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We have studied plasma waveguiding in soft x-ray laser experiments. A curved-slab target was used in order to compensate for x-ray refraction in a gradient-density plasma. In addition to significant increase in the x-ray laser intensity in single-pass amplification, a small beam divergence of less than 1 mrad has been stably generated in double-pass amplification with the curved target. The angular intensity distribution of the double-passed beam indicates that the x-ray laser has been amplified in a plasma waveguide. PACS numbers: 42.55.Vc, 41.50.+h, 52.40.Fd X-ray lasers with high coherent power are required for various applications such as holography [1,2], plasma diagnostics [3], holographic nanolithography [4], and nonlinear x-ray optics [5]. Most of the x-ray laser schemes now operating, especially the collisional excitation x-ray lasers [6], use laser-produced plasmas with high electron density. The major difficulty in improving the spatial co-

The XUV spectra from laser-produced germanium plasma are investigated.The energy levels of the 2s22p531 and 2s2p631 states with I = 0, 1 , 2 and the oscillator strengths for the transitions between these levels are calculated by the relativistic configuration-interaction (RCI) method. A high-resolution (AjAA -16000) grazing-incidence spectrometer is used to measure the spectra in 185-3008L The wavelengths of the five lasing lines in Ne-like Ge and the wavelengths of the nonlasing lines have been determined with k0.04A accuracy. The observed lines are assigned to the 3-3 transitions in Ne-like and Na-like Ge ions. The energies of the excited states in Ne-like Ge ions derived from the spectroscopic measurements are compared with the RCI and other theoretical calculations.

We describe significant improvement in the efficiency and brightness of the J = 0-1 19.6-nm line of the neonlike germanium laser. A curved slab target was irradiated by two laser pulses of 100-ps duration separated by 300 ps with a total energy of 200-300 J on target. Compared with that obtained by 1-ns single-pulse pumping with 1.1 kJ of energy, the J = 0-1 line has a 12 times higher peak intensity, with a smaller beam divergence of 2 mrad, corresponding to a factor-of-25 improvement in brightness.