Free-electron laser (FEL) oscillators require pulse trains of high peak brightness and, in some applications, high average power. Recent developments in the technology of photoemissive and thermionic electron sources in RF cavities for electron linac-injector applications offer promising advances over conventional electron injectors. Reduced emittance growth in high peak current electron injectors may he achieved by using high field strengths and by linearizing the radial component of the cavity electric field at the expense of lower shunt impedance. Harmonically excited cavities have been proposed for enlarging the phase acceptance of linac cavities and thereby reducing the energy spread produced in the acceleration process. Operation of injector linacs at a subharmonic of the main linac frequency is also proposed for enlarging the phase acceptance.

High-brightness injectors for RF-driven free-electron lasers

A free-electron laser (FEL) oscillator, driven by an rf linac, requires a train of electron bunches delivered to an undulator. The electron-beam brightness requirement exceeds that available from a conventional buncher. The demonstrated high peak brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely without the usual large loss in beam brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm2 is located on an end wall of an rf cavity to accelerate a 60-ps bunch of electrons to 1 MeV as rapidly as possible. Preliminary experimental work and simulation calculations are presented.

High-Brightness Photoemitier Injector for Electron Accelerators

Free electron lasers (FELs) require electron beams of high peak brightness. In this presentation, we describe the design of a compact high-brightness electron source for driving short-wavelength FELs. The experiment uses a laser-illuminated Cs 3 Sb photoemitter located in the first rf cavity of an injector linac. The photocathode source and associated hardware are described. The doubled YAG laser (532 nm), which is used to drive the photocathode, produces 75 ps micropulses at 108 MHz repetition rate and peak powers of approximately 300 kW. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a 4 ps resolution streak camera. Present experiments give the following results: micropulse current amplitudes of 100 mA to 400 A, beam emittances ranging from 10 π mm mrad to 40 π mm mrad, an energy spread of ± 3%, and peak current densities of 600 A/cm 2 The design of experiment has now been changed to include a separately phased rf cavity immediately following the first cavity. This modification enables us to study the effects of phasing with the possibility of improving the injector performance. Also, this change will improve the vacuum conditions in the photoelectron source with a consequent improvement in lifetime performance. A brief discussion on the possible applications of this very bright and compact electron source is presented.

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The los alamos photoinjector program

From basic laws governing the flow of electrons in a plane diode, we obtain combined relations about boundaries of space‐charge‐limited current density variations in an electronic‐injector diode when duration of emission is varying. Although diode geometry of effective injectors is generally more complicated than that of a plane diode, we point out how the simplest relations can be used with a relatively good approximation. Then typical temporal modes of diode operation are analyzed. For each one, we give characteristic equations connecting peak intensity, maximum charge, and current duration in the diode, and we determine their applicability. Then we define a time that is a typical limit between the two principal temporal operating regimes. Utilization of reduced variables permits application of the main results to many practical situations such as comparison between working performances of photoinjectors assisted by laser, and prediction of maximum peak current available at the diode output for a given duration.

Space‐charge‐limited current density as a function of electron flow duration in an emissive diode

PHOTOCATHODES IN ACCELERATOR APPLICATIONS* J. S. Fraser, R. L. Sheffield, E. Gray, P. M.Giles, R. W. Springer and A. Loebs, MS-H825 Los Alamos National Laboratory, Alamos, NM 87545 Abstract Some electron accelerator applications require bursts of short ulses at high microscopic repetition rates and high pea E brightness. A illuminated by a mode-locked laser, is well suited to filling this need. The intrinsic brightness of a photoemitter beam is high; experiments are under way at Los Alamos to study the brightness of short bunches with hi h s ace charge after acceleration. A laser-illuminated E s,S t photoemitter is located in the first rf cavity of an injector linac. Diagnostics include pep er-pot emittance analyzer, a magnetic spectrometer, an 2 a streak camera. Introduction Electron accelerator applications that require a beam of high peak brightness as well a high pulse-repetition rate put severe demands on the electron source and injector. The conventional solution to this problem is to use a pulsed thermionic emitter of low peak brightness followed by a bunching, or phase compression, system that increases the peak current by a large factor. Ideally, the FJ eak brightness should increase in proportion to the unching factor; in however, the result always falls short of the ideal. An additional shortcoming; in the conventional buncher is that a repetition rate in excess of a few tens of megahertz is beyond the state of the art electronically swrtching a thermionic triode electron gun. There have been several reports of diode and triode guns operated in an rf cavity to produce electron pulses of width somewhat less than one-quarter of the rf period.1-3 The ultimate upper limit to this technique for triode guns may be the power dissi ation in the grid. In current app F lcations of rf-driven free-electron lasers, trains of over 300-A, 16-ps-wide (5nC charge) pulses are being used.4 Recent success 5 with the production of high peak currents from a laser-illuminated photocathode shows that high repetition rates in peak-brightness electron beams may be possible. At the Los Alamos National Laboratory, a program is under way to develop a high peak brightness, average current photoelectron injector. The Los Alamos Photoinjector Program 1985, the achievement of hi!h peak currents from a Cs,Sb uhotocathode was reported. It has been shown that thg laser-driven photocathode produces an intrinsically bright beam.‘j It remains to be demonstrated that short bunches can be accelerated to relativistic energies without loss of brightness. It is evident from simulation studies70f the acceleration of short bunches in an rf cavity that dense s lp ace charge the external rf field lead to a degradation o beam quality and, therefore, to a loss of brightness. Although pulses of only a few picoseconds can be produced in a photocathode, it now seems advisable to generate pulses that initially are about 100 ps long and thin, after acceleration to about 10 MeV, bunch them magnetically.8 Acceleration of the longer bunches is best done in a low-frequency linac at subharmonic of the main linac frequency.g Nevertheless, there is a strong incentive to accelerate the bunches as rapidly as possible, a condition that can be met only with high-frequency rf fields.‘O A study of the envelope equation Ref. 11 reveals that, for continuous beams, the dominance of space charge over emittance is adiabatically damped as Y-“~. A *Work performed under the auspices of U.S. Department of Energy and supported by the U.S. Army Strategic Defense Command. paper in these proceedinps’2 shows that for bunched beams, the damping dependence on energy is much stronger, namely y- . Therefore. the reauirement of maximum acceleratin gradient (hence a hiih frequency) to minimize the in if uence of space charge must be balanced against a conflicting need to accept long pulses (hence a low frequency to reduce the emittance growth associated with rf fields). The conceptual design of a subharmonic injector linac with an rf gun electron source has been described earlier.g The initial rf gun experiments, however, are being carried out at a high frequency because powerful klystron was available for use at 1300 MHz. A schematic diagram of the Los Alamos injector experiment is shown in Fig. 1. OtlnRTz SCHkCN -j Fig. 1. Plan view of the photoinjectorexperiment. Photocathode Design In recent years, photocathodes for sources have been made from wafers of B olarized electron aAs.13*14 Curre?! densities as high 180 A/cm2 have been reported. Photoemitters of Cs,Sb are less demanding o system cleanliness15 than are those of GaAs. An additional advantage of a positive electron affinity semiconductor like Cs,Sb lies in rapid emissio? of Jhe photoelectrons. l6 By contrast, the intrinsic emission-time uncertainty of GaAs has been measured in the range from 71 ps for active layers between 50 nm and 2 pm in thickness.16 A cesium antimonide (Cs Sb) photocathode was chosen for its ease of preparation within the vacuum environment linac and for its relative tolerance vacuum conditions in the injector linac.15 A photoinjector linac must be bakeable in its entirety to about 200°C ;nd be capable of maintaining a preferably lo-‘Otorr. If a Cs, b hotocathode LS damaged ,rrre below . lo- torr, in use, the damage can be erased y heating to 4OO”C, then a new one prepared in situ. The spectral response17 of Cs Sb extends from a quantum energy of 1.8 eV (A = B90 nm) to energies greater than 3.8 eV (A < 320 nm). Therefore, a Cs,Sb F hotocathode can be used with Nd:YAG laser doubled (A = 532 nm) or tripled (A = 355 nm). ?&%;;‘lG laser can readilv be mode-locked to deliver trains of 60-ps pulses at a microscopic repetition rate in a range from 120 MHz. Rf Gun Cavity Design The highest cavity is limited l!l ossible acceleration rate in rf .gu+n y the sparking breakdown characterlstlc of the cavity. A typical rf cavity, optimized for maximum effective shunt im edance ZT2, will have a ratio of peak surface electric fie acceleration gradientlo of about 6. A lower ratio, a larger maximum 1705 CH2387.9:87!000(- $ I .oO Q IEEE

Photocathodes in accelerator applications

High-current-density, bunched electron beams with low emittance are required for efficient operation of rf-linac-driven free-electron lasers (FELs). Laser-irradiated, photoemissive electron sources are suitable for this application. Currents of over 200 A have been generated in an ultrahigh vacuum chamber from a 1-cm2 Cs3Sb photocathode irradiated by a frequency-doubled, Q-switched pulse from a Nd: glass laser. These currents are over two times larger than previously reported from any photocathode. The duration of the electron pulse was 50 ns (FWHM), corresponding to the width of the 532-nm laser pulse.

J. H. Jacob

Papers

Electron Emission of over 200 A/CM2 from a Pulsed-Laser Irradiaied Photocaihode