lehnertu Annual Report 2000

Design of the Electron-Beam Transport to the ELBE Experimental Caves

U. Lehnert

For the FEL beamlines an S-shaped design with achromatic bends was chosen which performs a compression of the electron bunch on its way to the FEL undulators. Both bends of the S use a dipole-quadrupole-dipole layout. Position and focusing power of the quadrupoles have to be adjusted to achieve zero position and momentum dispersion. For the first bend this leads to an asymmetric quadrupole position, given the asymmetric pole-face angles of the used dipole magnets. For each bend a beam matching is possible which results in equal beam radii at the entrance and exit of the bend. Given the positions of the dipole magnets the phase shift in longitudinal phase space is fixed to R₅₆=93 mm. A preparation of a certain longitudinal phase-space correlation which leads to an upright phase-space ellipse at the undulator is done with a variable-R₅₆ chicane in between the two accelerator cryomodules.
The straight beamline section between the bends is mainly determined by mechanical limitations as the beam has to go through a 2.4 m thick concrete wall in front of which a beam shutter is placed. Considering this, the quadrupole-doublet magnetic lens has to be placed inside the concrete wall immediately behind the shutter in order to limit the beam magnification. Including two single quadrupoles at the ends of the straight section an imaging of the matched beams of the bends onto each other is possible.
The undulator of the mid-infrared FEL consists of two 1 m long segments. Their spacing varies from 245 to 345 mm determined by the resonance condition between the optical and the electron beam. Due to the field-free drift space beween the undulator segments an ideal matching which would yield a constant-radius envelope is impossible. Instead, a symmetric matching condition was choosen which has a beam waist between the undulator segments. The beam diameter at the waist is determined as to produce minimal betatron oscillations and thus a minimal average beam diameter over the undulator. The beamline to the undulator contains a quadrupole triplet, which does not allow an ideal matching, as there are only three degrees of freedom for four matching conditions. However, there exist decent matching compromises for all possible undulator settings, yielding a reasonably small beam diameter in comparison to the optical beam.

Fig. 1: The general layout of the ELBE beamlines

Space-charge effects on the electron beam transport were studied using the Parmela [1] beam transport code. Electron beams which should allow an FEL operation can be supplied down to 10 MeV beam energy. The longitudinal phase shift R₅₆ increases with decreasing energy from its design value R₅₆=93 mm at 20 MeV to R₅₆=160 mm at 10 MeV which has to be compensated for by tuning the chicane. At the same time, the longitudinal emittance and the energy spread of the beam are growing significantly. At 10 MeV the longitudinal emittance already triples but the beam quality still is just sufficient for the FEL operation.
The electron beam transport to the radiation physics cave has to be designed achromatic in order to conserve the low transverse emittance of the beam. This is achieved with a dispersion-crossover in the 45 deg section of the beamline. This crossover is placed asymmetrically in order to be situated outside the concrete wall and to be accessible for beam diagnostics. The necessary focussing is provided by quadrupole doublets before and after the wall that separates the accelerator hall from the radiation physics cave.
At the position of the channeling target a beam waist is created which diameter can be adapted to different experimental requirements. Either large focal spot sizes with very low beam divergence or rather narrow foci can be tuned using the two single quadrupoles situated before the switching magnet and after the first dipole in the radiation physics cave. The actual position of the focus is variable as well, thus allowing to use an alternative channeling target downstream the goniometer chamber, which design is optimized for the production of high-flux photon beams but lacks some of the flexibility of the goniometer.
For the nuclear physics beamline the last dipole magnet of the chicane is used in connection with one additional dipole and a quadrupole in between to achromatically deflect the electron beam by 45 degrees. A quadrupole doublet provides the necessary focusing onto the radiator target. A set of steerers is used to vary the angle of incidence of the electron beam with respect to the collimator axis, thus allowing experiments using partially polarized bremsstrahlung radiation emitted off the forward direction. The thickness of the radiator is limited by electron scattering and the according radiation background from electrons hitting the tubing. Therefore, special attention has to be paid to the design of the beam path from the radiator target to the beam dump, which has to be as short as possible and well shielded.