Development of a Cell-Coupled Drift Tube Linac (CCDTL) for Linac4

The 352 MHz CCDTL will accelerate the Linac4 beam from 50 to 102 MeV. It is the first structure of this kind that will be used in a proton linac. Three short DTL-type tanks, each having two drift tubes, are connected by coupling cavities and form a chain of resonators operating in the stable π/2 mode. The CCDTL section is made of 7 such 5-resonator chains, each fed by a 1.3 MW klystron. Focusing quadrupoles are placed between tanks, easing their alignment with respect to a conventional DTL thus making the structure less sensitive to manufacturing errors. In order to validate the design and to develop the production technology, two prototypes have been constructed and successfully tested. The first prototype, built at CERN, consists of two halfcavities and one coupling cell, whereas the second, larger one, having two full cavities and one coupling cell, was built at VNIITF and BINP in Russia within the frame of an R&D contract funded by the ISTC Organisation. Both prototypes have been tested at CERN slightly beyond their nominal power level, at the design duty cycle of 10%. In this paper we present the results of high-power tests, the results of the technological developments prior to production, and the final design of the CCDTL. INTRODUCTION AND BASIC DESIGN The Cell-Coupled Drift Tube Linac (CCDTL) was originally developed at LANL as a structure providing higher shunt impedance than conventional Drift Tube Linacs (DTL) for intermediate-velocity particles [1]. In the original design the CCDTL was used at twice the basic linac frequency (800 MHz) and when the principle was tested on a CW prototype it appeared that surface power density was too large for stable operation. To avoid these problems, CERN started to develop a CCDTL at the basic linac frequency of 352 MHz and for applications as the Superconducting Proton Linac (SPL), limited at a duty cycle of less than 10% [2]. Different combinations were analysed and tested, to finally adopt for the Linac4 project [3] the CCDTL configuration shown in Fig. 1. This CCDTL is made of 3-gap DTL-like accelerating tanks, connected by off-axis coupling cells bridging the focusing quadrupoles. Whereas the shunt impedance of this CCDTL configuration remains similar to that of a DTL with permanent quadrupoles, its main advantages are the easy access, alignment and cooling of the quadrupoles and the simpler construction and alignment of the tanks, the drift tube alignment tolerances being no longer dominated by the tight requirements of the quadrupoles. Figure 1: Linac4 CCDTL structure with indication of the electric field lines. The RF configuration of Linac4 limits the peak power per resonator to about 1 MW. For this reason, the CCDTL tanks are grouped in modules of 3 tanks connected by two coupling cells (Fig. 2). The basic Linac4 CCDTL resonator is therefore made of 5 coupled cells operating in the π/2 mode. The CCDTL starts at 50 MeV, an energy that allows placing quadrupoles within the 3/2 βλ distance between neighbouring gaps. The geometry of the coupling cell and coupling slot is kept constant for all modules to simplify construction. This is achieved by shifting the end-walls of the tanks. Figure 2: 3D view of a Linac4 CCDTL module with support structure and rectangular RF port. At higher energies the shunt impedance of the CCDTL falls considerably, together with the coupling factor between CCDTL cells, inversely proportional to the stored energy per tank. Both these factors impose an upper energy limit of about 100 MeV for this structure. Proceedings of LINAC08, Victoria, BC, Canada MOP008 Proton and Ion Accelerators and Applications 2D DTLs (Room Temperature)