Nonclassical hydrodynamic behavior of Sn plasma irradiated with a long duration CO2 laser pulse

It was found that the electron density scale length of Sn plasma irradiated with a long duration CO2 laser pulse is much shorter than that predicted by the classical isothermal model. The experimentally observed small dominant region of in-band (2% bandwidth) 13.5-nm extreme ultraviolet (EUV) emission coincides with this constrained hydrodynamic behavior. The lower hydrodynamic efficiency may come from the strongly inhibited ablation mass and makes a CO2-laser-produced Sn plasma suitable as an EUV radiation source. When an intense laser pulse arrives at the surface of a solid material placed in a vacuum, a thin layer of the material is ablated, heated, and expands into vacuum due to the thermal gradient. Such hydrodynamic expansion of a laser-produced plasma has been studied for more than 40 years motivated by a wide range of applications, such as efficient compression of a pellet in laser fusion, X-ray lasers, laser ion acceleration, and short wavelength radiation sources [1]. Isothermal expansion is a well-accepted model to describe the hydrodynamic expansion of laser-produced plasma in the corona Y. Tao ( ) · M.S. Tillack · S. Yuseph · R. Burdt · F. Najmabadi Center for Energy Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA e-mail: [email protected] M.S. Tillack Mechanical and Aerospace Engineering Department, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA S. Yuseph · R. Burdt · F. Najmabadi Electrical and Computer Engineering Department, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA within the laser pulse duration, in which energy and material are fed by the continuously deposited laser energy near the critical density and the materials introduced from the ablation region [2]. However, the hydrodynamic expansion depends on particular experimental conditions, such as laser wavelength. For the application of laser fusion, a lot of effort has been expended to enhance hydrodynamic efficiency in order to achieve efficient compression of the fusion pellet. It has been shown that short wavelength lasers could provide higher hydrodynamic efficiency as compared with long wavelength lasers [3]. However, for the application of radiation sources, less hydrodynamic efficiency is preferred to obtain higher radiation efficiency and less ablation mass. Recently, in-band (2% bandwidth) 13.5-nm extreme ultraviolet (EUV) emission from high Z plasma irradiated with a CO2 laser with wavelength of 10.6 μm has been increasingly viewed as the main candidate source used in EUV lithography [4]. However, most of the previous efforts to investigate hydrodynamic behavior in intense CO2 laser plasma interaction have focused on the preheating of the pellet induced by hot electrons, which come from nonclassical laser absorption mechanisms at the high laser intensities of interest to laser fusion [5]. The little previous efforts to characterize the thermal heat conduction inhibition in CO2 laser plasmas interaction were done with low-density (1016 cm−3) Z-pinch plasma [6]. In this paper, we present further understanding for the hydrodynamic behavior of a CO2-laser-produced Sn plasma under the most favorite conditions for efficient inband 13.5-nm EUV emission. Experiments are carried out using a home-built master oscillator and power amplifier (MOPA) CO2 laser system as the pumping laser pulse [7]. The CO2 laser pulse durations are varied from 25 to 85 ns. The laser is focused with a F/6 lens onto a planar target surface at normal incidence. The