Study On Transmission Curved Crystal Spectrometer For Laser-Plasmas Hard X-ray Diagnosis

Controlled thermonuclear fusion has become an important research task in theworld science, for its sustainable and clean merit as an energy resource. One effectiveway to realize controlled thermonuclear fusion is laser Inertial Confinement Fusion(ICF). There are many approaches to achieve ICF, among them is fast ignition whichshows a promising future and quickly becomes a hot research topic. According to thesupposition of fast ignition, nuclear fuel ignition is significantly functioned by the hotelectrons produced by the interaction of laser with plasma. Thus, the research of hotelectron plays an important role in the intensity field physics. Plentiful informationabout hot electrons is contained in the bremstruhlung spectrum emitted from theinteraction of laser with plasma, hence we can characterize the hot-electron energydistribution of the laser generated plasmas by diagnosing the continuum x-ray spectrum.The implementation of K-shell spectroscopy (mang energy above10keV) diagnostictechniques is helpful to understand the laser energy distribution, ionization distribution,and X-ray conversion efficiency in plasmas produced by an intense femtosecond laserpulse. With the building and development of many kinds of laser devices in China, hardX-ray spectroscopy diagnostic attracts more and more interests. Transmission curvedcrystal spectrometer has been broadly used in laser-plasmas hard X-ray diagnosisbecause of its wider energy range and higher resolving power compared to otherdiagnostic methods (such as the single-photon counting CCD). In this paper, atransmission curved crystal spectrometer for XG-Ⅲlaser device is developed.Firstly, the basic theories of transmission curved crystal spectrometer such as thecrystal X-ray diffraction and the basic schematic are introduced. This kind ofspectrometer is based on Cauchois-Johann type, and belongs to focusing spectrometer.The spectral line is focused on the Rowland circle. X-ray spectrum is recorded by theplanar detector that intersects the focusing circle whose diameter equals to the radius ofcurvature of crystal.Secondly, by means of the spectrometer optical geometry, theoretical analysis andnumerical simulation are performed to investigate the dependence of the energy rangeand resolving power on various design parameters (crystal lattice spacing, crystalbending radius, displacement of source to crystal). In addition, the influence of sourcesize, detector location and the effective detector resolution on the resolving power of spectra is analyzed. Also, the reasons of the shift of spectral line on the detection planeare discussed. Such theoretical analysis provides a theoretic guide for the system designand fabrication of the transmission curved crystal spectrometer.Thirdly，according to the theoretical analysis and practical requirements, thedetailed design of the spectrometer system is completed, including the optical designand mechanical design. The spectrometer system is divided into three parts: dispersionsystem, detection and recording system, and alignment system. The reasons for theselection of each element are illustrated. Ultimately, the fabricated transmission crystalsurvey spectrometer, covering a12～60keV energy range and showing a resolvingpower of E/ΔE≥300(in low energy range), is able to realize the online aiming andreal-time measuring in a vacuum. Here, the light beam used for aiming enters thevacuum chamber via a vacuum fiber laser system.At last, an X-ray source (with positive electrode Mo) based experiment and a500TW laser device based experiment are carried out to validate the spectrometer. Theresult presents that the spectral resolving power E/ΔE for spectrum line under energy of20keV is greater than377, showing the rationality and the compliance to the designrequirement.