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FELs rely on the accurate design of their driving high brightness linear accelerators

In linac-based FELs the magnetic transport and radio-frequency (RF) accelerating systems are closely intertwined with the dynamics of the electromagnetic field built up by the interaction of the bunch charge with its environment (wakefield) and with its own Coulomb field (space charge field). Notwithstanding the high beam rigidity at energies up to the GeV range, these “collective effects” may increase the beam 6-D emittance relative to the injection level, eventually degrading the beam brightness. It is therefore a remarkable achievement of the FEL community that the physics and the computer model of those effects, and of solutions to counteract them, have found a confirmation in the successful operation of linac-driven FEL facilities like FLASH in Germany, LCLS in California, SACLA in Japan and FERMI in Italy.
In spite of apparent similarities in accelerator layouts, the peculiarities of each FEL facility have required different solutions to basically similar challenges. As an example, the combination of one order of magnitude lower electron beam energy and one order of magnitude higher bunch charge in FERMI compared to LCLS and SACLA, makes the FERMI electron beam more sensitive to collective effects. Moreover, FERMI is presently the only FEL user facility seeded by an external laser, which ensures a higher degree of coherence of the output light compared to that produced by self-amplified spontaneous emission elsewhere. The interaction of the seeding laser with the electron bunch is efficient, provided that both high charge density and uniform charge distribution are achieved at the undulator. This implies an accurate control of the electron dynamics along all steps of beam delivery, from the linac injection point to bunch length compression, acceleration and beam transport to the undulator. Enlarging on the experience cumulated during the design of previous FEL facilities and linear colliders, the accelerator physicists and engineers of FERMI developed new theoretical models for electron beam control, and tested them experimentally, thereby enlightening new physical processes and ensuring a final beam brightness several orders of magnitude higher than in 3rd generation synchrotron light sources.
The electron beam brightness of designed, planned and existing FEL facilities around the world is shown in Fig. 1. Analytical models for the impact of collective effects on the brightness were developed, as shown in Fig. 2. They are used in combination with particle tracking simulations to predict the electron beam charge distribution at the undulator.

Figure 1. Peak brightness of FEL facilities in the EUV and x-ray wavelength range (data at 2013). The 6-D beam brightness is the ratio of bunch charge and the product of beam rms transverse size and longitudinal momentum spread. The slice brightness is a local property of one bunch’s longitudinal slice, the projected brightness is a property of the entire bunch and it is enlarged by correlations among different slices in the phase space. The closer the two brightness values, the more uniform the charge distribution and the more efficient the FEL process is along the bunch. We observe a linear correlation between the beam brightness and the minimum achievable FEL wavelength. The gap between slice and projected brightness is typically one order of magnitude. In a few cases the two values are closer to each other, but at the expense of the tunability (in wavelength, flux and spectral properties) of the FEL performance.

Figure 2 Analytical models for the collective effects such as geometric transverse wakefield (GTW) in accelerating structures and coherent synchrotron radiation (CSR) in bunch length magnetic compressors allow the evaluation of the final brightness as a function of the linac setting, for a given set of initial beam parameters. In the plot, the final brightness of a 250 pC beam in FERMI is evaluated as a function of the collective effects included in the beam dynamics (XW stays for GTW only, XCSR stays for CSR only), and of the bunch length compression factor. At relatively low compression factors, it is dominated by the GTW (the dashed-dot vertical line identifies the FERMI working point at 250 pC). This analytical estimate fits well with the experimental value.

           

This research was conducted by the following research team:

  • S. Di Mitri and M. Cornacchia, Elettra Sincrotrone Trieste, Trieste, Italy.

Contact persons:
Simone Di Mitri: simone.dimitri@elettra.eu

 

Reference

S. Di Mitri and M. Cornacchia, Electron beam brightness in linac drivers for free-electron-lasers”, Physics Reports 539, 1 (2014); DOI:10.1016/j.physrep.2014.01.005

 

Last Updated on Wednesday, 12 November 2014 16:37