Characterization of Ar and Kr K$\alpha $ self-emission in clustering gas jets

K$\alpha $ self-emission from Ar and Kr has been characterized by x-ray spectroscopy and x-ray imaging. The K$\alpha $ x-rays were produced by irradiating clustering Ar and Kr gas jets with ultra-high intensity (10$^{20}$ W/cm$^{2})$, ultra-short pulse (150-700 fs) lasers. These K$\alpha $ sources are very high contrast (defined as the ratio of K$\alpha $ to continuum radiation) and the K$\alpha $ emission area is millimeter-scale. Such qualities make these sources useful for high-contrast diffraction and for backlighting large objects in extended-source geometries. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory, through the Institute for Laser Science and Applications, under contract DE-AC52-07NA27344. The authors also acknowledge support from Laboratory Directed Research and Development Grant No. 08-LW-004.