Toroidal magnetic lens systems for intense ion beam focusing

Numerical calculations were performed to clarify the focusing property of annular cylindrical high intensity proton beams in toroidal magnetic lens systems. The ion beams were assumed to be emitted uniformly from the annular anode at the diode voltage in a toroidal-magnetically insulated diode and to be force-free except for the deflecting forces due to the magnetic fields in the lens system. A single toroidal field coil could be used as a diverging(D) lens or a focusing(F) lens corresponding to the two magnetic field directions. The beam focusing characteristics were compared for three kinds of lens system: the single lens, double lens and triple lens, changing the beam particle energies and lens parameters such as the thickness, major radius, magnetic fields strength of each coil, and its field direction and the distances between coils. Variations in focal length and focal spot size caused from the change of beam particle energy and spread of proton velocity direction at the anode were found to be minimized for the DFD triple lens system. From our experiments on toroidal-magnetically insulated ion diodes, we observed the beam orbit which was consistent with the path deflected only by the toroidal magnetic field. Concerning the mechanism of magnetic field deflection for plasma-like ion beams, shorting effects of polarized electric field induced in the magnetic field are now under examination.