At the Laser MégaJoule facility, we have recently developed a calibrated experimental platform to produce and characterize high-fluence and multi-kiloelectronVolt nanosecond x-ray sources. Such sources are used for radiographic imaging or for studying the effects of intense x-ray irradiation on materials. Broadband spectrometers provide time-resolved x-ray powers measurements; an x-ray spectrometer, recently implemented, provides well resolved x-ray emission spectra, which can be used to infer plasma characteristics such as the electron temperature, by relying on atomic physics calculations with the SAPHyR code. Xenon and krypton gas pipe targets were chosen for their peaked emission between 4 and 6 keV (L-shell line transitions) and at 13 keV (K-shell line transitions), respectively. After a proof-of-concept carried out in 2019, several experimental campaigns followed, in which the laser energy was gradually increased. We have conducted a comprehensive evaluation of all results, allowing us to evaluate the x-ray radiant energy and the source yields as a function of laser energy. For the xenon source, we demonstrated up to 3713 ± 740 J/sr in the L-band, i.e., a laser-to-x-ray conversion efficiency (CE) of 18%. For the krypton source, we demonstrated 381 ± 106 J/sr in the K-band, corresponding to CE equal to 2%, as the K-shell line excitations require more laser energy.
Baccou et al. (Wed,) studied this question.