Hard X-ray Spectral Diagnosis In Laser-driven Inertial Confinement Fusion

In laser-driven inertial confinement fusion, one of the obstacles to achieve high compression is preheat of the DT fuel by energetic electrons generated during laser-plasma interaction. By measuring the emitted hard x-rays of implosion targets, the number of energetic electrons can be quantified to infer the preheat level. The hard x-rays also provide a signature of the mixing, symmetry, temperature and density conditions of the fuel capsule. On the other side, novel high brightness hard x-ray sources produced in the short pulse laser-target interaction are widely used in ultrafast biomedical imaging, lattice dynamic probing, and radiography. In these applications, x-ray diagnostics, especially hard x-ray spectral measurement is an important method.In this work, design and primary experimental test of two hard x-ray spectrometers, the Filter Stack spectrometer and the transmission curved crystal spectrometer, were present.The Filter Stack spectrometer used a stack of metal filters and image plates to present a time and energy integrated x-ray measurement. In order to obtain the response of spectrometer to hard x-rays, the Monte Carlo simulation software Geant4was used to calculate the energy deposited in the image plates. To unfold the x-ray spectra from integrated signals, unfolding code were programed and verified. The algorithms of unfolding code based on the least-squares method and the principle of maximum entropy. Two-dimensional radiography of an aluminum target had been achieved using a W anode microfoucus x-ray source. The spectra of the x-ray source and the mass density of the target were obtained simultaneously. By now, we obtained the spectra of hard x-ray source with energy<100keV. However, by changing the stack of filters, the spectrometer can be applied to MeV gamma measurement.As the spectral resolution of Filter Stack spectrometer was poor, two transmission curved crystal spectrometers were designed for obtaining high-resolution spectra with characteristic line emission in the10-100keV range. The spectrometers implement two cylindrically curved quartz crystals with different radius, R=150mm for10-56keV and R=300mm for17-100keV, and named LCCS and HCCS, respectively. The distance between the crystal and the x-ray source can be changed over a broad distance from200to1500mm. Theoretical calculation of x-ray three-dimensional optics was presented, and also used in optical design and theoretical calculations of the spectrometer. The integral reflectivities of the curved crystals were about10"5. The sensitivities of the spectrometers using image plate were in the level of10’8PSL/photon. The spectral resolutions (E/AE) were331-57of LCCS and327-54of HCCS, respectively. We also presented mechanic design details, including method to bend crystal.In order to calibrate the energy scale of spectrometer only by one metal filter, new method of energy calibration was presented and verified by an application in the experimental spectra of an Ag anode x-ray source. Experimental results using a Mo anode x-ray source were also presented. High quality spectral images were obtained. We confirmed that the spectral resolution could be improved by increasing the distance between the recording medium and the Rowland circle. Finally the absolute intensity of the x-ray tube was estimated using the theoretical sensitivity.