Demonstration of a desktop size high repetition rate soft x-ray laser

We have demonstrated a new type of high repetition rate 46.9 nm capillary discharge laser that fits on top of a small desk and that it does not require a Marx generator for its excitation. The relatively low voltage required for its operation allows a reduction of nearly one order of magnitude in the size of the pulsed power unit relative to previous capillary discharge lasers. Laser pulses with an energy of ~ 13 μJ are generated at repetition rates up to 12 Hz. About (2-3) x 10 4 laser shots can be generated with a single capillary. This new type of portable laser is an easily accessible source of intense short wavelength laser light for applications. ©2005 Optical Society of America OCIS codes: (140.7240) UV, XUV, and X-ray lasers; (140.3460) Lasers; (140.3210) Ion lasers References and Links 1. B.J. MacGowan, L.B. Da Silva, D.J. Fields, C.J. Keane, J.A. Koch, R.A. London, D.L. Matthews, S. Maxon, S. Mrowka, A.L. Osterheld, J.H. Scofield, G. Shimkaveg, J.E. Trebes, and R.S. Walling, “Short wavelength x-ray laser research at the Lawrence Livermore National Laboratory,” Phys. Fluids B 4, 2326, (1992). 2. A. Carrillon et al. 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. Oneill, G.J. Pert, S.A. Ramsden, J.P. Raucourt, G.J. Tallents, and J. Uhomoibhi, “Saturated and near-diffraction-limited operation of an XUV laser at 23.6 nm,” Phys. Rev. Lett. 68, 2917, (1992) 3. J.J. Rocca, V.N. Shlyaptsev, F.G. Tomasel, O.D. Cortazar, D. Hartshorn, and J.L.A. Chilla, “Demonstration of a Discharge Pumped Table-Top Soft-X-Ray Laser,” Phys. Rev. Lett. 73, 2192 (1994). 4. J. Dunn, Y. Li, A.L. Osterheld, J. Nilsen, J.R. Hunter, V.N. Shlyaptsev, “Gain Saturation Regime for LaserDriven Tabletop, Transient Ni-Like Ion X-Ray Lasers,” Phys. Rev. Lett. 84, 4834 (2000). 5. S. Sebban,. R. Haroutunian, Ph. Balcou, et al., Phys. Rev. Lett. 86, 3004 (2001) and S. Sebban, T. Mocek, D. Ross, et al., “Demonstration of a Ni-Like Kr Optical-Field-Ionization Collisional Soft X-Ray Laser at 32.8 nm,” Phys. Rev. Lett. 89, 253901 (2002). 6. K.A. Jenulewicz, A. Lucianetti, G. Pruebe, W. Sadner and P.V. Nickles, “Saturated Ni-like Ag x-ray laser at 13.9 nm pumped by a single picosecond laser pulse,” Phys. Rev. A 68, 051802 (2003). 7. A. Butler, A.J. Gonsalves, C.M. McKenna, D.J. Spence, S.M. Hooker, S. Sebban, T. Mocek, I. Betttaibi and B. Cros, “41.8-nm Xe laser driven in a plasma waveguide,” Phys. Rev. A 70, 023821 (2004). 8. J.J. Rocca, D.P. Clark, J.L.A. Chilla, and V.N. Shlyaptsev, “Energy Extraction and Achievement of the Saturation Limit in a Discharge-Pumped Table-Top Soft X-Ray Amplifier,” Phys. Rev. Lett. 77, 1476 (1996). 9. B.R. Benware, C.D. Macchietto, C.H. Moreno and J.J. Rocca, “Demonstration of a High Average Power Tabletop Soft X-Ray Laser,” Phys. Rev. Lett. 81, 5804 (1998). 10. C.D. Machietto, B.R. Benware, and J.J. Rocca, “Generation of millijoule-levelsoft-x-ray laser pulses at a 4Hz repetitionrate in a highly saturated tabletop capillary dischargeamplifier,” Optics Lett. 24, 1115 (1999). 11. A. Ben-Kish, M. Shuker, R.A. Nemirowsky, A. Ron, and J.L. Schwob, “Plasma Dynamics in Capillary Discharge Soft X-Ray Lasers,” Phys. Rev. Lett. 87, 1 (2001). 12. A. Ritucci, G. Tomassetti, A. Reale, L. Palladino, L. Reale, F. Flora, L. Mezi, S.V. Kukhlevsky., A. Faenov, T. Pikuz, “Investigation of a highly saturated soft X-ray amplification in a capillary discharge plasma waveguide,” Applied Phys. B 78, 965 (2004). 13. J. Filevich, K. Kanizay, M.C. Marconi, J.L.A. Chilla, and J.J. Rocca., “Dense plasma diagnostics with an amplitude-division soft-x-ray laser interferometer based on diffraction gratings,” Optics Lett. 25, 356 (2000). 14. A. Artioukov, B.R. Benware, J.J. Rocca, M. Forsythe, Y.A. Uspenskii, A.V. Vinogradov, “Determination of XUV optical constants by reflectometry using a high-repetition rate 46.9-nm laser,” IEEE J. Sel. Top. Quantum Electon. 5, 1495 (1999). (C) 2005 OSA 30 May 2005 / Vol. 13, No. 11 / OPTICS EXPRESS 4050 #7073 $15.00 US Received 5 April 2005; revised 2 May 2005; accepted 17 May 2005 15. B.R. Benware, A. Ozols, J.J. Rocca, I.A. Artioukov, V.V. Kondratenko and A.V. Vinogradov, “Focusing of a tabletop sof t-x-ray laser beam and laser ablation,” Opt. Lett. 24, 1714 (1999). 16. M.Seminario, J.J. Rocca, R.Depine, B. Bach, and B. Bach, “Characterization of Diffraction Gratings by use of a Tabletop Soft-X-Ray Laser,” Appl. Optics 40, 5539 (2001). 17. G. Tomassetti, A. Ritucci, A. Reale, L. Palladino, L. Reale, L. Arriza, G. Baldacchini, F. Bonfigli, F. Flora, L. Mezi, R.M. Montereali, S.V. Kukhlevsky, A. Faenov, T. Pikuz, J. Kaiser, “High-resolution imaging of a soft-X-ray laser beam by color centers excitation in lithium fluoride crystals,” Europhys. Lett. 63, 681 (2003). 18. M.G. Capeluto, G. Vaschenko, M. Grisham, M.C. Marconi et al., “Nanopatterning with interferometric lithograph using a compact λ=46.9 nm laser,” (submitted to IEEE Transac. on Nanotechnology). 19. B. Luther, L. Furfaro, A. Klix, and J.J. Rocca, “Femtosecond laser triggering of a sub-100 picosecond jitter high-voltage spark gap,” Appl. Phys. Lett. 79, 3248-3250 (2001). 20. C.H. Moreno, M.C. Marconi, V.N. Shlyaptsev, B. Benware, C. Macchietto, J.L.A. Chilla, J.J. Rocca, “Twodimensional near-field and far-field imaging of a Ne-like Ar capillary discharge table-top soft-x-ray laser,” Phys. Rev. A 58, 1509 (1998). 21. J.L.A. Chilla and J.J. Rocca, “Beam optics of gain-guided soft-x-ray lasers in cylindrical plasmas,” J. Opt. Soc. Am. B 13, 2841 (1996). 22. S. Le Pape, Ph. Zeitoun, M. Idir, P. Dhez, J.J. Rocca, and M. François, “Electromagnetic-Field Distribution Measurements in the Soft X-Ray Range: Full Characterization of a Soft X-Ray Laser Beam,” Phys Rev. Lett. 88, 183901 (2002). The development of compact high repetition rate short wavelength laser sources is of interest for numerous applications in science and technology. Significant efforts have been devoted to reducing the size of saturated soft x-ray lasers from laboratory size [1, 2] to table-top [3-7]. The demonstration of laser amplification in transitions of Ne-like ions in a capillary discharge plasma [3, 7] opened the possibility to develop very compact short wavelength lasers for applications. Table-top size Ne-like Ar lasers operating at a wavelength of 46.9 nm have been developed making use of water capacitors that are pulsed charged to high voltage (200-700 kV) by Marx generators [8-12]. These lasers have been used in numerous applications, including interferometry of dense plasmas [13], the measurement of optical constants [14], materials ablation [15], the characterization of soft x-ray optics [16], excitation of color centers in crystals [17], and nanopatterning [18]. We report a new type of capillary discharge laser that is significantly more compact and less costly than its predecessors. It is to our knowledge the first soft x-ray laser to fit onto a small desk and also the first to be easily transportable (Fig. 1(a)). It emits >10 μJ pulses of λ= 46.9 nm light at 12 Hz repetition rate. The laser occupies a table area of approximately 0.4 × 0.4 m (0.4 X 0.8 m including the vacuum pump), smaller than that occupied by many widely used ultraviolet gas lasers. The reduced size of this capillary discharge device is achieved making use of a very low inductance co-axial discharge configuration illustrated in Fig. 1(b) that decreases the voltage necessary to generate the peak current required for laser excitation. This allows the excitation of the capillary discharge channel utilizing ceramic capacitors, which are charged at moderate voltages (< 90 kV). The reduced voltage eliminates the need of a Marx generator. As a result the volume of the pulsed power unit is ~ 9 times smaller than that of previous capillary discharge lasers [9] and can be accommodated in a small rack under a regular optical table. As in the case of the larger size Ne-like Ar 46.9 nm capillary discharge lasers previously demonstrated [3, 8-12], laser amplification is generated by fast discharge excitation of an Arfilled capillary. The magnetic force of the current pulse and large thermal pressure gradients near the wall rapidly compress the plasma to form a dense and hot column with a large density of Ne-like ions, with a very high axial uniformity and a length to diameter ratio of the order of 1000:1. Collisional electron impact excitation of the ground state Ne-like ions produces a population inversion between the 3p S0 and 3s P1 levels resulting in amplification at 46.9 nm [7]. Laser amplification is obtained in a Ne-like Ar plasma column generated in an aluminumoxide capillary 3.2 mm inside diameter and 21 cm in length filled with pre-ionized Ar gas at a (C) 2005 OSA 30 May 2005 / Vol. 13, No. 11 / OPTICS EXPRESS 4051 #7073 $15.00 US Received 5 April 2005; revised 2 May 2005; accepted 17 May 2005 Fig. 1. (a) Schematic representation of the 46.9nm capillary discharge laser. (b) Photograph of the laser device. A handheld multimeter is shown to provide a reference of scale. an optimized pressure of 700 mTorr. The plasma column is excited by current pulses of ≈22 kA peak amplitude that are monitored with a Rogowski coil. The excitation current pulse is produced by discharging a set of ceramic capacitors with a combined capacitance of 27 nF through a pressurized air high voltage spark-gap switch that is connected in series with the capillary load. The capacitors, which are placed in a ring configuration surrounding the sparkgap, are pulse-charged to 80-90 kV by a single-stage pulsed power unit that is enclosed in a separate box and which is connected to the laser head with a coaxial cable. The main current pulse through the capillary is initiated by triggering the spark-gap with a ~ 50 kV pulse of opposite polarity to that used to charge the capacitors. This allows the synchronization of the laser

The development of compact high repetition rate short wavelength laser sources is of interest for numerous applications in science and technology.Significant efforts have been devoted to reducing the size of saturated soft x-ray lasers from laboratory size [1,2] to table-top [3][4][5][6][7].The demonstration of laser amplification in transitions of Ne-like ions in a capillary discharge plasma [3,7] opened the possibility to develop very compact short wavelength lasers for applications.Table-top size Ne-like Ar lasers operating at a wavelength of 46.9 nm have been developed making use of water capacitors that are pulsed charged to high voltage (200-700 kV) by Marx generators [8][9][10][11][12].These lasers have been used in numerous applications, including interferometry of dense plasmas [13], the measurement of optical constants [14], materials ablation [15], the characterization of soft x-ray optics [16], excitation of color centers in crystals [17], and nanopatterning [18].
We report a new type of capillary discharge laser that is significantly more compact and less costly than its predecessors.It is to our knowledge the first soft x-ray laser to fit onto a small desk and also the first to be easily transportable (Fig. 1(a)).It emits >10 μJ pulses of λ= 46.9 nm light at 12 Hz repetition rate.The laser occupies a table area of approximately 0.4 × 0.4 m 2 (0.4 X 0.8 m 2 including the vacuum pump), smaller than that occupied by many widely used ultraviolet gas lasers.The reduced size of this capillary discharge device is achieved making use of a very low inductance co-axial discharge configuration illustrated in Fig. 1(b) that decreases the voltage necessary to generate the peak current required for laser excitation.This allows the excitation of the capillary discharge channel utilizing ceramic capacitors, which are charged at moderate voltages (< 90 kV).The reduced voltage eliminates the need of a Marx generator.As a result the volume of the pulsed power unit is ~ 9 times smaller than that of previous capillary discharge lasers [9] and can be accommodated in a small rack under a regular optical table.
As in the case of the larger size Ne-like Ar 46.9 nm capillary discharge lasers previously demonstrated [3,[8][9][10][11][12], laser amplification is generated by fast discharge excitation of an Arfilled capillary.The magnetic force of the current pulse and large thermal pressure gradients near the wall rapidly compress the plasma to form a dense and hot column with a large density of Ne-like ions, with a very high axial uniformity and a length to diameter ratio of the order of 1000:1.Collisional electron impact excitation of the ground state Ne-like ions produces a population inversion between the 3p 1 S 0 and 3s 1 P 1 levels resulting in amplification at 46.9 nm [7].
Laser amplification is obtained in a Ne-like Ar plasma column generated in an aluminumoxide capillary 3.2 mm inside diameter and 21 cm in length filled with pre-ionized Ar gas at  an optimized pressure of 700 mTorr.The plasma column is excited by current pulses of ≈22 kA peak amplitude that are monitored with a Rogowski coil.The excitation current pulse is produced by discharging a set of ceramic capacitors with a combined capacitance of 27 nF through a pressurized air high voltage spark-gap switch that is connected in series with the capillary load.The capacitors, which are placed in a ring configuration surrounding the sparkgap, are pulse-charged to 80-90 kV by a single-stage pulsed power unit that is enclosed in a separate box and which is connected to the laser head with a coaxial cable.The main current pulse through the capillary is initiated by triggering the spark-gap with a ~ 50 kV pulse of opposite polarity to that used to charge the capacitors.This allows the synchronization of the laser output with external events with a jitter of several ns, as required in some of the applications.Sub-nanosecond jitter can also be obtained using laser triggering of the sparkgap [19].A typical current pulse is shown in Fig.  about 35 ns after the initiation of the current pulse.The capillary discharge tube, the ceramic capacitors, and spark-gap are all contained in an Al enclosure that helps to shield the electromagnetic noise produced by the fast discharge.Biodegradable transformer oil is circulated for electrical insulation and also for cooling using a commercially available chiller unit.The laser light exits the cathode electrode that has a hole on axis and that is maintained at ground potential.Argon is continuously flown at the cathode end of the discharge, and is differentially pumped using the combination of a scroll pump and a 360 l/s turbomolecular pump to avoid significant attenuation of the laser beam by photoionization of Ar atoms.The laser output pulse energy was measured using a vacuum photodiode placed at 80 cm from the exit of the laser and the data were recorded and stored by a 5Gs/s digitizing oscilloscope.
The quantum efficiency of the Al photocathode used was previously calibrated with respect to a silicon photodiode of known quantum yield [8].The laser output was attenuated with several stainless steel meshes of measured transmissivity to avoid saturation of the photodiode.The laser was successfully operated at repetition rates up to 12 Hz.Figure 3a shows the shot to shot variation of the peak of the excitation current pulse for 1500 consecutive shots at 12 Hz repetition rate.Ablation of the capillary walls by the powerful discharge over a large number of shots increases their surface roughness, ultimately leading to the deterioration of the uniformity of the plasma column and to a consequent decrease of the laser output energy.Capillary lifetime tests were conducted at 12 Hz repetition rate recording the laser output energy for a large number of shots.The laser output energy was measured to decay by 2 times after about (2-3) x 10 4 shots (Fig. 4).This is to our knowledge the longest series of soft x-ray laser shot achieved to date.The full output pulse energy can be recovered by replacing the used capillary discharge tube by a new one, an operation that demands 30-40 minutes including the pumping time required to evacuate the system to a pressure of ~1×10 -5 Torr.
The far field laser output intensity distribution was measured using a microchannelplate/ phosphor screen read by a CCD array detector of 1024 X 1024 pixels placed at 157.5 cm from the exit of the laser.The microchannelplate was gated with a ~ 5 ns voltage pulse to be able to discriminate the laser light from the spontaneous light emitted by the plasma in hundreds of extreme ultraviolet transitions that while several orders of magnitude less intense than the Fig. 4. Variation of the laser output pulse energy as a function of the number of shots.The data was obtained operating the laser at 12 Hz repetition rate.The output energy is observed to degrade to half of the maximum value in about 2-3 10 4 discharge shots laser line, produce a significant background when temporally integrated over the duration of the discharge.The beam profile was observed to have an annular shape that is the result of refraction of the amplified rays by radial electron density gradients in the plasma column [20,21].Figure 5 shows a cross section of an output intensity pattern acquired in a single shot.The

Conclusions
In conclusion we have demonstrated high repetition rate (12 Hz) operation of a desk-top size 46.9 nm lasers that is easily transportable.This is to our knowledge the most compact soft xray laser demonstrated to date.This new type of portable short wavelength laser is interest for numerous application including experiments in photochemistry, materials characterization and patterning and high-resolution imaging.

Fig. 1 .
Fig. 1.(a) Schematic representation of the 46.9nm capillary discharge laser.(b) Photograph of the laser device.A handheld multimeter is shown to provide a reference of scale. 2 . The pulse has 10% to 90% rise time of approximately 60 ns, and first half cycle duration of 165 ns.A pronounced kink in the current is observed to occur about 40 ns after the beginning of the current pulse.This local minima of the current occurs at the time the plasma column reaches its minimum diameter of 200-300 μm, and is caused by the significant increase in the plasma column inductance that accompanies the reduction of the plasma column diameter.The laser pulse of 1.5 ns FWHM duration occurs shortly before the time of maximum plasma compression, which takes place b a b (C) 2005 OSA 30 May 2005 / Vol. 13, No. 11 / OPTICS EXPRESS 4052

Fig. 2 .
Fig. 2. Discharge current pulse (upper trace) and laser output pulse (lower trace).The kink in the current trace is caused by the abrupt increase of the plasma column inductance at the time of the pinch.

Figure 3 (
b) and Fig.3(c) illustrate the corresponding shot to shot variation of the laser output pulse energy as a function of the shot number and its statistical distribution respectively.The average pulse energy is 13 uJ and the standard deviation is +-1.3 uJ, corresponding to an average power of about 0.15 mW.The use of external triggering of the spark-gap in Fig.1(b) allowed to obtain relatively low jitter operation.Figure3(d) illustrates the statistical distribution of the time delay between the TTL trigger signal into the high voltage trigger unit that fires the spark-gap and the laser output pulse for the 1500 laser shots of Fig 3a.The standard deviation of the jitter is + -5 ns.

Fig. 3 .
Fig. 3. Data corresponding to 12 Hz repetition rate laser operation.The data is for 1500 shots of continuous operation.(a) Peak current; (b) Measured laser output pulse energy.The average pulse energy is 13 uJ ± 1.3 uJ; (c) distribution of the laser output pulse energy, (d) time delay statistics of the laser pulse respect to TTL signal into the high voltage trigger unit.
-peak divergence is about 5.2 mrad.While we have not yet characterized the wavefront, previous measurements of similar annular capillary discharge laser beams have shown good focusing properties[22].

Fig. 5 .
Fig. 5. (a) Far field image of the laser beam measured at 157.5 cm from the capillary exit.(b) corresponding intensity lineout.