Compact compressive arc and beam switchyard for energy recovery linac-driven ultraviolet free electron lasers

High gain free electron lasers (FELs) driven by high repetition rate recirculating accelerators have received considerable attention in the scientific and industrial communities in recent years. Cost-performance optimization of such facilities encourages limiting machine size and complexity, and a compact machine can be realized by combining bending and bunch length compression during the last stage of recirculation, just before lasing. The impact of coherent synchrotron radiation (CSR) on electron beam quality during compression can, however, limit FEL output power. When methods to counteract CSR are implemented, appropriate beam diagnostics become critical to ensure that the target beam parameters are met before lasing, as well as to guarantee reliable, predictable performance and rapid machine setup and recovery. This article describes a beam line for bunch compression and recirculation, and beam switchyard accessing a diagnostic line for EUV lasing at 1 GeV beam energy. The footprint is modest, with 12 m compressive arc diameter and $\sim 20\mathrm{m}$ diagnostic line length. The design limits beam quality degradation due to CSR both in the compressor and in the switchyard. Advantages and drawbacks of two switchyard lines providing, respectively, off-line and on-line measurements are discussed. The entire design is scalable to different beam energies and charges.

Figure 1

Conceptual scheme of an ERL-FEL; beam moves clockwise. The scheme is not to scale, and a portion of the straight lines is omitted in order to make the arc lattice more evident.

Figure 2

Plan view of the FODO-arc lattice (left) and optics functions along it (from top to bottom, betatron functions, dispersion functions, first and second order transport matrix coefficients, magnetic elements).

Figure 3

Schematic of CSR kick model. Left: particle’s trajectory is distorted by the local change of longitudinal momentum in a dispersive region, due to CSR emission. Right: double-bend achromatic cell used for comparison of Elegant result and analytical prediction of CSR-induced emittance growth, see Table 2.

Figure 4

Left: particle transverse displacement in the horizontal phase space due to CSR energy kicks in the FODO-compressive arc dipoles of Fig. 2. The numbers correspond to CSR kicks at the eighteen consecutive dipoles. Right: normalized projected horizontal emittance growth as a function of Twiss parameters at the end of the beam line, calculated with the CSR kick model. Compressive arc and beam parameters used for this study are listed in Table 1.

Figure 5

Results from Elegant simulation for current profile at the exit of the CA (left), and projected normalized betatron emittances (right) along the CA. Slice emittances (not shown) are preserved at the injection level.

Figure 6

Top view of the off-line diagnostic line as a branch of the CA. The beam moves from right to left.

Figure 7

Linear optics functions along the CA and the diagnostic branch line; the beam moves from left to right. The position of elements along the lattice and optics constraints for optimum diagnostic resolution is shown. The transport matrix for the horizontal and vertical betatron motion of the line portion, labeled “Extraction” in the figure, is the $4\times 4$ identity matrix indicated in Fig. 6.

Figure 8

Top: lateral view of the off-axis line, at the height of 1 m from the production line. The legend is as follows: ${\mathrm{B}}_{\mathrm{ARC}}=\mathrm{last}$ bending magnet of the compressive arc (dc dipole); $\mathrm{B}=\text{bending}$ magnet (dc dipole); $\mathrm{SP}=\text{spectrometer}$ (dc dipole); $\mathrm{LS}=\text{Lambertson}$ septum; $\mathrm{K}=\mathrm{fast}$ kicker; $\mathrm{Q}=\text{quadrupole}$ magnet; $\mathrm{SCR}=\text{screen}$ system; $\mathrm{BPM}=\mathrm{beam}$ position monitor; $\mathrm{TCAV}\text{-V}=\mathrm{rf}$ transverse deflecting cavity, vertical; $\mathrm{BLM}=\text{bunch}$ length monitor; $\mathrm{BAM}=\text{bunch}$ arrival time monitor; $\mathrm{CM}=\text{charge}\text{monitor}$; $\mathrm{D}=\mathrm{dump}$. Bottom: top view of the off-axis line. The beam moves from left (exit of CA) to right. Plots are not to scale.

Figure 9

Linear optics functions along the off-axis diagnostic line. Position of elements along the lattice and optics constraints for optimum diagnostic resolution are shown. The starting point is end of the CA, the beam moves from left to right.

Figure 10

From top to bottom: longitudinal phase space at the end of the CA (reference), at the end of the horizontal CA branch line, and at the exit of the vertical dogleg.