Minxi Wei

Calibration of a gated flat field spectrometer as a function of x-ray intensity

We present an experimental determination of the response of a gated flat-field spectrometer at the Shenguang-II laser facility. X-rays were emitted from a target that was heated by laser beams and then were divided into different intensities with a step aluminum filter and collected by a spectrometer. The transmission of the filter was calibrated using the Beijing Synchrotron Radiation Facility. The response characteristics of the spectrometer were determined by comparing the counts recorded by the spectrometer with the relative intensities of the x-rays transmitted through the step aluminum filter. The response characteristics were used to correct the transmission from two shots of an opacity experiment using the same samples. The transmissions from the two shots are consistent with corrections, but discrepant without corrections.

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.

Spectroscopic studies of shell mix in directly driven implosion on SGIII prototype laser facility

We study shell mix during implosion using a capsule with a Cl-doped gas-shell interface that is also filled with Ar-doped deuterium gas. The Ar and Cl K-shell emissions are recorded with a flat crystal spectrometer. The He-β complex is analyzed to infer the electron temperature and density in the hotspot. Two regions of different hydrodynamic states are observed: a region of Te ∼ 1.2 ± 0.2 keV and ne ∼ (4 ± 0.5) × 1023 cm−3 and another of Te ∼ 0.6 ± 0.2 keV and ne ∼ (4 ± 0.5) × 1022 cm−3 probed by Ar and Cl ions, respectively. The neutron yield was also recorded and found to have reduced by a factor of ten for the Cl-doped capsule. By attributing the degradation of the neutron yield to the enhanced radiation loss due to the presence of mixed Cl ions and using the conservation of hotspot internal energy, the amount of mixed Cl ions is estimated to be ∼1.4 × 1014, corresponding to a 0.1 μm thickness of the initial shell material and a mixing width of about 16 μm.

Different approaches to precise wavelength calibration of a flat-field grating spectrometer for laser-produced plasmas

Accurate mathematical models called one-line calibration and parameter fitting are presented for wavelength calibration of a flat-field grating spectrometer. The models precisely establish the relationship between wavelength and pixel position of the detector, since geometry parameters and the grating equation of the spectrometer are taken into account. Compared with the commonly used polynomial fitting, the models presented here provide more reliable calibration results, especially in the extended region away from the calibration points. In addition to the high precision of calibration, the parameter fitting procedure provides a helpful way to obtain the actual parameters of the spectrometer.

Time-Resolved Transmission Measurements of Warm Dense Iron Plasma*

Transmission measurements of warm dense iron plasma are reported over the photon energy range of 400–1200 eV, including the strong 2p–3d structures. One-dimensional hydrodynamic simulation is performed to estimate the plasma conditions: temperatures of several eV and densities of about 0.1 g/cm3. By using the simulated temperature and density, the calculations of the transmission spectra are performed and compared with the time-resolved experimental results.

Calibration of the linear response range of x-ray imaging plates and their reader based on image grayscale values.

X-ray imaging plates are one of the most important X-ray imaging detectors and are widely used in inertial-confinement fusion experiments. However, their linear response range, which is the foundation of their quantitative data analysis, has not been sufficiently deeply investigated. In this work, we develop an X-ray fluorescer calibration system and carefully explore the linear response range of X-ray imaging plates. For the first time, nearly the entire grayscale range of the X-ray imaging plate linear response-7819-64 879 in the range of 0-65 535-has been observed. Further, we discuss the uncertainties involved in the calibration process. This work demonstrates the excellent linear response qualities of X-ray imaging plates and provides a significant foundation for expanding their quantitative applied range.

Fluorescence based imaging for M-band drive symmetry measurement in hohlraum

We describe an experimental technique to measure the drive symmetry of M-band radiation on the capsule in hohlraum. M-band radiation from the corona of the laser-produced gold plasma, especially the laser spot regions in the cavity, was used to pump x-ray fluorescence of a thin layer of Si-tracer coated on a solid CH-ball. The fluorescence images were time resolvedly recorded by an x-ray framing camera and the drive asymmetry due to M-band radiation was deduced from these fluorescence images. Moreover, a Si-doped gold cavity was used with the initial purpose of maximizing the fluorescence signal through resonance transitions. Since the Si-plasma expands more rapidly than the gold-plasma, the evolution of drive asymmetry was accelerated in Si-doped hohlraum.

Diagnostic for determining the mix in inertial confinement fusion capsule hotspot

A diagnostic is developed for determining the hotspot mix in inertial confinement fusion experiments. A multi-channel pinhole camera measures Bremsstrahlung emissions from implosion capsules ranging from 6 keV to 30 keV and records an image of the hotspot. Meanwhile, a planar crystal spectrometer measures Ar line emissions used to deduce the electron density of the hotspot. An X-ray streaked camera records the burn duration. With the Bremsstrahlung spectrum, electron density, hotspot volume, and burn duration, the mix quantity is determined by solving a pair of linear equations. This inferred mix amount has an uncertainty due to the uncertainty of the electron density, but with the help of the measured neutron product, the most likely mix quantity value can be determined. This technique is applied to experimental images to infer the quantity of CH ablator mix into the hotspot.

New two-dimensional space-resolving flux detection technique for measurement of hohlraum inner radiation in Shenguang-III prototype

The space-resolving measurement of X-ray flux from a specific area (laser spot, re-emitting wall, or capsule) inside the hohlraum is an ongoing and critical problem in indirectly driven inertial-confinement fusion experiments. In this work, we developed a new two-dimensional space-resolving flux detection technique to measure the X-ray flux from specific areas inside the hohlraum by using the time- and space-resolving flux detector (SRFD). In two typical hohlraum experiments conducted at the Shenguang-III prototype laser facility, the X-ray flux and radiation temperature from an area 0.2 mm in diameter inside the hohlraum were measured through the laser entrance hole (LEH). The different flux intensities and radiation temperatures detected using the SRFD from the inner area of the LEH were compared with the result measured using the flat-response X-ray detector from the entire LEH. This comparison was also analyzed theoretically. The inner area detected using the SRFD was found to be the re-emitting wall area alone. This important improvement in space-resolving X-ray flux measurement will enhance the current X-ray flux space characterization techniques, thereby furthering the quantitative understanding of X-ray flux space behavior in the hohlraum.

A time-gated multi-channel x-ray crystal spectrometer on the Shenguang-III laser facility

An eight-channel x-ray flat crystal spectrometer was developed for high energy density physics research at the Shenguang-III (SG-III) laser facility. The spectrometer uses trihydroxymethylaminomethane crystals (2d = 8.78 Å) to record Ti K-shell emission in the photon energy range of 4.65-5.05 keV. The spectrometer couples to an x-ray framing camera to achieve time-resolution. This has four microstrips, and each strip records two snapshots of the emission image. Based on the intersection positioning system with a dual-charge coupled device, the alignment system is easily operated and efficient. The instrument was tested and used for Au hohlraum plasma diagnosis experiments on SG-III. The He-α line and its Li-like satellites and the Ly-α line of a Ti tracer were detected, from which the spectral resolution of the instrument was analyzed. The spectral resolution E/ΔE at the Ti He-α line ranges from about 500 to 880 and mainly limited by the x-ray source size.