The main accelerator consisting of two sections - linac 1 and linac 2 - is operated with standing wave rf cavities. The cavities are superconducting Nb structures with operation temperatures of 1.8 K (superfluid helium). These structures have been developed for the future TESLA accelerator at DESY and will be delivered from there for the ELBE project. Accelerating field gradients of up to 15 MV/m have been obtained routinely with these cavities. The cryostats and mechanical tuning systems of the TESLA project cannot be used at ELBE as both objectives differ considerably. These systems have, therefore, been developed and built exclusively for the ELBE project in close collaboration with the Stanford University. Two nine-cell cavities are housed in each cryostat, so that 20 MeV output energy will be reached by each cavity-cryostat module without problems.
Schematic diagram of the cryostat with Nb cavities
The clean-room technology required up to class 10 for vacuum components and assembly work at the superconducting structures are part of the ELBE project.
Following requirements are easy to meet with the accelerating structures:
- Fast and simple change of the electron energy, as the necessary rf power is relatively small (extremly low power losses in (/high quality factors Q of) the superconducting accelerating structures) and the rf amplifiers (klystrons) are operated in a linear regime.
- Easy regulation (feedback) via the rf (and, thus, the electron beam energy) in order to stabilize the FEL wave length.
- cw mode as well as macro pulses of any length can be realized.
- The iris diameter of the accelerating structures of about 70 mm enables relatively large tolerances in the alignment.
- Little/negligible influence of the transversal resp. longitudinal wake fields.
- Large bunch charges resp. peak currents can be obtained.