Using Tune Shifts to Evaluate Electron Cloud Effects on Beam Dynamics at CesrTA

The proposed future International Linear Collider (ILC) will collide electrons and positrons. Because positrons are antimatter, they are harder to produce than electrons and need to be accumulated and conditioned in a damping ring. The Cornell Electron Storage Ring Test Accelerator (CesrTA) at the Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE) is similar to the damping ring of the proposed ILC and is being used as a model to study beam dynamics [1] [2]. Accelerating charges radiate, and for a circular damping ring, the radiated photons knock electrons off the walls of the beampipe. These photoelectrons can be accelerated by subsequent bunches in the beam and continue to knock off more electrons, forming a cloud, which can affect the beam dynamics [2]. Figure 1 shows a model of the nominal and actual paths of a beam around the beampipe. The motion of the particles in a beam can be described to first order by a damped simple harmonic oscillator. In this model, the particles are displaced from the nominal path and oscillate about it. The tune (Q) is defined as the number of oscillations of a particle about its nominal path, per turn in the ring. One method of characterizing the effect of the electron cloud on beam dynamics is to study the beam itself. The presence of the electron cloud will affect the forces on the beam, causing a shift in the tune. The tune shift (∆Q) is defined as the difference in the tune caused by the electric field of the electron cloud, and measuring it can allow for a better understanding of the electron cloud growth.