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

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294 C Traoier / Rf guns bright injectors for FEL 5 . Rf guns vs conventional injectors Conventional injectors including a do gun and one or several bunchers have already produced very bright beams (e .g., SLC [31], Boeing [32]) . Therefore several new FEL designs make use of a conventional injector (e .g ., CLIO [33], FELIX [34], CDRL Berkeley [35]) . Meanwhile several other recent designs rely on rf guns (e.g ., Duke [10], Los Alamos [15], Twente University [361, Milan [37], etc .) . Both technologies have advantages and drawbacks that are summarized in table 3 . The two technologies are in a very different state of development . The statements presented in table 3 are thus subject to evolution in the next few years . Infrared FEL users' facilities to be built now are very likely to use a conventional injector since its performances are sufficient and its reliability much greater . For operation in the visible regime, rf guns are advisable owing to the fact that the considerable efforts made to improve their performances (stability, reliability) should succeed soon . For even shorter wavelengths, they are probably the only choice to date . Pbotoinjectors are also very interesting for compact infrared FELs that might be developed for commercial use . Compactness allows to reduce the amount of concrete necessary for shielding thus making the FEL cheaper. 6 . Design philosophy From the Kim equations [38], it can be shown that the minimum emittance expressed in iT mm mrad for an rf gun beam is equal to e mm - 1 .57 X 10-_ ~faxa6~2 where I is the peak current to A, f the rf frequency in MHz, a, the rms transverse beam size in mm, a b the rms bunch length in ps . This equation is valid for short pulses and Gaussian distributions, This minimum emittance is obtained for an optimum accelerating field expressed in MV/m : 0 _J w w 0 I = 100 A - sigma . . = 3 mm 10 100 1000 FREQUENCY (MHz) Fig . 9 . Optimum electric field emittances are shown in fig. 10 . When the optimum field is higher than the practical limit, the emittance is calculated for this practical field limit . Figs . 9 and 10 provide a quick estimate of rf gun performances . For higher current, the results are easily scaled since both E.Pt and f r , are proportional to the square root of the current. For the rms bunch length of interest (less than 10 ps), fig . 10 shows that the best frequency to choose is around 1 GHz which corresponds to L-band. If very short pulses can be produced from the laser, higher frequencies are possible . If only long pulses are possible or if very high charges are needed, lower frequencies are to be chosen . In this case the rf gun has to be followed by a magnetic compressor . This scheme is probably the best way to obtain currents larger than 500 A. In order to minimize the emittance growth occurring I = 100 A - sigma x = 3 mm Eopt - 7.2 x 10 5 f7Y a b Sax_+ l .506 Fig . 9 shows the optimum field as a function of the frequency for a current of 100 A, a bunch size ax of 3 mm and different pulse lengths . Since the above equations are only valid for short pulses, fractions of the rf period are shown to indicate what are the reasonable bunch lengths to consider . The practical limit of two times the Kilpatrick value is also represented showing that it is not always possible to reach the optimum field. From this picture it appears that low frequencies require a very high field. The corresponding minimum E E C w U Z Q E- ui 100 10 100 1000 FREQUENCY (MHz) Fig . 10 . Minimum enuttance .
C. Traurer / Rf guns . bright injectorsfor FEL 295 during drifts at low energy, it is better to use several cells thus producing a beam with an energy larger than 10 MeV at the end of the gun . The above equations did not include the use of a magnetic lens to reduce the emittance as suggested by Carlsten . However, Kim [39] showed that this method can be applied only if In yq < 1, arid 30 .7Ezr 2 where yq is the electron energy (m units of MC Z ) at the location of the lens, I is the peak current in A, E the gradient in MV/m and r the beam radius in mm . For typical values of current, beam radius and electric field, the above conditions show that emittance reduction due to the magnetic lens is only possible for low frequencies . Several other methods to reduce the emittance are being investigated such as laser pulse shaping [40] and time-dependent rf focusing [41,42] . Computer simulations show significant reduction of the beam emittance but the practicality of such methods is to be demonstrated . With these schemes, the residual emittance is mainly due to the thermal emittance of the cathode . 7 . Conclusion Between 1983 and 1988, it was proven that thermionic and laser-driven rf guns could produce bright electron beams. Starting in 1989, rf guns began to be widely studied and used in different laboratories . Efforts are made to push the technology to the limit while trying to improve its reliability . The first routine operation of an rf gun is starting at SSRL [43] . Starting in the fall of 1990, several laboratories will be ready to experiment their rf guns (CEA, BNL, CERN, DUKE, etc.) . It will then be possible to test the different types of cathodes in an accelerator environment . Long hours of operation experience will give birth to new ideas to bring the rf gun technology to a mature state . Acknowledgements I would like to thank very much J. Gao, H. Liu and B . Carlsten for helpful discussions . S.V . Benson, S.C . Chen, A. Enomo, G.J . Ernst, W. Gai, S. Joly, K.J . Kim, A. Michalke, R.L . Sheffield, Y. Tur and S . Zhong are acknowledged for providing me with a parameter list for their projects. Finally, I am indebted to J. Le Duff who gave me the opportunity to work on this subject . References . . . . . . C. Travier, Proc . ICFA Workshop on Short Pulse High Current Cathodes, Bendor, France, 1990 (Ed . Frontières) to be published . [21 G.A . Westenskow and J.M .J . Madey, Laser and Particle Beams 2 (1984) 223 C.H Lee et al ., Proc. IEEE Particle Accelerator Conf Vancouver, Canada, 1985, IEEE Trans Nucl Sci . NS-32 (5) (1985) 3045 [4] J.S. Fraser et al ., ibid ., p 1791 . [51 R.L . Sheffield, E.R. Gray and J.S . Fraser, Nucl . Instr and Meth . A272 (1988) 222 . [6] M. Curtin et A., Proc. 11th Int. FEL Conf., Naples, FL, USA, 1989, Nucl . Instr and Meth . A296 (1990) 127 . [71 As known to the author by September 1990 . [8] C.B . McKee and J.MJ Madey, Proc. 11th Int. FEL conf ., Naples, FL, USA, 1989, Nucl. Instr. and Meth, A296 (1990) 716 . S.V Benson, private communication . S.V . Benson et al ., Proc . 11th Int. FEL Conf., Naples, FL, USA, 1989, Nucl . Instr. and Meth . A296 (1990) 110. [11] J.S. Fraser et al ., Proc. IEEE Particle Accelerator Conf Washington, DC, USA, 1987, IEEE 87 CH2387-9, p . 1705 . [12] R.L . Sheffield et al., Proc. Linear Accelerator Conf., Williamsburg, VA, USA, 1988, CEBAF-Report-89-001 (1989) P . 520 . [131 W.D . Cornelius et al ., Proc. 11th Int. FEL Conf ., Naples, FL, USA, 1989, Nucl . Instr . and Meth . A296 (1990) 251 . [14] D.W . Feldman et al ., these Proceedings (12th Int . FEL Conf ., Pans, France, 1990) Nucl . Instr . and Meth . A304 (1991) 224, R.L. Sheffield, private communication . [15] J.C . Goldstein et al ., Proc . 11th Int . FEL Conf., Naples, FL, USA, 1989, Nucl. Instr and Meth . A296 (1990) 282 . [16] B.E . Carlsten and R.L . Sheffield, Proc . Linear Accelerator Conf., Williamsburg, VA, USA, 1988, CEBAF-Report-89- 001 (1989) p 365 . [17] R. Dei-Cas, Nucl Instr. and Meth . A285 (1989) 320 [18] S . Joly, private communication . [19] R. Dei-Cas et al ., Proc . ICFA Workshop on Short Pulse . . . High Current Cathodes, Bendor, France, 1990 (Ed . Frontières) to be published. [20] K. Batchelor, Proc. Linear Accelerator Conf ., Albuquerque, NM, USA, 1990, to be published. [21] K.T . McDonald, Design of the Laser-Driven RF Electron Gun for the BNL Accelerator Test Facility, DOE/ER/ 3072-43 (Princeton University, 1988) [221 J. Fisher, Proc . ICFA Workshop on Short Pulse High Current Cathodes, Bendor, France, 1990 (Ed Frontières) to be published . [231 L. Falce, ibid ., to be published. [24] P.G. O'Shea et a] ., Proc . Linear Accelerator Conf. Albuquerque, NM, USA, 1990, to be published [251 R.L. Sheffield, ibid, [26] H. Bergeret, M. Boussoukaya, R. Chehab, B . Leblond and J. Le Duff, Proc . ICFA Workshop on Short Pulse High Current Cathodes, Bendor, France, 1990 (Ed Frontières) to be published . V. ACCELERATOR TECHNOLOGY
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