Fixed field phase shifters for a multipass recirculating superconducting proton linac

The multi-pass recirculating proton linac can significantly improve the usage efficiency of rf superconducting cavities by accelerating the proton beam through the same cavity multiple times. However, in order to achieve acceleration for different energy proton beams, longitudinal phase synchronous conditions among these beams have to be satisfied. In this paper, we propose a fixed field phase shifter system based on two superconducting bending magnets with an optimized field distribution to meet the synchronous conditions. Such a phase shifter provides both longitudinal phase delay and transverse focusing of multiple energy beams. Self-consistent macroparticle simulations including space-charge effects through the system show that the proton beam emittances can be well preserved after the phase shifter for multiple energy beams.

Figure 1

A schematic plot of the two fixed field bending magnet phase shifter. Here, $y$ is the vertical direction pointing out of the plane.

Figure 2

Vertical magnetic field middle plane distribution after attaining the longitudinal synchronous conditions (Table 1).

Figure 3

Horizontal centroid ($⟨x⟩$) trajectories of four energy passes through the phase shifter using the field distribution from the Table 1.

Figure 4

Proton beam transverse and longitudinal RMS emittance evolution through the phase shifter (Table 1) with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and zero current.

Figure 5

Proton beam transverse and longitudinal RMS size evolution through the phase shifter (Table 1) with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and zero current.

Figure 6

Proton beam transverse and longitudinal RMS emittance evolution through the phase shifter with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and including the space-charge effects of 40 mA current.

Figure 7

Proton beam transverse and longitudinal RMS size evolution through the phase shifter with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and 40 mA current.

Figure 8

Proton beam transverse and longitudinal RMS emittance evolution through the phase shifter with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Gaussian distribution, and 40 mA current.

Figure 9

Proton beam transverse and longitudinal RMS size evolution through the phase shifter with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Gaussian distribution, and 40 mA current.

Figure 10

Vertical magnetic field middle plane distribution (Table 2).

Figure 11

Proton beam transverse and longitudinal RMS emittance evolution through the phase shifter (Table 2) with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and 40 mA current.

Figure 12

Proton beam transverse and longitudinal RMS size evolution through the phase shifter (Table 2) with 150 MeV (top left), 262.5 MeV (top right), 375 MeV (bottom left), 487.5 MeV (bottom right) kinetic energy, initial Waterbag distribution, and 40 mA current.

Figure 13

Vertical magnetic field distribution in the middle plane of the bending magnet (Table 3).

Figure 14

Proton beam transverse and longitudinal RMS emittance evolution through the phase shifter (Table 3) with 251.25 MeV (top left), 363.75 MeV (top right), 476.25 MeV (bottom left), 588.75 MeV (bottom right) kinetic energy, initial Waterbag distribution, and 40 mA current.

Figure 15

Proton beam transverse and longitudinal RMS size evolution through the phase shifter (Table 3) with 251.25 MeV (top left), 363.75 MeV (top right), 476.25 MeV (bottom left), 588.75 MeV (bottom right) kinetic energy, initial Waterbag distribution, and 40 mA current.