Longitudinal emittance in high-current linear ion accelerators

Results of numerical beam-dynamics studies for both the radiofrequency quadrupole (RFQ) and the drift-tube linac (DTL) are presented. The scaling of longitudinal emittance produced during the adiabatic bunching in an RFQ is discussed. The benefits of using ramped DTL accelerating field designs to maintain high longitudinal focusing strength with increasing particle energy are shown. For the RFQ bunching, it is found that: (1) nonlinear RF and space-charge fields are both important, (2) ξ₁ (longitudinal emittance) scales as the product of zero-current separatrix area times a current-dependent factor that decreases with increasing current, and (3) the decrease of ξ₁ with current is correlated with transverse emittance growth. For the DTL, both rapid and slow charge-redistribution emittance growth mechanisms are observed. The rapid growth is consistent with the charge redistribution mechanism studied previously. The slow growth is caused by a gradual weakening of the longitudinal focusing force with increasing beam velocity and can be controlled if the accelerating field can be ramped to compensate