X-ray lasers

Click to see animated view of X-ray laser generation

The soft X-ray lasers (XRL) we develop use as the active medium a column of laser-produced plasma. The pump device is a high-power infrared laser delivering a sequence of two pulses onto the surface of typically a metallic slab target. While the first pulse vaporizes the target and creates a weakly ionised plasma, the second pulse keeps up generating further plasma and heats it to an appropriate temperature to produce the required ionisation and achieve population inversion.

The ionic species hosting population inversion in the soft X-ray lasers developed here are neon-like (1s22s22p6, 10 bound electrons remaining) or nickel-like (1s22s22p63s23p63d10, 28 bound electrons remaining). These isoelectronic configurations have the completed principal shell (n = 1, 2 for Ne-like and n = 1, 2, 3 for Ni-like), and are highly stable in laser plasmas with respect to a fairly large interval of plasma densities and/or electron temperatures.

Multimillijoule zinc soft X-ray laser at 21.2 nm

A particular example is neon-like zinc soft X-ray laser, which is the backbone of our experimental programme. Here, the population inversion is produced between 2p53p and 2p53s fine-structure levels of Ne-like ions, through electron collisional excitation from the ground state 2p6. The inversion is maintained by strong radiative dipole decay from the 3s levels back to the ground state, while the upper 3p levels are metastable with respect to this process. Under appropriate plasma conditions, the strongest population inversion is generated (due to the highest monopole electron excitation rate from the ground) for the transition (2p51/23p1/2)J=0 to (2p51/23s1/2)J=1, corresponding in Ne-like zinc to a wavelength of 21.22 nm.

The pumping sequence consists of a separately delivered and focused prepulse (with energy of a few joules), and the main pulse (energy 400 to 600 J), both with identical length of about 400 ps. The plasma column generated has the nominal length of 3 cm. The soft X ray laser operates in double pass, which is achieved by a Mo:Si multilayer mirror located about 1 cm near one end of the plasma.

The emerging X-ray beam may be switched between typically three alternative optical paths, by making it reflect off the respective retractable multilayer mirror. One of these paths is employed as the application beamline, delivering for instance a probe coherent radiation for X ray interferometry of surfaces, a pulsed source for studies of X-ray ablation of materials, dense astrophysical plasmas, molecular microbiology, etc.

The zinc soft X-ray laser is generated by a sequence involving a loosely focused prepulse (typically 700-µm wide line focus), delivered 10 to 50 ns ahead of the main heating pulse. This latter pulse is narrowly focused by a special cylindrical composite optics down to a line of width comparable to ~100 µm. In this configuration, implemented for the first time ever by our group, the amplifying column is produced within a much wider preplasma, which allows generating a smooth ellipsoidal X-ray beam that exhibits high spatial quality and coherence.

Plasma generated by the prepulse		Amplifying plasma created by the main pulse

		The X-ray beam of the zinc laser, generated by the half cavity, delivers 4 to 10 mJ per pulse according to the prepulse delay applied (10 to 50 ns). This corresponds to peak power 40 to 100 MW, given the pulse duration of ~100 ps. Both the pulse energy and peak power are the highest values ever achieved for a laboratory X-ray laser. The divergence of the half-cavity zinc X-ray laser beam is 3 × 5  milliradians (horizontal × vertical).
Footprint of the half-cavity XRL beam at 21.2 nm

Example of application of the zinc X-ray laser at 21.2 nm

Besides the development of soft X-ray lasers, our group is involved in several application projects using these extremely bright radiation sources. Most notably, we develop the technique of soft X-ray interferometry/holography of surfaces, based on advanced Lloyd’s mirror interferometer configuration and having nanometric resolution capability. Another application being prepared is generation of volumetrically heated superdense plasmas by a focused X-ray laser beam. We are further involved in collaborative projects on nanometric-resolution interferometric probing of solid surfaces subjected to strong electric or radiation fields, and on soft X-ray ablation of solid state materials relevant to microfabrication technologies.

Interferometric probing of niobium surfaces subjected to strong electrical fields: modification of the surface occurring at 50 MV/m, Nb layer on positive potential with respect to the knife electrode (collaboration with LIXAM/IOTA, Université Paris-Sud, France).