Fermi Machine Description
LINAC: Acceleration, Compression and Transport to the Undulators
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At the exit of the photoinjector, the ~ 100 MeV electrons enter the L1 linac (four S-band, i.e. 3 GHz, accelerating sections) where they are accelerated to ~ 250 MeV. Acceleration off-crest creates the correlated energy spread along the bunch needed to compress it in the first compressor, BC1. An X-band rf structure tuned at the 4th harmonic of the main (3 GHz) linac frequency is placed half-way between the four sections of L1. The function of the structure is to provide the non-linear quadratic and, when operated off-crest, cubic corrections of the correlated momentum distribution along the bunch in presence of the photoinjector and the magnetic compressors non-linearities and of longitudinal wakefields. The L2 and L3 linac structures, located between the first and second bunch compressors, accelerate the beam from ~ 250 MeV to ~ 650 MeV. They also provide the residual momentum chirp needed for the second compressor, BC2. After BC2 the beam is accelerated to its final FEL-1 ~1.2 GeV energy in the L4 structure (FEL-2 energy is 1.5 GeV). The rf phases of the linac sections following BC1 are chosen to provide the necessary momentum spread for compression and also to cancel the linear part of the longitudinal wakes. The non-linear correlated momentum spread at the end of the linac is fine-tuned by acting on the amplitude and phase of the x-band structure. |
Magnetic Bunch Compressor 3D model |
The linac focusing system is designed to minimize transverse emittance dilution due to transverse wakefields, momentum dispersion and coherent synchrotron radiation in bends. Two transfer lines, one assigned to FEL-1 and the other to FEL-2, transport the electron beam from the linac end to the undulators. This system, called the “Spreader”, starts with two, three degree bending magnets that deflect the beam away from the linac. In the line that leads to the FEL-2 undulator, two more, three degree bend dipoles of opposite polarity bring the beam back parallel to the linac at a distance from it of 1 m.
When operating the FEL-1 line, one of the afore-mentioned dipoles is switched off and the beam proceeds to a second pair of dipoles that again bend the beam parallel to the linac and displaced from it by 2 m. The two undulator lines are thus parallel and separated by 1 m. The electron optics is designed to cancel any emittance blow up due to the emission of coherent synchrotron radiation in the bends by a suitable choice of the (small) bending angles and of the phase advances between dipoles. The lattice of the spreader is flexible, and allows to switch from the configuration for photon delivery to a configuration less suitable for operation but optimized for electron beam diagnostics purposes.
