Nuclear Instruments and Methods in Physics Research A 528 (2004) 316–320 A two-Frequency RF Photocathode Gun D.H. Dowell a, *, M. Ferrario b , T. Kimura c , J. Lewellen d , C. Limborg a , P. Raimondi b , J.F. Schmerge a , L. Serafini e , T. Smith c , L. Young f a Stanford Linear Accelerator Center, Mail Stop 18, 2575 San Hill Rd., Menlo Park, CA 94025-7015, USA b INFN-Frascati, via E. Fermi 40, 00040 Frascati, Italy c Hansen Labs, Stanford University, Stanford, CA 94305-4085, USA d Advanced photron Source, Argonne National Laboratory, Argonne, IL 60439, USA e INFN-Milan-LASA, via Fratelliceni 201, 20090 Segrate (MI), Italy f Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA Abstract In this paper we resurrect an idea originally proposed by Serafini (Nucl. Instr. and Meth. A 318 (1992) 301) in 1992 for an RF photocathode gun capable of operating simultaneously at the fundamental frequency and a higher frequency harmonic. Driving the gun at two frequencies with the proper field ratio and relative phase produces a beam with essentially no RF emittance and a linear longitudinal phase space distribution. Such a gun allows a completely new range of operating parameters for controlling space charge emittance growth. In addition, the linear longitudinal phase space distribution aids in bunch compression. This paper will compare results of simulations for the two-frequency gun with the standard RF gun and the unique properties of the two-frequency gun will be discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.Bx; 29.27.Ac; 41.60.Cr; 41.85.Ar Keywords: Electron beam; Photocathode RF gun; RF harmonics; Emittance 1. Introduction The RF gun transverse emittance is predomi- nately due to space charge forces, RF fields and thermal emittance. Emittance compensation does well to remove the linear space charge contribu- tion, but the gun parameters are still constrained to operate between the space charge and RF limits. This is illustrated in Fig. 1, where the uncompen- sated emittance at the gun exit is plotted in the plane defined by beam size and bunch length. The operating region is shown for the LCLS gun and is typical of most guns. They perform best in the saddle between the space charge and RF domi- nated regimes. Emittance compensation extends the region into the space charge regime, and does a remarkably good job of recovering the emittance where the correct combination of solenoid, drift and linac gradient and phase, reduce the projected emittance from three microns to one micron after ARTICLE IN PRESS *Corresponding author. Tel.: +1-650-9262494; fax: +1-650- 926-8533. E-mail address: dowell@slac.stanford.edu (D.H. Dowell). 0168-9002/$-see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.078