Rf guns: bright injectors for FEL - Triangle Universities Nuclear ...

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290 C. Tracter / Rf guns- bright injectorsfor FEL T QUADRUPOLE VERTICAL STEERING -i `- HORIZONTAL STEERING Fig. 3 . Stanford/Duk e rf gun layout (from ref. [44]) . Cs 3Sb photocathode illuminated by a frequency doubled Nd : YAG laser [4] . The cavity shape was designed to minimize the nonlinear components of the radial electric field, assuming a do approximation . This gun produced a 1 MeV beam of 70 ps pulses with a peak current of 200 A and a normalized enuttance of 40m mmmrad [I1]. A second cavity independently powered and phased was then added to the original one . Meanwhile, the adjunction of a pulse compressor to the laser allowed to generate 16 ps optical pulses . This new gun tested in 1987 and 1988, provided a 2.7 MeV beam . The maximum extracted charge was 13 .2 nC corresponding to a peak current of 600 A. No enuttance measurements were possible due to the drift space between the second cavity and the "pepper-pot" device that led to excessive space-charge emittance growth [12] . During this period, the photocathode lifetime was significantly improved by replacing Cs 3Sb with CsK 2 Sb . This lifetime remained very dependent though on the operating conditions . Table 2 summarizes the performances of the two experiments named for convenience phase I and phase 11 . Based on the gained experience, a new photoinlector was built to serve as an electron source for the HIBAF infrared FEL [13] . This new gun which replaced the previous conventional injector provides a 350 A, 16 ps and 351T mm mrad beam. It consists of six cells where the first cell contains the CsK 2Sb photocathode (fig . 4) . This photoinlector has been designed for maximum flexibility in order to do parametrization studies for LOS ALAMOS NATIONAL LABORATORY ACCELERATOR TECHNOLOGY DIVISION GROI.P AT-1 1300 Mi; PHOTOINJECTOR LINAC Fig . 4 . Los Alamos HIBAF rf gun (from ref . [14]) .
C. Travier / Rf guns : bright injectors for FEL 291 Fig. 5 . CEA photoinjector (from ref . [19]). simulation codes verification . Nominal performances are given in table 2 [14]. Because of the possible industrial use of FEL, it is interesting to develop a very compact device. This work has been undertaken by LANL [15] . A compact FEL named AFEL is now under design . The photoinjector will consist of ten cells powered by one rf source. All flexibility will be removed and the parameters will be chosen for an optimum design according to simulations . This injector should be able to produce a beam brightness of the order of 10 12 A/(m2 rad 2 ) (table 2) . Carlsten proposed a very powerful way to reduce the emittance of a photoinjector [161 . After identifying the four different emittance growth mechanisms : (a) linear space charge, (b) nonlinear space charge, (c) nonlinear time-independent rf effect, (d) linear time-dependent rf effect, he shows that for a pulse produced by a laser, it is possible to remove most of the correlated emittance (variations in the transverse phase space correlated with longitudinal position) by a proper choice of position and strength of magnetic lenses . Such a theory is not applicable for conventional injectors since bunching ~F TUNER Fig . 6 . BNL rf gun cavity (from ref. [21]). V ACCELERATOR TECHNOLOGY
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Rf guns: bright injectors for FEL - Triangle Universities Nuclear ...

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