μs, the number of excited atoms remaining in the
`gas by the beginning of the next pulse is sufficient to
`initiate plasma formation at a radiation intensity of
`If ∼ 107 W cm–2, which is two orders of magnitude
`lower than the breakdown intensity. Strong nonlinear
`absorption required for plasma formation in the next
`pulse is probably ensured by fast gas heating due to
`radiation absorption by excited xenon atoms in 5p56s
`
`PLASMA PHYSICS REPORTS
`
` Vol. 41
`
` No. 10
`
` 2015
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`
`RUDOY et al.
`
`
`
`
`
`1,,arb.units
`
`.°.°NA
`
`-500
`
`0 3
`
`00
`
`400
`
`Fig. 3. Waveforms ofthe (a) plasma radiation intensity Ip(t)
`and (b) spectral brightn lS,,(}.) for (I) POD and
`(2) COD in xenon at a pressure 0fp3 = ll i 1 bar and
`focusing parameter of F= 8. The POD parameters are as
`follows: Pi“ = 232 W and fmp = 2.7 kHz, where Pin is the
`incident peak pulse laser power. The incident laser power
`for COD is Pin = 230 W.
`
`xenon pressure: p3 = ll i l bar for Fig. 3 and p4 = l9 i
`1 bar for Fig. 4. The pressure value influences the role
`of refraction and the observable distinctions in the
`
`waveforms and spectra ofCOD and POD plasma radi-
`ation.
`
`At the pressure p4 = l9 i 1 bar, the POD plasma
`dynamics is significantly affected by the refraction of
`laser radiation, which is reflected in the shape of the
`light pulse in the course of plasma formation. In
`Fig. 4a, one can see pronounced peaks corresponding
`to the passages of two plasma fronts during one laser
`pulse. At the pressure p4, the pulsed brightness of POD
`plasma at the first front is nearly one order of magni-
`tude higher than the brightness of COD plasma,
`whereas at the pressure p3 = II i 1 bar, the brightness
`of POD plasma only slightly exceeds that of COD
`plasma. The reason for this difference is that, at the
`increased pressure p4, the brightness of COD plasma is
`reduced due to refraction. In the initial stage of its for-
`mation, POD plasma is in the region with a high
`intensity of the laser beam and the beam refraction is
`weak due to the small thickness of the plasma front.
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`PLASMA PHYSICS REPORTS
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`Vol. 41
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`No. 10
`
`2015
`
`Energetiq Ex. 2020, page 3 - |PR2015-01277
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`Fig. 2. Tirne-averaged distributions of the brightness of
`(a) POD and (b) COD plasmas (the contour lines of the
`brightness are drawn with a step of 10% of its maximum
`value). The working gas is xenon at a pressure ofp = 1] i
`1 bar. The focusing parameter is F= 5.6. The POD param-
`eters are as follows:/‘M, = 2.7 kill, the incident peak pulse
`laser power is Pin = 268 W; the time-averaged incident and
`output laser powers are (Pin) = 140 W and (Pom) = 53 W,
`respectively; the plasma length is L’, = 2.3 mm; and the
`plasma diameter is D, = 0.33 mm. The COD parameters
`are as follows: the incident laser power is Pin = 230 W, the
`output laser power is P0“, = 65 W, L, = 2.8 mm, and D, =
`0.38 mm.
`
`of the maximum brightness in the time—averaged
`plasma image shown in Fig. 2.
`
`When measuring the waveform of the POD total
`brightness, the output ofthe fiber guide was connected
`to a photomultiplier with a broadband photocathode,
`whereas when recording the time-integrated spec-
`trum, it was connected to a survey CCD spectrometer
`with an operating spectral range of 200-1100 nm and
`an average wavelength resolution of 0.25 nm. The
`spectrometer was precalibrated in the absolute bright-
`ness of the light source by using a UV deuterium lamp
`and a ribbon tungsten lamp emitting in the visible and
`IR regions. To obtain the pulsed spectral brightness of
`POD, the time-integrated spectrum was multiplied by
`the average duty factor determined from the waveform
`of the total brightness. In so doing, the dependence of
`the duty factor on the wavelength was ignored. Actu-
`ally, the duty factor ofplasma radiation pulses varies in
`the range of l0—l5%, growing with decreasing radia-
`tion wavelength.
`
`3. COMPARISON OF COD VERSUS POD
`
`Figures 3a and 4a compare the waveforms of the
`integral plasma radiation intensity Ip(t) at the point of
`the maximum brightness for COD and POD, while
`Figs. 3b and 4b show the spectral brightnesses IS,,(}.)—
`the continuous one for COD and the peak pulse one
`for POD, respectively. The results were obtained for
`the same focusing parameter of F = 8 and different
`
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`
`XENON PLASMA SUSTAINED BY PULSE-PERIODIC LASER RADIATION
`
`861
`
`4. CONCLUSIONS
`For the first time, the conditions have been deter-
`mined for sustaining POD xenon plasma that is close
`to COD plasma in the threshold power, stability, and
`reproducibility, but surpasses COD plasma in the tem-
`perature and brightness due to specific features of
`pulse-periodic excitation. The existence of such a
`regime is explained by the preservation of the non-
`equilibrium density of 5p56s excited xenon states
`between laser pulses, due to which the gas is able to
`absorb laser radiation for more than 100 μs after termi-
`nation of the laser pulse.
`This kind of discharge may be used to generate
`plasma with an enhanced density (close to nonideal),
`as well as to develop broadband light sources with a
`very high brightness and a small size of the emitting
`region. POD with a pulse duration on the order of
`10 μs and a sufficiently high repetition rate may
`be used for the same purposes as a COD, but with
`a higher efficiency.
`
`2
`
`500
`t, μs
`
`1
`
`2
`
`ACKNOWLEDGMENTS
`The authors are grateful to NTO IRE-Polyus (IPG
`Photonics branch) for providing them with continu-
`ous and pulse-periodic fiber ytterbium lasers.
`
`600
`λ, nm
`
`(а)
`
`1
`
`500
`
`0
`
`(b)
`
`2.0
`
`1.5
`
`1.0
`
`0.5
`
`0
`
`101
`
`100
`
`Ip, arb. units
`
`ISp, W сm2 nm1 sr1
`
`101
`
`300
`
`400
`
`500
`
`Fig. 4. Waveforms of the (a) plasma radiation intensity Ip(t)
`and (b) spectral brightness ISp(λ) for the (1) POD and
`(2) COD in xenon at a pressure of p4 = 19 ± 1 bar and
`focusing parameter of F = 8. The POD parameters are as
`follows: Pin = 270 W and frep = 2 kHz, where Pin is the inci-
`dent peak pulse laser power. The incident laser power for
`COD is Pin = 230 W.
`
`The high temperature of the propagating POD plasma
`is indicated by the sharp enhancement of spectral lines
`of xenon ions as compared to those in COD (cf.
`Figs. 3b and 4b).
`At the increased pressure p4, the difference in the
`spectral brightnesses between COD and POD is
`reduced as F decreases, i.e., as the optical compensa-
`tion for the refraction effects is introduced. This
`occurs mainly due to an increase in the COD bright-
`ness with decreasing F. As far as the refraction is com-
`pensated for by a decrease in F, the POD brightness
`increases only slightly, because, in the pulsed mode,
`refraction insignificantly affects the brightness of the
`first plasma front.
`In our experiments, the maximum spectral bright-
`ness of POD plasma of about 8 W cm–2 nm–1 sr–1
`within the wavelength range 450–500 nm was
`achieved for the focusing parameter F = 3.3 and xenon
`pressure p = 19 ± 1 bar. Such a spectral brightness is
`twice as large as the maximum brightness of COD
`plasma under similar conditions.
`
`PLASMA PHYSICS REPORTS
`
` Vol. 41
`
` No. 10
`
` 2015
`
`REFERENCES
`1. Yu. P. Raizer, Sov. Phys. Usp. 23, 789 (1980).
`2. D. K. Smith, Patent US No. 7435982 (2008).
`3. D. K. Smith, W. M. Holber, and J. A. Casey, Patent US
`No. 8309943 (2012).
`4. S. Horne, D. Smith, M. Besen, M. Partlow, D. Stol-
`yarov, H. Zhu, and W. Holber, Proc. SPIE 7680, 76800
`(2010).
`5. V. P. Zimakov, V. A. Kuznetsov, N. G. Solovyov,
`A. N. Shemyakin, A. O. Shilov, and M. Yu. Yakimov,
`Proc. SPIE 8600, 860002 (2013).
`6. P. S. Antsiferov, K. N. Koshelev, V. M. Krivtsun, and
`A. A. Lash, Patent Application No. RU2013116408/07
`(2013), Patent RF No. 2534223 (2013).
`7. U. Arp, R. Vest, J. Houston, and T. Lucatorto, Appl.
`Opt. 53, 1089 (2014).
`8. J. Uhlenbusch and W. Viol, J.Quant. Spectros. Radiat.
`Transfer 44, 47 (1990).
`9. P. K. Tret’yakov, G. N. Grachev, A. I. Ivanchenko,
`V. L. Krainev, A. G. Ponomarenko, and V. N. Tish-
`chenko, Doklady Phys. 39, 415 (1994).
`10. G. N. Grachev, A. G. Ponomarenko, A. L. Smirnov,
`V. N. Tischenko, and P. K. Tret’jacov, Laser Phys. 6,
`376 (1996).
`
`Translated by M. Samokhina
`
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