At the Electron Stretcher Facility ELSA of Bonn University, an upgrade of the maximum stored beam current from 20 mA to 200 mA is planned. The storage ring operates applying a fast energy ramp of 6 GeV/s from 1.2 GeV to 3.5 GeV and an afterwards slow extraction over a few seconds to the hadron physics experiments. The intended upgrade is mainly limited due to the excitation of multibunch instabilities. As a countermeasure, we succesfully commissioned state-of-the-art bunch by bunch feedback systems in the longitudinal and the two transverse dimensions.
To achieve the maximum feedback gain, the beam phase with respect to the reference signal and the synchrotron frequency have to stay constant. A high performance low level RF system has been commissioned which is able to stabilize voltage and phase of the RF station while in addition taking care of resonator resonance tuning using a fast FPGA based fully digital approach.
At highest energies the beam current also is limited by the maximum available RF power of the stretcher rings RF station. A second RF station including a PETRA type 7 cell RF cavity is under construction to increase the maximum accelerating voltage and to compensate beam loading effects up to 3.2 GeV and 200 mA.
A new high current and single bunch injector is needed to decrease the injection time of the stretcher ring in the standard post acceleration mode. A thermionic triode gun for high intensities is followed by a buncher section and the main linac for 20 MeV. The beam is transfered to an energy compressor system to ensure a proper energy spread for injection into the booster. Here, a new high power RF waveguide system for the buncher and energy compressor sections has been developed. A single bunch can be produced for detailed studies of the stretcher rings instabilities.