Design and construction of an injector for an electron/positron Linac optimized for positron yield and minimal particle loss

The Linac II is the first part of the accelerator chain supplying PETRA III. Since the start of PETRA III operation, highest reliability is demanded and several updates are required. Part of these is the new injection system. Beam loss at high energies and the associated activation have to be avoided. At energies above 80 MeV particle loss of 20% occurred. Additionally, an alternative to the old gun, operating in an oil bath and for which cathode preparation is not available, is required. The new system will be commissioned while the old bombarder gun injector is kept for redundancy. In order to obtain the space for joining the beam lines of both electron sources, one accelerator section must be removed. Electron pulses of 6 A beam current and 2 to 30 ns length are provided by the new injection system. The gun uses a thermionic cathode, 100 kV voltage for acceleration and is built as a triode. Longitudinal focusing is performed by a prebuncher and a hybrid buncher structure, both operating at 3 GHz. The buncher is a traveling wave structure to which a short cell has been added, operated in π mode with a standing wave. That way, better electron capture is achieved. A magnetic chicane serves for energy filtering. The design of the injection system, as well as the old injector, have been optimized in simulations and transmission in the linac has been compared. Possible reasons for beam loss are beam loading and misaligned components. For the bombarder gun particle tracking, a loss of 1% at high energies was observed due to beam loading. The additional beam optics and steering options in the beam line allow for compensation of the misalignment of preceding and succeeding components. The complete new injection system has been operated in a test stand and has undergone extensive tests. After successive enhancement of technically critical components, reliable operation was possible. Investigations of the electron capture and bunching procedure have been carried out by energy measurements in the magnetic chicane. For beam optics measurements, quadrupole scans for different rf settings were executed. Beam loading compensation showed promising results. Bunched beam of up to 3.5 A was transmitted through the buncher structure and 2.6 A through the energy filter to the terminating Faraday cup. The tests revealed an insufficiently focused beam with a beam size of up to 13 mm directly behind the buncher structure. This necessitates a better cooled buncher's solenoid coil in order to achieve a higher magnetic field. For those reasons, the new injection system is going to replace the second section of the main linac instead of the first. In this manner, the second buncher structure is not needed for the bombarder gun injector and thus its performance is not risked. Since March 2014 the Linac II has been shut down and the installation of the new injection system and commissioning as electron source for the injector complex for PETRA III is ongoing.