Abstract
The efficient conversion of optical laser light into bright ultrafast x-ray pulses in laser created plasmas is of high interest for dense plasma physics studies, material science, and other fields. However, the rapid hydrodynamic expansion that cools hot plasmas has limited the x-ray conversion efficiency (CE) to 1% or less. Here we demonstrate more than one order of magnitude increase in picosecond x-ray CE by tailoring near solid density plasmas to achieve a large radiative to hydrodynamic energy loss rate ratio, leading into a radiation loss dominated plasma regime. A record 20% CE into hν>1 keV photons was measured in arrays of large aspect ratio Au nanowires heated to keV temperatures with ultrahigh contrast femtosecond laser pulses of relativistic intensity. The potential of these bright ultrafast x-ray point sources for table-top imaging is illustrated with single shot flash radiographs obtained using low laser pulse energy. These results will enable the deployment of brighter laser driven x-ray sources at both compact and large laser facilities.
Highlights
Intense ultrashort bursts of x-ray radiation are essential for backlighting the implosion of capsules in inertial confinement fusion experiments [1,2]
We show that an increase of more than one order of magnitude in optical to picosecond x-ray conversion efficiency (CE) can be achieved by tailoring the plasma characteristics to reach a smaller radiative cooling time, τrad, than the hydrodynamic cooling time, τhydro, resulting in a larger radiative to hydrodynamic cooling rate ratio that effectively overcomes hydrodynamic cooling
We experimentally demonstrate that this approach results in an ∼20% CE of optical laser light into hν > 1 keV x-rays in 4π sr, more than one order of magnitude increase in CE with respect to previous work
Summary
Intense ultrashort bursts of x-ray radiation are essential for backlighting the implosion of capsules in inertial confinement fusion experiments [1,2]. They are of significant interest for fundamental studies that include laboratory opacity measurements in matter at the conditions of stellar interiors [3], and for probing ultrafast changes in material with high spatial and temporal resolution [4,5]. Efforts to increase the ultrafast x-ray yield have largely focused on addressing the first of these two limitations by improving the coupling of the laser energy into the material using structured targets. Targets investigated include micro-lithographic gratings [6,7,8], nanometer-size dielectric spheres or ellipsoids [9,10], “smoked”
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