Tuning Section 2 of the Leda Ccdtl*

As part of the Low-Energy Demonstration Accelerator (LEDA) portion of the Advanced Accelerator Applications (AAA) project we have fabricated and will perform high-power RF tests on Section 2 of the 700MHz Coupled-Cavity Drift-Tube Linac (CCDTL). This CCDTL section contains six, two-gap accelerating cells. This portion of the CCDTL was designed to accelerate the 100-mA, LEDA RFQ proton beam to 7.3 MeV. This paper reports on the process and results of tuning Section 2 leading up to high-power RF testing. 1 DESCRIPTION OF SECTION 2 Section 2 of the LEDA CCDTL is comprised of six two-gap accelerating (AC) (cell no. 7-12) and five transverse coupling (TC) cavities. Each end-AC cell also has half of a longitudinal coupling (LC) cell attached. The TC and LC cell designations refer to the orientation of their axis with respect to the Section-2 beam axis. Bridge coupling (BC) cavities that adjoin LC half-cells are then used to combine adjacent sections together. A cut-away view of Section 2 is shown in Figure 1. The RF properties of the CCDTL structure are explained elsewhere [1]. Each AC cavity is built up from two half cells joined to a ring that supports the drift tube and also provides a means to tune the cavity to the desired frequency. Prior to the initial tuning process, the internal cells were combined into subassemblies built from adjacent-AC half-cells brazed to the lower half of a TC cell. A cutaway view of this subassembly and a drift-tube ring assembly is shown in Figure 2. The exterior subassemblies consisted of an AC and LC half-cell brazed together. For the high-power tests, a BC cavity with a single LC half-cell was built. This BC cavity contains the waveguide coupling iris. The Section 2 cavity dimensions were determined in an iterative fashion using SUPERFISH [2] to calculate the cavity frequency and field distribution and analytical expressions for estimating the intercell coupling strength [3] and frequency effects [4] of the coupling slots. Based upon the field tilt requirements originating from beam dynamics simulations, the relative cavity locations and slot dimensions were iterated on until a solution was found. Except for the AC cell tuning ring inner diameter and the TC, LC and BC nose-to-nose gaps, all cavities were fabricated to their expected final design dimensions. Also, prior to any tuning, all the coupling slots were cut to their calculated final design dimensions. The structure was designed to operate at 700 MHz at 90°F.