OptimizedKαx-ray flashes from femtosecond-laser-irradiated foils

Abstract

We investigate the generation of ultrashort Kalpha pulses from plasmas produced by intense femtosecond p-polarized laser pulses on Copper and Titanium targets. Particular attention is given to the interplay between the angle of incidence of the laser beam on the target and a controlled prepulse. It is observed experimentally that the Kalpha yield can be optimized for correspondingly different prepulse and plasma scale-length conditions. For steep electron-density gradients, maximum yields can be achieved at larger angles. For somewhat expanded plasmas expected in the case of laser pulses with a relatively poor contrast, the Kalpha yield can be enhanced by using a near-normal-incidence geometry. For a certain scale-length range (between 0.1 and 1 times a laser wavelength) the optimized yield is scale-length independent. Physically this situation arises because of the strong dependence of collisionless absorption mechanisms-in particular resonance absorption-on the angle of incidence and the plasma scale length, giving scope to optimize absorption and hence the Kalpha yield. This qualitative description is supported by calculations based on the classical resonance absorption mechanism and by particle-in-cell simulations. Finally, the latter simulations also show that even for initially steep gradients, a rapid profile expansion occurs at oblique angles in which ions are pulled back toward the laser by hot electrons circulating at the front of the target. The corresponding enhancement in Kalpha yield under these conditions seen in the present experiment represents strong evidence for this suprathermal shelf formation effect.

Acoustic response of a laser-excited polycrystalline Au-film studied by ultrafast Debye–Scherrer diffraction at a table-top short-pulse x-ray source

“…Short bursts of Cu K α radiation at 8 keV were produced by focusing femtosecond laser pulses (repetition rate 10 Hz, pulse energy 150 mJ, pulse duration 120 fs, wavelength 800 nm) onto the surface of a moving 10 µm thick copper tape housed in a small vacuum chamber. A pre-pulse scheme is employed to optimize X-ray production 26,27 , resulting in a total Cu K α -flux of more than 10 10 photons per pulse.…”

Theoretical analysis of hard x-ray generation by nonperturbative interaction of ultrashort light pulses with a metal

“…In the laser-plasma scenario, a pre-pulse 23,24 or the leading part of the main pulse creates a very hot ( k B T  ≈ 500 eV) overdense plasma at the surface of the metal target [Fig. 2(a)].…”

Ultraintense Lasers as a Promising Research Tool for Fusion Material Testing: Production of Ions, X-Rays and Neutrons

“…Thus, K alpha generation is known to be optimized when the laser intensity produces electrons with temperatures a few times higher than the K alpha energy and when the laser energy conversion into hot electrons is maximized (in this case, pre-plasmas play an important role [46]). Energy conversion efficiency from laser to K alpha can achieve values of 10 −4 -10 −3 , generating X-ray pulses of >10 12 photons per laser shot.…”