Luca Serafini

Laser-driven electron beamlines generated by coupling laser-plasma sources with conventional transport systems

Laser-driven electron beamlines are receiving increasing interest from the particle accelerator community. In particular, the high initial energy, low emittance, and high beam current of the plasma based electron source potentially allow generating much more compact and bright particle accelerators than what conventional accelerator technology can achieve. Using laser-generated particles as injectors for generating beamlines could significantly reduce the size and cost of accelerator facilities. Unfortunately, several features of laser-based particle beams need still to be improved before considering them for particle beamlines and thus enable the use of plasma-driven accelerators for the multiple applications of traditional accelerators. Besides working on the plasma source itself, a promising approach to shape the laser-generated beams is coupling them with conventional accelerator elements in order to benefit from both a versatile electron source and a controllable beam. In this paper, we perform start-to-end simulations to generate laser-driven beamlines using conventional accelerator codes and methodologies. Starting with laser-generated electrons that can be obtained with established multi-hundred TW laser systems, we compare different options to capture and transport the beams. This is performed with the aim of providing beamlines suitable for potential applications, such as free electron lasers. In our approach, we have analyzed which parameters are critical at the source and from there evaluated different ways to overcome these issues using conventional accelerator elements and methods. We show that electron driven beamlines are potentially feasible, but exploiting their full potential requires extensive improvement of the source parameters or innovative technological devices for their transport and capture.

Beam dynamics in rf guns and emittance correction techniques

Abstract 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 Kims 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.

The Project Plasmonx for Plasma Acceleration Experiments and A Thomson X-Ray Source at SPARC

We present the status of the project PLASMONX, recently approved by INFN. This project, based on a collaboration between INFN and CNR-IPCF, aims at a long term upgrade of the SPARC system with the goal to develop at LNF an integrated facility for advanced beam-laser-plasma research in the field of advanced acceleration techniques and ultra-bright X-ray radiation sources and related applications. The project, in its first phase, foresees the development at LNF of a High Intensity Laser Laboratory (HILL) whose main component is a 100 TW-class Ti: Sa laser system synchronized to the SPARC photo-injector. Experiments of self-injection and acceleration of electrons into laser driven plasma waves will be conducted at HILL-LNF, early in this first project phase. Eventually an additional beam line will be built in the SPARC bunker in order to transport the SPARC electron beam at an interaction point, where a final focus system will allow to conduct experiments either of laser-beam co-propagation in plasma waves for high gradient acceleration, or experiments of laser-beam head-on collisions to develop a Thomson source of bright ultra-short X-ray radiation pulses, with X-ray energies tunable in the range 20 to 1000 keV and pulse duration from 30 fs to 20 ps. Preliminary simulations of plasma acceleration with self-injection are illustrated, as well as external injection of the SPARC electron beam.

Mapping the transverse coherence of the self amplified spontaneous emission of a free-electron laser with the heterodyne speckle method

The two-dimensional single shot transverse coherence of the Self-Amplified Spontaneous Emission of the SPARC_LAB Free-Electron Laser was measured through the statistical analysis of a speckle field produced by heterodyning the radiation beam with a huge number of reference waves, scattered by a suspension of particles. In this paper we report the measurements and the evaluation of the transverse coherence along the SPARC_LAB undulator modules. The measure method was demonstrated to be precise and robust, it does not require any a priori assumptions and can be implemented over a wide range of wavelengths, from the optical radiation to the x-rays.

The design for the LCLS RF photoinjector

Abstract We report on the design of the RF photoinjector of the Linac Coherent Light Source. The RF photoinjector is required to produce a single 150 MeV bunch of ∼1 nC and ∼100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance of ∼1 π mm-mrad. The design employs a 1.6-cell S-band RF gun with an optical spot size at the cathode of a radius of ∼1 mm and a pulse duration with an rms sigma of ∼3 ps. The peak RF field at the cathode is 150 MV/m with extraction 57° ahead of the RF peak. A solenoidal field near the cathode allows the compensation of the initial emittance growth by the end of the injection linac. Spatial and temporal shaping of the laser pulse striking the cathode will reduce the compensated emittance even further. Also, to minimize the contribution of the thermal emittance from the cathode surface, while at the same time optimizing the quantum efficiency, the laser wavelength for a Cu cathode should be tunable around 260 nm. Following the injection linac the geometric emittance simply damps linearly with energy growth. PARMELA simulations show that this design will produce the desired normalized emittance, which is about a factor of two lower than has been achieved to date in other systems. In addition to low emittance, we also aim for laser amplitude stability of 1% in the UV and a timing jitter in the electron beam of 0.5 ps rms, which will lead to less than 10% beam intensity fluctuation after the electron bunch is compressed in the main linac.

The short bunch blow-out regime in RF photoinjectors

A new beam dynamics regime of RF Photoinjectors is presented here, dealing with a violent bunch elongation under the action of longitudinal space charge forces. It is shown that such a blow-out expansion of the electron bunch can lead to highly linear behaviors of both the longitudinal and the transverse space charge field, a well known prerequisite to achieve minimum emittance dilution in photoinjectors. If operated in the ultra-short pancake-like bunch regime, such an effect can be very beneficial to the emittance correction mechanism, making it effective also for ultra-short pancake like bunches. The anticipated performances are presented: kA peak current beams can be generated directly out of the photoinjector (10 to 20 MeV exit energy) with rms normalized emittances below 1u2009mm⋅mrad.

High brightness, long pulse, electron beam production with SC photo-injectors

Abstract In this paper we discuss the design of a possible superconducting photoinjector able to produce long pulses of high charge electron bunches, as required for the TESLA project. An extrapolation to high brightness cw electron beams for inertial fusion and X-UV FEL experiments is also given, together with an analysis of the R&D needed for the actual injector development. The requirements for the photocathode which has to deliver high current density in a SC environment are discussed on the basis of the preliminary results of the Wuppertal experiment, performed in the context of an international collaboration.

Neutralization of the emittance blowup induced by rf time dependent forces in rf guns

Abstract In this paper a new method is presented to neutralize the rf-induced emittance blow-up generated inside rf electron guns. The method is based on a multi-mode operation of the rf gun cavity, which must be able to support both the accelerating mode (TM010-π) and a higher harmonic mode. The analytical study of the beam dynamics, which has been found in good agreement with the numerical simulations, shows that the growth of the normalized rms emittance, produced by the time-dependent rf forces during the acceleration in the gun, can be cancelled up to fourth-order terms. This is of great relevance for the improvement of rf gun performances, since the rf field contribution to the emittance blowup becomes negligible and no more dependent on bunch sizes, allowing in this way a natural damping also of the space charge induced emittance simply by using longer bunches, i.e., decreasing the charge density.

Asymmetric lateral coherence of betatron radiation emitted in laser-driven light sources

We show that the radiation emitted by betatron oscillations of a high-energy electron beam undergoing wake-field acceleration is endowed with peculiar coherence properties which deliver quantitative information about the electron trajectories. Such results are achieved by means of accurate numerical simulations and a simple geometrical model gives a clear physical interpretation.

Long multi-cell rf guns as full scale injectors

Abstract The behavior of single particle beam dynamics in long rf guns is discussed, mainly concerning the effect of rf focusing, relevant in a high gradient standing wave structure. An analytical transport matrix is derived and compared to simulations, showing that it is possible to transport the beam through the gun up to full scale injector energy (1 GeV) without the need for any other external focusing.