We report about nonlinear growth of x-ray emission intensity emitted from plasma generated by femtosecond petawatt laser pulses irradiating stainless steel foils. X-ray emission intensity increases as ∼ I 4.5 with laser intensity I on a target. High spectrally resolved x-ray emission from front and rear surfaces of 5 μm thickness stainless steel targets were obtained at the wavelength range 1.7-2.1 Å, for the first time in experiments at femtosecond petawatt laser facility J-KAREN-P. Total intensity of front x-ray spectra three times dominates to rear side spectra for maximum laser intensity I ≈ 3.21021 W/cm2. Growth of x-ray emission is mostly determined by contribution of bremsstrahlung radiation that allowed estimating bulk electron plasma temperature for various magnitude of laser intensity on target

Alkhimova, M.A.

Andreev, A.

Bulanov, S V.

Dover, N.P.

Esirkepov, T.

Faenov, A.Ya.

Fukuda, Y.

Kando, M.

Kiriyama, H.

Kodama, R.

Kondo, K.

Kondo, Ko.

Miyahara, T.

Nishitani, K.

Nishiuchi, M.

Ogura, K.

Pikuz, S.A.

Pikuz, T.A.

Pirozhkov, A.S.

Sagisaka, S.

Sakaki, H.

Skobelev, I.Yu.

Watanabe, Y.

Zhidkov, A.

English

Repositorium für Naturwissenschaften und Technik

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSX-ray emission from stainless steel foils irradiatedby femtosecond petawatt laser pulsesTo cite this article: M A Alkhimova et al 2018 J. Phys.: Conf. Ser. 946 012018 View the article online for updates and enhancements.You may also likeReview of laser-driven ion sources andtheir applicationsHiroyuki Daido, Mamiko Nishiuchi andAlexander S Pirozhkov-A broadband frequency-tripling scheme foran Nd:glass laser-based chirped-pulseamplification system: an approach forefficiently generating ultraviolet petawattpulsesYing Chen, Peng Yuan and Liejia Qian-Stretchers and compressors for ultra-highpower laser systemsI.V. Yakovlev-This content was downloaded from IP address 194.95.157.29 on 01/06/2023 at 07:491Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018 doi :10.1088/1742-6596/946/1/012018X-ray emission from stainless steel foils irradiated byfemtosecond petawatt laser pulsesM A Alkhimova2,3, A Ya Faenov1,2, T A Pikuz2,4, I Yu Skobelev2,3,S A Pikuz2,3, M Nishiuchi5, H Sakaki5, A S Pirozhkov5, S Sagisaka5,N P Dover5, Ko Kondo5, K Ogura5, Y Fukuda5, H Kiriyama5,T Esirkepov5, S V Bulanov5, A Andreev6,7, M Kando5, A Zhidkov8,K Nishitani9, T Miyahara9, Y Watanabe9, R Kodama1,4,8 andK Kondo51 Institute for Academic Initiatives, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871,Japan2 Joint Institute for High Temperatures of the Russian Academy of Sciences, Izhorskaya 13Bldg 2, Moscow 125412, Russia3 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute),Kashirskoe Shosse 31, Moscow 115409, Russia4 Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871,Japan5 Kansai Photon Science Institute, National Institutes for Quantum and Radiological Scienceand Technology, Kizugawa-shi 8-1-7, Umemidai, Kyoto 619-0215, Japan6 Max Born Institute, Max-Born-Straße 2A, Berlin 12489, Germany7 Extreme Light Infrastructure—Attosecond Light Pulse Source, Dugonics 13, Szeged 6720,Hungary8 Photon Pioneers Center, Osaka City University, 2-1 Yamadaoka, Suita, Osaka 565-0871,Japan9 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1Kasugakoen, Kyushu, Fukuoka 816-8580, JapanE-mail: maalkhimova@mephi.ruAbstract. We report about nonlinear growth of x-ray emission intensity emitted from plasmagenerated by femtosecond petawatt laser pulses irradiating stainless steel foils. X-ray emissionintensity increases as ∼ I4.5 with laser intensity I on a target. High spectrally resolved x-rayemission from front and rear surfaces of 5 µm thickness stainless steel targets were obtained atthe wavelength range 1.7–2.1 Å, for the first time in experiments at femtosecond petawatt laserfacility J-KAREN-P. Total intensity of front x-ray spectra three times dominates to rear sidespectra for maximum laser intensity I ≈ 3.2× 1021 W/cm2. Growth of x-ray emission is mostlydetermined by contribution of bremsstrahlung radiation that allowed estimating bulk electronplasma temperature for various magnitude of laser intensity on target.1. IntroductionRecent years petawatt laser (PWL) facilities become available in several laboratories worldwide.PWLs allow to study various issues of particle beams acceleration [1–4] simulate astrophysicalphenomena in laboratory conditions [5,6] and create extremely bright short x-ray sources [7,8].21234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/012018Figure 1. Experimental setup. Laser beam is focused on the foil surface at the angle 45◦ tocreate plasma. Generated plasma emitted x-ray radiation measured by two x-ray spectrometersinstalled at front and rear sides of target surface about 20 degree above the laser radiation plane.The spectrometers are equipped with mica spherical crystals, permanent magnet slits and Andorx-ray CCD detectors.Correspondingly dense plasma created by femtosecond PWL pulse irradiation of a thin-foil targetwas continually considered as a powerful x-ray source [9–11].The idea of ultra-bright x-ray generation in PWL experiments is as follows. Ultra-intenselaser pulse focused onto thin metal foil creates highly ionized plasma at the laser focal spot. Dueto optical field ionization valence electrons are quickly accelerated up to MeV energies. Suchelectrons multiply passing through the target emit x-ray photons up to high energies. Thesex-ray photons irradiate peripheral target area that results in heating and inner-shell transitionpumping in atoms or ions, and hollow atoms excitation as a consequence [9, 12]. Hollow atomemission lines could be applied for determination of powerful x-ray radiation source parameters[13]. It was shown in [11] and [14] that for hollow ions emission appearance in x-ray spectra ofelements with Z ∼ 10, x-ray pumping power has to be higher than Ix-ray ∼ 1017 W/cm2.In general, laser intensity increase should results in x-ray intensity growth but in practicelaser intensity is rather limited due to complexity of laser facility construction, laser energytransport losses and optic aberrations. As was previously described, the growth of x-ray sourceintensity has non-liner dependence on laser pulse energy expressed as ∼ En where n ≈ 4–5[11]. The most common way to study the dependence on laser intensity in experiment is tovary the focusing of laser beam on a target. The influence of laser focusing on x-ray emissionintensity from dense plasma generated by high-Z foils irradiated by femtosecond laser pulsespossesses remarkable interest. In this paper we consider how x-ray yield from stainless steel(SUS) depends on the magnitude of laser intensity on the target surface. We compare x-rayemission obtained from both sides of the target and contained highly spectrally resolved K-shelland bremstrahlung radiation.31234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/0120182. Experimental set-up: Measurements of x-ray spectraThe measurements were made at the J-KAREN-P facility, which is optical parametric chirped-pulse amplification (OPCPA) Ti: Sapphire hybrid laser system at Kansai Photon ScienceInstitute [15]. Laser pulses with duration of 40 fs (full width at half maximum, FWHM, withrespect to intensity) and wavelength of 0.8 µm focused onto target by f/1.4 off-axis parabolicmirror at an incidence angle of 45◦ producing d = 1.5–2 µm (FWHM measurements) focalspot. Laser energy on target reached ≈ 10 J that corresponds to maximum laser-peak intensityI ∼ 3.2 × 1021 W/cm2. The intensity on target is varied by shifting the position of the targetalong the laser beam axis, changing the focal spot size. To register x-ray emission from SUS foil-targets of 5 µm thickness radiated by femtosecond PWL pulses two x-ray focusing spectrometerswith high spectral resolution (FSSR) were implemented (see figure 1 for setup).X-ray spectrometers were installed in front side (irradiated by laser pulse) and rear sideof the target surface, at the distance L = 2600 and 1900 mm from plasma respectively. Bothspectrometers were equipped with spherically bent mica crystals (curvature radius R = 150 mm,lattice spacing 19.94 Å). These crystals were aligned to register K-shell emission spectra of multi-charged (Heα line of Fe XXV and Lyα line of Fe XXVI) and neutral (i.e. Kα, Kβ lines) Fe ionsin the wavelength range 1.6–2.1 Å. However, mica crystal ability to reflect radiation in differentreflection orders leads to appearance of intense neutral Cr Kα line and Fe Kα line from 7th and8th reflection orders simultaneously (figure 2). X-ray emission from 1st to 5th reflection ordershas been attenuated by 100 µm Mylar (C10H8O4) films located in front of the crystals to protectfrom debris pollution. X-ray spectra were measured by an x-ray Andor DX420 back-illuminatedcharged coupled device (CCD) with 26 µm pixel size. CCD apertures were protected againstexposure to visible light using 2 layers of 1 µm thick polypropylene coated both sides with0.2 µm Al. X-ray emission observed from front and rear sides of the target surface for varioustarget positions offset from the best focus position are shown in figure 2. Raw x-ray spectrawere averaged over three shots to increase signal-to-noise ratio. By changing the target positionalong the laser axis we varied laser intensity on target surface keeping the laser energy on targetElt ≈ 10 J for all shots. Figure 2 shows that at maximum laser intensity He-like transitionsof Fe predominate in front side spectrum, while in rear side spectrum Fe Heα lines appearsto be of very low intensity, and Cr Heα and neutral Fe Kα lines keep recognizable intensitiesonly. The short shift of the target to lf ∼ 10 µm from the best focus position leads to adistinguishable changes in both the spectra and x-ray yield. For such focusing position the totalintensity of front side x-ray spectrum in the spectral range 1.6–2.1 Å decreases more notablythan rear side spectrum intensity (figure 4). Meanwhile Cr Heα line has already disappeared inrear side spectrum while Fe Heα and Cr Heα stay dominant in front side spectrum. At furtherlaser intensity lowering mostly neutral Kα and Kβ transitions in Fe and Kα line in Cr ions areobserved from both sides of the target.In order to reveal the change in actual laser intensity regarding the target displacement,the intensity distribution across the attenuated laser beam was carefully measured in differentcross sections using optical microobjective technique. Data examples are shown in figure 3demonstrating the achievement of ≈ 2 µm tight focusing conditions in the case of zerodisplacement, and approximately 6 µm at FWHM focal spot diameter at +100 µm away fromthe best focus. The obtained dependence of the laser intensity regarding the target displacementis given in figure 3(c). From here it can be seen that just a 20 µm target displacement leadsto 3 times laser intensity decrease, due to a sharp focusing with f/1.4 off-axis parabolic mirror.However, along ∼ 100 µm displacement range the laser intensity decreases rather slowly thanit might be expected from general consideration for f/1.4 focusing system, demanding furtherstudies.X-ray yield was measured by the integration of FSSR data over the full observation rangecovering 1.7 to 2.1 Å: wavelength band. Using the data on laser focusing conditions described41234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/012018Figure 2. X-ray emission spectra from 5 µm SUS foils measured for various target displacement.(a) Front side x-ray spectra observed for laser energy on target Elt ≈ 10 J and differentmagnitudes of laser intensity on target surface. (b) Corresponding rear side spectra measuredin same shots.Table 1. Bulk electron temperature estimation for the 2×1020 to 3×1021 W/cm2 range of laserintensities on target. Temperature estimation is made with the assumption of bremsstrahlungradiation to be registered from 5th and 7th mica reflection orders.Target displacement lf Laser intensity on target Front side Te Rear side Te(µm) (1021 W/cm2) (eV) (eV)0 3.2 850 45010 2.7 780 40050 0.9 320 210100 0.4 160 160150 0.23 140 140above, the dependence of x-ray yield from in SUS 5 µm target was obtained versus thelaser intensity. Figure 4 shows that x-ray yield, as seen both from the front and the rearside of the target foil, is quite sensitive to the laser intensity, especially in the range ofI ≈ (1–3)× 1021 W/cm2 intensities. From here it can be revealed that x-ray emission intensityIx-ray increases with laser intensity as Ix-ray ∼ I4.5lt and I3.5lt for front and rear sides of targetrespectively, where Ilt is the magnitude of laser intensity on target.The contribution of continuum radiation brightly observed in the spectra measured near thefocus (lf ≈ 0± 10 µm) becomes very weak at lf ∼ 100 µm and further. This probably could be51234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/012018Figure 3. Focal spot image obtained for the best focusing displacement along the laser axislf = 0 (a) and +100 µm (b). (c) Laser intensity on the target vs target displacement. Laserintensity calculation include the focal spot size measurements data for various lf and laserpulse duration τ ∼ 40 fs. (d) X-ray spectra measured at best focusing conditions, and bulkelectron temperature estimation with the assumption of bremsstrahlung radiation contributionin measured x-ray spectra.attributed to the lowering of collisional processes efficiency. This assumption allows estimatingbulk plasma electron temperature by determination of bremsstrahlung radiation contribution tothe measured spectra.Bremsstrahlung spectrum is given as Ibrs ∼ A exp−12.4/(λTe) /(λ2), where λ is the wavelength(Å), Te is the bulk electron temperature (keV), A is a constant. The measured x-ray spectra(see i.e. figure 3) was fitted to estimate Te for different laser intensities on target given in table1 along with the revealed laser intensities for the particular target displacements. It is necessaryto notice a certain roughness in such bulk temperature estimation allowing to determine Tevalues not better than with 20% accuracy (140 ± 30 eV) for the lowest electron temperature.Plasma generated at the front side target surface has the bulk electron temperature Te ≈ 850 eVfor maximum laser intensity I ≈ 3.2 × 1021 W/cm2 while rear side plasma temperature Te isof ≈ 450 eV. For lowest studied laser intensities I ≈ 2.3 × 1020 W/cm2. The bulk plasmatemperature was of ≈ 140 eV.61234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/012018Figure 4. Total x-ray emission intensities as a function of different laser intensities on target.Intensity of x-ray emission integrated over the wavelength range 1.7–2.1 Å.3. ConclusionWe demonstrated that x-ray yield in the range from 1.7 to 2.1 Å from femtosecond-duration-PWL-produced plasma is sensitive to the laser intensity on target ranged from 2 × 1020 to3 × 1021 W/cm2 and increases as Ix-ray ∼ I4.5lt . X-ray emission measured by front sidespectrometer was three times more intense than rear side for maximum laser intensity. Thedifference could be explained by different bulk electron temperatures Te. The estimated valuesof Te are indicative for plasma created in full plasma area including the peripheral target region[11,14]. Peripheral target region was heated to high electron temperatures keeping solid densityby x-ray pumping or hot electrons from focal spot region. Meanwhile in the case of SUS targetsthe plasma temperature in the focal spot region could reach Te ≈ 1700 eV, as was required forFe and Cr He-like transition excitation [3,16]. Though the measured x-ray emission spectra forboth front and rear sides of the target displayed no hollow atom transition lines, x-ray sourceIx-ray ∼ 1017 W/cm2 [11] should be formed in the central plasma region. However, such highintensities are appeared to be not enough effective to pump deep lying electrons in iron andchrome atoms, and to create significant populations of hollow atom states.AcknowledgmentsX-ray diagnostics and data analysis were made at JIHT RAS under financial support from theRussian Science Foundation, grant No. 14-50-00124.References[1] Schwoerer H, Pfotenhauer S, Jäckel O, Amthor K U, Liesfeld B, Ziegler W, Sauerbrey R, Ledingham K W Dand Esirkepov T 2006 Nature 439 445–8[2] Nishiuchi M et al 2015 Phys. Plasmas 22 033107[3] Gonzalez-Izquierdo B et al 2016 Nat. Commun. 7 12891[4] Daido H, Nishiuchi M and Pirozhkov A S 2012 Rep. Prog. Phys. 75 05640171234567890 ‘’“”ELBRUS 2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 946 (2018) 012018  doi :10.1088/1742-6596/946/1/012018[5] Bulanov S V, Esirkepov T Z, Kando M, Koga J, Kondo K and Korn G 2015 Plasma Phys. Rep. 41 1–51[6] Chen H et al 2015 Phys. Rev. Lett. 114 215001[7] Weisshaupt J, Juvé V, Ku S, Holtz M, Woerner M, Elsaesser T, Alǐsauskas S, Pugžlys A and Baltuška A2014 Table-top hard x-ray source driven by sub-100 fs mid-infrared pulses Proc. 2014 Conf. Lasers andElectro-Optics (CLEO)—Laser Science to Photonic Applications (San Jose, CA) pp 1–2[8] Miaja-Avila L, O’Neil G C, Uhlig J, Cromer C L, Dowell M L, Jimenez R, Hoover A, Silverman K L andUllom J N 2015 Struct. Dyn. 2 024301[9] Colgan J et al 2013 Phys. Rev. Lett. 110 125001[10] Shiraga H et al 2011 Plasma Phys. Controlled Fusion 53 124029[11] Faenov A Ya, et al 2015 Sci. Rep. 5 13436[12] Pikuz S A et al 2013 High Energy Density Phys. 9 560–7[13] Colgan J et al 2016 Europhys. Lett. 114 35001[14] Pikuz S A, Faenov A Ya, Fortov V E and Skobelev I Yu 2014 Phys. Usp. 57 702–7[15] Kiriyama H et al 2015 IEEE J. Sel. Top. Quantum Electron. 21 232–49[16] Stafford A, Safronova A S, Faenov A Ya, Pikuz T A, Kodama R, Kantsyrev V L, Shrestha I and ShlyaptsevaV V 2017 Laser Part. Beams 35 92–9

X-ray emission from stainless steel foils irradiated by femtosecond petawatt laser pulses

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X-ray emission from stainless steel foils irradiated by femtosecond petawatt laser pulses

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