Abstract
FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) and was designed to deliver high quality, transversely and longitudinally coherent radiation pulses in the extreme ultraviolet and soft x-ray spectral regimes. The FERMI linear accelerator includes a laser heater to control the longitudinal microbunching instability, which otherwise is expected to degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. In this paper, we present the results of the FERMI laser heater commissioning. For the first time, we show that optimizing the electron beam heating at an upstream location (beam energy, 100 MeV) leads to a reduction of the incoherent energy spread at the linac exit (beam energy, 1.2 GeV). We also discuss some of the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL output intensity.
Highlights
The photon pulse generated by an externally seeded freeelectron laser (FEL) inherits the spectral quality of the seed laser and in principle can be nearly fully longitudinally coherent
We show that optimized laser heater” (LH) heating reduces the compressed electron beam energy modulations and, for the first time, this optimization reduces the incoherent energy spread at the linac exit
When the laser energy was increased to ≈ 3 μJ or greater, even the longer scale structures are nearly smeared out and the high frequency structures are essentially eliminated [Fig. 8(c)]. Another indication of strong microbunching instability (MBI) modulation of the FERMI electron beam was the presence of coherent optical transition radiation (COTR) [36,37] produced when the beam passed through OTR screens, as previously reported [38]
Summary
The photon pulse generated by an externally seeded freeelectron laser (FEL) inherits the spectral quality of the seed laser and in principle can be nearly fully longitudinally coherent. This coherence can be spoiled by several collective processes occurring during the preparation of the electron beam before its entrance to the FEL undulator. As a means to control the MBI in high brightness linacs, Saldin et al [7] proposed the addition of a particular device commonly referred to as a “laser heater” (LH) This device adds a controlled amount of incoherent energy spread to the electron beam and suppresses further MBI growth via energy Landau damping [5]. We comment upon the LH system’s positive effects upon the intensity and the spectral quality of the output FEL pulses
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