Observation of Synchrotron Radiation from Electrons Accelerated in a Petawatt-Laser-Generated Plasma Cavity

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.

Figure 1

(a) Electron energy distribution and (b) x-ray photon yield $N_{\mathrm{ph}}$ (solid square) and $N_{\mathrm{ph}}/N_e$ (open circle) for different $a_0$ with $f/3$ focusing, $D=2\mathrm{mm}$, and $n_e=1.6\times {10}^{19}{\mathrm{cm}}^{-3}$.

Figure 2

X-ray photon signal as a function of plasma density (a) and interaction length $D$ (b) for $f/5$ focusing and $a_0=10$.

Figure 3

(a) Measured and modeled x-ray signal for different filters for $n_e=3.7\times {10}^{19}{\mathrm{cm}}^{-3}$. The filters, from left to right, are V $20\mu \mathrm{m}$, Ti $30\mu \mathrm{m}$, Ni $20\mu \mathrm{m}$, Fe $30\mu \mathrm{m}$, Al 1.5 mm, and Cu $270\mu \mathrm{m}$. Modeling is based on a SAL spectrum with different $E_{\mathrm{crit}}$. (b) Scaling of $E_{\mathrm{crit}}$ and $T_{\mathrm{eff}}$ with $n_e$. Shots were taken with $f/5$ focusing on a $D=5\mathrm{mm}$ nozzle at $a_0=10$.

Figure 4

Electron trajectories from simulation showing transverse deflection $y$ and axial momentum $p_x$.