Development of high repetition rate X-ray lasers

XRL have great potential to extend current optical laser applications to shorter space and timescales, while probing much deeper into the core of matter. X-ray time-resolved imaging of biological samples, nanolithography, or laboratory astrophysics are typical examples of applications that are just starting to be explored. However, the use of XRL in these novel applications is seriously limited by the parameters of the laser driver creating population inversion. Fortunately, plasma-based XRL are now being perfected to the point that they can become truly tabletop. In 2005, an extension of the transient pumping scheme was demonstrated using a grazing-incidence angle for pumping (GRIP), and lasing at 18.9 nm close to saturation with as little as 250 mJ of optical laser was achieved. Recently, an optical field ionization XRL amplifier produced by femtosecond laser excitation of a krypton gas cell was seeded with the 25th harmonic of a Ti:sapphire laser to generate saturated amplification in the 32.8 nm laser line of nickel-like krypton.

Recently, it has become possible to envisage the production of a state-of-the-art, high-repetition rate XRL using the recently completed 20 TW Ti:sapphire laser system, operated by our group, which provides about 1 J in 40 fs at 10 Hz. From now on, our main goal is to explore diverse schemes and build a compact, highly monochromatic laser in the short-wavelength region of 10-45 nm, operating at high repetition rate, suitable for widespread applications. In particular, we are investigating physical phenomena constituting a base for understanding and development of table-top XRL with a special emphasis on the medium excitation and the consecutive generation, amplification and propagation of coherent X-ray radiation. Two specific XRL inversion schemes are being studied: longitudinal pumping by optical field ionization of gaseous targets (e.g. Pd-like Xe at 41.8 nm, Ni-like Kr at 32.8 nm), and grazing incidence pumping using solid targets (e.g. Ne-like Ti at 32.6 nm, Ni-like Mo at 18.9 nm, Ni-like Ag at 13.9 nm, Ni-like Cd at 13.2 nm). In both cases we plan to seed the XRL plasma amplifier with a HHG to improve significantly the output beam quality (i.e. divergence, coherence, photon number). These advanced, high-repetition rate, extremely monochromatic XRL can be used in application experiments such as X-ray interferometry or coherent imaging.