High current betatron experiments and theory

A conventional Betatron has been modified by introducing a toroidal magnetic field to control space charge. Electrons are injected contiuously from a thermionic emitter while both toroidal and Betatron fields are rising. Electrons have been contained and typically accelerated for about 3000 turns to about 600 keV. The beam current is 150 A. The usual theory of Betatrons based on the paraxial approximation is not applicable. A new theory based on adiabatic particle dynamics has been developed which is consistent with the experimental results to date. Small changes in the Betatron field suggested by this theory have raised the final energy to 1 Mev. In a second Betatron experiment the current is increased by elongation. It is a small Astron with Larmor radius 6 cm and mirror spacing 80 cm. Rapid data acquisition is facilitated by a 1 Hz repetition rate. The purpose of the experiment is basic studies of injection, trapping and extraction. The injection is nearly tangential compared to inductive charging and it is expected that this will result in a lower emittance beam. Multi-turn injection has been demonstrated with electrostatic and magnetic inflectors. The trapped charge limit is about 10 nC which is contained for about 10 µs with no toroidal field. When a toroidal field is added and the electrons are accelerated to 1 Mev, a 10 nC beam is contained for 200 µsec.