Development of Rebunching Cavities at Iap*

A focus of work at IAP has been the development and optimization of spiral loaded cavities since the 1970s [1]. These cavities feature a high efficiency, a compact design and a big variety of possible fields of application. They find use both as bunchers and post accelerators to vary the final energy of the beam. In comparison to other available designs, the advantage of these structures lies in their small size. Furthermore they can easily be tuned to the required resonance frequency by varying the length of the spiral. Due to the small size of the cavities the required budget can also be kept low. Here, two slightly different types of spiral loaded cavities, which were built for the REX-ISOLDE project at CERN and the intensity upgrade program at GSI are being discussed. 1 COMMON FEATURES 1.1 General Remarks The variation of the final energy of accelerators cannot be optained with long linear accelerators like alvarez, wideroe or IH structures alone. Therefore, short additional resonators are very often used. They allow changing the energy of the particles as well as the velocity distribution of the beam. Figure 1: Schematic Drawing of spiral loaded cavity In a cylindrical external tank one or more spiral arms are placed. At their open end each spiral carries a drift tube running along the symmetry axis of the cavity and thus forming several accelerating gaps. * Work supported by the BMBF Different designs of the spiral arms are possible, depending on the required resonance frequency. With Splitring structures resonance frequencies of up to 300 MHz can be achieved, whereas archimedic type spirals find their application at resonance frequencies as low as 27 MHz [2]. 1.2 Characteristic Parameters The transittime factor T indicates the flexibility of an accelerator as the quotient of the effectively experienced voltage and the total gap voltage. The Rp-value is defined as the effective cavity voltage squared over the power P put into the cavity. P U R U U T 2 . Cav , eff p tot Gap , eff = = With n as the number of gaps, the Rp-value is proportional to n in case of the single-spiral structure and proportional to n for a splitring structure. The transittime factor is the product of a velocity dependent and a geometry based term [1]