Preliminary Concept for the Project X CW Radio Frequency Quadrupole (RFQ)

PRELIMINARY CONCEPT FOR THE PROJECT X CW RADIO- FREQUENCY QUADRUPOLE (RFQ)* S. P. Virostek, M . D. Hoff, D. L i , and J. W. Staples Lawrence Berkeley National Laboratory, Berkeley, C A , USA Abstract Project X is a proposed multi-MW proton facility at Fermi National Accelerator Laboratory [1]. It is the key element for future accelerator complex development intended to support world-leading High Energy Physics (HEP) programs. The Project X front-end would consist of an H - ion source, a low-energy beam transport ( L E B T ) , a radio-frequency quadrupole (RFQ) accelerator, and a medium-energy beam transport (MEBT). To support current and future H E P experiments at Fermilab, a C W RFQ is required. One of the chosen R F Q designs has a resonant frequency of 325 M H z . A 162.5 M H z option is also being considered but is not presented here. The R F Q provides bunching of the 10 mA H - beam with acceleration from 30 keV to 2.5 M e V and wall power losses of less than 250 kW. Lawrence Berkeley National Laboratory ( L B N L ) is currently developing the early designs for various components in the Project X front-end [2]. The R F Q design concept and the preliminary thermal analyses are presented here. INTRODUCTION The Project X baseline R F Q design is 2.66 m long and will accelerate a 10 mA H - beam to 2.5 M e V , with a 64 kV vane-to-vane voltage (corresponding to a 1.55 Kilpatrick peak field). Most of the RF input power is dissipated on the cavity walls to establish the needed RF field with only about 17% of the total power transferred to the beam. Each of the two 1.33 m long R F Q modules will consist of four solid O F H C copper vanes that are modulated prior to being brazed together. A brazed copper structure has been chosen due to the high power, C W operation. A 304 stainless steel outer shell is to be bolted to the cavity by means of thread inserts in the copper. A series of 32 water-cooled pi-mode rods provides quadrupole mode stabilization, and a set of 48 evenly spaced fixed slug tuners is used for final frequency adjustment and local field perturbation correction. The Project X R F Q design incorporates technology validated by recent RFQ's developed at L B N L , including for the Spallation Neutron Source (SNS) Front End [3] as well as a recent design completed for the Accelerator Driven Neutron Source (ADNS) [4]. The use of proven and reliable fabrication and assembly methods permits construction using readily available machinery incorporating previously proven techniques. The bolt-on, stainless steel outer stiffening plates provide the necessary structural rigidity as well as a means for reliably applying vacuum and RF sealing forces for the tuners, couplers, * This work was supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231. sensing loops and vacuum pumping manifolds. The outer shell also provides for a relatively simple method to interconnect the modules. A preliminary 3-D C A D model of the RFQ conceptual design has been developed and is used here to present a description of the design characteristics. A n overall view of a single R F Q module is shown in Fig. 1. Figure 1: C A D model of a single RFQ module. RFQ DESIGN DETAILS Cavity Body Each of the four vanes in a module are to be machined from a single piece of copper and will include simple cooling channels produced using an established gun boring technique. The R F Q vane tips are to be modulated by means of a fly cutter technique previously developed at L B N L using a commonly available programmable mill. Fiducial surfaces that also act as mating surfaces will be machined directly onto the vanes to provide high precision during both machining and assembly. Two vane geometries will be used (major and minor) with the opposing vanes being identical. Other features such as tuner ports, RF coupling ports, vane cut back cooling passages, cooling taps, vacuum pumping ports, pi-mode rod penetrations, sensing loop ports and tapped holes for the stainless steel backing plates are to be machined prior to finish machining of the cavity surfaces and vane tips. Note that all vacuum seals to the cavity for penetrations are recessed beyond the outer layer of stainless steel and are to be applied directly to the O F H C . The finished vanes are to be brazed together along axially running joints. A zero-thickness brazing process will be used in order to maintain the tight vane tip-to-vane tip tolerance, which is dictated by the high dependence of cavity frequency on vane tip spacing. Wire braze alloy will be loaded into grooves in the joint surfaces such that the alloy spreads throughout the joint during the braze