CBETA: First Multipass Superconducting Linear Accelerator with Energy Recovery

Energy recovery has been achieved in a multipass linear accelerator, demonstrating a technology for more compact particle accelerators operating at higher currents and reduced energy consumption. Energy delivered to the beam during the first four passes through the accelerating structure was recovered during four subsequent decelerating passes. High-energy efficiency was achieved by the use of superconducting accelerating cavities and permanent magnets. The fixed-field alternating-gradient optical system used for the return loop successfully transported electron bunches of 42, 78, 114, and 150 MeV in a common vacuum chamber. This new kind of accelerator, an eight-pass energy recovery linac, has the potential to accelerate much higher current than existing linear accelerators while maintaining small beam dimensions and consuming much less energy per electron.

Figure 1

The major components of CBETA are the electron gun (GUN), the injector cryomodule (ICM), the main linac cryomodule (MLC), the diagnostic line (DL), the four splitter-combiner lines (SX), the FFA arc consisting of the first arc (FA), first transition (TA), straight section (ZX), second transition (TB), second arc (FB), and the four splitter-combiner lines (RX). The fully decelerated beam is absorbed in the beam stop (BS).

Figure 2

Measured orbits through the FFA and arrival time before and after the MLC. Arrival time is shown in units of rf phase, and the phases of the decelerating passes are shown relative to 180°.

Figure 3

Measured tunes in the FFA arc sections (left) and straight section (right) as a function of beam energy. The lines show the result of a field map-based model calculation.

Figure 4

Left: Image of the beam on the view screen before beam stop. Right: Transmission for each of the seven passes through the FFA arc. Blue bars are a scaled reading of charge from individual BPMs, red circles are an average of that data over each pass. Red lines are included as a guide to the eyes.

Figure 5

Measured (left) and simulated (right) transverse phase spaces after the injector at 5 pC.

Figure 6

Measured vertical orbits of the first three passes through the FFA return loop before (left) and after (right) application of a simultaneous orbit correction algorithm. Orbits are offset for clarity.