Multiple channel space lattice focusing and features of its use in applied RF linac

Abstract

Nowadays the use of multiple channel accelerator systems is well known with some hundred channels helps us to increase total beam intensity proportional to the number of channels while the divergence of the total beam is roughly equal to the divergence of single channel. The accelerator structure for multiple beam linac must provide both transversal and longitudinal stability for every small beam taking into account Coulomb interactions of all the micro beams. The most convenient for accelerator structures with 100 and more beams are the systems that use RF focusing such as RFQ, APF and DTL with rectangular profiles. The common disadvantage of all those systems is connected with decreasing of focusing forces of RF field with particle velocity increase. Our analysis shows that the disadvantage may be overcome in structures with rectangular profiles. For this purpose some additional thin (3–5 mm) focusing electrodes called space lattices (SL) must be arranged within accelerator gaps. The distance between these electrodes is chosen roughly equal to the thickness of additional electrodes. The number of the electrodes must be increased with length of accelerator gaps and may be equal n=1,2,…,6 and even more. The arrangement of n thin electrodes in accelerator gaps helps us to reach qualitative change of accelerator structure parameters. Firstly, they make n times amplification of the sign-alternate component of RF focusing field without appreciable influence to phasing action of accelerating field. Secondly, introducing of additional electrodes that divide the gap on n small accelerator gaps provides beams shielding from each other within the region of beam acceleration in RF fields between drift tubes. The analysis shows that if n=4–6, it is possible to reach transversal stability of all particles independently of their input phases in RF field. On the other hand, the analysis shows that adiabatic change of synchronous phase at the input stage of acceleration helps us to increase both longitudinal and transversal capture of particles into acceleration process. The basic feature of multiple channel structure with SL focusing is concerned with the necessity of arrangement of some additional electrodes within accelerator gaps. It might be principally easily realized by means of insulator stems while it is practically impossible because of their low surface electric durability. So there is a necessity to develop special RF voltage dividers to provide appropriate potentials at additional electrodes. Some variants of designs were supposed and tested.