Laser plasma soft X-ray source based on cryogenic target

Abstract

Laser plasma soft X-ray sources that operate in the water window and are based on cryogenic targets have been studied. The cryogenic targets used are composed of solid layers of inert gases and are deposition-free targets, which is advantageous for continuous long-lifetime operation using repetitive pulses. The developed laser plasma source has a translating substrate system with a closed He gas cryostat that can continuously supply the cryogenic target, and the source can generate repetitive X-ray pulses continuously. Cryogenic solid Ar and N2 targets were compared for the soft X-ray source when operating in the water window between 2.3 nm and 4.4 nm. The intensity of the water window Xrays from the Ar target was approximately eight times stronger than that from the N2 target at a laser intensity of 5×1012 W/cm2 . At this laser intensity, the Ar plasma source also demonstrated a higher conversion efficiency of 14% and an average power of 140 mW in the water window for a laser energy of 1 J at 1 Hz. This power compares favorably with that of X-rays produced using a synchrotron radiation source. The results also indicated that our source satisfied the requirements for use as a source for a contact X-ray microscope. The developed laser plasma source can be used not only for soft X-ray microscopy but also may be used to replace synchrotron facilities in a variety of other applications.