Beam dynamics studies of the HIE-LINAC at CERN

We present a beam dynamics study of the superconducting (SC) HIE-LINAC proposed to replace the existing normal conducting REX-ISOLDE accelerating infrastructure at CERN. The Linear Accelerator Numerical Analysis (LANA) code was used to run first-order simulations of the HIE-LINAC in order to study the beam quality during acceleration. A resonance in the transverse emittance growth at ejection from the HIE-LINAC was discovered and understood as a parametric coupling between the longitudinal and transverse dynamics. The dangerous effect of this resonance can be avoided for all mass-to-charge states in the range 2.5 ≤ A/q ≤ 4.5, if the linac is operated with a transverse phase advance higher than 70 degrees. The transverse emittance growth is minimised along the HIE-LINAC if operated above a transverse phase advance of 90 degrees per focusing period. Without a dedicated matching region between the two sections of the HIE-LINAC a solution for matching the beam was found by using the solenoids in the low-energy section. In the optimum case and with an A/q = 4.5, the RMS and 99 % transverse emittance growth at ejection can be kept at 2 % and 8 %, respectively. The effect of an increased longitudinal emittance at injection from REX was studied and the HIE-LINAC found to tolerate up to four times the nominal value of 2 π keV/u ns. The design specification for the focusing system was finalised – demanding SC solenoids with an active length of 20 cm and a peak field larger than 8.5 T.