Experimental exploration of compressed beam dynamics in an energy recovery linac with comparison to simulations

The compact energy recovery linac (cERL) has been developed for industrial applications since 2017. Applications such as free electron laser require a compressed beam with a small energy spread and transverse emittance. The typical operation energy of cERL is an intermediate energy region close to 17.5 MeV; therefore, the electron bunch is easily affected by the longitudinal space-charge effects and the coherent synchrotron radiation wakefield effects. Bunch compression is demonstrated by optimizing a combination of a longitudinally chirped electron bunch and the arcs with nonzero $R_{56}$ parameters. When the bunch compression procedure is applied for a bunch charge of 60 pC, an increase in energy spread is observed at the short bunch length. We systematically explored the chirp phase to determine the best condition. The measurement results of the energy spread, bunch length, and transverse emittance were compared with the tracking simulation results to understand the compressed beam dynamics.

Figure 1

Schematic layout of the compact ERL of cases 1 and 2.

Figure 2

Beta function ${\beta }_x$, ${\beta }_y$, and dispersion function ${\eta }_x$ of isochronous optics. ${\beta }_x$ and ${\beta }_y$ are minimized using elegant simulation code without the CSR wake and space-charge effects. The positive sign of the $x$-direction is inside the arc.

Figure 3

Example of dispersion function ${\eta }_x$ of the arc section with the lattice layout and positions of BPMs.

Figure 4

Horizontal dispersion function ${\eta }_x$ of the arc section during $R_{56}$ scan. Left: results from elegant are compared with those from gpt-tracking simulation. Right: results of experimental measurements.

Figure 5

gpt-tracking results of the longitudinal distribution at cam13 at several chirp phases of ML2 cavity. $\mathrm{The}+0°$ chirp phase is at the minimum energy spread at the arc entrance. (a) $-8°$, (b) $+0°$, (c) $+8°$, and (d) $+25°$ (the on-crest acceleration). The positive value of the $z$-axis is the traveling direction.

Figure 6

Left: beam energy spread estimated from the rms horizontal beam size at cam14 and gpt-tracking simulation results. Center: experimental and gpt simulation results of the rms horizontal beam size. Right: shift of accelerating gradient $E_{\mathrm{acc}}$ of the cavity ML2 to maintain the beam energy of 17.7 MeV at off-crest acceleration.

Figure 7

gpt-tracking results of the longitudinal distribution at cam25 at the chirp phase of $+8°$. $R_{56}$ of the arc section are (a) $-0.34\mathrm{m}$, (b) $-0.27\mathrm{m}$ at the minimum bunch length at THz-CTR, (c) $-0.2\mathrm{m}$, (d) $-0.13\mathrm{m}$, respectively. The smaller $z$ means the bunch head contrary to Fig. 5 at the north straight section (cam13).

Figure 8

Energy spread ${\sigma }_{\mathrm{\Delta }E/E_0}$ and bunch length ${\sigma }_z$ at cam26 during $R_{56}$ scan. Left: gpt-simulation results. Center: measurement results of rms beam size. Right: beam center position.

Figure 9

Transverse emittances ${ϵ}_x$, ${ϵ}_y$ at cam18 during $R_{56}$ scan. Left: gpt-simulation results. Right: measurement results.

Figure 10

Comparison between experimental and simulation results of the main beam parameters at case 1 before/after bunch compression at the several chirp phase of ML2. Transverse emittances before bunch compression are measured at only $+8°$.

Figure 11

gpt-simulation results of the bunch length ${\sigma }_z$ and energy spread ${\sigma }_{\mathrm{\Delta }E/E_0}$ at the several chirp phase of Fig. 10. The direction of $z$ is defined as the same direction at the exit of the main linac; therefore, ${\sigma }_z$ in the arc section (gray zone) does not correspond to the bunch length. Locations of THz-CTR measurement and cam26 are at $s=35.7\mathrm{m}$ and $s=67.8\mathrm{m}$, respectively.

Figure 12

Longitudinal phase space distribution and histogram at the chirp phases of $+8°$ and $-8°$. The locations in Fig. 1 are (a) exit of the main linac, (b) cam13, (c) THz-CTR, and (d) cam26, respectively. The definition of the $z$ direction is reversed after passing through the 180-degree-arc section.