Research on table-top plasma soft x-ray lasers

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In 1994, we have obtained the significant improvement of a lasing performance using double pulse pumping with a curved slab target at 19.6 nm wavelength of neon-like germanium J=0-1 line. The double pulses of 100 ps individual pulse width separated by 300 ps with a total energy of 300 J results in a soft x-ray beam of 12 times higher peak intensity with a smaller beam divergence of 2 mrad compared with those produced by the 1 ns single pulse pumping with 1.1 kJ energy.In 1995, we have demonstrated efficient soft x-ray lasing in nickel-like lanthanide elements (Nd, Sm, Gd, Tb and Dy) covering the spectral range between 6 nm and 8 nm. A curved slab target was pumped by 1.053 mum-wavelength multiple laser pulses ; two or three 100 ps duration pulses separated by 400 ps. A gain coefficient of 3.1 cm-1 and a gain-length product of 7.8 have been achieved at 7.97 mn in the Nd ions with 250 J pumping energy on a 2.5 cm length target. Possibility for further reduction of the pumping energy is discussed. We have also demonstrated efficient soft x-ray lasing in nickel-lik-elements i.e., Ag through Ta covering the spectral range between 14.3nm and 4.4 nm. A curved slab target was pumped by 1.053 mum-wavelength quadruple laser pulses ; the pulse to pulse separation for the first 3 pulses was 400 ps and that between the third and the fourth pulses was 1.6 ns, respectively. Each pulse duration and irradiances on a target were 100 ps and -7 * 10^<13> W/cm^2 for low atomic number elements and -2 * 10^<14> W/cm^2 for high atomic number elements, respectively. A gain coefficient of 2 cm^<-1> and a gain-length product of 5 have been achieved at 11 nm and 14 nm wavelengths for the Te and Ag lasers.