Abstract
The longitudinal-transverse coupling is an important source for space-charge-induced emittance growth in high intensity proton and ion linacs. Different from the equipartitioning method which tries to avoid the longitudinal-transverse coupling, a new design approach has been developed to minimize emittance growth via the low emittance transfer enabled by holding the ratio of longitudinal emittance to transverse emittance in the range of 0.9--1.4. Using a high intensity radio-frequency quadrupole accelerator as an example, a comparison between the new approach and the equipartitioning method has been made. Furthermore, input beams with non-nominal beam intensities and emittances have been applied to the designed accelerator. The simulation results show that the design obtained by following the new approach has a large tolerance for the off-design situations.
Highlights
Space-charge-induced emittance growth is a big concern for designing high intensity proton and ion linacs, especially the radio-frequency quadrupole (RFQ) accelerators which are typically designed for the acceleration of lowvelocity beams in the range of ∼0.01 to 0.06 times the speed of light in vacuum c [1]
From the longitudinal beam dynamics point of view, the input beam will go through three sequential stages in an RFQ designed using the new four section procedure (NFSP) [16]: (i) The first stage of bunching with maximum separatrix starts from a continuous beam to form an initial bunch with a full 360° phase acceptance
For the J-PARC RFQ III, Pc 1⁄4 400 kW, U 1⁄4 81 kV, and L 1⁄4 3.623 m [20,27]. Based on all these data, the calculated equivalent intervane voltage for the J-PARC epRFQ is 85.7 kV which is ∼14% higher than that adopted for the minimizing emittance growth via low emittance transfer (MEGLET) RFQ
Summary
Space-charge-induced emittance growth is a big concern for designing high intensity proton and ion linacs, especially the radio-frequency quadrupole (RFQ) accelerators which are typically designed for the acceleration of lowvelocity beams in the range of ∼0.01 to 0.06 (nowadays extended to 0.08) times the speed of light in vacuum c [1]. In different positions of the RFQ, one can adjust a, m, U, and the synchronous phase φs to adapt the transverse and longitudinal electricfield components for meeting different demands on focusing, bunching, and acceleration, respectively. The motion of the beam particle in the RFQ satisfies the equation of a simple harmonic oscillator in both transverse and longitudinal planes. The calculation was performed using the Vlasov equation for an initial Kapchinskii-Vladimirskii distribution with arbitrary emittance ratios, tune ratios, and intensity [11] These thresholds had been originally obtained for continuous beams in the two transverse directions, but it was found that they could be applied to investigate the longitudinaltransverse emittance transfer in bunched beams [12]. 1.0 resonance peak disappears on the Hofmann t t chart, but it grows again on the neighboring charts gradually (see Fig. 2)
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