Drifts and equilibrium of relativistic electron current rings in toroidal geometry

A theoretical and experimental study of the equilibrium of an electrostatically neutralized, relativistic electron beam (30 kA, gamma =1.75) in a metal wall torus (R0=75 cm, r0=7.5 cm) with an applied toroidal field (Btheta =100-1000 G) is presented. Since complete electrostatic and partial magnetic neutralization of the beam is assumed in the analysis, its equilibrium position is a stable orbit shifted outside the minor axis. First order orbit theory is used to calculate the guiding center drifts which develop when the beam is not launched on this equilibrium orbit. This motion is a result of the gradient and curvature of the magnetic field as well as a wall force. In addition, computer solutions for more exact trajectories are presented. Experimental results obtained from a 90 degrees torus sector show displacements which are in good agreement with the calculated drifts.