It is shown experimentally that a high intensity subpicosecond laser impinging on a solid target generates a plasma with an electron density exceeding 10 23 cm 23 . Depending on the electron density, femtosecond high-order harmonics from a gas jet are transmitted through a subpicosecond laserproduced plasma while lower order harmonics are absorbed or reflected. In the present experiment, electron densities up to 4.3 3 10 23 cm 23 at mean electron energies of 50 to 150 eV are measured and the plasma expansion is mapped out in time with subpicosecond resolution. [S0031-9007(96)

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A novel spectrograph comprised of a transmission grating, toroidal mirror, and microchannel plate detector is presented here. It is capable of detecting spectra in the wavelength range from 5 to 150 Å with a resolution of about 1.5 Å. Owing it to high efficiency, even an x-ray spectrum of a low-energy laser produced plasma, i.e., emission of the order of 10−10 J/sr, can be measured in a single shot.

Hard x-ray emission from intense short pulse laser plasmas Phys. Plasmas 2, 3106 (1995); 10.1063/1.871142Plasmas for short-wavelength lasers driven by ultra-short, high-intensity laser pulses AIP Conf.The interaction of high-intensity subpicosecond KrF*-laser pulses with aluminum plasmas has been investigated. The laser pulse absorption and the x-ray production have been related at a series of intensities up to 10 17 W/cm 2 and different angles of incidence. Soft x-ray spectra were measured in the wavelength range from 7 to ISO A with various spectrographs. The spectra show distinct dependences on the laser intensity, the polarization, and the angle of incidence. The observed dependence of the spectra on the intensity and on the angle of incidence is the result of both the angular dependence of the laser pulse absorption and that of the conversion efficiency of the absorbed energy into soft x-ray emission. The spectra are influenced by the size of the absorption volume, the electron density where the absorption mainly takes place, and other parameters, such as the electron temperature.