Figure 1

The experiment scheme (right) and He plasma density profile (left) for the shots in Figs. 2, 3.Reuse & Permissions

Figure 2

Typical single-shot comblike spectra. (a) Raw data for $P_0=9\mathrm{TW}$, $n_e=2.7\times {10}^{19}{\mathrm{cm}}^{-3}$. (b) Curves for two regions marked in (a) by dashed brackets. Dotted vertical lines are for harmonics of the base frequency ${\omega }_f=0.885{\omega }_0$. The highest resolved order is $n_H^{*}=126$. (c) Spectra for the shot shown in (a) (red) and for $P_0=120\mathrm{TW}$, $n_e=1.9\times {10}^{19}{\mathrm{cm}}^{-3}$ obtained from the 2nd (blue) and 3rd (green) diffraction orders. Inset: harmonics in the 2nd diffraction order; similarly for the 3rd order. (d) The spectral region embracing the “water window” for $P_0=9\mathrm{TW}$, $n_e=4.7\times {10}^{19}{\mathrm{cm}}^{-3}$. The drop at the carbon $K$ absorption edge, $K_C$, is due to hydrocarbon contamination. Dashed curves in (c),(d) are for background due to the CCD dark current, readout noise, and scattered photons.Reuse & Permissions

Figure 3

Modulated spectra with (a) resolved and (b) nearly unresolved harmonic structure for $P_0=9\mathrm{TW}$, $n_e=4.7\times {10}^{19}{\mathrm{cm}}^{-3}$. Curves in (c),(d) show spectra denoted by dashed brackets in (a),(b), respectively.Reuse & Permissions

Figure 4

(a) 3D PIC simulation. Electron density $n_e$ (top, upper half removed) and its cross section at $z=0$ (bottom). The electromagnetic energy density for $\omega \ge 4{\omega }_0$ is represented by red arcs. (b) Catastrophe theory model. Singularities in the electron density are created by foldings of the electron phase space $(x,y,⟨p_y⟩)$, where $p_y$ is averaged over the laser period.Reuse & Permissions

Figure 5

2D PIC simulation. (a) The electron density (blue), laser envelope (curves for $a=1,4,7,10$), and electromagnetic energy density, $W_H$, for frequencies from 60 to $100{\omega }_0$ (red). (b) The upper spike emission spectrum for the dashed rectangle in (a). (c) The electron density profile 10 laser cycles earlier than (a) and (d) the right spike structure.Reuse & Permissions