Phase stable net acceleration of electrons from a two-stage optical accelerator

In this article we demonstrate the net acceleration of relativistic electrons using a direct, in-vacuum interaction with a laser. In the experiment, an electron beam from a conventional accelerator is first energy modulated at optical frequencies in an inverse-free-electron-laser and bunched in a chicane. This is followed by a second stage optical accelerator to obtain net acceleration. The optical phase between accelerator stages is monitored and controlled in order to scan the accelerating phase and observe net acceleration and deceleration. Phase jitter measurements indicate control of the phase to $\sim 13°$ allowing for stable net acceleration of electrons with lasers.

Figure 1

Monte Carlo simulation of the evolution of a microbunch in the net acceleration experiment. Shown are the longitudinal phase space (top row) and energy spectrum (bottom row) starting before the IFEL and finishing with accelerating and decelerating cases for the ITR interaction.

Figure 2

Layout of the net acceleration experiment. The laser is split at a polarizing beamsplitting cube to drive both the ITR acceleration and the IFEL. The ITR path includes a piezo driven mirror for varying the optical phase. Each arm is $\sim 5\mathrm{m}$ in path length. Interferometric noise measurements indicate the optical paths are stable to within $\sim 20\mathrm{nm}$ or 2 parts per billion of the total path.

Figure 3

Phase stability of the net acceleration experiment. The figure on the left shows the linearity of the phase monitor to the experiment laser phase while the phase is actively scanned using the piezo delay mirror. When the phase is fixed the slow drift of the phase can be observed. Over time the experiment laser phase can wander away from the phase of the monitor system. The fast jitter is 13° rms.

Figure 4

Frequency domain analysis of net acceleration experiment. The phase is scanned with a saw-tooth function as seen by the phase monitor (a). The electron centroid energy (b), energy spread (c), and asymmetry (d) show variation at the drive frequency of 0.1 Hz, confirming that net acceleration is occurring in the staged experiment.

Figure 5

Drift of the phase offset between the acceleration phase versus the monitor over time. Left: Two short subsets of a long run show differing phase offset in the energy spread versus phase correlation plot. Right: The phase offset at each subset of a long data scan shows the time evolution of the offset during the full data scan.

Figure 6

Correlation of electron spectrum centroid energy (left), energy spread (center), and asymmetry (right) with the corrected phase subtracting slow drift. The error bars are the deviations of the means for the binned data. Also shown are sinusoidal fits to each set.