Space-charge-neutralized ion induction linacs with cusp insulation

The space charge of ions limits the current that can be accelerated in a conventional ion linear accelerator (linac). Furthermore, the accelerating electric field must be kept low enough to avoid the generation and acceleration of counterstreaming electrons. These limitations have resulted in heavy ion fusion accelerator designs that employ long beam lines and would be expensive to build. Space charge neutralization and magnetic insulation of the acceleration gaps could substantially reduce these two limitations, but at the expense of increasing the complexity of the beam physics. We consider it necessary to keep all structures out of the beam path to maintain beam symmetry and to avoid beam-induced outgassing. Using only field coils outside of the beam results in a cusp field to insulate the accelerating gaps and a solenoidal field between acceleration gaps, which provides a containment force. Electrons can enter the beam region along the magnetic field lines and provide fairly good space charge neutralization. We have performed three-dimensional particle-in-cell simulations, which indicate that electrons remain tied to magnetic field lines, despite the development of electromagnetic fluctuations. Thus, the center of the beam is not neutralized, resulting in a non-linear net focusing force. We have developed a simple model to determine the beam emittance induced by the non-linearity of the focussing force. We discuss the constraints this model puts on the choice of ion and the number of stages required for the accelerator.