Figure 1

Schematic of the experimental setup, showing the $f/8$ off-axis parabola focusing the laser pulse on the gas cell target and the transverse interferometry probe. The interferometer has two beam splitters (B) and four mirrors (M). An example of an interferogram is shown above the interferometer CCD camera. The electrons (dashed line) are dispersed in energy by the magnet centered on the laser axis and located 3.5 cm from the source inside the target chamber and then recorded on image plates (${\mathrm{IP}}_a$ and ${\mathrm{IP}}_b$, located 37.2 cm and 111.1 cm away from the magnet exit, respectively). The corresponding retrieved electron beam spectrum is shown on top. Dot with error bar indicates maximum energy of $268\pm 25\mathrm{MeV}$. After exiting the vacuum chamber, x rays (solid line) propagate through 7.3 cm of air and through ${\mathrm{IP}}_a$ onto the 15-channel x-ray spectrometer, below which are shown raw images ($25\mu \mathrm{m}$ pixels) from channels 1–6. LP indicates the laser polarization direction on image plates.Reuse & Permissions

Figure 2

X-ray spectrometer signal (dots with error bars), in channels 1–6 (photostimulated luminescence [PSL] per pixel). Calculated dose (normalized to the first data point) for betatron spectra with critical energies of 15 and 20 keV (dotted and dashed lines, respectively) and radiation produced by an electron injected $5\mu \mathrm{m}$ off-axis and accelerated up to 268 MeV (red solid line). The inset shows the inferred normalized energy distribution spectrum for each case. Peak energy is shown for the spectrum with $\hslash {\omega }_c=20\mathrm{keV}$.Reuse & Permissions

Figure 3

(a) Measured and simulated betatron x-ray beam profiles, recorded on ${\mathrm{IP}}_a$, scanned with a $200\mu \mathrm{m}$ pixel size. LP indicates the laser polarization direction. The number of electrons around the propagation axis (positive $z$ direction) is shown in (b) for four distinct groups of electrons accelerated up to $\gamma =100$, 200, 440, and 520 (dashed, dotted, solid, and dot-dashed lines, respectively). $\theta =0$ is along the $x$ (vertical) axis and the laser is polarized along $\theta =\pi /2\mathrm{rad}$ ($y$ horizontal axis). (c) Sample three-dimensional trajectories are shown for the four groups of electrons in the plasma.Reuse & Permissions

Figure 4

Peak experimental betatron x-ray energy as a function of the angle of observation (dots with error bars). The spectrum was measured on axis and at 7, 14, and 28 mrad corresponding to respective vertical positions 1, 2, 3, 4 indicated in the inset showing the beam profile on the first spectrometer channel. The two curves show the theoretical peak energy for Eq. (3) (dashed line) and the full set of electron trajectories (dotted line).Reuse & Permissions