Characterization of laser wakefield acceleration efficiency with octave spanning near-IR spectrum measurements

We report on experimental measurements of energy transfer efficiencies in a GeV-class laser wakefield accelerator. Both the transfer of energy from the laser to the plasma wakefield and from the plasma to the accelerated electron beam was diagnosed by simultaneous measurement of the deceleration of laser photons and the acceleration of electrons as a function of plasma length. The extraction efficiency, which we define as the ratio of the energy gained by the electron beam to the energy lost by the self-guided laser mode, was maximized at $19\pm 3\%$ by tuning the plasma density and length. The additional information provided by the octave-spanning laser spectrum measurement allows for independent optimization of the plasma efficiency terms, which is required for the key goal of improving the overall efficiency of laser wakefield accelerators.

Figure 1

Laser and electron spectrum at the exit of the 27-mm gas cell as functions of electron density (a) and (b) and as functions of accelerator stage length for $n_e=1.25\pm 0.06\times {10}^{18}{\mathrm{cm}}^{-3}$ (c) and (d). The red-bordered region in the laser spectra plots indicates the gap between the measurement ranges of the two spectrometers, which was filled by interpolation. Each column of each image shows the average from three to five shots at the same conditions.

Figure 2

Laser energy loss, electron beam energy, and extraction efficiency as functions of plasma density for $z=27\mathrm{mm}$ (a)–(c), and length (d)–(f) with $n_e=1.25\times {10}^{18}{\mathrm{cm}}^{-3}$.

Figure 3

(a) Laser and (b) electron spectra as functions of propagation distance taken from a PIC simulation with a plateau density of $n_e=1.25\times {10}^{18}{\mathrm{cm}}^{-3}$. (c) The extraction efficiency, calculated as the ratio of the electron beam energy to the laser energy loss, for simulations with $n_e=(1.25,1.5)\times {10}^{18}{\mathrm{cm}}^{-3}$ and the experimental measurements (blue points) at $n_e=1.25\times {10}^{18}{\mathrm{cm}}^{-3}$.

Figure 4

(a) Loaded and (b) unloaded longitudinal electric field and injected electron bunch position (green) over the propagation axis $z$ of PIC simulations with $n_e=1.25\times {10}^{18}{\mathrm{cm}}^{-3}$. The unloaded fields were extracted from a simulation without the ionization injection species. The maximum accelerating field position in the first plasma period (black line) is overlaid.