Experimental evidence of nonthermal acceleration of relativistic electrons by an intensive laser pulse

Nonthermal acceleration of relativistic electrons is investigated with an intensive laser pulse. An energy distribution function of energetic particles in the universe or cosmic rays is well represented by a power-law spectrum, therefore, nonthermal acceleration is essential to understand the origin of cosmic rays. A possible candidate for the origin of cosmic rays is wakefield acceleration at relativistic astrophysical perpendicular shocks. The wakefield is considered to be excited by large-amplitude precursor light waves in the upstream of the shocks. Substituting an intensive laser pulse for the large amplitude light waves, we performed a model experiment of the shock environments in a laboratory plasma. An intensive laser pulse was propagated in a plasma tube created by imploding a hollow polystyrene cylinder, as the large amplitude light waves propagated in the upstream plasma at an astrophysical shock. Nonthermal electrons were generated, and the energy distribution functions of the electrons have a power-law component with an index of $~2$. We described the detailed procedures to obtain the nonthermal components from data obtained by an electron spectrometer.

Figure 1

Schematic of the ESM.

Figure 2

(a) Intensities of PSL of the electron side and the ion side against electron energy in MeV. (b) Intensity of PSL of the ion side against the distance from a point source of $\gamma$ rays.

Figure 3

(a) Reevaluation of ESM data using logarithmic bins. $E_{\mathrm{imp}}=1.9\hspace{0.5em}\mathrm{kJ}$. (b) Expanded plot of high-energy region of (a). (c) Same as (b) except for $E_{\mathrm{imp}}=2.0\hspace{0.5em}\mathrm{kJ}$. (d) Same as (b) except for $E_{\mathrm{imp}}=2.3\hspace{0.5em}\mathrm{kJ}$.

Figure 4

(a) ESM signals after subtracting the ion-side values from the electron-side values. (b) IP sensitivity obtained from absolute calibrations (open circles) and the fitted line. (c) Energy distribution functions of electrons.