Intense High-Contrast Femtosecond $K$-Shell X-Ray Source from Laser-Driven Ar Clusters

Bright Ar quasimonochromatic $K$-shell x ray with very little background has been generated using an Ar clustering gas jet target irradiated with a 30 fs ultrahigh-contrast laser, with a measured flux of $2.2\times {10}^{11}\mathrm{\text{photons}}/\mathrm{J}$ into $4\pi$. This intense x-ray source critically depends on the laser contrast and intensity. The optimization of source output with interaction length is addressed. Simulations point to a nonlinear resonant mechanism of electron heating during the early stage of laser interaction, resulting in enhanced x-ray emission. The x-ray pulse duration is expected to be only 10 fs, opening the possibility for single-shot ultrafast keV x-ray imaging applications.

Figure 1

X-ray spectrum of Ar when the laser beam was focused on the cluster target. The x-ray spectrum of a solid Cu target is also shown (dotted line) with $\times 10$ times in amplitude. The inset shows the photon flux dependence upon laser intensity.

Figure 2

Channeled emission characteristics. The $K\alpha$ photon emission dependence as a function of the target displacement $\Delta x$ from the best laser focus. The upper right inset shows the shadowgraph obtained in the case of the nozzle located on the focal spot position (short channel) and a position $\Delta x=+6\mathrm{mm}$ (long channel). The positive displacement means the nozzle moves closer to the OAP.

Figure 3

Simulation snapshots of the electron density distribution (a) of cluster at various times (with unit of laser optical cycle) in laser intensity $1\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$. The energy spectra and phase spectra ($P_x,P_y$) of outer electrons at various time are shown on the early stage of laser pulse (b) and on the peak of pulse (c).

Figure 4

Laser pulse shape dependent. The simulation temporal distribution of number of inner electrons (a), energy of total electrons (b), energy of inner electrons (c), and energy of outer electrons (d). The laser pulse with rising (down) edge $5/5$ (red line), 5 (10) (black line), 10 (5) (blue line) are used.

Figure 5

Ar $K$-shell x-ray emission as a function of laser pulse width with negatively skewed pulse shape (red line) and positively skewed pulse shape (black line) for (a) a full energy pulse and (b) low-energy pulse irradiation.