Nuclear Instruments and Methods in Physics Research A 528 (2004) 71–77 Overview of the 100mA average-current RF photoinjector D.C. Nguyen a, *, P.L. Colestock a , S.S. Kurennoy a , D.E. Rees a , A.H. Regan a , S. Russell a , D.L. Schrage a , R.L. Wood a , L.M. Young a , T. Schultheiss b , V. Christina b , M. Cole b , J. Rathke b , J. Shaw c , C. Eddy c , R. Holm c , R. Henry c , J. Yater c a Los Alamos National Laboratory, MS H851, Los Alamos, NM 87545, USA b Advanced Energy Systems, Medford, NY 11763, USA c Naval Research Laboratory, Washington, DC 11763, USA Abstract High-average-power FELs require high-current, low-emittance and low-energy-spread electron beams. These qualitieshavebeenachievedwithRFphotoinjectorsoperatingatlow-dutyfactors.Todate,ahigh-average-currentRF photoinjectoroperatingcontinuouslyat100%dutyfactorisyettobedemonstrated.Theprincipalchallengesofahigh- duty-factornormal-conductingRFphotoinjectorarerelatedtoapplyingahighacceleratinggradientcontinuously,thus generatinglargeohmiclossesinthecavitywalls,coolingtheinjectorcavitywallsandthehigh-powerRFcouplers,and findingaphotocathodewithreasonableQ.E.thatcansurvivethepoorvacuumoftheRFphotoinjector.Wepresentthe preliminary design of a normal-conducting 700MHz photoinjector with solenoid magnetic fields for emittance compensation. The photoinjector is designed to produce 2.7MeV electron beams at 3nC bunch charge and 35MHz repetitionrate(100mAaveragecurrent).Thephotoinjectorconsistsofa2 1 2 -cell, p-mode,RFcavitywithon-axiselectric coupling, and a non-resonant vacuum plenum. Heat removal in the resonant cells is achieved via dense arrays of internal cooling passages capable of handling high-velocity water flows. Megawatt RF power is coupled into the injector through two tapered ridge-loaded waveguides. PARMELA simulations show that the 2 1 2 -cell injector can produce a 7 mm emittance directly. Transverse plasma oscillations necessitate additional acceleration and a second solenoid to realign the phase space envelopes of different axial slices at higher energy, resulting in a normalized rms emittanceof6.5 mmand34keVrmsenergyspread.Wearedevelopinganovelcesiatedp-typeGaNphotocathodewith 7%quantumefficiencyat350nmandacesiumdispensertoreplenishthecathodewithcesiumthroughaporoussilicon carbide substrate. These performance parameters will be necessary for the design of the 100kW FEL. r 2004 Published by Elsevier B.V. PACS: 41.60.Cr; 29.17.+w; 29.27.Bd; 41.75.Fr Keywords: FEL; Photoinjector; High current; High brightness; cw; Photocathode ARTICLE IN PRESS *Corresponding author. Tel.: +1-505-667-9385; fax: +1-505-667-8207. E-mail address: dcnguyen@lanl.gov (D.C. Nguyen). 0168-9002/$-see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.nima.2004.04.021