Design of the Beam Transport System for the Radiation Physics Cave at ELBE
The beamline, which transports the accelerated electrons to the radiation physics target chamber has to be achromatic in order to conserve the transverse emittance of the electron beam. Additionally, it has to provide a versatile focussing of the beam onto the target allowing to achieve either a minimal beam spot size or a minimal beam divergence.
Fig. 1: The placement of electron optical elements on the radiation physics beamline
The general outline of the design is shown in figure 1. Its main features have to meet given spacial limitations. The center of the beamline from the switching magnet to the first dipole lies inside a concrete wall and is not accessible for optical elements, preventing a symmetrical design. In order to provide enough space for experimental setups the beamline segments between the dipoles and the target chamber are kept very short, precluding the insertion of quadrupoles in these positions. Thus, all necessary focussing has to be provided by the first dipole and the preceding quadrupole triplett.
Fig. 2: The transverse phase space at the target position for an 18 MeV electron beam with an initial emittance of 0.8 p mm mrad
a) for minimal divergence of the electron beam
b) for minimal beam spot size and a maximum allowed divergence of 1 mrad
RMS and 90% phase space ellipses are shown.
As shown in figure 2, the requirements of a narrow focussing and a low divergence, conserving the low initial emittance of the beam, can be met with the proposed design.