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 at PALS 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.
Animated view of X-ray laser generation
A particular example is neon-like zinc XRL, 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.
Laser scheme of zinc soft X-ray laser
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.
Experimental setup for X-ray laser propagation
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).
X-ray laser beam footprint
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