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28 6 C Travier / Rf guns bright injectors for FEL 2.1 . Thermionic rf gun Electrons are continuously emitted by a thermionic cathode but can only be extracted and accelerated during half an rf cycle. The electrons, emitted during the accelerating half cycle, but with a too large phase (typically larger than 100') do not have enough energy to reach the cavity output and are accelerated backward to the cathode . The pulse that actually comes out the cavity is very long (about one fourth of the rf period) and has a very large energy spread . It is thus necessary to place behind the gun a magnetic bunching system (e .g ., an "a-magnet") with energy slits to reduce the pulse length, increase,the peak current and select a given spectrum . 2 .2 . Laser-driven rf guns A photocathode placed in an rf cavity is illuminated by a laser which delivers short pulses . Therefore as they leave the cathode, electrons are already bunched . Besides the obvious advantages (high field, bunched beam), laser-driven guns have other attractive features . The pulse format is more flexible than that of conventional injectors and that of thermionic rf guns. It depends essentially on the laser pulse format which can be varied over a wide range. Photocathodes can deliver much higher current densities than thermionic cathodes : more than 400 A/cm2 has been reported [3] . 2 .3 Field emission rf guns When placed in a high field, sharp needles can produce electrons by the so-called field emission process . This principle can be used in an rf gun . The difficulty is to find stable operating conditions . When high current is emitted, the needle is heated and easily destroyed . 3 . Worldwide review Since 1983, many laboratories have launched R&D programs on rf guns . Some are already producing experimental results while others are still at the preliminary design stage. Reviewing all these projects is a difficult task, due to the fast changing situation and the lack of literature for the most recent projects . Thus, the review presented here is probably incomplete and for some cases not up-to-date . May those who will not find their work reported here or who will find it reported inaccurately, be comprehensive and believe that they are not the subject of a special "censure" . Another difficulty met during this review was to produce consistent lists of parametersmorder to fairly compare the different projects . Each author uses his own units and definitions of the parameters and often all the parameters are not given for the same operating conditions . The following conventions are assumed . Design data for emittance are always given as four times the rms normalized emittance . When an author uses a different definition, necessary adjustments are made . Experimental data are reported as given in the publications . Since in some cases real beams have much longer tails than ideal beams and have not necessarily Gaussian or uniform profile, the "4 rms" emittance may not be appropriate to describe them . For laser-driven guns, the rmcropulse length is taken as 4a b , where ab is the rms bunch length . Rms definition (lab) is adopted for thermionic guns because of the pulse shape which has a long tail and a very sharp front end . In this case, la b includes most of the particles . When only the charge Q and the bunch length T are given, the peak current I is calculated in a conservative way assuring a uniform distribution (I = Q/T ) . Table 1 lists the different projects for each type of rf gun . Status of the projects and their main purpose are given . In table 2, the main parameters of each project are given . According to the project status, these parameters correspond to experimental or design values . A more detailed description of each project can be found in ref. [1] . 3.1 . Stanford (HEPL) - Duke (DFELL) The first thermionic rf gun was built at Stanford High Energy Physics Laboratory (HEPL) by Westenskow and Madey [2] to serve as an injector for the Mark III accelerator dedicated to FEL studies . The design work started in 1983 . The whole system (rf gun + lmac + wiggler) was operated as a laser oscillator in September 1985 . The system was then disassembled and moved to the Stanford Photon Research Laboratory where it was lasing again in September 1986 . In August 1987, a new momentum filter which increased the peak current was installed . In September 1988, a new gun cavity allowed a more stable operation with longer rf macropulses (8 Ws) repeated at a higher frequency (15 Hz) . The thermionic gun consists of a LaB6 cathode placed in an rf cavity operated at 2857 MHz . The cavity is followed by a transport system including an "a-magnet" with momentum filter and several quadrupoles, as shown in fig. 3 . In order to control the backbombardment, a transverse magnetic field at the cathode was added [8] . A summary of the gun's main parameters and performances is given in table 2 [9] . After the "ci-magnet", 2 to 3 ps electron pulses corresponding to 20 to 40 A peak current are obtained with emittances as good as 41T mm mrad in the vertical plane . In collaboration with Rocketdyne division of Rockwell Corporation, the same gun was operated as a
C. Trainer / Rf guns - bright injectors for FEL 28 7 photoinjector and led in November 1988 to the first successful operation of a photoinjector driven FEL [6] . The LaB6 cathode was illuminated by a tripled Nd : YAG laser. Because the laser pulse was quite long (100 ps), the "a-magnet" was still necessary. Although the experiment was run for only a very short time, this photoinjector gave better results (in terms of current and emittance) than the thermionic gun [10] . The FEL performances suggest rms emittances as good as 2m mm mrad in the vertical plane . The parameters and performances are given in table 2 [9] . The whole system was then disassembled again in December 1988 to be moved to Duke University Free Electron Laser Laboratory (DFELL) where it was scheduled to be operating again in 1990 . 3.2. Los Alamos National Laboratory Since at least 1984, Los Alamos National Laboratory (LANL) is involved in R&D programs about laserdriven rf guns and related topics like lasertron . In 1985, Lee et al . [3] demonstrated the possibility of extracting high current density (over 200 A/cm2) from CS 3 Sb photocathode illuminated by a frequency doubled Nd : glass laser. The first rf gun design consisted of one cavity operating at 1 .3 GHz and incorporating a Table 1 List of rf gun projects Laboratory Country Status Beginning Beginning Purpose September 90 of design of tests Therauonic rf guns Stanford - Duke [2,10] USA tested 1983 2/85 FEL IHEP Bering [451 China tested 6/86 2/88 FEL Mitsubishi [461 Japan tested IPT Kharkov [47] USSR tested ? storage ring injector AET/Varian/SSRL [431 USA under tests 1988? 1989 storage ring injector KEK [48] Japan construction 1989 1989 FEL/other North Carolina Central USA construction ? 1991 microwave University (NCCU) [49] generator Stanford University [50] USA design 1990 ? FEL Laser-driven rf guns Los Alamos [12] USA tested/stopped 1984? 1985 rf gun Los Alamos (HIBAF) [24] USA operating 9/88 5/89 FEL Stanford -> Duke [6] USA tested 1987? 9/88 FEL CEA [17,19] France under tests 3/86 1989 FEL Brookhaven [20,21] USA under tests 1987? 1989 new accelerator methods/FEL CERN [511 Switzerland rf tests 1988? 1990 collider Argonne [52] USA construction 1988 ? wakefield accelerator Wuppertal [53] Germany construction 1987? ? bright injector Twente [36] Netherlands construction ? ? FEL Duke [9] USA construction 9/89 6/91 storage ring injector Boeing [54] USA construction ? ? FEL Rockwell [55] USA construction ? ? FEL Los Alamos (AFEL) [151 USA design 1988? ? FEL INFN Milan [37] Italy design 1988? ? FEL LAL Orsay [56] France design 9/89 1991 rf gun INFN Frascati [57] Italy design ? 9 FEL MIT [58] USA design 1990 ? collider KEK [591 Japan design ? ? collider UCLA [601 USA ? ? ° FEL Vanderbilt University [9] USA ? ? ? FEL LLNL/SLAC/LBL [611 USA stopped 1987 - high gradient accelerator Field emission rf guns IHEP Beijing [62] China preliminary design 1988? ? FEL LAL Orsay [63] France stopped 1988 1/89 rf gun V ACCELERATOR TECHNOLOGY
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