Laser-driven Rf Gun Research Articles

An overview of beam diagnostic and control systems for 50 MeV AREAL Linac

Advanced Research Electron Accelerator Laboratory (AREAL) is an electron linear accelerator project with a laser driven RF gun being constructed at CANDLE Synchrotron Research Institute. After the successful operation of the gun section at 5 MeV, a program of facility energy enhancement up to 50 MeV is launched. In this paper the current status of existing diagnostic and control systems, as well as the results of electron beam parameter measurements are presented. The approaches of intended diagnostic and control systems for the upgrade program are also described.

AREAL test facility for advanced accelerator and radiation source concepts

Advanced Research Electron Accelerator Laboratory (AREAL) is a 50MeV electron linear accelerator project with a laser driven RF gun being constructed at the CANDLE Synchrotron Research Institute. In addition to applications in life and materials sciences, the project aims as a test facility for advanced accelerator and radiation source concepts. In this paper, the AREAL RF photoinjector performance, the facility design considerations and its highlights in the fields of free electron laser, the study of new high frequency accelerating structures, the beam microbunching and wakefield acceleration concepts are presented.

AREAL low energy electron beam applications in life and materials sciences

The AREAL laser-driven RF gun provides 2–5MeV energy ultrashort electron pulses for experimental study in life and materials sciences. We report the first experimental results of the AREAL beam application in the study of molecular-genetic effects, silicon-dielectric structures, ferroelectric nanofilms, and single crystals for scintillators.

Optimizations of transverse projected emittance at the photo-injector test facility at DESY, location Zeuthen

High brightness electron sources for linac based short-wavelength free-electron lasers are developed and optimized for small transverse projected emittance at the photo-injector test facility at DESY, location Zeuthen (PITZ). A major part of the measurement program at PITZ is dedicated to transverse phase space optimization in order to fulfill the requirements of the European X-ray free-electron laser (European XFEL). A laser-driven RF-gun, treated with a dry-ice sublimation-impulse cleaning technique, a new photocathode laser system allowing short rise and fall times of the flat-top temporal distribution as well as several new diagnostic components have been installed at PITZ in 2008. The electrons generated via the photo-effect at a cesium telluride (Cs2Te) cathode are accelerated by a 1.6 cell L-band RF-gun cavity with a maximum accelerating gradient at the cathode of about 60MV/m. The transverse projected emittance is measured using a single slit scan technique. In the 2008–2009 run period, a detailed characterization of the projected transverse emittance was performed at different operating conditions. Optimizations and measurement results as well as simulation predictions of the transverse projected emittance for bunch charges of 1, 0.5, 0.25 and 0.1nC are presented and discussed in this paper. The geometric mean of the normalized projected rms emittance in both transverse directions for an electron bunch charge of 1nC was measured to be 0.89±0.01mmmrad for a 100% rms phase-space distribution.

Improvement of an S-band RF gun with a Cs 2Te photocathode for the KEK-ATF

A laser-driven RF gun with a Cs 2Te photocathode, a device modeled after the BNL-IV RF gun, has been in development at KEK since 2002. This gun has been operated as an electron source for the Accelerator Test Facility (ATF) and generates a beam with an operational intensity of up to 3.2 nC per bunch. Based on successful results at the ATF, this gun has been applied several times for new research at universities. From 2002 onwards, successive improvements have been incorporated into newer models of the RF gun. Most recently, in 2008, a new gun incorporating all of the earlier modifications was produced for the ATF. Tests have confirmed a significant improvement of the Q-value of the latest model. A typical transverse emittance of 1.3 π mm mrad has been obtained under the following conditions: solenoid field of 0.18 T, beam intensity of 1.6 nC per bunch, and RF power of 9 MW. This RF gun equipped with the Cs 2Te photocathode is being used for the advanced research programs at the ATF. A survey of its performance has recently been completed.

Running experience with the laser system for the RF gun based injector at the TESLA Test Facility linac

During the run 1998/1999, the new injector based on a laser driven RF gun was brought into operation at the TESLA Test Facility Linac (TTFL) at DESY. A key element of the injector is the laser system to illuminate the RF gun cathode to produce short (ps) electron bunches of high charge (nC). This electron beam is used to perform various experiments for the future TESLA linear collider, and to drive the free electron laser TTF-FEL. The laser design is challenged by the unusual requirement of providing synchronized ps UV pulses in 0.8 ms long trains with ambitious stability requirements. The design was also driven by the requirement to have an operational system with a high reliability. The system is based on a mode locked solid-state (Nd:YLF) pulse train oscillator followed by a linear amplifier chain. In a first phase, a laser pulse rate of 1 MHz within the train has been realized, 2.25 MHz and 9 are in preparation. Performance and running experiences with the laser system during the last TTF run are reported.

First operation of cesium telluride photocathodes in the TTF injector RF gun

During the run 1998/1999 a new injector based on a laser-driven RF gun was brought in operation at the TESLA Test Facility (TTF) linac at DESY, in order to produce the beam structure and quality required either by TeV collider and SASE FEL experiments. High quantum efficiency cesium telluride photocathodes, prepared at Milano and transferred to DESY, have been successfully operated in the RF gun. A bunch charge of 50 nC, only limited by space charge effects, was achieved. The photocathodes have shown an operative lifetime of several months. A new cathode surface finishing has showed a promising decrease of the photocathode dark current. Measurements of dark current, quantum efficiency and lifetime are reported.

Micro-bunch production with radio frequency photoinjectors

In this paper we discuss the generation of ultra-short electron bunches using laser-driven RF guns. The designs are tailored for future plasma accelerators. Second generation plasma accelerators are expected to be very demanding in terms of bunch length, since the accelerated beam is expected to be short with respect to the wavelength of the excited Langmuir space-charge plasma wave. Since the anticipated wavelength ranges from 100 to 300 /spl mu/m, 10-50 /spl mu/m-long bunches are required with a bunch population of the order of 10/sup 8/ particles. The laser-driven RF gun is a promising candidate to attain such beams. The rationale for this choice as well as the main limitations in terms of minimum bunch length will be analyzed and discussed in the following. Two possible configurations are evaluated: the direct production at the photocathode surface of ultra-short electron bunches by illumination of the cathode with 160-fs-long laser pulses and the acceleration of a 1-ps electron bunch with further magnetic compression in a wiggler.

A laser-driven gun for electron-positron factories

Future electron-positron factories require very high luminosity and hence a sufficiently large number of injected particles. The injector system must be capable of supplying the circulating current at a rate equal to or greater than the rate at which it is lost from the stored beams by beam-beam and beam-gas bremsstralhung processes. A laser driven rf gun appears to be the most convenient electron source able to supply very high intensity. Technological problems connected with the rf source are discussed in this paper.

Quality of the self-field dominated beam, with arbitrary aspect ratio, of a high-intensity magnetically focused rf gun

In a high-intensity, laser-driven rf gun, such as that designed to feed the high-power rf-FEL of the CEA at Bruyères-le-Châtel, self-field effects are dominant in electron-beam emittance growth. For beams with an aspect ratio of the order of 1 (as are the energetic beams of the forementioned FEL), this emittance growth takes place mainly during extraction or at the beginning of acceleration. It is attended by radial expansion which makes it necessary to focus the beam as soon as possible, i.e. with a magnetic lens located at the exit of the first cavity, before the beam enters further accelerating cavities. We study theoretically the influence of such magnetic focusing on the beam quality, measured by various whole-beam emittances: radial, transverse, longitudinal, 3D, as well as by energy spread. Theoretical modelling treats the self electromagnetic field by Liénard-Wiechert's equations. This provides a rigorous relativistic description of early beam transport and emittance growth for beams of any aspect ratio, in which the velocity spread is enormous at the beginning, so that monokinetic — or space charge — approximations like Kim's cannot be used.

Beam dynamics in rf guns and emittance correction techniques

In this paper we present a general review of beam dynamics in a laser-driven rf gun. The peculiarity of such an accelerating structure versus other conventional multi-cell linac structures is underlined on the basis of the Panofsky-Wenzel theorem, which is found to give a theoretical background for the well known Kim's model. A basic explanation for some proposed methods to correct rf induced emittance growth is also derived from the theorem. We also present three emittance correction techniques for the recovery of space-charge induced emittance growth, namely the optimum distributed disk-like bunch technique, the use of rf spatial harmonics to correct spherical aberration induced by space charge forces and the technique of emittance filtering by clipping the electron beam. The expected performances regarding the beam quality achievable with different techniques, as predicted by scaling laws and simulations, are analyzed, and, where available, compared to experimental results.