Demonstration of Nonlinear-Energy-Spread Compensation in Relativistic Electron Bunches with Corrugated Structures

High quality electron beams with flat distributions in both energy and current are critical for many accelerator-based scientific facilities such as free-electron lasers and MeV ultrafast electron diffraction and microscopes. In this Letter, we report on using corrugated structures to compensate for the beam nonlinear energy chirp imprinted by the curvature of the radio-frequency field, leading to a significant reduction in beam energy spread. By using a pair of corrugated structures with orthogonal orientations, we show that the quadrupole wakefields, which, otherwise, increase beam emittance, can be effectively canceled. This work also extends the applications of corrugated structures to the low beam charge (a few pC) and low beam energy (a few MeV) regime and may have a strong impact in many accelerator-based facilities.

Figure 1

(a) Schematic of the nonlinear energy chirp compensation experiment (not in scale); (b) The detailed layout of the vacuum chamber where the CS is housed (the yellow arrow indicates the beam direction); (c) geometry of the planar CS (the red ellipse represents a beam propagating along the $z$ axis).

Figure 2

Time-resolved measurements of beam distribution at screen $P1$ with (a) 1 crystal, (b) 2 crystals, and (c) 3 crystals inserted (bunch head to the left); the corresponding beam current distribution with 3 crystals inserted is shown in (d).

Figure 3

Time-resolved measurements of beam longitudinal phase space distributions (bunch head to the up) at screen $P2$ (a) with the two structures open, (b) with one of the CS gap reduced to 3 mm, and (c) with the gaps of the two CS both set at 3 mm; (d) the corresponding projected beam energy distribution.

Figure 4

(a) Measured and simulated wake potential of the CS. The bunch distribution, with the head to the left, is also shown with the red line.

Figure 5

Time-resolved measurements of beam distribution at screen $P1$, showing (a) no quadrupole wake when the gaps of the two structures are open, (b) time-dependent defocusing in $x$ from the quadrupole wake of the upstream CS, (c) time-dependent focusing in $x$ from the quadrupole wake of the downstream CS, and (d) again, almost no quadrupole effect with the gaps of the two structures both set at 3 mm.