The Stellatron -- A Strong-Focusing, High-Current Betatron

Electrons are accelerated in a betatron by the electric field induced by the changing magnetic flux linking the electron orbit. If the average magnetic field within the orbit is twice the value at the orbit, the radius will remain constant during the acceleration. In a radially decreasing field specified by Bz = B0(ro/r)n a particle will experience restoring forces toward an equilibrium orbit provided only that n lies between zero and unity. Bo is the value of the magnetic field at the equi-librium orbit ro. These are the principles upon which the first successful betatron as well as the highest energy betatron were designed. During the 1950's the FFAG accelerator concept added strong focusing fields to a betatron to achieve a configuration having large energy mismatch bandwidth but low current capability. More recently, efforts have been made to extend the current-carrying capability of the betatron by the addition of a toroidal magnetic field. The resulting "modified betatron" configuration provides high-current capability, but is limited to the same energy mismatch bandwidth as a conventional betatron. Field errors due to the toroidal field and the beam self fields require the modified betatron to have a larger energy mismatch bandwidth than a conventional betatron. By adding both a toroidal magnetic field and a strong focusing field to a betatron, we can achieve high current capability together will the required energy mismatch tolerance. The resulting configuration is a combination of stellarator and betatron (stellatron) fields, as shown in Figure 1.