High Brightness Photoinjector Research Articles

Production of quasi ellipsoidal laser pulses for next generation high brightness photoinjectors

The use of high brightness electron beams in Free Electron Laser (FEL) applications is of increasing importance. One of the most promising methods to generate such beams is the usage of shaped photocathode laser pulses. It has already demonstrated that temporal and transverse flat-top laser pulses can produce very low emittance beams [1]. Nevertheless, based on beam simulations further improvements can be achieved using quasi-ellipsoidal laser pulses, e.g. 30% reduction in transverse projected emittance at 1nC bunch charge.In a collaboration between DESY, the Institute of Applied Physics of the Russian Academy of Science (IAP RAS) in Nizhny Novgorod and the Joint Institute of Nuclear Research (JINR) in Dubna such a laser system capable of producing trains of laser pulses with a quasi-ellipsoidal distribution, has been developed. The prototype of the system was installed at the Photo Injector Test facility at DESY in Zeuthen (PITZ) and is currently in the commissioning phase.In the following, the laser system will be introduced, the procedure of pulse shaping will be described and the last experimental results will be shown.

Cesium telluride cathodes for the next generation of high-average current high-brightness photoinjectors

We report on the performances of a Cs2Te photocathode under extreme conditions of high peak time-dependent accelerating fields, continuous wave operations, and MHz pulse extraction with up to 0.3 mA average current. The measurements, performed in a normal conducting cavity, show extended lifetime and robustness, elucidate the main mechanisms for cathode degradation, and set the required system vacuum performance for compatibility with the operations of a high average power X-ray free electron laser user facility, opening the doors to the next generation of MHz-scale ultrafast scientific instruments.

Photocathodes for High Brightness Photo Injectors

The development of the photo injector has become a key technology for accelerator-based light sources and for the electron collider. There are a lot of opportunities to improve the injector quality, as well as photocathode function. A better photocathode can reduce the specification requirements of the drive laser system. The identification of better photocathodes is always a principal technical challenge, especially for the high brightness injectors. In this contribution, we will briefly review the status of photocathode research and the developing trends that are foreseen for the future.

Design of a compact C-band high brightness photoinjector for an ultra-fast electron diffraction

A compact C-band high brightness photoinjector for ultra-fast electron diffraction (UED) is under development at Tsinghua University. The C-band photoinjector, operating at 5.172 GHz, uses a 1.45-cell C-band RF gun as the electron source. The design of the RF gun has addressed the physics, beam dynamics and mechanical aspects, and based on these design studies, a prototype has been fabricated. In order to obtain optimized performance for the UED, beam dynamics simulation has been carried out using the particle tracking code Astra for the photoinjector consisting of an alternative multi-cell RF gun design and a C-band compressor downstream. The beam dynamics parameters that are optimized include the bunch length, peak current, horizontal emittance, and position of the minimum bunch length.

Optimization of a high brightness photoinjector for a seeded FEL facility

The FERMI@Elettra project is a seeded free electron laser (FEL) source, based on the High Gain Harmonic Generation (HGHG) scheme. It is designed to supply photons in a spectral range from 65 to 20 nm with the first undulator line (FEL-1) and from 20 nm to 4 nm with the second undulator line (FEL-2).After a first period of commissioning of the electron beam up to 100 MeV at low charge, started in August 2009, several phases of installations and beam commissioning periods have been alternated through the 2010. On December 2010 the first FEL light in the FEL-1 line was obtained at 65 nm and 43 nm by adopting as an initial seed the third harmonic of a TiSa laser. The complete optimization and commissioning of the photo-injector has been carried on in parallel with the Linac and FEL commissioning, from a conservative set-up to the final designed configuration. This paper reports the electron beam characterization in the injector area, the comparison with the theoretical expectations and the experimental process which resulted in a high brightness electron beam. This beam was optimized to be compressed and then transported through the undulators of FEL-1 where intense photons ranging from 65 nm to 20 nm were generated [1,2].

Monte Carlo charge transport and photoemission from negative electron affinity GaAs photocathodes

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. Here, photoemission from such photocathodes is modeled using detailed Monte Carlo electron transport simulations. Simulations show a quantitative agreement with the experimental results for quantum efficiency, energy distributions of emitted electrons, and response time without the assumption of any ad hoc parameters. This agreement between simulation and experiment sheds light on the mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance.

Record high-average current from a high-brightness photoinjector

High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.

Effect of nanoscale surface roughness on transverse energy spread from GaAs photocathodes

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the wavelength of incident light.

Emittance control and RF bunch compression in the NSRRC photoinjector

The high-brightness photoinjector being constructed at the National Synchrotron Radiation Research Center is for testing new accelerator and light-source concepts. It is the so-called split photoinjector configuration in which a short solenoid magnet is used for emittance compensation. The UV-drive laser pulses are also shaped to produce uniform cylindrical bunches for further reduction of beam emittance. However, limited by the available power from our microwave power system, the nominal accelerating gradient in the S-band booster linac is set at 18 MV/m. A simulation study with PARMELA shows that the linac operating at this gradient fails to freeze the electron beam emittance at low value. A background solenoid magnetic field is applied for beam emittance control in the linac during acceleration. A satisfactory result that meets our preliminary goal has been achieved with the solenoid magnetic field strength at 0.1 T. RF bunch compression as a means to achieve the required beam brightness for high-gain free-electron laser experiments is also examined. The reduction of bunch length to a few hundred femtoseconds can be obtained.

Laser pulse shaping for generating uniform three-dimensional ellipsoidal electron beams

A scheme of generating a uniform ellipsoidal laser pulse for high-brightness photoinjectors is discussed. The scheme is based on the chromatic aberration of a dispersive lens. Fourier optics simulation reveals the interplay of group velocity delay and dispersion in the scheme, as well as diffractions. Particle tracking simulation shows that the beam generated by such a laser pulse approaches the performance of that by an ideal ellipsoidal laser pulse and represents a significant improvement from the traditionally proposed cylindrical beam geometry. The scheme is tested in an 800-nm, optical proof-of-principle experiment at lower peak power with excellent agreement between the measurement and simulation.

BEAM DYNAMICS OPTIMIZATION FOR THE XFEL PHOTO INJECTOR

The main challenge for the European XFEL photo injector is the production of 1 nC electron beams with a normalized transverse emittance of 0.9 mm mrad. The photo injector setup consists of a 1.5-cell L-band rf gun cavity supplied with solenoids for beam focusing and emittance compensation and the first accelerating section with 8 TESLA superconducting cavities. The first 4 cavities are used as a booster to provide by proper choice of its position, gradient and phase matching conditions for the emittance conservation. For optimization of the beam dynamics in the photo injector, a staged algorithm, based on ASTRA simulations, has been developed. The first stage considers the emission of electrons from a photo cathode. The cathode laser energy and its transverse parameters are adjusted to produce a bunch charge of 1 nC in presence of space charge forces (including image charge at the cathode) and Schottky-like effects. The second stage contains rf gun cavity and solenoid optimization. The booster position, gradient and initial phase are optimized at the third stage yielding the minimum emittance at the photo injector exit. Results of the XFEL photo injector optimization will be presented. Besides simulations experimental studies towards XFEL photo injector are carried out. The photo injector test facility at DESY in Zeuthen (PITZ) develops photo injectors for FELs, including FLASH and the European XFEL. A thorough comparison of measured data with results of beam dynamics simulations is one of the main PITZ goals. Detailed experimental studies on photo emission processes, thermal emittance, transverse and longitudinal phase space of the electron beam are being performed together with beam dynamics simulations. This aims to result in better understanding of beam dynamics in high brightness photo injectors. Experimentally obtained photo injector characteristics (like thermal emittance) have to be used in an additional optimization of the photo injector resulting in more realistic beam dynamics simulations. Results of these studies will be reported as well.

Efficient temporal shaping of electron distributions for high-brightness photoemission electron guns

To achieve the lowest emittance electron bunches from photoemission electron guns, it is essential to limit the uncorrelated emittance growth due to space charge forces acting on the bunch in the vicinity of the photocathode through appropriate temporal shaping of the optical pulses illuminating the photocathode. We present measurements of the temporal profile of electron bunches from a bulk crystal GaAs photocathode illuminated with 520 nm wavelength pulses from a frequency-doubled Yb-fiber laser. A transverse deflecting rf cavity was used to make these measurements. The measured laser pulse temporal profile and the corresponding electron beam temporal profile have about 30 ps FWHM duration, with rise and fall times of a few ps. GaAs illuminated by 520 nm optical pulses is a prompt emitter within our measurement uncertainty of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{ps}\text{ }\mathrm{rms}$. Combined with the low thermal emittance of negative electron affinity photocathodes, GaAs is a very suitable photocathode for high-brightness photoinjectors. We also report measurements of the photoemission response time for GaAsP, which show a strong dependence on the quantum efficiency of the photocathode.

Criteria for emittance compensation in high-brightness photoinjectors

A critical process in high-brightness photoinjectors is emittance compensation, which brings under control the correlated transverse emittance growth due to the linear space-charge force. Although emittance compensation has been used and studied for almost two decades, the exact criteria to achieve emittance compensation is not as clear as it should be. In this paper, a perturbative analysis of slice envelopes and emittance evolution close to any reference envelope is developed, via which space-charge and chromatic effects are investigated. A new criterion for emittance compensation is found, which is complementary to the well-known matching condition for the invariant envelope and agrees very well with simulations.

EMITTANCE DEGRADATION DUE TO WAKE FIELDS IN A HIGH BRIGHTNESS PHOTOINJECTOR

Wake fields effects in addition to space charge forces may have an important impact during the emittance compensation process in a high brightness photo-injector. To study this effect we developed an upgraded version of the Homdyn code including off axis beam dynamics and wake fields. Homdyn describes a bunch as a uniformly charged cylinder, divided in cylindrical slices; in the upgraded version each slice's centroid can be transversally displaced from the nominal axis thus inducing wake fields. When the bunch is short as compared to the beam pipe radius, wake fields for a single cavity are calculated using methods of diffraction theory; instead we use, for a periodic collection of cavities, an asymptotic wake field obtained numerically at SLAC and then fitted to a simple function. As a first application we studied and verified a correction scheme for the SPARC photo-injector to control the bunch trajectory and angle at the entrance of the undulator. The correction scheme consists of a number of steering magnets and beam position monitors placed along the photo-injector. Two different steering approaches are analyzed and the emittance degradation is studied. The code demonstrates the steering positions and number do correct the bunch's orbit and angle and gives good results concerning the emittance degradation. The emittance and energy spread degradation due to wake fields in the emittance meter experiment is also discussed.

SUMMARY OF WORKING GROUP 1: ELECTRON SOURCES

This report summarizes the presentations and discussions in the Working Group "Electron Sources" during the ICFA Advanced Accelerator Workshop on "The Physics and Applications of High-Brightness Electron Beams", held at the Ettore Majorana Foundation and Center for Scientific Culture, Erice, October 9-14, 2005. Latest developments towards a better understanding of high-brightness photo injectors physics and technology are reported.

Hamiltonian analysis of transverse beam dynamics in high-brightness photoinjectors

A general Hamiltonian suitable for perturbative analysis of rapidly accelerating beams is derived from first principles. With the proper choice of coordinates, the resulting Hamiltonian has a simple and familiar form, yet is able to take into account the rapid acceleration, rf focusing, magnetic focusing, and average space-charge forces in rf photoinjectors. From the linear Hamiltonian, the beam-envelope evolution is solved and analyzed, which better illuminates the invariant-envelope solution as well as the theory of emittance compensation. The third-order nonlinear Hamiltonian is derived and analyzed to some extent. To make the analysis systematic and self-contained, alternative derivations are given for the smoothed ponderomotive rf focusing and the transfer matrix of a rf cavity.

Velocity bunching of high-brightness electron beams

Velocity bunching has been recently proposed as a tool for compressing electron beam pulses in modern high brightness photoinjector sources. This tool is familiar from earlier schemes implemented for bunching dc electron sources, but presents peculiar challenges when applied to high current, low emittance beams from photoinjectors. The main difficulty foreseen is control of emittance oscillations in the beam in this scheme, which can be naturally considered as an extension of the emittance compensation process at moderate energies. This paper presents two scenarios in which velocity bunching, combined with emittance control, is to play a role in nascent projects. The first is termed ballistic bunching, where the changing of relative particle velocities and positions occur in distinct regions, a short high gradient linac, and a drift length. This scenario is discussed in the context of the proposed ORION photoinjector. Simulations are used to explore the relationship between the degree of bunching, and the emittance compensation process. Experimental measurements performed at the UCLA Neptune Laboratory of the surprisingly robust bunching process, as well as accompanying deleterious transverse effects, are presented. An unanticipated mechanism for emittance growth in bends for highly momentum chirped beam was identified and studied in these experiments. The second scenario may be designated as phase space rotation, and corresponds closely to the recent proposal of Ferrario and Serafini. Its implementation for the compression of the electron beam pulse length in the PLEIADES inverse Compton scattering (ICS) experiment at LLNL is discussed. It is shown in simulations that optimum compression may be obtained by manipulation of the phases in low gradient traveling wave accelerator sections. Measurements of the bunching and emittance control achieved in such an implementation at PLEIADES, as well as aspects of the use of velocity-bunched beam directly in ICS experiments, are presented.

Overview of FERMI@ELETTRA: a proposed ultra-bright coherent X-ray source in Italy

Together with the INFM and other Italian institutes, Sincrotrone Trieste has proposed and developed FERMI@ELETTRA in response to the Italian government's call for proposals from national institutes for a multipurpose pulsed laser X-ray source. The proposed FEL source will be located at the third-generation synchrotron radiation facility ELETTRA and will utilize the fully available 1.0-GeV normal conducting linac. The project is organized along three stages of development, allowing gradual improvements for the production of 40-, 10-, and 1.2-nm radiation. The first two phases involve upgrading the linac with a high-brightness photo-injector and bunch compressor for the generation of increasingly higher quality electron beams. The third phase requires both an increase in peak current and beam energy to 3.0GeV. The requirements of the user-community necessitate reproducible radiation sources with controllable polarization and wavelength. For this APPLE-type undulators will be implemented and seeding schemes will be used where possible. An overview of the proposal is presented elucidating linac upgrades and overall layout, electron beam simulations, and laser seed schemes.

Conceptual design of a high-brightness linac for soft X-ray SASE-FEL source

FELs based on SASE are believed to be powerful tools to explore the frontiers of basic sciences, from physics to chemistry to biology. Intense R&D programs have started in the USA and Europe in order to understand the SASE physics and to prove the feasibility of these sources. The allocation of considerable resources in the Italian National Research Plan (PNR) brought about the formation of a CNR–ENEA–INFN–University of Roma “Tor Vergata” study group. A conceptual design study has been developed and possible schemes for linac sources have been investigated, leading to the SPARX proposal. We report in this paper the results of a preliminary start to end simulation concerning one option we are considering based on an S-band normal conducting linac with high-brightness photoinjector integrated in an RF compressor.

Initial experimental results of CAEP photoinjector

An RF photocathode gun which, along with a compact linac, forms the injection system for a planned FIR FEL amplifier experiment, has been commissioned in the high power radiation laboratory. This high-gradient gun has emitted 60 ps, 10 A electron beams of up to 3 MeV energy. These beams have been characterized by a variety of diagnostics. The quantum efficiency of Cs 2Te has been measured at normal incidence. The energy and energy spread of the beam were tested using a dipole spectrometer, while the time structure was examined with a picosecond resolution streak camera. The emittance of the beam was measured using the pepper pot technique and its dependence on space charge and RF phase were found. The impact of these results on improving the design and operation of this high brightness photoinjector is discussed. This injector will be applied to the FIR FEL project in future.