Brightness Electron Beams Research Articles

Analysis on effects of transverse electric field in an injector cavity of compact-ERL at KEK

For a future synchrotron light source based on a linac, e.g. an X-ray free electron laser and an energy recovery linac (ERL), an injector is a key component to generate a high brightness electron beam. For the acceleration and transportation of the electron beam in the injector, the adjustment of beam orbit inside the cavity is important to avoid the deterioration of the beam quality due to the transverse electric field of it, which causes the transverse emittance growth. To adjust the beam orbit, an investigation of the electromagnetic center of the cavity is required in the beam operation. This paper shows a new method for measuring the electromagnetic center of the cavity, and describes an analytical model of emittance growth due to a combination of transverse electric field and orbit offset. The validation of the method was confirmed by the emittance measurement in the compact ERL (cERL) injector at KEK.

External-injection Experiment at SPARC_LAB

At the SPARC_LAB facility of INFN-LNF we are installing two transport lines for ultra-short electron bunches and an ultra- intense laser pulses, generated by the SPARC photo-injector and by the FLAME laser in a synchronized fashion at the tens of f s level, to co-propagate inside a hydrogen filled glass capillary, in order to perform acceleration of the electron bunch by a plasma wave driven by the laser pulse. The main aim of this experiment is to demonstrate that a high brightness electron beam can be accelerated by a plasma wave without any significant degradation of its quality. Motivations of the technical choices made and expected performances are reported.

The External-Injection experiment at the SPARC_LAB facility

At the SPARC_LAB facility of INFN-LNF we are installing a transport lines for ultra-short electron bunches and another for ultra-intense laser pulses, generated by the SPARC photo-injector and by the FLAME laser in a synchronized fashion at the tens of fs level, to co-propagate inside a hydrogen filled glass capillary, in order to perform acceleration of the electron bunch by a plasma wave driven by the laser pulse. The main aim of this experiment is to demonstrate that a high brightness electron beam can be accelerated by a plasma wave without any significant degradation of its quality. Motivations of the technical choices are made and expected performances are reported.

Theoretical and experimental analysis of a linear accelerator endowed with single feed coupler with movable short-circuit

The front-end injection systems of the FERMI@Elettra linac produce high brightness electron beams that define the performance of the Free Electron Laser. The photoinjector mainly consists of the radiofrequency (rf) gun and of two S-band rf structures which accelerate the beam. Accelerating structures endowed with a single feed coupler cause deflection and degradation of the electron beam properties, due to the asymmetry of the electromagnetic field. In this paper, a new type of single feed structure with movable short-circuit is proposed. It has the advantage of having only one waveguide input, but we propose a novel design where the dipolar component is reduced. Moreover, the racetrack geometry allows to reduce the quadrupolar component. This paper presents the microwave design and the analysis of the particle motion inside the linac. A prototype has been machined at the Elettra facility to verify the new coupler design and the rf field has been measured by adopting the bead-pull method. The results are here presented, showing good agreement with the expectations.

A brightness exceeding simulated Langmuir limit

When an excitation of the first lens determines a beam is parallel beam, a brightness that is 100 times higher than Langmuir limit is measured experimentally, where Langmuir limits are estimated using a simulated axial cathode current density which is simulated based on a measured emission current. The measured brightness is comparable to Langmuir limit, when the lens excitation is such that an image position is slightly shorter than a lens position. Previously measured values of brightness for cathode apical radii of curvature 20, 60, 120, 240, and 480 μm were 8.7, 5.3, 3.3, 2.4, and 3.9 times higher than their corresponding Langmuir limits, respectively, in this experiment, the lens excitation was such that the lens and the image positions were 180 mm and 400 mm, respectively. From these measured brightness for three different lens excitation conditions, it is concluded that the brightness depends on the first lens excitation. For the electron gun operated in a space charge limited condition, some of the electrons emitted from the cathode are returned to the cathode without having crossed a virtual cathode. Therefore, method that assumes a Langmuir limit defining method using a Maxwellian distribution of electron velocities may need to be revised. For the condition in which the values of the exceeding the Langmuir limit are measured, the simulated trajectories of electrons that are emitted from the cathode do not cross the optical axis at the crossover, thus the law of sines may not be valid for high brightness electron beam systems.

Theoretical study and simulation for a nanometer laser based on Gauss–Hermite source expansion

Recently there has been worldwide interest in constructing a new generation of continuously tunable nanometer lasers for a wide range of scientific applications, including femtosecond science, biological molecules, nanoscience research fields, etc. The high brightness electron beam required by a short wavelength self-amplified spontaneous emission FEL can be reached only with accurate control of the beam dynamics in the facility. Numerical simulation codes are basic tools for designing new nanometer laser devices. We have developed a MATLAB quasi-one-dimensional code based on a reduced model for the FEL. The model uses an envelope description of the transverse dynamics of the laser beam and full longitudinal particle motion. We have optimized the LCLS facility parameters, then given the characteristics of the nanometer laser.

Maximum brightness of linac-driven electron beams in the presence of collective effects

Linear accelerators capable of delivering high brightness electron beams are essential components of a number of research tools, such as free electron lasers (FELs) and elementary particle colliders. In these facilities the charge density is high enough to drive undesirable collective effects (wakefields) that may increase the beam emittance relative to the injection level, eventually degrading the nominal brightness. We formulate a limit on the final electron beam brightness, imposed by the interplay of geometric transverse wakefield in accelerating structures and coherent synchrotron radiation in energy dispersive regions. Numerous experimental data of vacuum ultraviolet and x-ray FEL drivers validate our model. This is then used to show that a normalized brightness of $\ensuremath{\sim}{10}^{16}\text{ }\text{ }\mathrm{A}/{\mathrm{m}}^{2}$, promised so far by ultralow charge beams ($\ensuremath{\sim}1--10\text{ }\text{ }\mathrm{pC}$), can in fact be reached with a 100 pC charge beam in the 1.2 GeV FERMI@Elettra accelerator, with the existing machine configuration.

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].

Electron slicing for the generation of tunable femtosecond soft x-ray pulses from a free electron laser and slice diagnostics

We present the experimental results of femtosecond slicing an ultrarelativistic, high brightness electron beam with a collimator. In contrast to some qualitative considerations reported in Phys. Rev. Lett. 92, 074801 (2004), we first demonstrate that the collimation process preserves the slice beam quality, in agreement with our theoretical expectations, and that the collimation is compatible with the operation of a linear accelerator in terms of beam transport, radiation dose, and collimator heating. Accordingly, the collimated beam can be used for the generation of stable femtosecond soft x-ray pulses of tunable duration, from either a self-amplified spontaneous emission or an externally seeded free electron laser. The proposed method also turns out to be a more compact and cheaper solution for electron slice diagnostics than the commonly used radio frequency deflecting cavities and has minimal impact on the machine design.

Impact of radiator length in the emitted power for a high gain harmonic generation free-electron laser

We present a numerical study that characterizes the dependence on the radiator length of the output power produced by a free-electron laser (FEL) operated in the high gain harmonic generation (HGHG) configuration. Using the main parameters of the FERMI@Elettra FEL, numerical simulations of the FEL process have been performed for different lengths of the radiator. Our results show that in the case of HGHG the achievable output power has a dependence on the radiator length that is linear. The impact of the electron beam parameters on the achievable maximum power vs radiator length dependence is also studied. A normalization of the results to the FEL saturation power and to the FEL gain length shows that this dependence can be expressed by a universal linear equation that, in some conditions, is independent on the electron beam current and brightness. The reported results could be useful for the design of future FELs based on the HGHG scheme and could be used for a quick estimate of the best undulator length.

Using the Relativistic Two-Stream Instability for the Generation of Soft-X-Ray Attosecond Radiation Pulses

In this Letter we discuss a novel method for generating ultrashort radiation pulses using a broadband two-stream instability in an intense relativistic electron beam. This method relies on an electron beam having two distinct two-energy bands. The use of this new high brightness electron beam scenario, in combination with ultrashort soft x-ray pulses from high harmonic generation in gas, allows the production of high power attosecond pulses for ultrafast pump and probe experiments.

Electron beam collimation with a 40 000 tip metallic double-gate field emitter array and in-situ control of nanotip sharpness distribution

The generation of highly collimated electron beams from a double-gate field emitter array with 40000 metallic tips and large collimation gate apertures is reported. Field emission beam measurements demonstrated the reduction of the beam envelope down to the array size by applying a negative potential to the on-chip gate electrode for the collimation of individual field emission beamlets. Owing to the optimized gate structure, the concomitant decrease of the emission current was minimal, leading to a net enhancement of the current density. Furthermore, a noble gas conditioning process was successfully applied to the double-gate device to improve the beam uniformity in-situ with orders of magnitude increase of the active emission area. The results show that the proposed double-gate field emission cathodes are promising for high current and high brightness electron beam applications such as free-electron lasers and THz power devices.

Cancellation of Coherent Synchrotron Radiation Kicks with Optics Balance

Minimizing transverse emittance is essential in linear accelerators designed to deliver very high brightness electron beams. Emission of coherent synchrotron radiation (CSR), as a contributing factor to emittance degradation, is an important phenomenon to this respect. A manner in which to cancel this perturbation by imposing certain symmetric conditions on the electron transport system has been suggested.We first expand on this idea by quantitatively relating the beam Courant-Snyder parameters to the emittance growth and by providing a general scheme of CSR suppression with asymmetric optics, provided it is properly balanced along the line. We present the first experimental evidence of this cancellation with the resultant optics balance of multiple CSR kicks: the transverse emittance of a 500 pC, sub-picosecond, high brightness electron beam is being preserved after the passage through the achromatic transfer line of the FERMI@Elettra free electron laser, and emittance growth is observed when the optics balance is intentionally broken. We finally show the agreement between the theoretical model and the experimental results. This study holds the promise of compact dispersive lines with relatively large bending angles, thus reducing costs for future electron facilities.

Experimental verification of the 3-step model of photoemission for energy spread and emittance measurements of copper and CsBr-coated copper photocathodes suitable for free electron laser applications

This paper presents measurements and analysis of the quantum efficiency (QE) and intrinsic emittance of Cu and CsBr coated Cu photocathodes. The data analysis uses expressions for the quantum efficiency and the intrinsic emittance for metal cathodes previously derived from Spicer's three-step model of photoemission. Data taken with a 257 nm CW laser on (100) Cu crystals indicate an emittance of 0.77 (μm/mm-rms) for CsBr coated and 0.42 (μm/mm-rms) for uncoated cathodes. The high quantum efficiency and low emittance observed for CsBr coated cathodes have applications in free electron laser and other devices requiring high brightness electron beams.

Photon flux and spectrum of [formula omitted] Compton sources

We analyze the characteristics of the γ radiation produced by Compton back-scattering of a high brightness electron beam produced by a photoinjector and accelerated in a linac up to energies of 360–720MeV and a laser operated at about 500nm, by comparing classical and quantum models and codes. The interaction produces γ rays in the range 4.9–18.8MeV. In view of the application to nuclear resonance fluorescence a relative bandwidth of few 10−3 is needed. The bandwidth is reduced by taking advantage of the frequency–angular correlation typical of the phenomenon and selecting the radiation in an angle of tens of μrads. The foreseen spectral density is 20–6 photons per eV in a single shot, a number that can be increased by developing multi-bunch techniques and laser recirculation. In this way a final value of 104 photon per eV per second can be achieved.

Influence of longitudinally tapered collimators on a high brightness electron beam

This article presents the design and operation of a longitudinally tapered collimator in a single-pass $S$-band linac driving a high brightness electron beam. Measurements were done for the transverse emittance growth induced by the collimator wakefield as a function of the lateral displacement of the beam inside the collimator and the energy acceptance provided by an identical collimator installed in a dispersive region. The measurements demonstrate that: (i) the proposed design allows very precise and reproducible motion down to the micron level of the compact, four-hole collimator; (ii) the collimator does not degrade the beam emittance in the presence of standard trajectory control; (iii) the measured kick factor and energy acceptance are in agreement with the theoretical expectations. These measurements were made using 500 pC, 2.4 ps long bunches at the FERMI@Elettra free electron laser facility.

Three dimensional analysis of longitudinal plasma oscillations in a thermal relativistic electron beam: Application to an initial value problem

In this paper, we study the initial value problem of longitudinal plasma oscillations in a relativistic electron beam. Our analysis is based on the formalism developed in Marinelli et al. [Phys. Plasmas 18, 103105 (2011)]. We study the evolution of an arbitrary six-dimensional phase-space perturbation under the effect of longitudinal space-charge forces, with the inclusion of three-dimensional effects due to the finite size of the beam, transverse betatron motion, and longitudinal thermal motion induced by both energy-spread and transverse emittance. We expand the phase-space perturbation in a series of eigenmodes of a Schrödinger-like equation, corresponding to a set of propagating space-charge waves. We develop a general formalism, which we use to find explicit expressions for the evolution of an initial perturbation coupled to the fundamental plasma eigenmode. This work has important applications in the theory of space-charge instabilities in high brightness electron beams and control of shot-noise in seeded free-electron lasers. We discuss the application of the present theory to a specific experimental scenario corresponding to a shot-noise suppression scheme at optical wavelengths.

Y thin films by ultrashort pulsed laser deposition for photocathode application

In this work, the deposition of Y thin films by laser beams with 0.5ps and 5ps pulse durations at different laser fluences (1.2–6.4J/cm2) is reported. The morphology of the deposited films and of the ablated target surface is investigated by scanning electron microscopy analyses. The present results show that the films, well adherent to the substrates, are characterized by a high abundance of sub-micrometric particulates with average size less than 0.3μm, whose density decreases with increasing laser fluence. The formation of columnar structures observed on the target surface seems to be responsible of the poor film homogeneity. Acceptable deposition rate in the range of 0.08–0.16Å/pulse with 5ps pulse duration is found; on the contrary with 0.5ps pulse duration, it is not possible to get information on deposition rate as a function of the laser fluence due to the high non-uniformity of the films. A comparison with the results previously obtained in ns regime is presented and discussed. The achievements of our investigation will be useful to optimize the synthesis of photocathodes based on Y films for the production of bright electron beams in radio-frequency photoinjectors.

Experimental investigation of thermal emittance components of copper photocathode

With progress of photoinjector technology, thermal emittance has become the primary limitation of electron beam brightness. Extensive efforts have been devoted to study thermal emittance, but experiment results differ between research groups and few can be well interpreted. Besides the ambiguity of photoemission mechanism, variations of cathode surface conditions during cathode preparation, such as work function, field enhancement factor, and surface roughness, will cause thermal emittance differences. In this paper, we report an experimental study of electric field dependence of copper cathode quantum efficiency (QE) and thermal emittance in a radio frequency (rf) gun, through which in situ cathode surface parameters and thermal emittance contributions from photon energy, Schottky effect, and surface roughness are extracted. It is found the QE of a copper cathode illuminated by a 266 nm UV laser increased substantially to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ after cathode cleaning during rf conditioning, and a copper work function of 4.16 eV, which is much lower than nominal value (4.65 eV), was measured. Experimental results also show a thermal emittance growth as much as $0.92\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}/\mathrm{mm}$ at $50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.

Precision linac and laser technologies for nuclear photonics gamma-ray sources

Tunable, high precision gamma-ray sources are under development to enable nuclear photonics, an emerging field of research. This paper focuses on the technological and theoretical challenges related to precision Compton scattering gamma-ray sources. In this scheme, incident laser photons are scattered and Doppler upshifted by a high brightness electron beam to generate tunable and highly collimated gamma-ray pulses. The electron and laser beam parameters can be optimized to achieve the spectral brightness and narrow bandwidth required by nuclear photonics applications. A description of the design of the next generation precision gamma-ray source currently under construction at Lawrence Livermore National Laboratory is presented, along with the underlying motivations. Within this context, high-gradient X-band technology, used in conjunction with fiber-based photocathode drive laser and diode pumped solid-state interaction laser technologies, will be shown to offer optimal performance for high gamma-ray spectral flux, narrow bandwidth applications.