PHYSICAL REVIEW A 89, 053820 (2014) High-average-power, 100-Hz-repetition-rate, tabletop soft-x-ray lasers at sub-15-nm wavelengths Brendan A. Reagan, 1, 2, * Mark Berrill, 1, 3 Keith A. Wernsing, 1, 2 Cory Baumgarten, 1, 4 Mark Woolston, 1, 2 and Jorge J. Rocca 1, 2, 4 1 NSF Engineering Research Center for Extreme Ultraviolet Science and Technology, Colorado State University, Fort Collins, Colorado 80523, USA 2 Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523, USA 3 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA 4 Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA (Received 31 March 2014; published 15 May 2014) Efficient excitation of dense plasma columns at 100-Hz repetition rate using a tailored pump pulse profile produced a tabletop soft-x-ray laser average power of 0.1 mW at λ = 13.9 nm and 20 μW at λ = 11.9 nm from transitions of Ni-like Ag and Ni-like Sn, respectively. Lasing on several other transitions with wavelengths between 10.9 and 14.7 nm was also obtained using 0.9-J pump pulses of 5-ps duration from a compact diode-pumped chirped pulse amplification Yb:YAG laser. Hydrodynamic and atomic plasma simulations show that the pump pulse profile, consisting of a nanosecond ramp followed by two peaks of picosecond duration, creates a plasma with an increased density of Ni-like ions at the time of peak temperature that results in a larger gain coefficient over a temporally and spatially enlarged space leading to a threefold increase in the soft-x-ray laser output pulse energy. The high average power of these compact soft-x-ray lasers will enable applications requiring high photon flux. These results open the path to milliwatt-average-power tabletop soft-x-ray lasers. DOI: 10.1103/PhysRevA.89.053820 PACS number(s): 42.55.Vc, 52.38.Ph I. INTRODUCTION The great interest in sources of bright coherent soft-x- ray radiation that has motivated the construction of free- electron lasers [1,2] (FELs) also motivates the development of more readily accessible tabletop soft-x-ray laser (SXRL) sources. Despite the significant progress recently made in both compact high-harmonic-generation-based sources [3,4] and plasma-based SXRLs [5–15], their average power is at present lower than that delivered by soft-x-ray FEL facilities [1,2]. The average power of laser-pumped SXRLs has been limited by the relatively low repetition rate of the high-energy optical wavelength pump lasers used to drive them and by low pumping efficiency. In contrast, capillary discharge lasers are capable of milliwatt average power [16,17], but currently their wavelengths are limited to values longer than 46 nm [18]. For laser-pumped soft-x-ray lasers operating in the (10–15)-nm wavelength range, thermal effects within the flash-lamp-pumped solid-state driver lasers have limited the repetition rate to 10 Hz or less, resulting in maximum average powers ranging from 1 to 20 μW[8,10]. Recently, we reported a tabletop SXRL capable of 100-Hz-repetition-rate operation, generating 0.15-mW average power on the λ = 18.9-nm line of Ni-like Mo [19], including the hour-long operation of this laser at high repetition rates [20]. Here we report the demonstration of a 0.1-mW-average-power, gain- saturated, λ = 13.9-nm soft-x-ray laser operating at 100-Hz repetition rate made possible by combining a high-energy chirped pulse amplification (CPA) infrared laser pumped by laser diodes that delivers pulses of up to 1 J of energy at this repetition rate, with a temporally tailored infrared driver laser pulse that more efficiently pumps the plasma amplifier. The plasma amplifiers were excited by a single-shaped pump pulse. Although the use of a single pulse has been * brendan.reagan@colostate.edu used to drive tabletop soft-x-ray lasers before [9,13,19], the pulse employed here has different characteristics. The advantage of shaping the temporal sequence of pulses used to efficiently create and heat soft-x-ray plasma amplifiers has been recognized [7,13]. In particular it was shown that the addition of a short-duration [2-ps full width at half maximum (FWHM)] prepulse with a fraction of the intensity of the main pulse can significantly increase the soft-x-ray laser pulse energy produced by a λ = 13.9-nm Ni-like Ag laser when pumping with relatively low driving laser pulse energies [7]. In another paper this work was expanded upon to allow the generation of 4.7-μJ pulses at λ = 13.9 nm using less than 2 J of total pump laser energy [21]. In recent previous work we used a single tailored pump laser pulse to drive strong lasing in the λ = 18.9-nm transition of Ni-like Mo at 100-Hz repetition rate that resulted in an average power of 0.15 mW [19]. Here, in order to more efficiently pump the soft-x-ray amplifier plasma and allow generation at shorter wavelengths, we made use of a tailored temporal pulse profile that consists of an 2-ns-long, low-amplitude ramp that creates and ionizes the plasma and a short-duration intense pulse that precedes the main peak of the pulse as shown in Fig. 1(b). Hydrodynamic and atomic physics plasma simulations show that the pump pulse profile, consisting of a nanosecond ramp followed by two peaks of picosecond duration, creates a plasma with an increased density of Ni-like ions at the time of maximum temperature that results in a larger gain coefficient over a temporally and spatially enlarged region leading to a threefold increase in the soft-x-ray laser output pulse energy. This increase in pumping efficiency combined with the increased repetition rate results in extremely high average power at these wavelengths from a tabletop device. Using this approach, bright lasing was also demonstrated at a number of wavelengths between 10.9 and 14.7 nm, including the high-repetition-rate, gain- saturated operation of a λ = 11.9-nm laser from Ni-like Sn. These results will enable technologic applications and basic 1050-2947/2014/89(5)/053820(7) 053820-1 ©2014 American Physical Society