Shisheng Chen

Evolution of spectrally discriminated spatial uniformity of line‐shaped plasmas

A space‐resolved soft x‐ray (SXR) transmission grating spectrometer and a SXR streak camera are used to investigate the evolution of spectrally discriminated spatial uniformity of line‐shaped plasmas produced by uniform laser illumination in line focus. It is found that the spatial nonuniformity of SXR emissions and ion turbulence occur during optical laser’s heating. Various instabilities can be excited due to the long scale length in line‐shaped plasmas to produce the phenomenon.

A simple in situ calibration technique for soft x‐ray film

A simple valid in situ relative intensity calibration technique for soft x‐ray film is described. This is based on film exposure measurements of the uniform line‐shaped distributed soft x‐ray monochromatic irradiation transmitted through a step‐wedge absorption filter. Fitting the calibration data with Henke’s semiempirical equation for thick‐emulsion film, the characteristic curves for Shanghai 5F soft x‐ray film without supercoat (SIOM‐5FW) have been obtained in the wavelength region from 50 to 80 A.

Soft X-ray amplification by Li-like Al10+ and Si11+ ions in recombining plasmas

Experimental studies of soft X-ray lasers were carried out on the six-beam laser facility and the LF 12 laser facility of SIOM. Using a home-made one-dimensional spatially resolved grazing incidence grating spectrograph, XUV amplification has been observed in Li-like aluminum and silicon ions, by irradiation of slab targets with a line-focused laser. Based on time-integrated measurement, gain coefficients are 3.1 cm−1 for the 105.7 Å 5f−3d transition in Li-like Al ions, and 1.5 cm−1 and 1.4 cm−1 for the 88.9 Å 5f−3d and the 87.3 Å 5d−3p transitions in Li-like Si ions, respectively. The maximum gain × length products (GL) are about 2.5.

Spatial uniformity and its time evolution of linear laser plasmas

The effect of laser illumination conditions on the uniformity of the linear laser plasmas produced is investigated with time-integrated and time-resolved soft x-ray diagnostics. Evident nonuniformities along the line focus resulting from nonuniform laser intensity distributions have been observed with a conventional simple line-focusing technique. Specifically, the nonuniformity of higher charge-state ions is more remarkable than that of lower ones. A uniform linear plasma can be obtained with a new line-focusing technique that produces uniform laser illumination. Nonuniformities under uniform laser illumination are found to occur primarily in the laser heating period. Further development of time-matched and spatial uniform line-focused laser illumination will produce more uniform linear plasmas.

Laser plasma x‐ray emission studies using a pinhole transmission grating spectrometer

The powerful diagnostic capabilities of the pinhole transmission grating spectrometer (PTGS) have been utilized in studying laser plasma soft x‐ray (SXR) emission and the following results are presented: (1) Studies of the SXR emission from C, Al, SiO2, Cu, Se, Mo, Sn, Er, W, and Au targets irradiated by a 1.06‐μm laser beam with average power density 5×1014 W/cm2 and 300 ps duration indicate the strong dependence of the SXR emission on the atomic number Z. (2) Comparative experiments of Al or Au plasma SXR emission show that changes of the laser intensity have a more remarkable influence on low‐Z plasma SXR emission than on high Z’s. (3) Experiments on target configurations (foil, multilayer, and half‐cylindrical‐groove target) reveal that the foil target is least effective for short‐wavelength SXR emission, while the half‐cylindrical target not only converges the plasma in the groove region, but also most effectively enhances the short‐wavelength SXR emission or increases the electron temperature therei...

Spatial distributions of x‐ray lasing spectral linewidth and electron density in a lithiumlike silicon plasma

The spatial distribution of the spectral linewidth of lithiumlike silicon ions in the 5f‐3d transition line at 88.84 A is presented in this letter, and it shows that Stark broadening is the dominant line‐broadening mechanism for lithiumlike Si11+ ions on recombination x‐ray lasers. Spatial distribution of the electron density of the x‐ray lasing gain medium is calculated using the linear Stark theory, which shows that the electron density is ∼1019/cm3 in the gain region. A large linewidth at the gain peak (i.e., at 450 μm from the target surface) is 0.21 A and the corresponding electron density and population inversion are 2.9×1019/cm3 and 5.5×1016/cm3, respectively.

Large-scale jet structures in laser-produced plasmas

Abstract In line-focused laser-produced plasma experiments, we have observed large-scale plasma jets. These plasma jets are in appearance similar to “tips of ox horns”, and always appear at the boundaries between the hot plasmas and the cold target materials, or between two plasmas with different Z-numbers.

Novel self-mode-locking mechanism in narrow-band lasers

We suggest a novel self‐mode‐locking (SML) mechanism in narrow‐band lasers. The analysis for such a mechanism show that the gain‐line splitting (Stark splitting) induced by an intracavity laser field can form a dip or dips in gain‐line shape, and the gain medium with such a gain‐line shape can produce short pulses. The further analyses give the rough criterion for SML, i.e., 8 ln(2)Δω2s≳Δω2g for the gas lasers and 12Δω2s≳Δω2g for the solid or liquid lasers.

Investigation of soft X-ray spectra from laser plasma by using a transmission grating spectrometer

With the help of the transmission grating spectrometer (PTGS), we have obtained the real emission spectra of several elements. The band structures of the soft x-ray spectra from laser produced plasmas are extensively studied as the function of atomic number and laser intensity. Finally, the PTGS space-resolved spectra from the laser heated half-cylindrically grooved target show that this kind of semi-closed geometry is a promising candidate for x-ray laser targets.

Plasma physics in a half-cylindrical target and its applications in X-ray lasers

Laser plasma interactions, plasmas hydrodynamics, and x-ray emissions in half-cylindrical target have been studied. It is found that this kind of target geometry can converge plasma expanding along the radial direction and form uniform flat distributions of electron density. Based on the unique characteristics of this kind of target, we suggest it be used for x-ray laser research. We also suggest a new configuration of two laser heating for x-ray lasers.