Tuo Zhu

Study of x-ray radiant characteristics and thermal radiation redistribution in CH foam filling cylindrical cavities

Experiments are presented, which demonstrate the properties of x-ray radiation and redistribution of radiant thermal energy in high Z cylindrical cavities filled with low Z CH foam. Time integrated spectra records were obtained by a calibrated space-resolved transmission grating spectrometer. The x-ray radiation became weaker in intensity and was changed to a softer near-Planckian radiation light after a 1500 μm long transport in the foam filling cavity. The experimental redistribution of the radiant thermal energy was plotted and compared to the numerical results of a simplified model. Good agreements have been achieved.

Instantaneous x-ray radiation energy from laser produced polystyrene plasmas for shock ignition conditions

Laser target energy coupling mechanism is crucial in the shock ignition (SI) scheme, and x-ray radiation energy is a non-negligible portion of the laser produced plasma energy. To evaluate the x-ray radiation energy amount at conditions relevant to SI scheme, instantaneous x-ray radiation energy is investigated experimentally with continuum phase plates smoothed lasers irradiating layer polystyrene targets. Comparative laser pulses without and with shock spike are employed. With the measured x-ray angular distribution, full space x-ray radiation energy and conversion efficiency are observed. Instantaneous scaling law of x-ray conversion efficiency is obtained as a function of laser intensity and time. It should be pointed out that the scaling law is available for any laser pulse shape and intensity, with which irradiates polystyrene planar target with intensity from 2 × 1014 to 1.8 × 1015 W/cm2. Numerical analysis of the laser energy transformation is performed, and the simulation results agree with the e...

One-dimensional space resolving flat-field holographic grating soft x-ray framing camera spectrograph for laser plasma diagnostics

A 1D space resolving x-ray spectrum diagnostic system has been developed to study the radiation opacity of hot plasma on SG-II laser facility. The diagnostic system consists of a 2400 lines/mm flat-field holographic grating and a gated microchannel plate coupled with an optical CCD and covers the wavelength range of 5-50 Å. The holographic grating was compared with a ruled one by measuring the emission spectra from a laser-produced molybdenum plasma. The results indicate that the holographic grating possesses better sensitivity than the ruled grating having nearly similar spectral resolution. The spectrograph has been used in radiative opacity measurement of Fe plasma. Simultaneous measurements of the backlight source and the transmission spectrum in appointed time range in one shot have been accomplished successfully with the holographic grating spectrometer. The 2p-3d transition absorption of Fe plasma near 15.5 Å in has been observed clearly.

Note: X-ray streak camera sweep speed calibration.

X-ray streak cameras are extensively used to study transient x-ray processes in experiments carried out on various laser facilities at the Research Center of Laser Fusion. Precise calibration and clear description of the sweep speed of the x-ray streak camera are vital for obtaining precise temporal information to understand the fast physics phenomena. An x-ray streak camera named wide-slit x-ray streak camera has been calibrated using a small-scale laser facility with pulse duration of 8 ps. Sweep speeds on most part of the output screen (charge coupled device) are obtained. These calibration results can be used to correct the nonlinearity of sweep speed in measurement of temporal processes.

Experimental demonstration of laser to x-ray conversion enhancements with low density gold targets

The enhancement of laser to x-ray conversion efficiencies using low density gold targets [W. L. Shang, J. M. Yang, and Y. S. Dong, Appl. Phys. Lett. 102, 094105 (2013)] is demonstrated. Laser to x-ray conversion efficiencies with 6.3% and 12% increases are achieved with target densities of 1 and 0.25 g/cm3, when compared with that of a solid gold target (19.3 g/cm3). Experimental data and numerical simulations are in good agreement. The enhancement is caused by larger x-ray emission zone lengths formed in low density targets, which is in agreement with the simulation results.

Accurate and efficient characterization of streak camera using etalon and fitting method with constraints

Streak camera is widely used in continuous time diagnostics in fast physical process. To produce accurate result, it requires delicate calibration and a reliable analysis method. High quality sweep-rate data with uncertainty smaller than 0.5% are obtained over the full record area by a constrained fitting method of peak position measurement, with a short pulse laser and an etalon as the fiducial source. The temporal response is linearized by this full-screen sweep-rate data, which eliminates errors in measurement due to nonlinearity and space-distortion inherent in streak camera.

Experimental study of the hydrodynamic trajectory of an x-ray-heated gold plasma

Hydrodynamic processes of radiation-ablated high-Z plasmas have great influence on the x-ray radiation transport both in a high-Z tube with low-Z foam filling and in a low-Z filling indirect drive cavity. Using the intense x-ray radiation to heat a low-Z foam-tamped high-Z gold plate from a half cavity, the hydrodynamic moving process of the x-ray-ablated gold plasma has been studied by an imaging method with Ti He-α line emission as the backlighter source. The hydrodynamic trajectory of the x-ray-heated gold plasma was obtained and the average trajectory velocity of (36.5±1.2) km/s was derived. The experimental trajectory was compared with the simulations using the one-dimensional (1D) RDMG [P. Gu et al., Sci. China, Ser. G 48, 345 (2005)] and two-dimensional (2D) LARED-R [K. Lan et al., Laser Part. Beams 23, 275 (2005)] codes separately. It is shown that the 2D LARED-R code simulated the measured plasma trajectory much better than the 1D RDMG code due to the fact that the lateral x-ray radiation loss wa...

High intensity x-ray line emission from aluminum plasmas generated by a 120 TW, 30 fs laser pulse

The K-shell x-ray spectra from the aluminum plasmas generated by the interaction of the 120TW, 30fs laser with aluminum targets have been measured by defocusing the laser beam. Laser energy-normalized intensity of the He-a line increases with the laser intensity approximately as a power law e∝ILγ with the much smaller exponent of γ=0.062 compared to γ=0.2∼0.5 in the previous experiments, which is caused by the stronger radial thermal diffusivity in the target for the smaller laser spot. Laser-to-He-a line conversion efficiency of up to 1.9×10−3 and as high as about 3×1013 photons/2π Sr aluminum He-a line x-ray source have been achieved for a single shot due to the preplasma effect and relatively large laser spot and energy for the single shot. The x-ray spectra as a function of the laser intensity are also analyzed to get the electron temperature and density.

A simple method to verify the opacity and equation of state of high-Z plasmas

Accurate opacity and equation of state (EOS) data are important parameters for reliable modeling and understanding many phenomena of high energy density physics. However, they are difficult to obtain either theoretically or experimentally especially for high-Z plasmas. In this paper, we proposed a simple method to verify the opacity and EOS of high-Z plasmas in rarefactive states, which is build on the two key aspects of ablation processes, i.e., the propagation of heatwave into materials and the hydrodynamic expansion of the blow-off plasmas. Based on the integrated analysis of the Au foil burnthrough and foam-confined plasma expansion experiments performed on the Shenguang II and Shenguang III prototype laser facilities, we suggested two semi-empirical multipliers to adjust the opacity and EOS of Au plasma. These adjusted opacity and EOS may then provide a foundation for other radiation hydrodynamic simulations under similar drive conditions.

Radiative cooling induced plasma collapse observed in laser irradiation of a CH-tamped gold micro-disk

Time-resolved x-ray self-emission imaging was used to study the dynamic evolution of a laser-produced gold plasma tamped by plastic (CH), and a significant plasma collapse was observed during the laser irradiation. The plasma collapse, a kind of transverse contraction, has been ascribed to the radial compression caused by the different radiative cooling rates and thus different pressures between the central high-Z gold plasma and the surrounding low-Z CH plasma, and this has been reproduced by numerical simulations using the two-dimensional radiation-hydrodynamics code Multi2D. The experimental results represent an observation of the radiative cooling induced plasma jet within a 1 ns laser pulse duration, much more quickly than those reported previously. In addition, our experiment design may offer a method to study the radiative cooling rates of high-Z plasmas. The measured cooling rate is a factor of 2 higher than the theoretical result [Post et al., At. Data Nucl. Data Tables 20, 397 (1977)], but is wi...