Laser wakefield acceleration with active feedback at 5 Hz

We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.

Figure 1

A schematic diagram of the experimental setup. The laser (light red) is focused into a gas jet (bottom left), accelerating an electron bunch (light blue). A holed glass wedge allows the transmitted laser beam to be diagnosed. Scintillator screens record the electron beam’s spatial profile and energy spectrum; the former screen can be removed to improve the resolution of the spectrometer. Betatron x rays are recorded by an x-ray CCD, which is protected from any residual laser light by a thin gold beam block. Data recorded from the scintillator screens and the x-ray CCD are sent to the control system (top right), which is running the optimization algorithm. This sends commands to the Dazzler and the deformable mirror, completing the feedback loop.

Figure 2

Five randomly selected images of the electron beam spatial profile, together with a 50-shot average. Each image is independently normalized. The average was taken after overlapping the centroids of the images, to remove the effects of pointing fluctuations. Data were taken using the temporal pulse shape which maximized the accelerated charge.

Figure 3

Five randomly selected electron energy spectra, together with a 50-shot average. The individual spectra correspond to the beam profiles of the same color shown in Fig. 2. Data were taken using the temporal pulse shape which maximized the accelerated charge.

Figure 4

Images of the electron beam spatial profile (a) before and (b) after optimization. Each image is an average of 50 shots, after removing pointing fluctuations. The color scales are different: The total charge in (b) is $2.1\times$ the total charge in (a).

Figure 5

(a) The electron spectra (averaged over 50 shots) produced at the beginning (dashed lines) and end (solid lines) of two optimization runs. Each optimization run is plotted in a different color, and each was targeted at a different energy interval (shaded regions). (b) The improvement factor: the ratio of the final spectrum to the initial spectrum.

Figure 6

The progress of optimization using the deformable mirror, described in more detail in the text. Only the best four measurements per generation are shown; these are the ones used to form the next generation. The dashed lines indicate two estimates of the goal function that would be achieved under similar conditions with the best focus.

Figure 7

Every measurement of the goal function during a single optimization run. The four best samples from each generation, used as parents for the next, are highlighted. The vertical axis is normalized to the first sample, taken with a fully compressed laser pulse.

Figure 8

Dazzler settings (second- and third-order phases only; the fourth-order phase is not shown) used during optimization. In both cases, the initial settings, giving the best pulse compression, are indicated by a dotted circle, while the optimal settings are indicated by a solid circle. (a) Charge optimization. The amount of accelerated charge is indicated by the color of the marker. (b) Energy spectrum optimization, as shown in Fig. 5. The value of the goal function (see the text) is indicated by the color of the marker. The shape of the marker indicates the target energy range. Some points have been omitted for legibility.

Figure 9

The temporal shapes of the initial (fully compressed) and optimal pulses from the charge optimization run. The optimal pulse is approximately twice as long and has a pronounced asymmetry. The shaded region indicates the root mean square variation from shot to shot.

Figure 10

The temporal shapes of two pulses optimized for different electron energy ranges, as well as the fully compressed pulse used as the start point of the optimization runs.