Comparison of Different RF-Structures for Post-Acceleration of Heavy Ions

Abstract

RF structures are well suited for upgrading of static machines. The following paper discusses normal-and superconducting resonators like spiral, splitring, helical loaded cavities as well as the interdigital structure. Their properties and advantages are treated, and relevant aspects such as flexibility, field strength, power consumption, and costs will be considered. General AspectsThe large number of Van de Graaff accelerators working in industry and university laboratories is operating in a wide range of research and applied science field. Upgrading of such machines can be done to a certain amount by improving ion sources, accelerator tubes, vacuum and charging systems. But significant upgrading can only be done by use of some kind of post-acceleration. For this purpose RF cavities are well suited. RF accelerators have a fixed or a variable velocity profile. In the first case one only needs one or two tanks and transmitters bringing down the investment costs per voltage gain. Fixed velocity profile structures (common field structures) like the Widerbe-or the Alvareztype are well known, they will not be treated here. Another structure in this group is the interdigital-H-structure, which has a really high efficiency. But all these structures suffer from lack of flexibility, whereas flexible short structures like normal-and superconducting helices, spirals, and split rings can easily give high voltage gains for a wide range of particles and energies typically accelerated in static machines.By proper bunching the energy resolution of the static machines, which is in general very high, can nearly be preserved. Since the linac can be built in a modular design, it can be extended stepwise according to financial possibilities and requirements in ion energies. The high flexibility has to be paid for by a large number of sections necessary (% 3 per 1 MV voltage gain in CW); this number can be lowered -at the expense of flexibilityby using 3 or 4 gap cavities as will be shown later (e.g. 3 electromagnetic coupled spirals in one tank).Unfortunately beam currents delivered by heavy ion sources are very small, thus leading to a relatively low efficiency of normal conducting heavy ion linacs (beam power/power consumption XV 1/1000). Therefore superconduc-'ting linacs are highly desirable for heavy ion acceleration. This sophisticated technique is studied at several labs, and two proposals (Argonne Nat. Lab., Stony Brook Univ.) have been funded until now. However, the application of RF superconductivity leads to complicated and pretentious systems; consequently for small linacs or linacs, which are not operating during a large fraction of the year, room temperature cavities are still preferable.Linac Structures Fig. 1 shows the schemes of post-accelerator structures, which will be considered in this paper. HELIX SPLIT RING E o QC INTERDIGITAL H STRUCTURE Fig. 1 RF post-accelerator structures The IH-structurel'2 consists of a cylindrical cavity, in which radial stems with drift tubes are mounted alternatively on opposite sides...