The Physics and Applications of High Brightness Electron Beams

PREFACE: THE PHYSICS AND APPLICATIONS OF HIGH BRIGHTNESS ELECTRON BEAMS.

Contents.

Working group A was devoted to high brightness beam production and characterization. The presentations and discussions could be categorized as cathode physics, new photoinjector designs, computational modeling of high brightness beams, and new experimental methods and results. Several novel injector and cathode designs were presented. However, a standard 1.5 cell rf photoinjector is still the most common source for high brightness beams. New experimental results and techniques were presented and thoroughly discussed. The brightest beam produced in a rf photoinjector published at the time of the workshop is approximately 2 10¹⁴ A/(m-rad)² at Sumitomo Heavy Industries in Japan with 1 nC of charge, a 9 ps FWHM long laser pulse and a normalized transverse emittance of 1.2 μm. The emittance was achieved by utilizing a temporally flat laser pulse which decreased the emittance by an estimated factor of 2 from the beam produced with a Gaussian pulse shape with an identical pulse length.

The relation between the emittance and the brightness of a charged-particle beam is reviewed, and several examples are used to illustrate the relationship for typical beams.

Laser beam with high stability and quality to illuminate the photocathode is of great importance to generate an electron beam of high brightness from the photocathode RF gun system. In order to stabilize the electron beam, the laser system and the synchronization between the laser pulse and RF signal were improved. The synchronization with the time jitter of 1.2 ps was realized with a frequency feedback of the Ti:Sapphire oscillator and the RF generation from the laser pulses. Considering the emittance growth caused by the space charge effect, the ideal laser profile should be cylindrical with uniform density. The disturbed profile of the laser pulse coming from the third harmonics generator of Ti:Sapphire laser was homogenized as quasi-flattop with a microlens array. The horizontal normalized rms emittance of the electron beam was reduced to 2πmm-mrad for a charge per bunch of 0.1 nC due to the improvement of spatial profile. The laser injection angle of 66 degrees causes a laser spot image deformation to an ellipsoidal image on the cathode. This leads to a lag of arriving time on the cathode as well as the expansion in horizontal diameter of the laser beam. Deformation of laser profile to an ellipsoidal image was also carried out with a pair of cylindrical lenses to get a round image on the cathode. The transverse beam emittance was improved to 5.4 πmm-mrad at 0.2 nC with the deformation optics, while it was 6.6 πmm-mrad at 0.1 nC without them.

Photo-injectors beam physics remains a fruitful and exciting field of research. New ideas have been recently proposed to achieve ultra-high brightness beams, as particularly needed in SASE-FEL experiments, and to produce flat beams as required in linear colliders. An overview of recent advancements in photo-injector beam physics is reported in this paper.

The high brightness required by a short wavelength SASE FEL may be reached only with an accurate design of the electron beam dynamics from the generation in the rf-injector up to the undulator. The beam dynamics is affected by strong self consistent effects at injection, in the compression stage and during the FEL process. The support of numerical simulations is extensively used in the predictions of the beam behaviour in these non linear dynamical conditions. We present a review of available simulation tools, currently exploited in the design of a short wavelength free electron laser.

Laser-triggered switching of MV DC voltages enables acceleration gradients an order of magnitude higher than in state-of-the-art RF photoguns. In this way ultra-short, high-brightness electron bunches may be generated without the use of magnetic compression. The evolution of the bunch during the critical initial part of the acceleration trajectory, the ‘pancake’ regime, where the space-charge induced deterioration is most severe, is investigated using a simple, but effective analytical model. We find an expression for the maximally achievable peak current that does not depend on the bunch charge. An expression for the normalized emittance is derived, which allows us to calculate the optimal beam radius. It is shown that both the peak current and the transverse emittance required for the most challenging applications can be attained without magnetic compression, if acceleration gradients of 1 GV/m can be realized. The results are confirmed by simulations with the GPT code, assuming a 1 GV/m acceleration field and a 50 fs laser pulse, generating 100 pC of charge. The model is complementary to simulations in the sense that it supplies useful scaling laws and improved understanding of the physics involved. Interestingly, we find that the highest brightness is achieved with the shortest photoemission laser pulses.

The generation of electron beam with high phase space density is one of the principal challenge for the next generation of ultra-short wavelength light sources and for the foreseen linear colliders projects. In this paper we review some of the recent experimental advances in the field of electron sources based on the photo-electric effect.

The TREDI Monte-Carlo program is briefly described, devoting some emphasis to the Lienard-Wiechert potentials approach followed to account for self-fields effects and the covariant technique devised to achieve regularization of electro-magnetic fields. Some guidelines to the choice of the correct parameters to be used in the simulation are also summarily sketched. The predictions obtained for the reference workpoint of the space-charge compensated SPARC project photo-injector are finally compared to those from other well-established simulation codes.

Proposed fourth generation light sources using SASE FELs to generate short pulse, coherent, X-rays require demonstration of high brightness electron sources. The Gun Test Facility (GTF) at SLAC was built to test high brightness sources for the proposed Linac Coherent Light Source at SLAC. The GTF is composed of an S-band photocathode rf gun with a Cu cathode, emittance compensating solenoid, single 3 m SLAC linac section and e-beam diagnostic section with a UV drive laser system. The longitudinal emittance exiting the gun has been determined by measuring the energy spectrum downstream of the linac as a function of the linac phase. The e-beam pulse width, correlated and uncorrelated energy spread at the linac entrance have been fit to the measured energy spectra using a least square error fitting routine. The fit yields a pulse width of 2.9 ps FWHM for a 4.3 ps FWHM laser pulsewidth and 2% rms correlated energy spread with 0.07% rms uncorrelated energy spread. The correlated energy spread is enhanced in the linac to allow slice emittance measurements by conducting a quadrupole scan in a dispersive section. The normalized slice emittance has been measured to be as low as 2 mm-mrad for beams with peak currents up to 150 A (300 pC with a laser pulse length of 1.8 ps) while the full projected emittance is 3 mm-mrad.

The RF gun based photoinjector of the TESLA Test Facility Linac (TTFL) at DESY has been built to produce a beam close to TESLA specifications in order to test superconducting accelerating structures. With the installation of the TTF Free-Electron Laser (TTF-FEL) in the TTF linac, the injector has been gradually optimized to improve the gain of the SASE lasing process and to achieve saturation in the VUV wavelength region. The report describes the performance of the optimized injector in terms of longitudinal and transverse phase space.

Systematic developments of the photoinjectors for ultrashort and high quality electron beam works are under way in Japan. Sumitomo Heavy Industries succeeded in transformation of the Gaussian shape to the trapezoidal one in the temporal and transverse profiles of the drive laser and achieved 0.9 πmm.mrad with 1 nC/bunch. It is the best data in the transverse aspect. Himeji Inst. Tech. is operating very unique needle-shaped photocathode RF gun with the original Nd/Glass laser for IR-FEL and Compton scattering X-rays. U.Tokyo/JASRI(SPring8)/KEK/NIRS/BNL/etc. are developing and operating the S-band photoinjectors with Cu, Mg and Cs₂Te cathodes, and transmission-type one in near future. Further, U.Tokyo/KEK/NIRS are designing and constructing a new X-band RF-gun/linac/laser system to generate inverse Compton scattering hard X-rays(33-50keV) for intravenous angiography.

Significant progress has been made in the physics of high-brightness beams during the few last years that has stimulated recent advances in the field of beam diagnostics. Known methods have been revised and new ones devised in order to meet specific needs of high-brightness beams. This paper reports the current status of optical methods in beam diagnostics. Techniques and measurements in transverse and longitudinal planes are reviewed. Advantages and limitations of different methods are briefly discussed.

The laser system proposed for SPARC photoinjector is described in this work. Lasers driving high brightness electron sources have to show specific performances: high pulse energy, uniform temporal and spatial distribution and low energy jitter from pulse to pulse. The emitted pulses have to be synchronized with a master oscillator within 1 ps time jitter to extract electron at exact phase of RF wave. Laser parameters tolerances have been studied with the aid of Parmela and Homdyn simulation code.

In working group B, the implications that collective effects have on the process of pulse compression were discussed. Various physical effects, from space-charge to coherent synchrotron radiation were examined, in the context of diverse scenarios ranging from magnetic pulse compression to velocity bunching. A particular emphasis was placed on computational results, with some notable new results.

We report detailed measurements of the transverse phase space distortions induced by magnetic chicane compression of a high brightness, relativistic electron beam to sub-ps length. A strong bifurcation in the phase space is observed when the beam is strongly compressed. This effect is analyzed using several computational models, and is correlated to the folding of longitudinal phase space. The impact of these results on current research in collective beam effects in bending systems, and implications for future short wavelength free-electron lasers and linear colliders are discussed.

In this paper we describe the rectilinear compression experiment at the Neptune photoinjector at UCLA. The electron bunches have been shortened to sub-ps pulse length by chirping the beam energy spectrum in a short S-band high gradient standing wave RF cavity and then letting the electrons undergo velocity compression in the following rectilinear drift. Using a standard Martin Puplett interferometer to characterize coherent transition radiation from the beam, we measured bunch length as short as 0.4 ps with compression ratio in excess of 10 for an electron beam of 7 MeV energy and charge up to 300 pC. We also measured slice transverse emittance via quad scan technique after a 45 degrees dispersing dipole. Three-dimensional simulations agree with the observed emittance growth.

An important necessary condition for transverse phase space damping in the optical stochastic cooling with transit-time method is derived. The longitudinal and transverse damping dynamics for the optical stochastic cooling is studied. We also obtain an optimal laser focusing condition for laser-beam interaction in the correction undulator. The amplification factor and the output peak power of the laser amplifier are found to differ substantially from earlier publications. The required power is large for hadron colliders at very high energy.

We present experimental results of a bunch compression scheme that uses a traveling wave accelerating structure as a bunch compressor. The bunch length issued from a laser-driven radio-frequency electron source was compressed by a factor >3 using an S-band traveling wave structure located immediately downstream of the electron source. Experimental data are found to be in good agreement with particle tracking simulations.

A self-amplified spontaneous emission free-electron laser (SASE FEL) is a device which is based on the creation of a very intense, relativistic electron beam which has very little temperature in all three phase planes. The beam in this system is described to as having “high brightness”, and when it is bent repetitively in a magnetic undulator, undergoes a radiation-mediated microbunching instability. This instability can amplify the original radiation amplitude at a particular, resonant wavelength by many orders of magnitude. In order to create high brightness beams, it is necessary to compress them to create higher currents than available from the electron source. Compression is accomplished by use of magnetic chicanes, which are quite similar to, if much longer than, a single period of the undulator. It should not be surprising that such chicanes also support a radiation-mediated microbunching interaction, which has recently been investigated, and has been termed coherent synchrotron radiation (CSR) instability. The purpose of this paper is to compare and contrast the characteristics of the closely related FEL and CSR microbunching instabilities. We show that a high gain regime of the CSR instability exists which is formally similar to the FEL instability.

We take a detour from the main theme of this volume and present a discussion of coherent synchrotron radiation (CSR) in the context of storage rings rather than single-pass systems. Interest in this topic has been revived by a series of measurements carried out at several light source facilities. There is strong evidence that the observed coherent signal is accompanied by a beam instability, possibly driven by CSR itself. In this paper we review a “self-consistent” model of longitudinal beam dynamics in which CSR is the only agent of collective forces. The model yields numerical solutions that appear to reproduce the main features of the observations.

Working group C, “Application to FELs,” of the Joint ICFA Advanced Accelerator and Beam Dynamics Workshop on July 1-6, 2002 in Chia Laguna, Sardinia, Italy addressed a total of nine topics. This summary will discuss the topics that were addressed in the stand-alone sessions, including Start-To-End Simulations, SASE Experiment, PERSEO, “Optics Free” FEL Oscillators, and VISA II.

The use of short laser pulses to generate high peak intensity, ultra-short x-ray pulses enables experimental capabilities that are otherwise unattainable. In principle, femtosecond-scale pump-probe experiments can be used to temporally resolve structural dynamics of materials on the time scale of atomic motion. Current research at Lawrence Livermore National Laboratory is leading toward such a novel x-ray source. The system is based on a low emittance photoinjector, a 100 MeV electron RF linac, and a 300 mJ, 35 fs solid-state laser system. The Thomson source will produce ultra-fast pulses with x-ray energies capable of probing into high-Z metals. A wide range of material and plasma physics studies with unprecedented time resolution will become possible.

In recent years a strong attention arose around the problem of the e.m. interaction of an ultra-relativistic beam with the residual roughness inside a beam tube, in particular in the framework of future 4th generation coherent light sources. The main concern was the effect of the wake-fields on the relative energy spread of the beam which has to be of the order of 10⁻⁴, as for example in the LCLS and TESLA case. Although the real roughness has a stochastic feature, most studies dealt with periodic structure, or dielectric-equivalent layer which are considered to be conservative with respect the stochastic case. In this paper we will review the main theoretical models, and the most significant measurements trying to provide to the reader a complete picture of the present status of understanding.

Free-Electron Lasers as high-brilliance radiation sources, rely on a high quality of the electron beam driving the FEL process. The amount of energy, transferred from the electrons to the radiation field, and thus the efficiency of the FEL depends on the provided beam parameters. The presentation discusses the impact of various beam parameter and how current designs of FEL injector try to accomplish the demands on the beam quality for reaching saturation.

At the end of 2001 the Italian Government launched a call for proposals to the national research institutions for the design and construction of an Ultra-Brilliant X-ray Laser, with a dedicated funding up to 94 million € for this initiative. The Italian community responded by submitting two proposals, both envisaging a Linac based SASE-FEL as a source of coherent X-ray radiation. The two anticipated machines are quite similar to each other, as they basically consist of a few GeV Linac driving a SASE-FEL operated at a minimum wavelength around 15 Å. Here we describe the essential features of the two proposals: SPARX has been prepared by a collaboration among CNR-ENEA-INFN and Universita’ di Roma “Tor Vergata”, while FERMI@ELETTRA by INFM and Sincrotrone Trieste. We will also illustrate the status of the R&D project SPARC, aiming at the design and construction of an advanced 150 MeV photo-injector for generating a high brightness electron beam to drive a SASE-FEL in the optical range. This project has been approved by the Italian Government to conduct an R&D activity aimed to be strategic on the way to the coherent X-ray source; it is pursued by a CNR-ENEA-INFN-Universita’ Tor Vergata-INFM-ST collaboration and will be located in the INFN National Laboratory at Frascati.

The work group D in application to advanced accelerators at the Sardinia workshop in “The Physics and Applications of High Brightness Electron Beams” focused on plasma acceleration this time. It held a series of meetings during the workshop. Presentations and discussion were performed on electron beam driven plasma wakefield acceleration, laser plasma wakefield acceleration and laser plasma X-ray generation. Finally, It discussed the integration to accelerator systems.

There has been much experimental and theoretical interest in blowout regime of plasma wakefield acceleration (PWFA), which features ultra-high accelerating fields, linear transverse focusing forces, and nonlinear plasma motion. Using an exact analysis, we examine here a fundamental limit of nonlinear PWFA excitation, by an infinitesimally short, relativistic electron beam. The beam energy loss in this case is shown to be linear in charge even for nonlinear plasma response, where a normalized, unitless charge exceeds unity, and relativistic plasma effects become important or dominant. The physical bases for this persistence of linear response are pointed out.

Plasma wakefield experiments are performed with the 28.5 GeV electron and positron beams of the Stanford Linear Accelerator Center. The lithium plasma is 1.4 m long and a density in the 0-2×10¹⁴ cm⁻³. Experimental results are presented which include the focusing of the electron and positron beams, the emission of x-ray radiation by the beam electrons resulting from their betatron motion in the plasma, the refraction of the electron beam at the plasma boundary, and the energy loss and gain by electrons and positrons. Finally, a brief look is given at the future of these experiments.

The energy loss and gain of a beam in the nonlinear, “blowout” regime of the plasma wakefield accelerator (PWFA), which features ultra-high accelerating fields, linear transverse focusing forces, and nonlinear plasma motion, has been asserted, through previous observations in simulations, to scale linearly with beam charge. In a new analysis that is the companion to this article¹, it has been shown that for an infinitesimally short beam, the energy loss is indeed predicted to scale linearly with beam charge for arbitrarily large beam charge. This scaling holds despite the onset of a relativistic, nonlinear response by the plasma, when the number of beam particles occupying a cubic plasma skin-depth exceeds that of plasma electrons within the same volume. This paper is intended to explore the deviations from linear energy loss using 2D particle-in-cell (PIC) simulations that arise in the case of finite length beams. The peak accelerating field in the plasma wave excited behind the finite-length beam is also examined, with the artifact of wave spiking adding to the apparent persistence of linear scaling of the peak field amplitude well into the nonlinear regime. At large enough normalized charge, the linear scaling of both decelerating and accelerating fields collapses, with serious consequences for plasma wave excitation efficiency. Using the results of parametic PIC studies, the implications of these results for observing the collapse of linear scaling in planned experiments are discussed.

Plasma density transition trapping is a recently purposed self-injection scheme for plasma wake-field accelerators. This technique uses a sharp downward plasma density transition to trap and accelerate background plasma electron in a plasma wake-field. This paper examines the quality of electron beams captured using this scheme in terms of emittance, energy spread, and brightness. Two-dimensional Particle-In-Cell (PIC) simulations show that these parameters can be optimized by manipulating the plasma density profile. We also develop, and support with simulations, a set of scaling laws that predict how the brightness of transition trapping beams scales with the plasma density of the system. These scaling laws indicate that transition trapping can produce beams with brightness ≥ 5x10¹⁴Amp/(m-rad)². A proof-of-principle transition trapping experiment is planned for the UCLA Neptune Laboratory in the near future. The proposed experiment and its status are described in detail.

We have studied generation of relativistic electrons by interaction between a high intensity ultra-short laser pulse (Ti:Sapphire, 12 TW, 50 fs, λ=790 nm) and gas jet. In the experiment, spatial and energy distribution of energetic electrons produced by an ultra-short, intense laser pulse in a He gas jet are measured. They depend strongly on the contrast ratio and shape of the laser prepulse. In the case of a proper prepulse the electrons are injected at the shock front produced by the prepulse and accelerated by consequent plasma wake-field up to tens MeV forming a narrow-coned ejection angle. In the case of non-monotonic prepulse, hydrodynamic instability leads to a broader, spotted spatial distribution. The numerical analysis based on a 2D hydrodynamics (for the laser prepulse) and 2D particle-in-cell simulation justify the mechanism of electron injection and acceleration.

With the development of table-top high-peak-power optical laser systems, it has become feasible to generate---for the first time---ultrashort-pulse-duration x-ray probes. Laser produced x-ray sources are promising on account of their compact size, reduced complexity and significant reduction in costs. There is currently a worldwide effort to produce a high brightness x-ray beam using an ultrafast optical pump, for myriad applications in chemistry and biology [1-8].

Investigations on the laser acceleration in vacuum are reviewed including up-date experimental results, projects of new experiments and theoretical prospects of vacuum acceleration schemes for high energies of accelerated particles. Most attention is devoted to analysis of the IFELs with focused laser beam drivers. The free space acceleration with the axicon and the Gaussian beam are also considered as prospects for the high energy particle acceleration. It is shown that at high quality of accelerated beams the vacuum laser accelerators could provide required high acceleration gradients as well as high energy gains and can be considered as possible prototypes of the future accelerators.

Slab-symmetric dielectric-loaded structures, consisting of a vacuum gap between dielectric-lined conducting walls, have become a subject of interest for high-gradient acceleration of high-charge beams due to their simplicity, relatively low power density, and advantageous beam dynamics. Such a structure can be resonantly excited by an external power source and is known to strongly suppress transverse wakefields. Motivated by the prospect of a high-power FIR radiation source, currently under construction at UCLA, we investigate a high-gradient slab-symmetric accelerator powered by up to 100 MW of laser power at 340 μm, with a predicted gradient near 100 MeV/m. Theory and simulation studies of the structure fields and wakes are presented, with an outline of a future experiment.

List of Participants.