A super-periodic radio frequency quadrupole focused structure for the general part of the ion linac

A spatially periodic radio frequency quadrupole focused structure is investigated with the aim of improving the RFQ accelerating efficiency at the second stage of the ion linac. Compared to the conventional RFQ, higher RF voltage is applied to the structure with the enlarged quadrupole aperture. The period of focusing longer than βλ (super-period) is necessary to keep the transverse beam stability because the focusing gradient is reduced due to aperture increase. The RFQ structure with the period length of 2βλ is studied in detail. The analytical theory promises the high values of accelerating rate and beam current; the results of numerical calculations are in good agreement with the theory prediction. That is, beam dynamics simulation in the 37 cell 5.4 m long section behind conventional RFQ gives the average accelerating gradient of around 3.3 MV/m for the medium ion beam with the current of 150 mA and normalized emittance of around 4 πmm mrad. The absence of magnetic lenses simplifies the design and reduces the total cost of the linac.