Zhiheng Fang

Eight-channel Kirkpatrick–Baez microscope for multiframe x-ray imaging diagnostics in laser plasma experiments

Because grazing-incidence Kirkpatrick-Baez (KB) microscopes have better resolution and collection efficiency than pinhole cameras, they have been widely used for x-ray imaging diagnostics of laser inertial confinement fusion. The assembly and adjustment of a multichannel KB microscope must meet stringent requirements for image resolution and reproducible alignment. In the present study, an eight-channel KB microscope was developed for diagnostics by imaging self-emission x-rays with a framing camera at the Shenguang-II Update (SGII-Update) laser facility. A consistent object field of view is ensured in the eight channels using an assembly method based on conical reference cones, which also allow the intervals between the eight images to be tuned to couple with the microstrips of the x-ray framing camera. The eight-channel KB microscope was adjusted via real-time x-ray imaging experiments in the laboratory. This paper describes the details of the eight-channel KB microscope, its optical and multilayer design, the assembly and alignment methods, and results of imaging in the laboratory and at the SGII-Update.

A pinhole camera for ultrahigh-intensity laser plasma experiments

A pinhole camera is an important instrument for the detection of radiation in laser plasmas. It can monitor the laser focus directly and assist in the analysis of the experimental data. However, conventional pinhole cameras are difficult to use when the target is irradiated by an ultrahigh-power laser because of the high background of hard X-ray emission generated in the laser/target region. Therefore, an improved pinhole camera has been developed that uses a grazing-incidence mirror that enables soft X-ray imaging while avoiding the effect of hard X-ray from hot dense plasmas.

Bow shocks formed by a high-speed laser-driven plasma cloud interacting with a cylinder obstacle*

A bow shock is formed in the interaction of a high-speed laser-driven plasma cloud with a cylinder obstacle. Its temporal and spatial structures are observed by shadowgraphy and interferometry. The width of the shock transition region is ~ 50 μm, comparable to the ion–ion collision mean free path, which indicates that collision is dominated in the shock probably. The Mach-number of the ablating plasma cloud is ~ 15 at first, and decreases with time resulting in a changing shock structure. A two-dimension hydrodynamics code, USim, is used to simulate the interaction process. The simulated shocks can well reproduce the observed.

Physical studies of fast ignition in China

Fast ignition approach to inertial confinement fusion is one of the important goals today, in addition to central hot spot ignition in China. The SG-IIU and PW laser facilities are coupled to investigate the hot spot formation for fast ignition. The SG-III laser facility is almost completed and will be coupled with tens kJ PW lasers for the demonstration of fast ignition. In recent years, for physical studies of fast ignition, we have been focusing on the experimental study of implosion symmetry, M-band radiation preheating and mixing, advanced fast ignition target design, and so on. In addition, the modeling capabilities and code developments enhanced our ability to perform the hydro-simulation of the compression implosion, and the particle-in-cell (PIC) and hybrid-PIC simulation of the generation, transport and deposition of relativistic electron beams. Considerable progress has been achieved in understanding the critical issues of fast ignition.

Elliptically bent crystal x-ray spectrometer for time-resolved laser plasma experiments

Measuring time-resolved spectra is crucial in inertial confinement fusion and radiation source development experiments. An elliptically bent crystal spectrometer is designed to measure X-rays in the range of 2.5-11.0 keV, which was achieved using four different lattice spacings of 0.8512, 0.6687, 0.4246, and 0.2749 nm with spectral resolution E/δE of ∼500. The X-rays emitted from a source at one focus of the ellipse undergo Bragg reflection off a crystal and pass through the second focus of the ellipse to a streak camera slit with 18-mm length and 80-μm width to generate a time-resolved spectrum. An alignment method for the time-resolved spectrometer was developed with the straight line connecting the centers of the two small holes on the fabricated substrate being the axis of the ellipse, thus allowing the spacing between the source and the elliptical crystal to be tuned to couple with the streak camera. The time-resolved spectrometers performance was experimentally tested at the Shenguang II laser facility. The results indicate that its performance is close to that predicted theoretically.