A new method to compensate the longitudinal component of the end-fields in 4-rod RFQ accelerators

摘要

In 4-rod RFQ accelerators, typically there is a non-zero on-axis longitudinal electric field component between the electrodes and the end-plates at both the beam entrance and exit. The longitudinal end-field component is caused by the violation of the quadrupole potential symmetry due to an uneven charging of the electrode overhang ends. This is a direct consequence of the design inherent geometric asymmetry of the conventional 4-rod structure. Because the unwanted end-fields will change the nominal beam properties e.g. energy, emittance and transmission, they can cause problems for the beam matching, especially to the downstream accelerating structures. In this paper, a novel method is being proposed to even out the potentials on all four electrode ends to ±|U_(vane)/2|, like in the case of 4-vane RFQs. Benefitting from this new method, a full compensation of the longitudinal component of the end-fields in the studied 4-rod RFQ model has been achieved.