Research On The Possibility Of Obtaining Ni-like Kr32.8nm Laser Pumped By Capillary Discharge

Ne-like Ar46.9nm laser is the shortest wavelength X-ray laser pumped bycapillary discharge, and the development of the research field is limited. It is significantto explore X-ray laser at the wavelength shorter than46.9nm. In order to obtain shorterwavelength x-ray laser, Kr can be used as the gain medium. It has larger Z value, andNi-like Kr32.8nm laser based on Optical-Field-Ionization scheme has already beenobtained in the laboratory successfully. In this paper, the possibility of obtaining Ni-likeKr32.8nm laser pumped by capillary discharge was explored both in theory andexperiment.In theory, the electron collisional excitation of ions and the monopole electronexcitation of metastable level were introduced to help understand the physical principleof Ni-like Kr32.8nm laser deeply. The related atomic parameters of Ni-like Kr systemwere calculated with the Cowan physical code, and the related energy level transitionsdiagram of Ni-like Kr32.8nm laser was given. The wavelength of the laser between theupper laser level3d94d1S0and the lower laser level3d94p1P1was calculated to be32.8nm, and the wavelength of transition between the lower laser level3d94p1P1and theground level3d101S0was calculated to be11.6nm. The laser rate equations of relatedenergy level were established and calculated with the method of quasi-steady-stateapproximation. The influence of the levels related to3d94f,3d94d and3d94pconfigurations on the output of the laser was researched, the characteristics ofpopulation distribution and the gain between the upper laser level and the lower laserlevel were analyzed as well. The optimum electron temperature and electron density oflaser generation were calculated to be75eV and1018cm-3, respectively. The electrontemperature and electron density was researched to be similar to Ne-like Ar46.9nmlaser pumped by capillary discharge. Hence, the possibility of Ni-like Kr32.8nm wasdemonstration in theory.In experiment, the spectra of Kr plasma pumped by capillary discharge in the rangeof5~70nm were measured with the Kr pressure in the domain of10~100Pa. Theexperiments were operated under the main current pulse of11kA and16kA, respectively.The11.6nm transition line between the lower laser level3d94p1P1and the ground level3d101S0was observed under the two current conditions, which was in agreement withthe theory calculation result. And it indicated that some particles were accumulated in the lower laser level. Because the electron temperature and electron density of theplasma through Z-pinch were both too low under11kA, Kr8+ions in the ground levelwere not pumped to the upper laser level through electron collision. Under the maincurrent of16kA and Kr pressure of40-50Pa, the Ni-like Kr32.8nm transition spectralline was observed, but not the laser line. This indicated that the partial Kr8+ions in theground level were pumped to the upper laser level under the main current pulse of16kA.However, because the electron temperature and electron density of the plasma throughZ-pinch were not high sufficiently, the gain between the upper laser level and the lowerlaser level was not large enough. Thus the plasma state of laser was not generated. Inaddition, the intensity variations of Kr7+29.7nm spectral line, Kr8+11.6nm spectralline and Kr9+9.9nm spectral line with different Kr pressure were analyzed. And thegeneral variation rules of Kr7+spectral line, Kr8+spectral line and Kr9+spectral line withpressure were obtained.In conclusion, the reason that Ni-like Kr32.8nm laser not observed in theexperiment is that the electron temperature and electron density of the plasma throughZ-pinch are not high sufficiently. However, the possibility of laser generation will beincreased by changing the experimental conditions, such as increasing main currentpulse and raising the Kr pressure to enhance the electron temperature and electrondensity of the plasma effectively.