Accelerator Test Facility
High Brightness Electron Beams
About Photoinjector R&D:
The generation of high quality beams of electrons has been made possible through the development of photoinjectors (also known as laser photocathode rf guns) and the technique of (linear) emittance compensation using a laminar-flow beam waist.
A high brightness means that the electron bunch has a high density in 6-D phase space. To achieve high brightness beams, it is necessary to master the production of such beams in special electron guns, to develop diagnostics that provide information of the 6-D distribution of electron bunches on sub-picosecond time scales, to control the 6-D distribution of the bunch in various ways and to be able to accelerate the electrons to high energies without diluting the brightness.
The state-of-the-art of photoinjectors allows direct injection into linear colliders, plasma wake-field laser accelerators, the construction of short wavelength FELs to the shortest desired wavelength, Compton backscattering for the production of femtosecond x-rays and more. Still, there is a powerful motivation to continue and improve the performance of these devices. Any improvement in the brightness of the electron beam will reduce the length of the accelerator and wiggler significantly and or make it possible to relax the requirements on the transport, acceleration and compression of the beam. As will be outlined below, there is a vast arena for research and development of photoinjectors that will require many years of R&D in many laboratories.
The development of high brightness electron beams is one of the main research initiatives of the ATF.
NSLS scientists working at the ATF measured the slice emittance of a 10 ps electron bunch with a 1 ps resolution (X. Qiu, K. Batchelor, I. Ben-Zvi and X.J. Wang, Demonstration of emittance compensation through the measurement of the slice emittance of a 10 picosecond electron bunch, Phys. Rev. Let. 76 No. 20, 3723, (1996) BNL 62386), achieved an unprecedentedly high 6-D electron phase-space density (X.J. Wang, X. Qiu and I. Ben-Zvi, Experimental Observation of High-Brightness Micro-Bunching in a Photocathode RF Gun, Phys. Rev. E54 No. 4, R3121, (1996)) and directly measured electron bunching on an optical scale (Y. Liu, X.J.Wang, D.B. Cline,M. Babzien, J.M. Fang,J. Gallardo, K. Kusche,I. Pogorelsky, J. Skaritka, A. van Steenbergen, Experimental Observation of Femtosecond Electron Beam Microbunching by Inverse Free-Electron-Laser Acceleration, submitted to PRL). Another diagnostic under development at the ATF is tomographic analysis of the distribution of electrons in transverse phase space (Ilan Ben-Zvi, Joe X. Qiu and Xijie Wang, Picosecond-resolution slice-emittance measurement of electron bunches, invited talk at the 1997 Particle Accelerator Conference, Vancouver, Canada). The slice emittance technique allows the direct observation of the emittance compensation process.The next step is to pursue non-linear emittance compensation. Laser photocathode RF guns have provided a major improvement in the brightness, which was further enhanced by the introduction of (linear) emittance compensation. The dream of another major improvement by the introduction of non-linear corrections has been brought within reach by the development of the slice-emittance diagnostic and the availability of lasers with longitudinal pulse shaping.
Compression of the electron beam at an early stage (immediately following the photoinjector) in a drift space (without the use of magnets) may be important for preservation of the beam quality past the photoinjector.
Research on photoinjectors can and should be done in a number of areas:
While many measurements of the emittance of photoinjectors has been done under a variety of conditions, there is no published exhaustive set of measurements exposing the dependence of emittance on the relevant gun and laser parameters such as phase, electric fields, laser spot size, laser pulse length, solenoid setting, and charge. These measurements should be carried out for both integrated and slice emittance to gain a complete understanding of the photoinjector. There are simulations that cover sub-sets of the above huge parameter space but even these are not complete.
The parametric study would not be complete or even completely meaningful unless the complete six dimensional phase space of the gun is measured (four dimensions if one can assure cylindrical symmetry) using tomographic techniques. This would be the only way to gain true understanding of the various emittance growth mechanisms and possible trade-off among parameters.
Having done this, the next challenge and perhaps most rewarding research in terms of potential improvement of the beam brightness would be through non-linear emittance compensation. This direction has been made possible by the advent of the slice-emittance diagnostic technique. Non-linear compensation can be done longitudinally by shaping the longitudinal power distribution of the photocathode laser. Transverse compensation can be done by shaping the transverse distribution of the laser power. Ultimately both longitudinal and transverse correction must be made simultaneously.
Bunch compression in and immediately following the photoinjector has been achieved recently. Partial compression of the electron bunch at his early stage can be important for the following reasons: Simulations show that the optimal bunch length under emittance compensation conditions is relatively long - enough so that energy spread due to the curvature of the linac RF waveform and transverse wake-field emittance growth may be a problem. A reduction of the bunch length immediately following the gun will help. It is very desirable to avoid magnetic compression at the low energy of the photoinjector. Emittance compensation under such compression has to be studied.
The emission properties of the photocathode covers many areas requiring extensive R&D. The thermal emittance of photocathodes has to be measured and its dependence on the photocathode material studied. The thermal emittance is the ultimate limit on what is the smallest emittance that may be achieved by photoinjectors. We must continue the study of the material properties of various photocathodes to improve the quantum efficiency and robustness. A better understanding of the Schottky effect, the field enhancement coefficient, the work function and quantum efficiency for the common cathode material such as copper, magnesium, cesium telluride is necessary. Furthermore, the most successful photocathode materials, magnesium and cesium telluride are quite recent. It is possible that materials that combine a better combination of high quantum efficiency and robustness wait to be discovered.
Superconducting photoinjectors have been tried with a very limited success. It is clear that the most promising accelerator for short wavelength FELs is the superconducting linac, particularly when a high average power, high energy stability and low wake-fields are desired. A superconducting photoinjector is the natural, best matched injector for such a machine. The development of such a device is very difficult due to the need of extremely high surface electric fields and the difficulty of maintaining a photocathode in the superconducting environment without compromising either
The use of multiple frequency RF in a photoinjector has been studied theoretically and can improve both longitudinal and transverse phase space characteristics. Due to the complications of tuning and powering a gun at multiple frequencies this has not been tried so far.
Finally, the performance of photocathode drive lasers has improved considerably in recent years but it is still an area where more progress is necessary. Areas of research include improved stability (energy, pointing, phase, long term and short term) and control of properties such as transverse and longitudinal shape, wavefront shaping and pulse length.
To be a little bit more precise, let us write down some expressions:
At the cathode, the normalized rms emittance is given by
For a uniform distribution in radius (up to a maximum of rc) and in the direction of p,
The thermal emittance, (cathode radius rc, temperature T) is
This is an extremely good emittance, but it is the limit rather than the norm. The emittance coming out of the gun is usually dominated by emittance growth stemming from space-charge forces and rf effects. The current I is related to the current density J,
Thus the normalized brightness, Bn is given by ;
and at the thermal limit:
Thermionic cathodes can provide J~10 A/cm2, but photocathode can go to J~105 A/cm2 or more. That is the reason why photoinjectors may reach a high brightness. But in order to realize this promise, we must be able to use sophisticated diagnostics such as the Slice Emittance and Phase Space Tomography and apply emittance corrections that will remove the phase-space correlations introduced by the space-charge and rf forces.
Last Modified: December 3, 2007