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Home > Research Topics > Laser plasma ultra-short X-ray sources

Ultrafast X-ray sources

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The interaction of an intense femtosecond laser pulse with a dense target leads to the ultra-fast production of a hot and dense plasma (a few 100 eV) inducing a strong and ultra-short X-ray emission. Different targets have been studied, operated at high repetition rate (1 kHz), with a view to offer a large variety of laser-based X-ray sources for application to ultra-fast X-ray science:

- Multi-keV X-ray source from laser – clusters interaction
- Multi-keV X-ray source from laser – solid interaction

Laser – clusters interaction

At high repetition rate, the laser – solid interaction leads to significant debris production that needs to be controlled in order not to pollute laser optics and X-ray detection. A clusters jet is a good alternative target, since it offers a high local density for efficient laser absorption (and then X-ray emission) together with a very low level of debris. It can simply be obtained from the condensation of an expanding rare gas trough a supersonic nozzle. It is then constituted by a mixing of residual gas and clusters with adjustable size from a few 10 to a few 100 Å. Several benefits are expected from this new regime of laser – matter interaction (cluster size << laser wavelength): fundamental physics from a well isolated nano-plasma, very high temperature achieved without thermal conduction loss, high conversion efficiency from laser energy to X-ray emission, easily renewable target for a high repetition rate multi-keV X-ray source without debris.

Many experimental works have recently been dedicated to ultra-intense laser-cluster interactions; while interesting per se, these interactions were demonstrated to yield efficiently multi-keV X-ray bursts, of great interest for ultrafast X-ray science applications. In CELIA, we focus intense (up to a few 1017 W/cm2) femtosecond (down to 40 fs) laser pulses onto a partially clusterized argon gas jet. An experimental set-up has been specifically designed to get both a spectral resolution of the X-ray emission ( 1 eV) and a picosecond temporal resolution. Our experiments have demonstrated, for the first time, that the K-shell emission duration is shorter than our temporal resolution (800 fs rms). Further, the ionization dynamics, that yield Ar ions up to Ar16+, have been observed to occur within a timescale shorter than 1 ps, which corresponds to our data analysis resolution. These observations have been quite well reproduced by simulations including "nano-plasma" model and collisional-radiative calculations, that indicate that the X-ray emission should occur within a few 100 fs scale (Cf. Fig. 1).

Figure 1. Top: X-ray emission duration measured as a function of the laser pulse duration. Ar clusters with 180 Å (circles), 275 Å (squares) and 350 Å (triangles) average radius are irradiated by 2.0 mJ, linearly (black) and circularly (white) polarized laser pulses. Full line : guiding line equating the x-ray pulse duration to the laser pulse duration. Dashed line : time-integrated K-shell x-ray emission from 275 Å Ar clusters. Below: same physical quantities (x-ray duration an emitted intensity) obtained from the calculation for a 275 Å radius Ar cluster.

©CELIA

Laser – solid interaction

Solid target irradiation leads to debris production at high repetition operation. Nevertheless, in the multi-keV range, it’s easy to protect the X-ray optics with thin plastic filters, without significant X-ray absorption. As another advantage over cluster targets is the very large variety of X-ray sources available, since most of the materials exist in solid phase. Depending on the target material, and function of the spectral range of interest, it’s possible to get a quasi-monochromatic X-ray source (line with 1 eV width) or a broadband multi-keV X-ray source over a few 100 eV spectral range. With such a technique, we have optimized a high repetition rate (1 kHz) ultra-short (a few ps) broadband X-ray source in the range from 1.5 to 1.75 keV.

We have realized a systematic study of X-ray burst emitted from high Z (atomic number) targets (samarium, gadolinium, dysprosium, erbium and ytterbium), as a function of the different interaction parameter, especially the laser pulse duration (from 30 fs to 5 ps). More specifically, the M-shell emission has been studied in the range 1.5 to 1.75 keV. This M-shell emission respectively corresponds to 4f – 3d, 5f – 3d, 3d – 3p transitions, presenting very emissive broadband spectral features (over a few 100 eV).

Other experiments have been realized for fundamental physics of hot and dense plasmas, on higher energy laser facilities, in collaboration with a team of LULI. An experiment has been performed on LULI2000, to study the temporal and spectral behavior of X-UV and X-ray emission (from 2 to 3.2 keV) of homogeneous ns gold plasmas. Another experiment has been realized on the LULI 100 TW laser to measure with spectral and temporal resolution the X-ray emission of different thin foils irradiated by 300 fs, 3 ps and 30 ps laser pulses: aluminum (K-shell emission), selenium (L-shell) and samarium (M-shell).

©CELIA

Rare gas cluster jet motorized device

©CELIA

Interior view of the X-ray Pulse Generation Chamber with solid target configuration