Includes bibliographical references (page 522).Extremely compact high repetition rate soft X-ray lasers based on capillary discharge excitation have demonstrated average powers of a few milliWatt at 46.9 nm, milli-Joule-level pulse energy, peak spectral brightness several orders of magnitude larger than third-generation synchrotron beam lines, and excellent spatial coherence. Examples of the use of a capillary discharge soft X-ray laser in dense plasma diagnostics and laser ablation of materials are summarized

Artioukov, I.

Benware, B.

Filevich, J.

Hammarsten, E. C.

Jankowska, E.

Luther, B.

Marconi, M. C.

Moon, S.

Rocca, Jorge J.

Shlyaptsev, V. N.

Vinogradov, A.

English

Extremely compact high repetition rate soft X-ray lasers based on capillary discharge excitation have demonstrated average powers of a few milliWatt at 46.9 nm, milli-Joule-level pulse energy, peak spectral brightness several orders of magnitude larger than third-generation synchrotron beam lines, and excellent spatial coherence. Examples of the use of a capillary discharge soft X-ray laser in dense plasma diagnostics and laser ablation of materials are summarized

Shlyaptsev, V. N., author

Moon, S., author

Artioukov, I., author

Vinogradov, A., author

Luther, B., author

Marconi, M. C., author

Benware, B., author

Jankowska, E., author

Hammarsten, E. C., author

Filevich, J., author

Rocca, Jorge J., author

Elsevier Science B. V., publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522Extremely compact soft X-ray lasers based oncapillary dischargesJ.J. Roccaa,*, J. Filevicha, E.C. Hammarstena, E. Jankowskaa, B. Benwarea,M.C. Marconia, B. Luthera, A. Vinogradovb, I. Artiukovb,S. Moonc, V.N. ShlyaptsevdaEngineering Research Center, Colorado State University, 1320 Campus Delivery, Fort Collins, CO 80523-1320, USAbLebedev Physical Institute, Moscow, RussiacLawrence Livermore National Laboratory, Livermore, CA, USAdUniversity of California at Davis, Livermore, CA, USAAbstractExtremely compact high repetition rate soft X-ray lasers based on capillary discharge excitation have demonstratedaverage powers of a few milliWatt at 46.9 nm, milli-Joule-level pulse energy, peak spectral brightness several orders ofmagnitude larger than third-generation synchrotron beam lines, and excellent spatial coherence. Examples of the use ofa capillary discharge soft X-ray laser in dense plasma diagnostics and laser ablation of materials are summarized.r 2003 Elsevier Science B.V. All rights reserved.PACS: 52.50.Jm; 42.55.Vc; 52.65.Kj; 52.70.LaKeywords: Capillary discharge; Soft X-ray interferometry; Soft X-ray applications1. IntroductionThere is significant interest in the developmentof compact sources of coherent soft X-ray radia-tion that can be supported by a single regularoptical table, yet one capable of producing high(milliWatt) average power. Capillary dischargeexcitation of Ne-like Ar plasmas has generatedsuch powers of coherent soft X-ray radiation andmilli-Joule-level pulses in a table-top setup [1,2].Multi-Hertz repetition rate operation generated anaverage power of E3.5mW at a wavelength of46.9 nm [2]. The advanced degree of developmentof this laser is summarized in the next section,followed by a summary of selected experimentalresults that demonstrate the use of this table-topsoft X-ray laser in several diverse areas of scienceand technology.2. Development status of capillary discharge soft X-ray laserLarge line amplification excited by collisionalelectron impact excitation of multiply ionized ionshas been demonstrated in Ne-like Ar (l ¼ 46:9 nm)ARTICLE IN PRESS*Corresponding author. Tel.: +1-970-491-2342; fax: +1-970-491-8671.E-mail address: rocca@engr.colostate.edu (J.J. Rocca).0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0168-9002(03)00908-2[3] Ne-like Cl (l ¼ 52:9) [4] and Ne-like S(l ¼ 60:8 nm) [5]. The Ne-like Ar capillary dis-charge laser is the most highly developed of theselasers. Table 1 summarizes the characteristics ofthis laser and its present output parameters. Theselaser output parameters were obtained utilizingaluminum oxide capillary channels 3.2mm indiameter filled with preionized Ar gas at anoptimized pressure of E460mTorr. The plasmacolumns were excited by current pulses of E26 kApeak amplitude with a 10–90% rise time ofapproximately 40 ns. Fig. 1 illustrates the size ofthe capillary discharge Ne-like Ar laser. Thiscapillary discharge-pumped laser occupies a tablespace of approximately 0.4m 1m, a size com-parable to that of many widely utilized visible orultraviolet gas lasers.In this laser, the excitation current pulse isproduced by discharging a water capacitorthrough a spark gap switch connected in serieswith the capillary load. The laser average outputpulse energy was measured to increase from0.075mJ for a plasma column 16 cm in length, to0.88mJ for the plasma column 34.5 cm in length.At the output of the longest capillary the laserbeam intensity approaches 1GW/cm2, and exceedsthe saturation intensity by more than an order ofmagnitude. Fig. 2a shows the shot-to-shot varia-tions of the measured laser output pulse energyand corresponding laser average power for a34.5 cm long discharge operated at 4Hz repetitionrate. The average laser output power is about3.5mW, corresponding to >8 1014 photons/s[2]. Fig. 2b shows the average laser output energyper pulse is 0.88mJ and that the energy of thehighest energy pulses exceeds 1mJ. More than5000 laser shots were obtained from a singlecapillary. The full-width at half-maximum(FWHM) of the corresponding laser pulse is1.570.05 ns, longer than the 1.2 ns pulsewidththat we measured for an 18.2 cm long amplifier [3].The average peak laser output power obtainedwith the longest plasma column is E0.6MW.The output beam intensity distribution has anannular shape. The peak-to-peak divergence wasmeasured to be about 4.6mrad for all capillarylengths between 18 and 34.5 cm. Recent measure-ments of the spatial coherence indicate that fullspatial coherence is approached with the longestcapillaries and that the peak spectral brightnessis about 2 1025 photons/(smm2mrad2 0.01%ARTICLE IN PRESSTable 1Summary of the parameters for the 46.9 nm capillary dischargetable-top soft X-ray laserLaser parameter Ref.Pulse energy 0.88mJ at 4Hz [2]Average power 3.5mW [2]Peak power 0.6MW [2]Divergence E4.6mrad [2,3]Pulse width 1.2–1.5 ns [2,3]Peak spectral brightness 2 1025 photons/(smm2mrad20.01% bandwidth)[6]Fig. 1. Photograph of capillary discharge soft X-ray laser (right) and applications chamber (left). A multimeter is shown in front of thelaser to provide size reference.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522516bandwidth) [6]. The possibility of extending thesecapillary discharge laser sources to shorter wave-lengths is currently under investigation [7].3. ApplicationsBelow, we discuss the use of the 46.9 nm laser indense plasma interferometry and material abla-tion. The application of this laser to plasmashadowgraphy [8] and soft X-ray reflectometryfor the determination of optical constants [9] hasbeen discussed in recent publications.3.1. Soft X-ray laser interferometry of denseplasmasThe Soft X-ray lasers allow the probing of large-scale high-density plasmas beyond the limits thatplasma refraction and absorption impose to visibleand ultraviolet laser probes. Da Silva et al.conducted the first soft X-ray laser interferometryexperiments using a laboratory-size 15.5 nm Ne-like Y laser pumped by the Nova laser inconjunction with a Mach–Zehnder interferometerbased on thin-film beam splitters [10]. Capillarydischarge-pumped soft X-ray lasers, with anextremely high brightness that is similar or higherthan that of their laboratory-size predecessors anda much higher repetition rate, offer the opportu-nity to develop portable soft X-ray tools for thediagnostics of a large variety of dense plasmas.Our group has realized the first demonstrations ofsoft X-ray interferometry using a table-top laser[11–13]. Experiments with the capillary discharge46.9 nm Ne-like Ar laser were conducted usingeither a wavefront-division interferometer basedon Lloyd’s mirror [11,12], or an amplitude divisioninterferometer in which diffraction gratings wereused as beam splitters [13].The soft X-ray laser interferometry system usedin our most recent experiments is schematicallyshown in Fig. 3. It consists of a table-top capillarydischarge laser emitting B0.1mJ pulses of B1.2 nsduration at 46.9 nm, the diffraction grating inter-ferometer (DGI), and a Nd:YAG laser that isfocused onto a polished Cu target to generate theplasmas that are the subject of this study.The DGI consists of a modified Mach–Zehnderconfiguration of rhomboidal shape that usesdiffraction gratings as beam splitters and elon-gated grazing incidence mirrors. The zero and firstdiffracted orders from the first grating are used toform the two arms of the interferometer. Theelongated gold-coated mirrors redirect the beamtowards the second grating where they arerecombined and the interference pattern is gener-ated. Advantages of the DGI scheme over otheramplitude-division soft X-ray interferometersbased on thin-film beam splitters [10,14] includea higher throughput (B6% percent per arm) and asignificantly increased resistance of the beamsplitters to plasma debris. A Si/Sc multilayersoptics system was used to image the plasma onto aCCD/MCP detector setup with 51.2 magnifica-tion. The alignment of the interferometer was donewith an 824 nm laser diode on a second set ofARTICLE IN PRESSFig. 2. Measured output pulse energy and average power of atable-top capillary discharge 46.9 nm laser operating at arepetition frequency of 4Hz: (a) Shot-to shot laser outputenergy and average output power computed as a walkingaverage of 60 continuous laser pulses; and (b) distribution ofthe output pulse energy. Average pulse energy is 0.88mJ andthe standard deviation is 0.06mJ.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522 517rulings on the gratings. In the elongated plasmacolumns generated by the capillary discharge, gainguiding and strong refractive anti-guiding resultsin the rapid buildup of spatial coherence [6]. Thisintrinsic mode selection mechanism makes itpossible to achieve a very high degree of spatialcoherence, which in turn allows for the generationof high-visibility interferograms. The interfero-grams obtained have a fringe visibility of B0.5over the entire 400 500 mm2 field of view. Below,we exemplify the use of this interferometry setup inthe characterization of a spot-focus laser-createdplasma.In this experiment, the plasmas were formedfrom a copper target using 0.62 J pulses of 13 nsFWHM duration from an Nd:YAG laser operat-ing at its fundamental wavelength of 1.06 mm. Thelaser was focused onto a B30 mm diameter spotusing a 15 cm focal length aspheric lens. Detailedseries of interferograms were obtained for plasmascreated by firing the heating laser either onto afresh polished copper target or into the craterformed by previous shots. In the latter case, theplasmas display a significant density increase atlarge distances from the target surface. This resultsfrom the fact that the crater constrains theplasma’s lateral expansion and guides its motioninto the direction normal to the target. In all cases,the measurements unveiled the formation of an‘‘inverted center’’ electron density profile with adensity minimum on axis, as we also observed inthe line-focus experiments at lower irradiationintensities [15]. The plasma evolution was mappedby taking interferograms at different times relativeto the initiation of the heating laser pulse, shownin Fig. 4.A concave shape is visible in the fringesthroughout the plasma evolution. In this axisym-metric geometry such fringe pattern is indicative ofa concave electron density profile with a significantminimum on axis. At the time of the maximumlaser intensity, this central minimum in the densityprofile is observed to extend through practicallythe entire sub-critical region of the plasma. Fig. 5shows the evolution of the electron densitydistributions derived from Abel inversion of theinterferograms of Fig. 4. The formation of aconcave electron density profile with pronouncedplasma sidelobes and a density cavity on theirradiation axis is observed. At 10.5 ns (notshown), the electron density of the sidelobes ismeasured to reach B9 1020 cm3 at a distance of27 mm from the target. This density corresponds to90% of the critical density of this l ¼ 1:06 mmpump laser.This observed ‘‘inverted’’ density profile isneither a result of the ponderomotive force northe effect of laser radiation refraction or filamen-tation. Hydrodynamic simulations show that theobserved two-dimensional profiles are caused byARTICLE IN PRESSFig. 3. Schematic representation of the experimental setup. The interferometer was at B2m from the exit of the soft X-ray laser. Thedetector was placed at B7m from the interferometer.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522518the generation of cold plasma sidelobes outside thelaser-irradiated target area and by the subsequentestablishment of pressure balance between thesidelobes and the hotter central plasma. Thesidelobes result from the buildup of cold materialgenerated by an increased ablated area causedmainly by XUV plasma radiation. Moreover, thesimulations indicated that this is essentially auniversal effect that should be observed over awide range of plasma parameters. Similar resultsfor line-focus plasmas are discussed in detail inanother paper [15].3.2. Soft X-ray laser ablationFocusing of these laser beams with multilayer-coated optics has potential to reach very highintensities and energy densities, opening newapplications for short-wavelength coherent radia-tion. Here, we summarize the characterization of afocused 46.9 nm laser beam generated by a Ne-likeAr capillary discharge soft X-ray laser, and theresults of the first laser ablation experiment with asoft X-ray laser [16].The soft X-ray laser pulses had an energy ofabout 0.13mJ and 1.2 ns FWHM duration andwere generated at a repetition rate of 1Hz byamplification in an 18.2 cm long Ar capillaryplasma. The laser beam was focused by a spherical(R ¼ 10 cm) Si/Sc multilayer-coated mirror ofE40% normal incidence reflectivity located in avacuum chamber at 256 cm from the exit of thecapillary amplifier. The focused laser beam wasobserved to have sufficient intensity to ablatealuminum, stainless steel and brass when thesamples were positioned within several hundredmicrometers of the focus. Series of imprints of thebeam for positions for different targets along theoptical axis were obtained by continuously movingthe translation stage while repetitively firing thelaser at a repetition rate of 1Hz.Fig. 6 is a scanning electron microscope (SEM)photograph of the surface of a brass targetshowing the progression of ablation patternsobtained as the target was moved away from themirror and towards the focus. Each ablationpattern is the result of a single laser shot. At anaxial distance of a few hundred micrometers fromthe focal region, the ablation patterns have theshape of thin annular disks. As the focal region isapproached, the thickness of the ablated ringsincreases and a central spot develops. The depth ofthe rings was measured to be E2 mm. Finally, verynear the focus, the patterns evolve into a singlespot with a deep hole on axis. The smallest spothas an outer diameter of about 17 mm and containsa deep central hole of about 2–3 mm diameter.ARTICLE IN PRESSFig. 4. Interferograms of plasmas generated focusing an Nd-YAG laser into a spot B30 mm, with an irradiation intensity of7 1012Wcm2. The interferograms correspond to plasmasgenerated firing the laser for a second time in the same targetlocation. The times delays are measured with respect to thebeginning of the laser pulse. The heating laser is incident fromthe right.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522 519Fig. 7 shows the radial cross-section of the beamintensity distributions in the focal region. Forcomparison with the experiment, the measuredboundaries of the ablated regions are representedas black dots in the same figure. All the majorfeatures of the observed ablation profiles of Fig. 6are well described by ray tracing computations.These features are the result of the sphericalaberration that causes the rays to converge andcross at those locations. Similarly, the sphericalaberration causes the central peak, which begins todevelop when the outermost rays converge on theaxis. Near the so-called ‘‘plane of minimumconfusion’’ the intensity distribution is dominatedby the sharp central peak, which is responsible forthe deep central hole observed in the SEM images.The average intensity within a 2mm diameter regionis estimated to be greater than 1 1011W/cm2 fromthe computed fraction of rays that intersect thisregion. Recent wavefront measurements indicateARTICLE IN PRESSFig. 5. Computed electron density corresponding to the interferograms of Fig. 4.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522520that the use of aspheric optics should allow focusingto intensities >1013W/cm2 [17].4. ConclusionsIn summary, very compact capillary discharge-pumped table-top soft X-ray lasers have reached alevel of development that allows their routine usein numerous applications. The average coherentpower per unit of spectral bandwidth of the 46.9Ne-like Ar laser is similar to that of a third-generation synchrotron beam line, and its high-peak spectral brightness makes it one of thebrightest sources of soft X-ray radiation in thisspectral region.ARTICLE IN PRESSFig. 6. SEM images of the ablation patterns obtained by moving a brass sample towards the focus. Each ablation pattern correspondsto a single laser shot. The composition of the brass used in this experiment was by weight 60.95% Cu, 39.05% Zn with small Alimpurities.Fig. 7. Computed radial cross-section of the beam in the focal region. The dots correspond to the measured boundaries of the ablatedpatterns.J.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522 521AcknowledgementsThe development of the soft X-ray laser wassupported by the National Science Foundation.The plasma interferometry work was supported bythe US Department of Energy Grant DE-FG03-02NA00062. We gratefully acknowledge a grantfrom the W.M. Keck Foundation, and the USCivilian Research and Development Foundation(CRDF).References[1] J.J. Rocca, Rev. Sci. Instrum. 70 (1999) 3799.[2] C.D. Macchietto, B.R. Benware, J.J. Rocca, Opt. Lett. 24(1999) 1115.[3] B.R. Benware, C.D. Macchietto, C.H. Moreno, J.J. Rocca,Phys. Rev. Lett. 81 (1998) 5804.[4] M. Frati, M. Seminario, J.J. Rocca, Opt. Lett. 25 (2000)1022.[5] F.G. Tomasel, J.J. Rocca, V.N. Shlyaptsev, C.D. Mac-chietto, Phys. Rev. A 55 (1997) 1437.[6] Y. Liu, et al., Phys. Rev. A 63 (2001) 033802.[7] S. Sakadzik, et al., Proceedings on ‘‘Soft X-ray Lasers andApplications IV’’, SPIE J., Vol 4505, San Diego, 1–3August 2001.[8] M.C. Marconi, et al., Phys. Rev. E 62 (2000) 7209.[9] I.A. Artioukov, et al., IEEE J. Sel. Topics QuantumElectron. 5 (1999) 1495.[10] L.B. Da Silva, et al., Phys. Rev. Lett. 74 (20) (1995)3991.[11] C.H. Moreno, et al., Phys. Rev E. 60 (1) (1999) 911.[12] J.J. Rocca, C.H. Moreno, M.C. Marconi, K. Kanizay,Opt. Lett. 24 (1999) 420.[13] J. Filevich, et al., Opt. Lett. 25 (2000) 356.[14] A.S. Wan, et al., Phys. Rev. E 55 (1997) 6293.[15] J. Filevich, et al., Proceedings of the Eighth InternationalConference on X-ray Lasers, Aspen, CO, USA, May 2002.[16] B.R. Benware, et al., Opt. Lett. 24 (1999) 1724.[17] S. Le Pape, et al., Phys. Rev. Lett. 88 (2002) 183901.ARTICLE IN PRESSJ.J. Rocca et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 515–522522

Extremely compact soft X-ray lasers based on capillary discharges

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Extremely compact soft X-ray lasers based on capillary discharges

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