CETAL-PW | Research | X-Rays sources

Plasma x-ray lasers (XRL) generate coherent x-ray emission in the range below 40 nm and down to 4 nm. Their operation principle is related to the generation of population inversion in laser produced plasmas. A common type of such laser system is based on population inversion generated in Ne-like and Ni-like plasmas produced with short pulse lasers, in the range of nanosecond and picosecond, on solid targets. Coherent and incoherent sources for extreme ultraviolet (EUV) and down to water window region (2.2 nm - 4.4 nm) are getting a well deserved place in the nowadays research and industry needs. The high flux of photons and brilliance are provided in large scale synchrotron and free electron lasers facilities; also a consistent effort is put in the development of laboratory scale sources. Typical applications of such short wavelengths laboratory sources are x-ray spectroscopy, EUV metrology and x-ray microscopy and imaging.

For coherent imaging techniques, sources with high spatial and temporal coherence are required. X-ray lasers (XRL), coherent sources in the EUV range, were initially developed at large scale laser facilities in 1980’s in single shot experiments and evolved as laboratory sources later on, with the introduction of the normal incidence, transient collisional excitation (NICE) pumping scheme in mid 1990’s.

With the use of one ns-long and one normal incidence ps-long laser pulse, the NICE XRL pump energy needs to be decreased from kJ range to the 10 J range. Later, with the introduction of the grazing incidence pumping (GRIP) scheme, this value decreased to the Joule level for the saturated plasma emission, e.g. in an Ag X-ray laser emitting at 13.9 nm, in the EUV domain.

As a consequence, the repetition rate of the XRL systems can be scaled with the repetition rate attainable with the actual high power laser system, increasing the average fluxes of EUV radiation in small size labs. To increase the average flux of EUV photons, higher repetition rate pump lasers are needed.

X-ray laser simulations based on Ehybrid code have shown that enhanced plasma x-ray laser emission can be achieved mastering the ionization dynamics and plasma temperature using one long and two short pulses. Thus, the introduction of the 1L2S pumping scheme, which uses one long pulse and two short pulses to pump XRL down to 13.9 nm with only 200 mJ on target come to demonstrate experimentally the results observed in simulations. The new method, 1L2S method can strongly affects the electron density gradients allowing the XRL emission down to 1-4 nm with a significantly low amount of laser pump energy than previously predicted.