Transport of high-current electron beams in a racetrack betatron

Abstract

This paper reports theoretical and experimental work on the transport of space-charge-dominated relativistic electron beams in a racetrack betatron with cusp focusing. High-efficiency propagation of 550-keV, 280-A beams through a complex optical system consisting of two 180° arcs with 36 focusing cells and a straight section was observed. More than 90% of the injected beam was captured and transported through the 7-m system. The experiments resolved several recent problems of beam injection and trapping in the machine and showed that electron beam transport at the space-charge limit is possible in a curved cusp array despite strong axial variations of focusing forces. We encountered no problems with the transitions between curved and straight focusing sections. The experiments demonstrated good beam containment in a cusp array with mixed focusing cell geometries as long as the axially averaged focusing force was the same for all cells. The ability to use long cells is a critical feasibility issue for other planned injection experiments into a closed racetrack geometry.