Conceptual Design Reports

CDR chapter 6 - Accelerator

The FERMI harmonic cascade FEL operates within a range of wavelengths from 100 to 10 nm, covered by two distinct undulator chains at the fixed energy of ~1.2 GeV. Two electron beam scenarios have been developed: the medium length bunch (MLB) mode with a bunch length of ~ 700 fs and the long bunch (LB) mode with a bunch length of ~1.4 ps. The electron peak current is 800 or 500 A, respectively. The accelerator was designed with sufficient flexibility to accommodate such variations in bunch parameters. Other important electron beam parameters include the normalized slice electron beam emittance and the slice energy spread, which are about 1.5 μm rad and 150 keV, respectively. A challenging aspect was the demand to produce an electron beam with as uniform as possible peak current and energy distributions along the bunch. For this reason, a new parameter, the “flatness”, defines the value of the quadratic component of energy variation along the bunch for which the increase in bandwidth of the x-ray signal due to this variation becomes equal to the Fourier transform limited bandwidth defined by the bunch length. Tracking results predict flatness of 0.8 MeV/ps2 for the MLB mode and of 0.2 MeV/ps2 for the LB mode. Since the RF photocathode gun produces 0.8 nC and 1 nC for the two options distributed over bunch length of 9 ps and 11 ps respectively, the bunch has to be compressed by a total factor of about 9 before it enters the undulator. The acceleration and compression is done in the main S-band linac. The two bunch compressors (BCs) consist of symmetric magnetic chicanes, each 8.0 m long. They include trim quadrupoles for a fine tuning of the dispersion bump. The locations and compression factor of the two chicanes were fixed in order to minimize the 6-dimensional emittance dilution of the electron bunch in presence of space charge forces and wake fields. The electron energy at BC1 is ~230 MeV in order to avoid space charge effects, while compressing the bunch early enough in the linac to reduce the effects of transverse wake fields. The energy of the second compressor is about 580 MeV, which balances the conflicting requirements of minimizing the transverse and longitudinal emittance dilution by coherent synchrotron radiation (CSR) and that of canceling the final correlated energy spread by means of the downstream longitudinal wake field. By using a weak chicane with a bending angle smaller than 0.07 rad per dipole and a large initial correlated energy spread within the range of 1.0% - 2.5%, the CSR effects can be reduced, but the chromatic aberrations make the tolerances on the magnets field quality tighter. ... Retrieve pdf