Novel bunch compressor chicane: The five-bend chicane

Four dipole magnetic chicanes are typically used in high-performance accelerators to compress electron bunches to their final peak currents but subject the beam to emittance dilution due to the coherent synchrotron radiation (CSR) effect. In this paper, we present a new magnetic bunch compressor geometry designed to locally suppress the CSR-induced emittance degradation called the five-bend chicane. We will include a simulation study comparing a five-bend chicane to the nominal four-bend chicane of the LCLS-II-HE’s second bunch compressor (BC2) beamline. The study will consist of emittance and energy spread growth due to CSR using a one-dimensional, elegant, and two-dimensional, csrtrack, CSR algorithm in free space, and the free-electron laser performance using genesis.

Figure 1

Layout of various generic four-bend (a–d) and six-bend (e) bunch compressor chicane (not to scale).

Figure 2

General diagram of the five-bend chicane (not to scale).

Figure 3

Various optical functions in the five-bend chicane (and 10 m after): (a) The Twiss $\beta$ function in the $x$ (red) and $y$ (blue) planes. (b) The Twiss $\alpha$ function in the $x$ (red) and $y$ (blue) planes. (c) The dispersion (red), $\eta$, and its derivative (blue), ${\eta }^{\prime }$, in the $x$ plane only. (d) The $R_{56}$ and curly $\mathcal{H}$-function along the chicane. The gray areas in all plots represent bending magnets.

Figure 4

Two-dimensional parameter scan of the final emittance for a 1.8-GeV Gaussian bunch with an initial emittance of $0.35\mu \mathrm{m}$ being compressed from 200 to $30\mu \mathrm{m}$ in a five-bend chicane with $R_{56}=-16\mathrm{mm}$. The chicane’s ${\theta }_3$ and ${\theta }_5$ angles are scanned while keeping the $R_{56}$ constant and the dispersion closed at the exit of the chicane.

Figure 5

Jitter of the linear energy chirp, $h$, (left) and the $R_{56}$ (right) versus the percent transverse emittance growth for a 1.8-GeV Gaussian bunch with an initial emittance of $0.35\mu \mathrm{m}$ being compressed from 200 to $30\mu \mathrm{m}$ in a five-bend chicane with $R_{56}=-16\mathrm{mm}$. The jitter for each parameter ranges from 1% to 110% of the nominal values.

Figure 6

The genesis simulation results (averaged over 20 shots) for the LCLS-II-HE for three different photon energies (3.25, 5, and 10 keV) using beams compressed by a four-bend (blue) and five-bend (red) chicane in BC2. (a) The pulse energy along the undulator axis. (b) The final power output along the bunch. (c) The spectrum of FEL radiation of the four-bend chicane. (d) The spectrum of FEL radiation of the five-bend chicane.