M. Cornacchia

Performance and design concepts of a free-electron laser operating in the x-ray region

We report on the design study of a free-electron-laser experiment designed to produce coherent radiation at the wavelength of 1.5 angstrom and longer. The proposed experiment utilizes 1/3 of the SLAC linac to accelerate electrons to 15 GeV. The high brightness electron beam interacts with the magnetic field of a long undulator and generates coherent radiation by self-amplified spontaneous emission (SASE). The projected output peak power is about 10 GW. The project presents several challenges in the realization of a high brightness electron beam, in the construction and tolerances of the undulator and in the transport of the x-ray radiation. The technical solutions adopted for the design are discussed. Numerical simulations are used to show the performance as a function of system parameters.

Measurements of nonlinear harmonic radiation and harmonic microbunching in a visible SASE FEL

The experimental characterization of nonlinear harmonic generation (NHG) and electron beam microbunching at saturation from a visible SASE FEL are presented in this report. The gain lengths, spectra and energies of NHG were experimentally measured up to the third harmonic, and agree with theoretical predictions. Electron beam microbunching in both the fundamental and the second harmonic as the function of the SASE output were experimentally observed over the full range of SASE gain. The bunching factors for both the fundamental (b1) and second harmonic (b2) were experimentally characterized at saturation. The microbunching data provides another test of SASE saturation as well as correlating the NHG and electron beam microbunching modes to the fundamental SASE.© 2003 Published by Elsevier Science B.V.PACS: 41.60. Cr;41.60. Ap;4185. Ja

Running fermi with one-stage compressor: advantages, layout, performance

Running FERMI with one-stage compressor: advantages, layout, performance. M. Cornacchia ∗ , P. Craievich*, S. Di Mitri*, G. Penco*,M. Venturini ∗∗ , S. Zholents** 1. Introduction CBP-Tech Note-345 (July 2005), devoted to a study of microbunching instability in FERMI@ELETTRA linac, quotes: “… the above analysis shows that the most of the gain in microbunching instability occurs after BC2, i.e. after transformation of the energy modulation to the spatial modulation that takes place in BC2. It is possible to avoid that if we use only BC1 for all our needs for bunch compression. There are also additional advantages for a mitigation of the microbunching instability related to that. First, we would need to increase R 56 in BC1 (for given energy chirp in the electron beam). Second, a relative energy spread is significantly larger at BC1 than at BC2. Both these factors would contribute to instability suppression due to increased Landau damping effect.” One additional argument was however missed in that report. Instability smearing due to finite emittance is stronger in BC1 simply because the geometrical emittance is larger than in BC2. In spite of the considerations in favor of a lattice with one-stage compressor, it was thought at the time that the two bunch compressors configuration was still preferable as it appeared difficult to obtain a flat-flat distribution at the end of the linac with only one bunch compressor. A flat-flat distribution has constant medium energy and a constant peak current along the electron bunch. Now, two years later and more studies behind, this problem is solvable. It has been demonstrated 1 that shaping the intensity of the electron bunch at the injector using intensity modulation of the photocathode laser allows to use the linac structural wake fields to advantage to obtain a flat-flat distribution at the end of the linac in a two-stage compressor. This report shows that, using the back- tracking technique, it is possible to obtain a flat-flat distribution also in a single-stage compressor. Preliminary results of a study of the microbunching instability applied to the FERMI lattice with one-stage compressor are shown in this report . There is concern that the effect of jitter in accelerator parameters is more pronounced with one bunch compressor: the results of jitter studies are given and are compared with the case of a two-stage compressor. Sincrotrone Trieste S.C.p.A., Trieste, Italy. Lawrence Berkeley National Laboratory, Berkeley, California, USA.

Operating synchrotron light sources with a high gain free electron laser

Since the 1980s synchrotron light sources have been considered as drivers of a high repetition rate (RR), high gain free electron laser (FEL) inserted in a by-pass line or in the ring itself. As of today, the high peak current required by the laser is not deemed to be compatible with the standard multi-bunch filling pattern of synchrotrons, and in particular with the operation of insertion device (ID) beamlines. We show that this problem can be overcome by virtue of magnetic bunch length compression in a ring section, and that, after lasing, the beam returns to equilibrium conditions without beam quality disruption. Bunch length compression brings a double advantage: the high peak current stimulates a high gain FEL emission, while the large energy spread makes the beam less sensitive to the FEL heating and to the microwave instability in the ring. The beams large energy spread at the undulator is matched to the FEL energy bandwidth through a transverse gradient undulator. Feasibility of lasing at 25 nm is shown for the Elettra synchrotron light source at 1 GeV, and scaling to shorter wavelengths as a function of momentum compaction, beam energy and transverse emittance in higher energy, larger rings is discussed. For the Elettra case study, a low (100 Hz) and a high (463 kHz) FEL RR are considered, corresponding to an average FEL output power at the level of ~1 W (~1013 photons per pulse) and ~300 W (~1011 photons per pulse), respectively. We also find that, as a by-product of compression, the ~5 W Renieris limit on the average FEL power can be overcome. Our conclusion is that existing and planned synchrotron light sources may be made compatible with this new hybrid IDs-plus-FEL operational mode, with little impact on the standard beamlines functionality.

High-brightness femtosecond x-ray source using an undulator in the SLAC Linac

A unique X-ray source, of exceptional brightness and with pulse widths as low as 30 fs rms, has been proposed at the Stanford Linear Accelerator Center. Named the Sub-Picosecond Photon Source (SPPS), the facility takes 30 Gev bunches from the linac and compresses them in three stages to achieve peak currents of 30 kA in the Final Focus Test Beam (FFTB) beamline. The existing FFTB can accommodate an undulator of up to 10 m in length which will deliver ~108 1.5 A photons per pulse in a 0.1% bandwidth with a peak brightness of ~1025 photons/sec/mm2/mrad2/0.1% BW, in a pulse width of ~80 fs FWHM. The short electron bunches are also ideal for plasma and wakefield studies as well as providing abundant R&D possibilities for verifying short bunch behavior in the future Linac Coherent Light Source (LCLS).

LCLS x-ray FEL at SLAC

The design status and R&D plan of a 1.5 angstrom SASE-FEL at SLAC, called the Linac Coherent Light Source (LCLS), are described. The LCLS utilizes one third of the SLAC linac for the acceleration of electrons to about 15 GeV. The FEL radiation is produced in a long undulator and is directed to an experimental area for its utilization. The LCLS is designed to produce 300 fsec long radiation pulses at the wavelength of 1.5 angstrom with 9 GW peak power. This radiation has much higher brightness and coherence, as well as shorter pulses, than present third generation sources. It is shown that such leap in performance is now within reach, and is made possible by the advances in the physics and technology of photo- injectors, linear accelerators, insertion devices and free- electron lasers.

Merit Functions for the Linac Optics Design for Colliders and Light Sources

Optics matching and transverse emittance preservation are key goals for a successful operation of modern high brightness electron linacs. The capability of controlling them in a real machine critically relies on a properly designed magnetic lattice. Conscious of this fact, we introduce an ensemble of optical functions that permit to solve the often neglected conflict between strong focusing, typically implemented to counteract coherent synchrotron radiation and transverse wakefield instability, and distortion of the transverse phase space induced by chromatic aberrations and focusing errors. A numerical evaluation of the merit functions is applied to the Pohang Accelerator Laboratory free electron laser.

FERMI&Elettra Accelerator Technical Optimization Final Report

This report describes the accelerator physics aspects, the engineering considerations and the choice of parameters that led to the accelerator design of the FERMI Free-Electron-Laser. The accelerator (also called the electron beam delivery system ) covers the region from the exit of the injector to the entrance of the first FEL undulator. The considerations that led to the proposed configuration were made on the basis of a study that explored various options and performance limits. This work follows previous studies of x-ray FEL facilities (SLAC LCLS [1], DESY XFEL [2], PAL XFEL [3], MIT [4], BESSY FEL[5], LBNL LUX [6], Daresbury 4GLS [7]) and integrates many of the ideas that were developed there. Several issues specific to harmonic cascade FELs, and that had not yet been comprehensively studied, were also encountered and tackled. A particularly difficult issue was the need to meet the requirement for high peak current and small slice energy spread, as the specification for the ratio of these two parameters (that defines the peak brightness of the electron beam) is almost a factor of two higher than that of the LCLSs SASE FEL. Another challenging aspect was the demand to produce an electron beam with as uniform as possible peak current and energy distributions along the bunch, a condition that was met by introducing novel beam dynamics techniques. Part of the challenge was due to the fact that there were no readily available computational tools to carry out reliable calculations, and these had to be developed. Most of the information reported in this study is available in the form of scientific publications, and is partly reproduced here for the convenience of the reader.

The coherent light source project at SLAC

Advances in the physics and technology of photo-injectors, linear accelerators, insertion devices and free-electron lasers make it now possible to generate coherent radiation in the x-ray region by means of the Self-Amplified-Spontaneous-Emission (SASE) process. This radiation has much higher brightness, shorter pulses and coherence than present 3rd generation sources. The status of the physics and technology involved in a radiation source based on SASE is reviewed. The design status of a 1.5 A SASE-FEL at SLAC, called the Linac Coherent Light Source (LCLS), is described.

Linac coherent light source at SSRL/SLAC: source characteristics and scientific opportunities

In this paper we describe the plans at Stanford University for an x-ray (1.5 - 15 angstroms) free-electron laser based on the last kilometer, 15 - 5 GeV, of the SLAC linear accelerator. The technical requirements are summarized as well as the novel scientific opportunities with this fourth generation source. It is concluded that with the present research and development efforts under way it should be possible to start the construction of such a facility as early as 2002 with a completion date in 2005.