Magnetic focusing of intense ion beams

Magnetic focusing of a uniform intense proton beam produced by a new magnetically insulated diode has been demonstrated using a single dipole coil. The new diode produces an intense (80 A/cm2 at 350 keV), low divergence (half-angle less than 4°), uniform proton beam that can be injected into a field-free vacuum drift region. Prompt space-charge neutralization by low energy electrons occurs almost immediately after the beam emerges from the diode. Experimental observations of the beam focal properties are in reasonable agreement with numerical calculations of the particle orbits and show that the protons follow essentially single particle trajectories. Calculations also show that for ion currents approaching the ion Alfvén current a net azimuthal diamagnetic current in the region of the dipole lens causes the beam defocusing. Finally, calculations and experimental observations indicate that the beam angular divergence may provide the most serious limitation on beam focusing for pellet fusion.