Impact of the pulse contrast ratio on molybdenum Kα generation by ultrahigh intensity femtosecond laser solid interaction

Abstract

We present an extended experimental study of the absolute yield of Kα x-ray source (17.48 keV) produced by interaction of an ultrahigh intensity femtosecond laser with solid Mo target for temporal contrast ratios in the range of 1.7 × 107–3.3 × 109 and on three decades of intensity 1016–1019 W/cm². We demonstrate that for intensity I ≥ 2 × 1018 W/cm² Kα x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the Kα photon number is measured and a value of ~2 × 1010 photons/sr/s is obtained at 10 Hz and I ~1019 W/cm². Furthermore, Kα energy conversion efficiency reaches the same high plateau equal to ~2 × 10−4 at I = 1019 W/cm² for all the studied contrast ratios. This original result suggests that relativistic J × B heating becomes dominant in these operating conditions which is supposed to be insensitive to the electron density gradient scale length L/λ. Finally, an additional experimental study performed by changing the angle of incidence of the laser beam onto the solid target highlights a clear signature of the interplay between collisionless absorption mechanisms depending on the contrast ratio and intensity.

Exploring phase contrast imaging with a laser-based Kα x-ray source up to relativistic laser intensity

“…Considering the real 2D beam profile, it corresponds to a maximum peak intensity of 1.3 × 10 19 W/cm 2 . The present study is done with a high temporal intensity contrast ratio (ICR) laser pulse 29 ~10 10 which is beneficial to produce a high x-ray flux 43 and at the same time a small x-ray source size 44 . Since x-ray flux and source size are critical parameters to enhance PCI, they are thoroughly characterized for all of these studies and investigated by varying the laser intensity in a large range (from ~10 17 to 1.3 × 10 19 W/cm 2 , see Fig.…”

High photon flux Kα Mo x-ray source driven by a multi-terawatt femtosecond laser at 100 Hz

“…For optimal use of K α sources for applications, the key parameters are a high photon flux, a pointlike x-ray source, and an ultrashort x-ray pulse duration. To get access to these characteristics and, especially, to reach a high photon flux per shot, operation of the driving laser at high intensity is needed [7], requiring the use of multi-terawatt femtosecond laser sources. The efficient generation of K α hard x-ray photons (>10 keV) necessitates even higher intensity when the atomic number Z of the target increases, as empirically scaled [8].…”

“…Many research groups developed K α sources at a high repetition rate reaching 1 kHz [3,11]. However, these systems are limited in laser energy per pulse which precludes the delivery of high intensity on target and, thus, reduces the conversion efficiency and the total hard x-ray flux [7,12]. At the opposite, high-energy ultrashort laser sources are limited to a low repetition rate [12,13] which is an obstacle when time exposure or sensitivity to x-ray peak dose is a concern.…”

“…Experimental details of the driving laser source, beam transport to the Mo target, and x-ray source diagnostics can be found in Refs. [7,14]. We recall hereafter only specific points related to running such experiments at 100 Hz.…”

“…Using an x-ray CCD camera (PIXIS-XB, Princeton Instruments −1024 × 1024 pixels of 13 μm × 13 μm pixel size) in the single-photon counting mode and a calibrated procedure [7], the K α photon number per shot is deduced from the reconstructed x-ray spectrum. The camera is located at 90 cm from the target at an angle of 45°with respect to the normal of the target front side and perpendicular to the laser beam.…”