EXPERIMENTAL STUDY OF BEAM HALO IN INTENSE CHARGED PARTICLE BEAMS

Abstract

Beam halo is a common phenomenon that occurs in most intense particle accelerators, and refers to collections of particles that stray far away from a well-defined central beam core. Often in high-intensity beams, the space charge force induces halo. Even for low intensity accelerators, the beam halo could occur in the injection section before the particles are accelerated to relativistic speed. The most severe effects from beam halo are emittance growth and beam loss. Emittance growth can cause the degradation of beam quality, and beam losses will impose restrictions on the beam current. Although one can use a larger aperture to compensate this, the overall cost will increase exponentially. In this dissertation, we address the halo phenomenon and formation mechanism in intense charged particle beams. Although most of the experiment and simulation study of halo is based on the University of Maryland Electron Ring, it is applicable to a wide range of accelerators in the same intensity regime.

We first discuss a matching procedure and rotation correction for the beam envelope. The gradients of four quadruples in the injection are independently adjust to match or mismatch the beam. The gradients of two skew quadruples in the injection are independently adjusted to correct the beam rotation. We succeed in matching the UMER beams and find out that the envelope mismatch and beam skewness are the major sources for halo formation in UMER. Halo could be drive out even in very early stage such as in 2 or 3 mismatch oscillations with large mismatch or beam rotation.

We simulate the halo formation in UMER lattice till about 10 mismatch oscillations with higher beam intensity in the frame of two envelope mismatch modes. In experiment, we generate envelope mismatch mode with different mismatch level (parameter) by adjusting the four quadrupoles in the injection. The agreement of the envelope between experiments and simulations is satisfactory for mismatch parameter in the range of 0.8-1.2. Emittance and beam width are obtained from tomography and adaptive optical masking and imaging method separately for comparisons with the simulation as well as the maximum emittance growth predicted by a free energy model and maximum particle radius predicted by a particle-core model. The experiments confirm the predictions from both the simulation and the theory with reasonable agreement.

We also further investigate the adaptive masking method for halo imaging, and apply it for halo diagnostics at JLAB FEL facility, and for imaging of the injected beam at the SLAC SPEAR3 storage ring.