Abstract
The size of a hard Kα x-ray source ({mathrm{E}}_{{rm{K}}_{rm{alpha }}} = 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.
Highlights
IntroductionWe 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
The size of a hard Kα x-ray source (E Kα = 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 ~ 1 017–2.8 × 1 019 W/cm2) and for four values of the temporal contrast ratio (6.7 × 1 07 < CR < 3.3 × 1 010)
The x-ray source size enlargement is experimentally studied by varying the laser intensity on target, the temporal contrast of the laser pulse, the target thickness and the laser focal spot size
Summary
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. Laser-plasma hard x-ray sources are a suitable alternative to conventional accelerator-based x-ray sources of high brightness[1,2] and for applications requiring a high x-ray flux and a micrometric size such as x-ray phase contrast imaging[3,4] Their characteristics and operating capabilities have been improved for decades[5,6,7,8] especially thanks to the continuous progress of femtosecond lasers in terms of intensity, spatial and temporal beam quality, repetition rate, reliability and compactness. Among these works only very rare studies have been published concerning the impact of different values of the temporal contrast ratio, at the fundamental wavelength of 800 nm, on the evolution of the x-ray source size for a large laser intensity r ange[13]
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