Experimental investigation of size broadening of a Kα x-ray source produced by high intensity laser pulses

Abstract

The size of a hard Kα x-ray source ($${\mathrm{E}}_{{\rm{K}}_{\rm{\alpha }}}$$ E K α = 17.48 keV) produced by a high intensity femtosecond laser interacting with a solid molybdenum target is experimentally investigated for a wide range of laser intensity (I ~ 1017–2.8 × 1019 W/cm2) and for four values of the temporal contrast ratio (6.7 × 107 < CR < 3.3 × 1010). Results point out the size enlargement of the x-ray source with the increase of laser intensity and with the deterioration of temporal contrast. It amounts up to sixteen times the laser spot size at the highest laser intensity and for the lowest temporal contrast ratio. Using hydrodynamic simulations, we evaluate the density scale length of the pre-plasma L/λ just before the main pulse peak. This allows us to show that a direct correlation with the laser absorption mechanisms is not relevant to explain the large size broadening. By varying the thickness of the molybdenum target down to 4 µm, the impact of hot electron scattering inside the solid is also proved irrelevant to explain the evolution of both the x-ray source size and the Kα photon number. We deduce that the most probable mechanism yielding to the broadening of the source size is linked to the creation of surface electromagnetic fields which confine the hot electrons at the solid surface. This assumption is supported by dedicated experiments where the evolution of the size enlargement of the x-ray source is carefully studied as a function of the laser focal spot size for the highest contrast ratio.

K[alpha] x-ray source size based on high-intensity femtosecond laser-solid interaction: generation and applications

“…Laser parameters are essential to optimize/enhance the emitted K X-ray source characteristics. To that goal, we conducted parametric studies [9,10] to have control on the flux yield and dimensions of the K X-ray source through management of the laser driving intensity and of its temporal characteristics before the main pulse (temporal contrast ratio).…”

“…From the spectrum obtained for every configuration of laser intensity and contrast ratio, it is further possible to deduce the Kα photon number/sr/shot. (b) Knifeedge technique used to measure the X-ray beam size [10]. (c) Example of Mo Kα xray spectrum [9].…”