Includes bibliographical references (page 1117).Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplification in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft-x-ray amplification in a highly saturated regime, resulting inefficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of ≈1 × 1023 photons/s mm2 mrad2 0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date

Benware, B. R.

Macchietto, C. D.

Rocca, Jorge J.

English

Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplification in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft-x-ray amplification in a highly saturated regime, resulting inefficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of ≈1 × 1023 photons/s mm2 mrad2 0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date

Benware, B. R., author

Macchietto, C. D., author

Rocca, Jorge J., author

Optical Society of America, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

August 15, 1999 / Vol. 24, No. 16 / OPTICS LETTERS 1115Generation of millijoule-level soft-x-ray laser pulsesat a 4-Hz repetition rate in a highlysaturated tabletop capillary discharge amplifierC. D. Macchietto, B. R. Benware, and J. J. RoccaDepartment of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523-1373Received March 8, 1999Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-passamplif ication in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain–length productof this plasma column allows for soft-x-ray amplif ication in a highly saturated regime, resulting inefficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MWwere obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of 1 31023 photonss mm2 mrad2 0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sourcesto date.  1999 Optical Society of AmericaOCIS codes: 140.7240, 140.3280, 140.1340.A characteristic that distinguishes soft-x-ray lasersfrom other sources of coherent soft-x-ray radiation istheir potential to generate high-energy pulses. Un-dulators at modern sychrotron facilities generate highaverage power beams of soft-x-ray radiation. How-ever, they operate at very high pulse repetition rates[e.g., 5 3 108 Hz (Ref. 1)], and the energy per pulse istypically of the order of nanojoules. In the case ofsoft-x-ray pulses that result from the generation ofhigh-order harmonics of optical lasers, the highestpulse energy that has been reported is 60 nJ.2 Theaverage power obtained by this method was recentlyincreased to the microwatt level as a result of thedemonstration of phase-matched harmonic conver-sion.3 In contrast, soft-x-ray lasers pumped by largeoptical lasers have produced output energies of asmuch as several millijoules.4 – 6 The largest pulseenergy reported to date is 8 mJ and was obtainedat a wavelength of 15.5 nm in a collisionally excitedNe-like Y laser pumped by the Nova laser.4 This lasercan operate at a repetition rate of approximately oneshot per hour. A more compact l  18.2 nm H-likeC recombination laser pumped by a 300-J CO2 lasergenerated laser pulses with an energy of 3 mJ at arate of one shot every 3 min.6 Recently importantprogress has been achieved in the development oftabletop soft-x-ray amplifiers.7 – 17 In particular, thehigh efficiency with which capillary discharges canpump soft-x-ray amplif iers with large gain volumescan potentially produce large laser pulse energies athigh repetition rates.8,14 – 16In this Letter we report what is to our knowledge thefirst demonstration of millijoule-level soft-x-ray laserpulses with a tabletop device. Efficient energy ex-traction from the laser amplifier was achieved by op-eration at intensities well above the gain-saturationintensity. Tabletop discharge pumped soft-x-ray am-plif iers that exceed the saturation intensity were pre-viously demonstrated,14,15 and recently the generationof a laser output pulse energy of 0.135 mJ at 46.9 nmin a capillary discharge Ar plasma column 18.2 cm inlength16 was reported. However, in all those saturated0146-9592/99/161115-03$15.00/0capillary discharge soft-x-ray laser experiments theamplif ier length was only slightly greater than the 12–15 cm at which gain saturation was observed. There-fore energy was efficiently extracted only from thelast few centimeters of the plasma column. Increasedlaser output energy can be achieved by double-passamplif ication by use of a mirror placed in close prox-imity to the end of the plasma column.5,14 However,rapid damage to the mirror by the plasma makesthis solution unsuitable for high-repetition-rate op-eration.14 Instead we obtained the results reportedherein by increasing the length of the plasma columnto 34.5 cm, which is more than twice the saturationlength. The length-to-diameter ratio of this plasmacolumn exceeds 1000:1. Operation in this highly gain-saturated amplification regime has allowed us to ef-ficiently extract the energy stored in the populationinversion from the majority of the amplif ier volume.Laser pulses with an average energy of 0.88 mJ .2 31014 photonspulse were generated at a wavelengthof 46.9 nm at a repetition frequency of 4 Hz; the en-ergy of the most intense pulses exceeded 1 mJ. Theaverage power obtained was 3.5 mW, to our knowl-edge the highest reported to date for a tabletop soft-x-ray laser.The experiments were conducted with aluminumoxide capillary channels 3.2 mm in diameter filledwith preionized Ar gas at an optimized pressure of460 mTorr. The plasma columns were excited bycurrent pulses of 26-kA peak amplitude with a 10–90% rise time of approximately 40 ns. The setup wassimilar to that used in previous experiments reportedin Ref. 16. The excitation current pulse was producedby discharge of a water capacitor through a sparkgap switch connected in series with the capillary load.The water served as a liquid dielectric for the capacitorand also cooled the capillary. The capacitor was pulsecharged by a compact four-stage Marx generator en-closed in a separate box and connected to the laser headwith a coaxial cable. The capillary discharge-pumpedlaser used in this experiment occupies an area ofapproximately 0.4 m 3 1 m on the top of a table, a size 1999 Optical Society of America1116 OPTICS LETTERS / Vol. 24, No. 16 / August 15, 1999comparable with that of many widely utilized visible orultraviolet gas lasers. The laser output pulse energyand power were measured with a vacuum photodiodeplaced 87 cm from the exit of the laser, and the datawere recorded and stored by a 2-Gbits digitizing oscil-loscope. The quantum efficiency of the Al photocath-ode had previously been calibrated with respect to a Siphotodiode of known quantum yield.14 The laser out-put was attenuated with several stainless-steel meshesof measured transmissivity to avoid saturation of thephotodiode. A measurement performed with the soft-x-ray laser beam verif ied that the transmittivity of thestack of meshes is in good agreement with that com-puted from the product of individual meshes.Figure 1 shows the average laser output pulse en-ergy obtained by use of capillary plasma columns 16,25, and 34.5 cm in length. The laser average out-put pulse energy was measured to increase from0.075 mJ for a plasma column 16 cm in length to0.88 mJ for a plasma column 34.5 cm in length. Esti-mates of the laser intensity based on these energies andon the pulse-width measurements discussed below in-dicate that the saturation intensity of 56 78 MW cm22(Ref. 14) is exceeded before the end of the 16-cm cap-illary and that the output of the longest capillary ex-ceeds the saturation intensity by more than an orderof magnitude, approaching 1 GWcm2. Figure 2(a)shows the shot-to-shot variations of the measured laseroutput pulse energy and the corresponding laser av-erage power for the 34.5-cm-long discharge operatedat a 4-Hz repetition frequency. The data correspondto 100 s of uninterrupted operation of the laser. Thesolid curve shows that the average laser power is3.5 mW, corresponding to .8 3 1014 photonss. Fig-ure 2(b) shows that the average laser output energyper pulse is 0.88 mJ and that the energy of the high-est energy pulses exceeds 1 mJ. More than 5000 lasershots were obtained from a single capillary.The temporal evolution of the laser pulse was mea-sured with a fast vacuum photodiode and a 1-GHz-bandwidth analog oscilloscope. A typical laser outputpulse that corresponds to the 34.5-cm-long amplifieris shown in Fig. 3. The full width at half-maximumof the laser pulse is determined to be 1.5 6 0.05 nsby correction of the measured pulse for the limitedbandwidth of the detection system. This laser pulsewidth is longer than the 1.2 ns that we measured for an18.2-cm-long amplif ier.16 The increase of the laserpulse width is the result of the increased transit time ofthe radiation through the amplif ier. With this pulsewidth taken into consideration, the average peak laseroutput power obtained with the longest plasma columnis estimated to be 0.6 MW.The far-field laser output intensity distributionsthat correspond to the three capillary plasma lengthsof Fig. 1 were measured with a phosphor screen anda CCD array detector of 1024 3 1024 pixels. In allcases the beam profile had a ring shape that wasthe result of refraction of the rays by plasma densitygradients in the plasma column.18,19 Figure 4 showscross sections of the output intensity patterns. Themeasurements are an average of five consecutive lasershots. The peak-to-peak divergence is 4.6 mrad inall three cases. The beam profiles are similar for allamplif ier lengths, with slightly more-pronounced peaksfor the longer plasma column lengths. The similarityof the profiles is a consequence of the waveguidenature of the amplifier column. The combination ofgain guiding and refraction antiguiding determines theintensity distribution,19 which for the plasma columnlength of 16 cm is already converging to that of theeingenmode of the waveguide. From the measuredFig. 1. Measured average laser output pulse energy as afunction of capillary plasma column length. Error barsrepresent the standard deviations of the shot-to-shot pulseenergy variations that correspond to 400 consecutive laserpulses obtained at a repetition rate of 4 Hz.Fig. 2. Measured laser output pulse energy and averageoutput power that correspond to the 34.5-cm-long amplifieroperating at a repetition rate of 4 Hz. The data are for100 s of continuous operation of the laser. (a) Shot-to-shotlaser output pulse energy. Solid curve, the average outputpower computed as a walking average of 60 contiguouslaser pulses. (b) Distribution of the output pulse energy.The average pulse energy is 0.88 mJ, and the standarddeviation is 60.06 mJ.August 15, 1999 / Vol. 24, No. 16 / OPTICS LETTERS 1117Fig. 3. Temporal characteristics of the laser pulse thatcorresponds to the 34.5-cm-long amplifier. Solid curve,the measured profile; dashed curve, result of correcting thedata for the limited bandwidth of the detection system.Fig. 4. Cross sections of the far-field laser intensity dis-tribution patterns that correspond to the capillary plasmacolumn lengths indicated. The two-dimensional far-fieldlaser intensity distribution pattern for the 34.5-cm-longamplifier is shown as an inset.laser beam parameters reported above for the 34.5-cm-long amplif ier, and assuming a beam diameter of300 mm at the exit of the laser,18 the peak spectralbrightness of this laser can be estimated to be 1 31023 photonss mm2 mrad2 0.01% bandwidth. Thisvalue makes this tabletop laser one of the brightestsoft-x-ray sources to date.20In summary, we have demonstrated the generationof millijoule-level laser pulses from a tabletop soft-x-ray amplifier. The results obtained at a repetitionfrequency of 4 Hz include the generation of what webelieve are the highest average output pulse energy(0.88 mJ) and average power (3.5 mW) obtained to datefrom a tabletop source of coherent soft-x-ray radiation.We thank J. L. A. Chilla, V. N. Shlyaptsev, and E.Bernstein for useful discussions. This research wassupported by the National Science Foundation undergrant DMR-9512282. J. J. Rocca’s e-mail address isrocca@engr.colostate.edu.References1. D. T. Attwood, K. Halbach, and N. J. Kim, Science 228,1264 (1985).2. T. Ditmier, J. K. Crane, H. Nyugen, L. B. Da Silva, andM. D. Perry, Phys. Rev. A 51, R902 (1995).3. A. Rundquist, C. G. Durfee III, Z. Chang, C. Herne, S.Backus, M. Murnane, and H. C. Kapteyn, Science 280,1412 (1998).4. L. B. Da Silva, B. J. MacGowan, S. Mrowka, J. A. Koch,R. A. London, D. L. Matthews, and J. H. Underwood,Opt. Lett. 18, 1174 (1993).5. A. Carrillon, H. Z. Chen, P. Dhez, L. Dwivedi, J. Jacoby,P. Jaegle, G. Jamelot, J. Zhang, M. H. Key, A. Kidd, A.Klishnick, R. Kodama, J. Krishnan, C. L. S. Lewis, D.Neely, P. Norreys, D. O’Neill, G. J. Pert, S. A. Ramsden,J. P. Raucourt, G. J. Tallents, and J. Uhomoibhi, Phys.Rev. Lett. 68, 2917 (1992).6. S. Suckewer, C. H. Skinner, D. Kim, E. Valeo, D.Voorhees, and A. Wouters, Phys. Rev. Lett. 57, 1004(1986).7. Y. Nagata, K. Midorikawa, S. Kubodera, M. Obara,H. Tashiro, and K. Toyoda, Phys. Rev. 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Lowenthal, and J. E. Balmer, Phys.Rev. A 59, 1577 (1999).18. C. H. Moreno, M. C. Marconi, V. N. Shlyaptsev, B. R.Benware, C. D. Macchietto, J. L. A. Chilla, J. J. Rocca,and A. L. Osterheld, Phys. Rev. A 58, 1509 (1998).19. J. L. A. Chilla and J. J. Rocca, J. Opt. Soc. Am. B 13,2841 (1996).20. T. N. Lee, S. Kim, and H. Shin, Proc. SPIE 3156, 250(1997).

Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier

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Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier

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