TRANSVERSE CHARACTERIZATION AND CONTROL OF BEAMS WITH SPACE CHARGE

Abstract

The characterization of the transverse phase space of beams is a fundamental requirement for particle accelerators. As accelerators shift toward higher intensity beam regimes, the transverse dynamics of beams becomes more influenced by interparticle forces known as the space charge forces. Therefore, it is increasingly important to take space charge into account in studying the beam dynamics. In this thesis, two novel approaches are presented for measurement of transverse emittance for beams with space charge, an important quality indicator of transverse phase space. It is also discussed the experimental work on orbit characterization and control done for space charge dominated beams of the University of Maryland Electron Ring (UMER).

The first method developed for measuring the emittance, utilizes a lens-drift-screen setup similar to that of a conventional quadrupole scan emittance measurement. Measurements of radius and divergence that can be obtained from beam produced radiation, e.g. optical transition, are used to calculate the beam cross correlation term and therefore the rms emittance. A linear space charge model is used in the envelope equations; hence the errors in the measurement relate to the non-uniformity of the beam distribution. The emittance obtained with our method shows small deviation from those obtained by WARP simulations for beams with high space charge, in contrast to other techniques.

In addition, a second method is presented for determining emittance that works for beams with intense space charge and, theoretically, does not require an a priori assumption about the beam distribution. In this method, the same lens-drift-screen setup as the previous method is used, except that the beam size and divergence are scanned to find the minimum of product of the measured quantities. Such minimum is shown to be equal to the rms emittance under specific conditions that usually can be satisfied by adjusting the experiment parameters such as the drift length. The result of numerical analysis of the method done for a realistic accelerator confirms the applicability of method for intense beams with nonuniform distribution.

Finally, the experimental work for characterization and control of beam centroid motions in UMER are discussed. Such work is important because at high space charge intensities, the nonlinearities of the lenses impose stricter constraints on the swing of beam centroid in the pipe. On the characterization side, we show new methods for more accurate measurements of the average orbit of particles, including inside the quadrupoles where there is no monitor. Based on this more precise orbit information, the beam orbit is corrected and its result is presented.