C. Vicario

The SPARC project: a high-brightness electron beam source at LNF to drive a SASE-FEL experiment

Abstract The Project Sorgente Pulsata e Amplificata di Radiazione Coerente (SPARC), proposed by a collaboration among ENEA–INFN–CNR–Universita’ di Tor Vergata–INFM–ST, was recently approved by the Italian Government and will be built at LNF. The aim of the project is to promote an R&D activity oriented to the development of a coherent ultra-brilliant X-ray source in Italy. This collaboration has identified a program founded on two main issues: the generation of ultra-high peak brightness electron beams and of resonant higher harmonics in the SASE-FEL process, as presented in this paper.

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

Abstract 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.

The SPARC/X SASE-FEL Projects

SPARC and SPARX are two different initiatives toward an Italian Free Electron Laser ~FEL! source operating in the Self Amplified Spontaneous Emission ~SASE! mode, in which several national research institutions are involved. SPARC is a high gain FEL project devoted to provide a source of visible and VUV radiation while exploiting the SASE mechanism. An advanced Photo-Injector system, emittance compensating RF-gun plus a 150 MeV Linac, will inject a high quality e-beam into the undulator to generate high brilliance FEL radiation in the visible region at the fundamental wavelength, ~;500 nm!. The production of flat top drive laser beams, high peak current bunches, and emittance compensation scheme will be investigated together with the generation of higher harmonic radiation in the VUV region. SPARX is the direct evolution of such a high gain SASE FEL toward the 13.5 and 1.5 nm operating wavelengths, at 2.5 GeV. To get the required value for the bunch peak current, Ipeak ’ 2.5 kA, the “hybrid” scheme, RF-compression stage plus magnetic chicane, is analyzed and compared with the more standard double stage of magnetic compression. The two options are reviewed considering the tolerance to the drive laser pulse phase jitter.

The Sparc project: a high brightness electron beam source at LNF to drive a SASE-FEL experiment

The Project SPARC (Sorgente Pulsata e Amplificata di Radiazione Coerente), proposed by a collaboration among ENEA-INFN-CNR-Universitadi Roma Tor Vergata- INFM-ST, was recently funded by the Italian Government. The aim of the project is to promote an R&D activity oriented to the development of a coherent ultra-brilliant X-ray source in Italy (SPARX proposal (1)). The SPARC collaboration identified a program based on two main issues: the generation of ultra-high peak brightness electron beams and experimental study of SASE-FEL process with generation of resonant higher harmonics. The SPARC project is being designed in order to encompass the construction of an advanced photo- injector producing a 150-200 MeV beam to drive a SASE-FEL in the optical range. The machine will be built at LNF, inside an underground bunker: it is comprised of an rf gun driven by a Ti:Sa laser, injecting into three SLAC accelerating sections. We foresee conducting investigations on the emittance correction(2) and on the rf compression techniques(3), which are expected to increase the peak current achievable at the injector exit up to kA level, with proper preservation of the transverse emittance. Although the system is expected to drive a FEL experiment, it can be used also to investigate beam physics issues like surface-roughness-induced wake fields, bunch-length measurements in the sub-ps range, emittance degradation in magnetic compressors due to CSR, and Compton backscattering production of sub-ps X-ray pulses.

Drive laser system for sparc photoinjector

In this paper we report the progress of the SPARC photoinjector laser system. In the high brightness photoinjector the quality of the electron beam is directly related to the photocathode drive laser. In fact the 3D distribution of the electron beam is determined by the incoming laser pulse. The SPARC laser is a 10 Hz frequency-tripled TW-class Ti:Sa commercial system. To achieve the required flat top temporal shape we perform a manipulation of the laser spectrum in the fundamental wavelength and in the third harmonic. The optical transfer-line has been implemented to limit the pointing instabilities and to preserve to the cathode the temporal and spatial features of the laser pulse. We present the recorded performances in terms of time pulse shape and rf-to-laser synchronization.

Simple scheme for ultraviolet time-pulse shaping

We present a method to generate high-energy flat-top UV laser pulses such as the ones needed to optimally drive high-brightness radio-frequency photoinjectors. In this scheme we believe to be novel, the longitudinal profile of a laser pulse from a Ti:sapphire master oscillator power amplifier system is controlled using a mechanical mask in the Fourier plane of a 4f stretcher located after the harmonic conversion crystals. Such a scheme allows us to overcome many of the difficulties faced by current state-of-the-art pulse-shaping designs. These are in fact based on various versions of preamplifier infrared shapers and hence suffer from the limitations set by the nonlinearities of chirped-pulse amplification and harmonic conversion. Beyond the clear advantages of simplicity and robustness, the proposed solution offers the possibility to deliver a pulse with very short rise and fall times and to freely change the output pulse length. We also note that, after proper calibration between spectral and temporal profiles, the shaper optical setup offers the possibility to retrieve the longitudinal profile of the laser pulse on a shot-to-shot basis.

Temporal E-Beam Shaping in an S-Band Accelerator

New short-wavelength SASE light sources will require very bright electron beams, brighter in some cases than is now possible. One method for improving brightness involves the careful shaping of the electron bunch to control the degrading effects of its space charge forces. We study this experimentally in an S-band system, by using an acousto-optical programmable dispersive filter to shape the photocathode laser pulse that drives the RF photoinjector. We report on the efficacy of shaping from the IR through the UV, and the effects of shaping on the electron beam dynamics.

Measurements of quantum efficiency of MG films produced by pulsed laser ablation deposition for application to bright electron sources

We report the results of preliminary tests on Mg film photocathodes deposited by pulsed laser ablation deposition on various substrates. A comparison with other deposition techniques is presented. We illustrate briefly the technique of deposition of thick films (up to several microns). Scanning electron microscope analysis of the films before and after laser cleaning has been performed to characterize the changes occurred on the surface. Measurements of the distribution and integrated value of quantum efficiency over the illuminated surface are reported. Plans for the deposition on the back flange of an RF gun are illustrated.

Status of the SPARX FEL project

The SPARX project consists in an X-ray-FEL facility jointly supported by MIUR (Research Department of Italian Government), Regione Lazio, CNR, ENEA, INFN and Rome University Tor Vergata. It is the natural extension of the ongoing activities of the SPARC collaboration. The aim is the generation of electron beams characterized by ultra-high peak brightness at the energy of 1 and 2 GeV, for the first and the second phase respectively. The beam is expected to drive a single pass FEL experiment in the range of 13.5-6 nm and 6-1.5 nm, at 1 GeV and 2 GeV respectively, both in SASE and SEEDED FEL configurations. A hybrid scheme of RF and magnetic compression will be adopted, based on the expertise achieved at the SPARC high brightness photoinjector presently under commissioning at Frascati INFNLNF Laboratories. The use of superconducting and exotic undulator sections will be also exploited. In this paper we report the progress of the collaboration together with start to end simulation results based on a combined scheme of RF compression techniques.

Recent Results from and Future Plans for the Visa II SASE FEL

The VISA II (Visible to Infrared SASE Amplifier) project, a consequent experiment to the succesful VISA enterprise, entails the use of a chirped electron beam to drive a high gain SASE FEL. The resulting ultra-short pulses will be characterized using an advanced FROG (Frequency Resolved Optical Gating) technique, as well as a double differential spectrum (angle/wavelength) diagnostic. Implementation of sextupole corrections to the longitudinal aberrations affecting the high energy-spread chirped beam during transport to the VISA undulator is studied. Start-end simulations, including radiation diagnostics, are discussed. Initial experimental results involving a highly chirped beam transported without sextupole corrections, the resulting high gain lasing, and computational analysis are briefly reported.