Accumulation and Clearing of Ions in Circular Electron Accelerators

Abstract

Modern sychrotron light sources have to provide stable storage for electron beams with high charge densities to make brilliant synchrotron radiation available to users. Since requirements on the brilliance of the radiation are constantly rising, amongst others the stored beam current in these accelerators has to be increased further. The electron beam, however, interacts with residual gas within the vacuum chamber of the accelerator and thereby continuously produces positively charged ions. These are attracted by the electron beam and accumulate in its beam potential. Besides changing the focusing strength for the electron beam, the ions' space charge also cause beam instabilities. Both effects significantly reduce the lifetime of the beam and limit the storable beam current. <br /> Accumulated ions are therefore an obstacle to the provision of highly brilliant synchrotron radiation in future accelerators, which are even more susceptible to these effects. Consequently, efforts are being made to reduce accumulated ions to a minimum in electron accelerators. Here, for example, gaps in the beam's filling pattern are applied to remove ions from the beam. Since the use of filling gaps in recent accelerator developments is not feasible or desirable anymore, the use of local clearing electrodes, which systematically draw ions away from the beam, gains in importance. The correct positioning of these electrodes is essential for reducing the ion density to even lower levels than previously achieved. It is therefore indispensable to understand the ion dynamics and the longitudinal transport mechanisms towards clearing electrodes in all areas of the accelerator, especially within the fields of guiding magnets and rf cavities. <br /> The thesis presented covers aspects of ion production and accumulation, their effects on the beam as well as their mitigation. These are studied by means of numerical simulations, modelling and deduced measurements at the ELSA storage ring, in which ion induced effects are observable with a large diagnostic variety. <br /> The motion of ions in the accelerator is determined by their space charge interaction with each other and with the electron beam. A considerable part of ions are produced within dipole magnets via photo ionization of residual gas by synchrotron radiation, emitted from the electron beam. Here, their motion becomes more complex as additional cross-field drifts emerge. To identify clearing electrode positions for an effective removal of ions in these magnets, the ions have to be tracked by sophisticated numerical simulations. To provide reliable results, local ion production maps are required for these simulations, which are now provided by a tool developed within the course of this work. <br /> The transversal ion production map is asymmetrical in the horizontal plane, as ions are also produced far outside the beam center by photo ionization. As the ions' spatial distribution determines their effect on the beam, this asymmetry also reflects in the ion induced change in focusing strength. <br /> Additionally, the longitudinal ion transport through multi-cell rf cavities is studied in consideration of the cavities' electromagnetic rf fields. Here, emerging ponderomotive forces drive ions into field-free regions, trap them and thus prevent an transport towards clearing electrodes. <br /> Furthermore, this work proposes measures to further reduce the ion density within the studied regions and provides an overview of ion phenomena and mitigation strategies at the ELSA storage ring.