Dense Plasmas for the Generation of Coherent Soft X-Ray Light J. J. Rocca 1,2 , S. Heinbuch 1,2 , D. Martz 1,2 , M. Grisham 1,2 , B. Reagan 1,2 , Y. Wang 1,2 , B.M. Luther 1,2 , M. Berrill 1,2 , D. Alessi 1,2 , M. Larotonda 1,2 , D. Gaudiossi 1,3 , T. Popmintchev 1,3 , M.M. Murnane 1,3 , H. Kapteyn 1,3 , V.N. Shlyaptsev 4 1 NSF Center for Extreme Ultraviolet Science and Technology 2 Colorado State University, Fort Collins, CO, USA 3 University of Colorado at Boulder, Boulder, CO, USA 4 University of California Davis at Livermore, Livermore, CA, USA There is significant interest in the development of compact sources of intense coherent soft x-ray light for a variety of applications. We review three different type of plasmas that we have successfully used in the generation of short wavelength coherent light. Two of them are plasmas generated by different types of compact capillary discharges: highly ionized plasma columns created by fast high current pulses for lasing in soft x-ray laser transitions of neon-like ions, and capillary plasmas with a tailored degree of ionization created by relatively low current pulses for high harmonic generation from ions. The third are highly ionized laser-created plasma columns that are efficiently heated by picosecond laser pulses impinging at grazing incidence. These laser- created plasmas have generated laser amplification at 5 Hz repetition rate at wavelengths down to 10.9 nm by collisional excitation of nickel-like ions. 1. Introduction Plasmas are relatively transparent media at soft x-ray wavelength in which is possible to concentrate large powers for energy conversion into intense beams of coherent soft x-ray light. These characteristics makes them the media of choice for the generation of coherent soft x-ray light using table-top devices. We will review recent advances in compact sources of coherent soft x-ray light obtained using different types of discharge–driven and laser-driven plasmas which differ several orders of magnitude in density and temperature. 2. Fast high current capillary discharge plasmas for soft x-ray lasers For more than a decade fast capillary discharges have been the most compact source of high power coherent soft x-ray radiation [1]. This approach uses a fast high current pulse to create a highly ionized plasma column in a gas filled capillary channel in which lasing is obtained by electron collisional excitation of neon-like ions. The magnetic force of the current pulse rapidly compresses the plasma (Fig. 1) to form a dense and hot (60-100 eV) column with a large density of Ne-like ions, and very high axial uniformity, where 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 0 ) levels. When Ar gas is used this approach results in lasing at 46.9 nm wavelength. Recently, we have developed a very compact version of this capillary discharge laser, that operates at repetition rates up to 12 Hz with an average laser output pulse energy of ~ 13 µJ, corresponding to an average power of 0.15 mW [2]. This desk-top size laser uses a low inductance discharge to excite capillary channels 3.2 mm in diameter and ~ 20 cm in length filled with pre-ionized Ar gas with current pulses of ~20 kA peak amplitude and 10% to 90% rise time of approximately ~ 60 ns. Capillary lifetime tests indicate that the laser output pulse energy decreases to about half the initial value in about Fig.1. Compact argon capillary discharge soft x-ray laser at λ=46.9 nm. Top: Interferograms of the rapidly compressing plasma column 28 ICPIG, July 15-20, 2007, Prague, Czech Republic th Workshop B Invited 65