Abstract
The new design for a spatially periodical rf quadrupole focusing linac is proposed. It consists of accelerating gaps formed between conventional cylindrical drift tubes, between drift tubes and rf quadrupoles with nonzero axial potential, and inside these rf quadrupoles, formed in the same way as in a conventional radio-frequency quadrupole (RFQ) linac with modulated electrodes. Such a combination provides both higher energy gain rate than conventional RFQ and stability of transverse motion for ion beams. The structure can be designed using various combinations of quadrupoles and drift tubes. Some options are considered in the paper using the smooth approximation method and computer simulation of beam dynamics. Transverse stability of particles has been studied. The proposed structure can provide suppression of rf defocusing effects on transverse beam dynamics. Some limitations of the spatially periodic rf quadrupole structure are mentioned.
Highlights
At the present time accelerating structures with spatially uniform radio-frequency quadrupole focusing (RFQ) are the best choice for the front end of ion linacs [1,2,3]
Acceleration for higher energies in a RFQ is inefficient due to significantly low acceleration efficiency of the structure with sinusoidal modulation of vanes compared with drift tube structures (DTL)
RFQs are currently used only as an initial part of ion linacs due to the following reasons: (a) acceleration efficiency of the RFQ cell is still lower than drift tube structures; (b) effective shunt impedance of an RFQ becomes comparatively low at higher energies, mainly because the energy gain rate of all structures based on H-mode cavities with constant voltage along the linac decreases as longitudinal velocity increases
Summary
At the present time accelerating structures with spatially uniform radio-frequency (rf) quadrupole focusing (RFQ) are the best choice for the front end of ion linacs [1,2,3]. Acceleration for higher energies in a RFQ is inefficient due to significantly low acceleration efficiency of the structure with sinusoidal modulation of vanes compared with drift tube structures (DTL). Alternative focusing lattices (for example, FOODOO, FFDD, etc.) are applied to provide stability of transverse motion [6,7,8,9,10] These modifications increase the nonlinear effects of the accelerating field on the beam dynamics. The energy range of an rf quadrupole focusing can be expanded in the case of combining accelerating gaps formed by drift tubes and rf quadrupoles Such structures with spatially periodic focusing are known for many years [11] and employed for acceleration of protons up to 30 MeV. This paper presents one more option of spatially periodical rf quadrupole structure with significantly reduced limitations that can be built on the base of the well-known resonant structures
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