RFQ [radio-frequency quadrupole] acceleration section and its optimization

The acceleration section is a crucial component of any radio- frequency quadrupole (RFQ). It is common a practice to design this section with a constant modulation factor equal to its value at the end of the gentle buncher. A new method of design is proposed in this paper. The algorithm is based on the fact that the transverse space-charge current limit (TCL) is approximately proportional to the instantaneous velocity of the accelerated particle and the longitudinal space-charge current limit (LCL) is nearly independent of the velocity in the acceleration section. The modulation factor is increased such that the TCL is slightly larger than the double of the design current. Simulation using this method shows that transmission efficiency and emittances are the same as the conventional design. The advantage gained is a 50-75% increase in accelerating rate. The optimization of the length of this section is also discussed. 3 refs., 4 figs., 1 tab