Particle Accelerators, 1991, Vol. 36, pp.33-74 © 1991 Gordon & Breach Science Publishers, S.A. Reprints available directly from the publisher Printed in the United Kingdom. Photocopying permitted by license only REVIEW OF MICROWAVE GUNS C. TRAVIER Laboratoire de I'Accelerateur Lineaire, IN2P3-CNRS et Universite de Paris-Sud, 91405 Orsay, Cedex, France (Received 15 October 1990; in final form 30 July 1991) Free Electron Lasers and future high energy linear colliders require very bright electron beams. Since conven­ tional injectors made of DC guns and RF bunchers have intrinsic limitations, an important R&D endeavour has been undertaken worldwide to develop a new type of bright electron source: the microwave or RF gun. After describing briefly the conventional injector and its limitations, the basic principles of RF gun are pre­ sented showing that there exist three types of gun. The technology state of the art is then shortly addressed. The thirty currently on-going RF gun projects are reviewed. Their specific characteristics are outlined and their nominal performances are given. Finally, the basic ideas of RF gun design are exposed. 1. INTRODUCTION The advent of free electron lasers and the perspective of high energy linear colliders boosted the development of high-brightness electron sources. During the last decade, the arrival of RF guns and photocathodes have offered a promise to reach the expected performances. In 1984, G.A. Westenskow and 1.M.l. Madey put a thermionic cathode directly in an RF cavity. 48 This new type of gun was named "RF gun" or "microwave gun". Meanwhile, Lee et al. reported that very high current densities could be obtained from semiconductor photocathodes. 75 Then, 1.S. Fraser and R.L. Sheffield experimented the use of such photocathodes in an RF gun. 74,78 In 1988, the "first demonstration of a FEL driven by electrons from a laser irradiated photocathode" was made at Stanford. 52 Since that time, many laboratories have begun to study both thermionic and laser­ driven RF guns. Almost thirty projects are identified in this review showing the strong interest manifested for this new bright electron source. Fig. 1 shows the exponential evolution of the number of projects over the past 8 years. It also shows the increasing preponderance of photoinjectors. From Fig. 2, it can be seen that half of the RF guns are developed for FEL purposes. After a brief summary of the beam quality requirements for the different applications, the basic principles of conventional injectors and RF guns are described and their merits are compared. A review of the different projects is then presented including a short description of the most advanced ones. The last part deals with the present understanding of RF gun design and introduces some novel ideas recently proposed. Previous bright 33 P.A.-B 34 c. TRAVIER 35 ......-----------.-........I""""P""'..................,......... _ THERMIONIC RF GUNS en 30 ~ LASER-DRIVEN RF GUNS b w 25 o FIELD EMISSION RF GUNS ~ t-----...----...-........-----,....---....--.I a.. 20 LL o 15 a: w co 10 ~ :::> z 5 83 84 85 86 87 88 89 90 YEAR FIGURE 1 Current number of RF gun projects. • FEL ~ OTHER PURPOSES o~~~-...­ 83 84 85 86 87 88 89 90 YEAR FIGURE 2 Projects purpose. REVIEW OF MICROWAVE GUNS 35 ... .... TM : macropulse length fM : macropulse repetition frequency Tm : micropulse length f m : micropulse repetition frequency FIGURE 3 Pulse format. injector reviews can be found in references. 6,7, 8, 9, 10, 11 2. HIGH QUALITY BEAM REQUIREMENTS 2.1. Some definitions Many parameters are used to characterize the quality of an electron beam produced by an electron source or injector. The most important are: • the pulse format: the beam is made of pulses of a certain length (micropulse length) repeated at a given frequency (micropulse repetition rate) and with some timing jitter. This train of pulses has a certain duration (macropulse length) and is also repeated (macropulse repetition rate). An example of pulse format is shown in Fig. 3. • the peak current defined as the current in the micropulse. • the energy spread: the dispersion in energy within a micropulse. It can be ex­ pressed either in relative or absolute values. • the emittance which is a measure of the distribution of the beam in the phase space. There are several definitions of emittance. 1 In order to be consistent and unless otherwise mentioned, all the emittances quoted in this paper are r.m.s. transverse normalized emittances 2, 3 defined as: where x is the coordinate of a particle in the beam, Px is the particle's momentum component in the x direction and < > indicates averaging over the electron 36 C. TRAVIER TABLE 1 FEL beam requirements. IR Visible XUV X-RAY Wavelength (I£m) 1 - 500 0.1 - 1 0.01 - 0.1 < 0.01 Energy (MeV) 10- 100 100 - 200 200 - 500 > 500 Micropulse length (ps) 1 - 20 1 - 20 1 - 20 1 - 20 Micro. repetition (MHz) 10 - 100 10 - 100 10 - 100 10 - 100 Jitter (ps) <: pulse length Peak current (A) > 20 > 50 > 100 > 200 Norm. emit. (1r mm mrad) 60 - 500 20 - 60 3 - 20 <3 Energy spread (%) < 0.5 < 0.2 < 0.1 < 0.1 Macropulse length (I£S) > 10 > 10 > 10 > 10 distribution. This emittance is equal to the phase space area for a Kapchinskij­ Vladimirskij distribution 4 or to the area of the phase space ellipse which contains around 90% of the particles ("20" emittance") of a beam which distribution is gaussian in each coordinate (x, Px, y, py). The unit is 'IT mm mrad (e.g.: En = 10 'IT mm mrad = 31.4 mm mrad). • the normalized peak brightness defined as B n == 21/(ExnEyn ) where 1 is the peak current and Exn , Eyn are the normalized emittances in both transverse directions. 2.2. Beam requirements Over the recent years, the demand for high-brightness injectors producing very short micropulses has increased. Essentially two types of accelerators require such injectors: linear accelerators for free-electron lasers (FEL) and future high energy linear colliders. 2.2.1. FEL beam requirements Since the first FEL 12 ever "lased" in 1976, there has been a constant growing interest for this new instrument among the scientific community. The performances of an FEL being closely related to those of its driving accelerator, the necessity of very bright electron sources quickly appeared. In this paper, Free Electron Lasers will be classified into four categories according to their wavelength, as shown in Table 1. So far, linac driven FEL have only been operated in infra-red and visible light regimes. It is very difficult to explicit general beam requirements for FEL since these require­ ments depend on the specific design of the electron beam transport system, the undulator and the optical cavity. They also depend on the operating mode of the FEL (eg: oscillator or amplifier). General guidelines can though be given but they have only an indicative value. REVIEW OF MICROWAVE GUNS 37 • The electron beam energy is determined by the laser wavelength in relation with the undulator characteristics. • The micropulse length should be the same as the desired laser pulse length. • The micropulse repetition period needs to be a multiple of the round-trip time in the optical cavity. • The timing jitter between micropulses should be much less than the micropulse length in order to preserve a good synchronism between electron bunches and optical pulses. • The macropulse length is imposed by the time necessary to build-up the oscillations. • The macropulse repetition rate is related to the FEL output power. • High peak current is needed to obtain a high gain and a good extraction efficiency. • The relative energy spread should be less than 1/4N where N is the number of periods of the undulator, in order to assure a good extraction efficiency. An optical gain close to maximum is obtained when there is a spatial overlap of the optical beam and the electron beam through the interaction region. This constraint is fulfilled when the unnormalized transverse emittance is less than the optical wavelength A; this can be written as Cxn,yn :S A{31, where {3 == v / c is the particle velocity and 1 == (1 - (32)-1/2. Crude estimations of the electron beam requirements for the different categories of FEL are given in Table 1. Except for the total energy and the relative energy spread, all these requirements are mainly applying to the injector part of the linac. This statement assumes that emittance is preserved during acceeration in the linac which might not always be absolutely true, especially for high currents. A more detailed description of FEL beam requirements can be found in references. 13, 14, 15, 16, 17 2.2.2. Linear colliders beam requirements Electron-positron colliders with energies much higher than LEP (Large Electron-Positron Collider at CERN) will have to be linear. Therefore a lot of R&D work is underway to find new acceleration schemes. 18 Whatever scheme will require very bright injectors. In the two-beam accelerator proposed by CERN, 19 the drive-linac also needs an injector capable to generate very short intense bunches. 20 For the collider linac, all the requirements at the collision point come from the neces­ sity to obtain a very high luminosity. If such requirements could be met by an electron source, the need for a damping ring could be suppressed. For the drive linac, a very high current is needed to produce the RF power for the collider; the pulse format is determined by the collider operating frequency and filling time. A tentative list of parameters is given in Table 2, for both the collider linac and the drive linac.