Experimental Demonstration of Laser Guiding and Wakefield Acceleration in a Curved Plasma Channel

Curved plasma channels have been proposed to guide intense lasers for various applications, such as x-ray laser emission, compact synchrotron radiation, and multistage laser wakefield acceleration [e.g. J. Luo et al., Phys. Rev. Lett. 120, 154801 (2018)]. Here, a carefully designed experiment shows evidences of intense laser guidance and wakefield acceleration in a centimeter-scale curved plasma channel. Both experiments and simulations indicate that when the channel curvature radius is gradually increased and the laser incidence offset is optimized, the transverse oscillation of the laser beam can be mitigated, and the stably guided laser pulse excites wakefields and accelerates electrons along the curved plasma channel to a maximum energy of 0.7 GeV. Our results also show that such a channel exhibits good potential for seamless multistage laser wakefield acceleration.

Figure 1

(a) Structure of the discharged capillary to produce the curved and straight plasma channel. (b) Spectrum distribution and calculated profile of the plasma density along the radial direction at the entrance of the discharged capillary. (c) Experimental setup for the measurements of laser guiding and electron acceleration.

Figure 2

Laser guiding at $3\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ in the curved plasma channel. (a) Beam spot at the capillary entrance. (b) and (c) Beam spots at the capillary exit for $r_{\mathrm{os}}=20$ and $r_{\mathrm{os}}=90\mathrm{\mu }\mathrm{m}$, respectively. (d) Dependence of energy concentration ($\mathrm{\Psi }$) and transmitted power ($P$) on the offset distance $r_{\mathrm{os}}$, the fitted smoothing spline curves show their evolution tendency.

Figure 3

Electron bunches obtained by laser wakefield acceleration in the curved plasma channel with different laser incidence offsets. (a,b,c) correspond to offsets of 0, 100 and 150 $\mathrm{\mu }\mathrm{m}$, respectively. The spatially integrated energy spectrum ($dN/dE$) and relative root-mean-squared (rms) energy spreads ($\delta E/E$) of each bunch are marked on the graph.

Figure 4

Simulation results of laser evolution, wakefield generation, and electron acceleration. (a) The dashed curves represent the simulated trajectories of the laser centroid from ray tracing for different laser incidence offsets. (b) Spatial distributions of laser fields and electron density when the laser propagates to 9.6 mm with $r_{\mathrm{os}}=100\mathrm{\mu }\mathrm{m}$, where electrons located in the first plasma wave bucket with energy higher than 10 MeV are marked with magenta dots. (c) Evolution of laser intensity ($a_0$), where the black dotted line represents the threshold electric field ($E_{\mathrm{th}}$) for $N^{5+}$ ionization. (d) Evolution of electron charge in the trapped bunches for different laser offsets, where the beam charge for the case of $r_{\mathrm{os}}=100$ and $150\mathrm{\mu }\mathrm{m}$ should be multiplied by a factor of 10.