Robert Rimmer

A Non-Invasive Magnetic Momentum Monitor Using a TE011 Cavity

The Jefferson Lab Electron-Ion Collider (JLEIC) design relies on cooling of the ion beam with bunched electron beam. The bunched beam cooler complex consists of a high current magnetized electron source, an energy recovery linac, a circulating ring, and a pair of long solenoids where the cooling takes place. A noninvasive real time monitoring system is highly desired to quantify electron beam magnetization. The authors propose to use a passive copper RF cavity in TE011 mode as such a monitor. In this paper, we present the mechanism and scaling law of this device, as well as the design of the prototype cavity which will be tested at Jlab Gun Test-Stand (GTS). INTRODUCTION Non-invasive measurement of the magnetic moment of a charged particle beam has long been on the wish-list of beam physicists. The previous efforts were mainly focused on measuring the beam polarization [1, 2, 3], which is in the order of ħ/2 per electron or proton. Enhanced by the Stern-Gerlach polarimetry, the RF signal in the cavity generated by the beam is still extremely hard to measure. The magnetic moment per particle of the magnetized beam is typically a few orders of magnitude higher. As a demonstration of the source for the JLEIC e-cooler, the magnetized beam generated at JLab GTS [4] can have a magnetic moment M=200 neV-s or 3.0×108 ħ. The JLab GTS beam also has a typical energy of 300 keV and a low γ, as well as a beam current of 5mA. These parameters make the magnetic moment more likely to be detected with an RF cavity. INTERACTION BETWEEN PILLBOX TE011 MODE AND MAGNETIZED BEAM The angular momentum and magnetic momentum of a charged particle is determined by its motion in azimuthal direction, as shown in Fig. 1, left. == (1) In a perfect pillbox RF cavity, the electric field of TE011 mode has only azimuthal component, and will be zero in other directions (radial or longitudinal), as shown in Fig. 1, right. For a pillbox with thickness d and radius a, when ρ/a<0.3, the TE011 mode azimuthal E-field’s amplitude can be approximated (within 1% error) as = sin ⁄ 2 ⁄ (2) TE011 mode will only have energy exchanging interaction with the azimuthal motion of a particle, making it an ideal candidate for magnetic moment measurement. To estimate the excited RF power analytically, we assume that the beam-cavity interaction has negligible perturbation on beam trajectory. By integrating E-field tangential to the particle trajectory, the cavity transverse R/Q can be calculated as