Abstract
High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the center of a dispersive chicane, it is possible to manipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2D spectrotemporal domain at the Fermi FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs.
Highlights
Laser heaters have proven to be invaluable components of high-brightness, short wavelength free-electron lasers (FELs) [1], utilized in order to suppress the microbunching instability [1,2,3], a collective effect that can develop due to shot noise [4] in the injector of such a machine, and undergo amplification due to space-charge [1,5] and coherent synchrotron radiation effects [6,7,8]
By applying this technique to the laser pulse used for the laser heater in its nominal configuration, the beating wavelength of this modified pulse can be increased by orders of magnitude, generating a laser pulse with an effective wavelength of a similar order to the length of the electron bunch
In this paper we study the effect of imposing such a modulated laser pulse on the electron beam in the Fermi laser heater for three bunch compression scenarios, and for a range of laser pulse modulation frequencies and laser pulse energies
Summary
Laser heaters have proven to be invaluable components of high-brightness, short wavelength free-electron lasers (FELs) [1], utilized in order to suppress the microbunching instability [1,2,3], a collective effect that can develop due to shot noise [4] in the injector of such a machine, and undergo amplification due to space-charge [1,5] and coherent synchrotron radiation effects [6,7,8]. Recent experiments at the Fermi FEL [30] have investigated the possibility of using a laser pulse with a nonuniform longitudinal intensity profile to impose energy spread modulations on the bunch in the laser heater, thereby seeding the microbunching at a known single frequency [31,32]. This study builds on previous work done on premodulated beams in storage rings [34] By applying this technique to the laser pulse used for the laser heater in its nominal configuration, the beating wavelength of this modified pulse can be increased by orders of magnitude, generating a laser pulse with an effective wavelength of a similar order to the length of the electron bunch. VII the microbunching parameters for each of these bunch compression schemes are analyzed, and compared with results from simulations
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