Three-hump curve of laser plasma X-ray emission vs. varied focusing conditions

The dependence of X-ray flux as a function of the position of the target relative to the laser focus of x-ray emission from laser plasma is not always single peaked which could occur at a certain position of the target. Two-hump curves are sometimes observed, especially when measuring the emission in the soft part of the emitted X-ray spectrum. In most cases, the accompanying hard emission is one-humped and it occurs close to the position of setting the target surface in the exact optical focus. As it was suggested in some publications, the reasons for appearing the two-hump curve could be different in dependence of the kind of plasma considered and experimental conditions. It is also possible that the two-hump curve can arise as a result of improper use of diagnosing detectors (e.g., as a result of overloading the detectors) or as a result of a fault in the optical focusing system or bad quality of the laser beam. As the problem of X-ray emission is important for the explanation of physical phenomena in plasma and also for optimisation of X-ray output, some special investigation was started on the 1-kJ PALS laser, Prague, to solve this problem. In these measurements, semiconductor detectors of two special types were used. They are effective in good separation of the two X-ray components: at about 1 keV and at about 15 keV. The measurements were carried out with the use of Ta, Ag and Cu targets at different laser energy, but always with the same optical focusing system. The obtained curves of X-ray emission are smooth, as the laser has operated with good repeatability. One- and two hump curves were mostly observed. The details of shapes were clearly seen, which allow drawing some additional conclusions. An original case of a three-hump curve of the changes of X-ray emission was noticed for the emission from Cu target at irradiating with 250-J pulses. The emission was measured with the use of two types of detectors: semiconductor and TLD. The three-hump curve and some details in the shapes of the two-humped curves are interpreted in this work as a result of superposition of two types of X-ray emissions that comes from two, radically different mechanisms of X-ray production. (author)