Emittance growth in the DARHT Axis-II Downstream Transport

TL;DR

This study uses PIC simulations to analyze emittance growth in the DARHT Axis-II transport system, finding minimal growth due to initial distribution effects and assessing measurement accuracy depending on beam shape.

Contribution

It provides new insights into emittance growth mechanisms and evaluates the accuracy of solenoid focal scan measurements in the system.

Findings

Emittance growth is less than 6% even with mismatched beams.

Initial hard edge distributions soften, reducing emittance growth.

Measurement accuracy depends on the beam shape after quadrupoles.

Abstract

Using a particle-in-cell (PIC) code, we investigated the possibilities for emittance growth through the quadrupole magnets of the system used to transport the high-current electron beam from an induction accelerator to the bremsstrahlung converter target used for flash radiography. We found that even highly mismatched beams exhibited little emittance growth (< 6%), which we attribute to softening of their initial hard edge current distributions. We also used this PIC code to evaluate the accuracy of emittance measurements using a solenoid focal scan following the quadrupole magnets. If the beam is round after the solenoids, the simulations indicate that the measurement is highly accurate, but it is substantially inaccurate for elliptical beams.