Space-charge dominated beam transport in magnetic quadrupoles with large aperture ratios

摘要

This memo summarizes the results of calculations of the usable aperture ratio for short period, magnetic quadrupoles for both ILSE magnet designs and the more general, heavy-ion fusion (HIF) ''driver'' case. From both analytic decomposition of the magnetic field in a periodic lattice and particle code simulations of beam transport, we find that fringe field nonlinearities and associated emittance growth become quite large when the beam radius a(sub b) exceeds one-quarter or so of the half-lattice period L. For ILSE, there are a number of magnet designs which can transport the specified line charge without any significant difficulties, primarily because the ratio a(sub b)/L is of order 0.1 or less. For larger sized beams such as one might employ in the low energy part of a driver, there are problems with properly matching the beam to the transport lattice in both the macroscopic and microscopic sense as a(sub b)/L exceeds 0.2. For even larger aperture ratios, particle loss can occur, with the threshold in beam radius roughly scaling inversely with (sigma)(sub o), the single particle phase advance per lattice period.