Nonparaxial pulse broadening in a solenoid focusing element

• Main Authors: D. P. Grote, A. Friedman, E. P. Lee
• Format: Article
• Language: English
• Published: American Physical Society 2012-10-01
• Series: Physical Review Special Topics. Accelerators and Beams
• Online Access: http://doi.org/10.1103/PhysRevSTAB.15.104001

In a standard scenario for focusing an ion beam onto a target, for example with ion beam driven inertial fusion energy, the beam is compressed longitudinally by a velocity ramp to enhance the current and then directed through a transverse focusing system to produce a small, bright spot on the target. To reach the highest levels of compression, the space-charge of the beam is neutralized, typically by the presence of a plasma with a density greater than the beam density. The system is arranged so that the peak longitudinal compression is coincident with the minimum transverse spot size. In this scenario, it has been discovered that nonparaxial effects can lead to degradation in the amount of compression. The transverse focusing causes a radially dependent variation in the axial velocity of the ions, leading to a radially dependent time delay that degrades the peak compression. This effect, nonparaxial pulse broadening, can become significant for short pulses and large focusing fields—the time delay can be comparable to the final pulse length. This pulse broadening will be present in both solenoid and quadrupole focusing systems. This paper describes this effect in solenoids, with some examples. It is expected that the size of the effect will be comparable with quadrupole focusing.