0021-3640/03/7803- $24.00 © 2003 MAIK “Nauka/Interperiodica” 0115 JETP Letters, Vol. 78, No. 3, 2003, pp. 115–118. Translated from Pis’ma v Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, Vol. 78, No. 3, 2003, pp. 146–149. Original Russian Text Copyright © 2003 by Fukuda, Yamakawa, Akahane, Aoyama, Inoue, Ueda, Abdallah, Jr., Csanak, Faenov, Magunov, Pikuz, Skobelev, Boldarev, Gasilov. 1. In recent years, there has been rapid progress in the spectroscopy of X-ray emission from microplasma sources formed as a result of the interaction of ultrashort multiterawatt laser pulses with cluster tar- gets. One of the goals consists in elucidation of the mechanisms of interaction between radiation and mat- ter under such extreme conditions, which would pro- vide for the creation and development of high-intensity coherent and incoherent X-ray sources for various applications. Experiments using high-intensity laser radiation (10 17 –10 18 W/cm 2 ) were performed mostly with small clusters 10–100 nm in diameter [1–7]. The spectra of X-rays generated by plasma formed in the case of large (~1 μm) clusters were studied previously using femtosecond laser pulses of lower intensity [8]. Hard X-ray and hot electron production by microdrop- lets of water under the action of femtosecond laser pulses was studied in [9]. The results of these investiga- tions showed that irradiation of a gas jet target enriched with clusters of a medium size (100–200 nm) leads to soft X-ray emission with a considerable yield. The aim of this work was to study the high-resolu- tion X-ray emission spectra of plasma formed from large argon clusters (containing over 10 10 atoms) under the action of superintense laser radiation. 2. The experiments with argon clusters were per- formed at JAERI (Kyoto, Japan) using a 100-TW Ti:sapphire laser system based on the chirped pulse amplification technique. The system generates 20-fs pulses at a repetition rate of 10 Hz and is capable of col- limating output radiation with intensities after focusing up to 10 20 W/cm 2 [10, 11]. The initial pulses (λ las = 800 nm; τ las = 10 fs; repetition rate, 82.7 MHz) were generated by a Ti:sapphire oscillator. Stretched to 10 ns and frequency modulated, the pulses were amplified by a regenerative amplifier and two multipass amplifiers. The amplified pulses were compressed to 20 fs by a vacuum compressor providing a maximum output energy of 1.9 J. The compressed pulses were directed to a vacuum target chamber by two gold-coated plane mir- rors and focused by an f/3 gold-coated off-axis para- bolic mirror. The focal spot size was 11 μm at 1/e 2 , which was only 10% greater than the diffraction limit. A Gaussian spot accounts for 64% of the total laser out- put energy. For a laser pulse duration of 30 fs and an energy of 300 mJ used in our experiments, the peak intensity amounted to 1.2 × 10 19 W/cm 2 . A prepulse arriving 1 ns before the main pulse was suppressed by passing the output signal through two double Pockels cells. The contrast ratio of the main pulse intensity to that of the prepulse was greater than 10 5 . 3. Argon clusters were produced by expanding a high-pressure (up to p = 60 bar) gas jet into vacuum via a specially designed pulsed supersonic conical nozzle with the input and output diameters of 0.5 and 2.0 mm, respectively, and a length of 75 mm. The nozzle shape and dimensions were optimized by numerical simula- tion of a two-phase jet with a maximum yield of clus- ters of required size (d cl ~ 1 μm) [12, 13]. X-ray Study of Microdroplet Plasma Formation under the Action of Superintense Laser Radiation Y. Fukuda 1 , K. Yamakawa 1 , Y. Akahane 1 , M. Aoyama 1 , N. Inoue 1 , H. Ueda 1 , J. Abdallah, Jr. 2 , G. Csanak 2 , A. Ya. Faenov 3, *, A. I. Magunov 3, *, T. A. Pikuz 3 , I. Yu. Skobelev 3 , A. S. Boldarev 4 , and V. A. Gasilov 4 1 Advanced Photon Research Center, Japan Atomic Energy Research Institute, 619-0215 Kyoto, Japan 2 Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3 Center for Data on Multicharged Ion Spectra, All-Russia Research Institute of Physicotechnical and Radioelectronic Measurements, Mendeleevo, Moscow region, 141570 Russia 4 Institute of Mathematical Modeling, Russian Academy of Sciences, Moscow, 125047 Russia *e-mail: misdc@vniiftri.ru Received June 10, 2003 We have measured the X-ray emission spectra of a plasma generated by laser radiation with an intensity above 10 19 W/cm 2 and a pulse duration of 30 fs acting upon an argon jet target with a large abundance of micron-sized clusters. The time variation of the X-ray yield from ions of various multiplicities, calculated within the frame- work of a nonstationary kinetic model, shows a good qualitative agreement with the experimental time-inte- grated spectrum. © 2003 MAIK “Nauka/Interperiodica”. PACS numbers: 52.38.Ph; 52.25.Os; 52.25.Dg; 52.50.Jm