IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, August 1983 THE FERMILAB RFQ PROJECT 0. Neuffer, C.D. Curtis, F.R. Huson, Q.A. Kerns, A. Martinez, C.W. Owen, C.W. Schmidt, E. Treadwell and G.J. Villa Fermi National Accelerator Laboratory* P.O. Box 500 Batavia, IL 60510 Abstract A 200 MHz Radio Frequency Quadrupole (RFQ) Linac is being designed and constructed for the acceleration of H- ions from 30 keV to 750 keV at a current of 30 mA. The RFQ is being developed as a possible re- placement for the Fermilab preaccelerator and as an R and 0 project to investigate RFQ properties for future accelerators. The current Status of the project is described. Introduction A new concept in low energy proton/ion accelera- tors, called the RFQ (Radio Frequency Quadrupole 1' linac, was proposed by Kapchinskii and Teplyakov in 1970 and developed at Los Alamos293, where design pro- cedures and an experimental "proof of principle" were obtained. More recently, other laboratories (INS, LBL, etc.) are contributing to this development, and in this paper we report the progress of an RFQ development pro- ject at Fermilab. The purposes of this project are to obtain local expertise in RFQ design, construction and operation, facilitating the transformation of this concept from an experimental device to a reliable accelerator com- ponent.. Our first RFQ is a relatively modest device: a 200 MHz structure to accelerate H- ions from source energy (30 keV) to 750 keV, which can function as a replacement for the Fermilab Cockcroft-Walton pre- accelerator. The RFQ field is obtained by excitation of a suitably shaped cavity. The potential is: cos(2$)tAIo(r) cos kz 1 . sin(ctit++) (1) where V is the peak intervane voltage, a is the inter- vane gap, I, is a Bessel function, k = ZIT/~X with D the ion speed and x, w are the RF wave length and fre- quency. X and A are defined by A= (m2-1) m210(ka)+ I,(mka) X = l-AIo(ka) (2) where m is the modulation factor (see Figure 1). which varies along the length of the RFQ. The first term of (1) is an electric quadrupole field providing alternate gradient transverse focusing. The second term provides acceleration. The RFQ cavity is a four-vane resonator designed to produce the TE (210) quadrupole of Equation (l), particularly in the intervane region through which the beam passes. The parameters V, a, m(z) and > are chosen to provide the desired beam dynamics, as described below. H- Source b?SCriptiOn The ion source for injection into the RFQ will be 3527 the same source used since 1978 as injector for the Fermilab linac. This is a surface-plasma source of the magnetron design which has previously been described in detail.4 This source has been proven capable of producing a >30 mA negative hydrogen ion beam of long duration with energies of a few tens of keV at the extraction electrode. We have matched the RFQ to a 30 mA, 30 keV source from these capabilities. Following the extraction electrode will be a transport region in which focusing magnets will be inserted and tuned to provide matched injection into the RFQ. The RFQ parameters will be chosen such that the RFQ can replace the 750 keV Cockcroft-Walton preaccelerator which currently follows the Ion Source at the Fermilab linac. Mechanical Desiqn The resonant cavity consists of four vanes en- closed in a cylinder. The resonant frequency (201.25 MHz) is set by the required match to the linac; the vane length (136 cm) meets the bunching and accel- eration requirements, and the mean gap radius (.45 cm) implies a relatively modest maximum field on the vane. Important design considerations are minimum cost, ease of assembly, tuning flexibility, and precision align- ment of the vane tips. A transverse view of the cavity showing a vane cross section is displayed in Figure 2A and a longi- tudinal view of a vane is shown in Figure 2B. A vane is shaped from l/8 inch steel plate, a 2 inch x 1 inch bar is welded to the vane top and precision machined to provide the modulated vane tip. This vane tip is reolaceable. The base of the vane is fastened to a curved copper arc which continues to the next vane, completing a quadrant. The vanes are supported by three rods connected to the vane base which can be precision aligned and adjusted for tuning. Each vane weighs approximately 50 lbs. The vanes are interrupted at three points for insertion of "vane coupling rings" which provide an electrical connection between opposite vanes, elimina- tine dioole modes.5 The ends of the vane are desisned to provide an inductive and variable capacitive co;p- ling. The inductance is provided by the open area of the vane base, which permits coupling of magnetic flux from quadrant to quadrant. The variable capacitance is provided by end tuners on each end plate near the vane tips. Variation of the end tuners should provide adequate field uniformity along the RFQ length and balancing of the four quadrants. The init in two ways: ial alignment of the vanes will be done 1. Each providing 4 ( meter. 2. Each vane has a 45" surface near the tip 1 gaps which can be measured by micro- vane contains crosshairs at each end for optical alignment. *Operate-the Universities Research Association, Inc., under contract with the U.S. Department of Energy. 0018-9499/83/0800-3527$01 .OO@ 1983 IEEE © 1983 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.