Laguerre-Gaussian Mode Laser Heater for Microbunching Instability Suppression in Free-Electron Lasers

Microbunching instability (MBI) driven by beam collective effects is known to be detrimental to high-brightness storage rings, linacs, and free-electron lasers (FELs). One known way to suppress this instability is to induce a small amount of energy spread to an electron beam by a laser heater. The distribution of the induced energy spread greatly affects MBI suppression and can be controlled by shaping the transverse profile of the heater laser. Here, we present the first experimental demonstration of effective MBI suppression using a ${\mathrm{LG}}_{01}$ transverse laser mode and compare the improved results with respect to traditional Gaussian transverse laser mode at the Linac Coherent Light Source. The effects on MBI suppression are characterized by multiple downstream measurements, including longitudinal phase space analysis and coherent radiation spectroscopy. We also discuss the role of ${\mathrm{LG}}_{01}$ shaping in soft x-ray self-seeded FEL emission, one of the most advanced operation modes of a FEL for which controlled suppression of MBI is critical.

Figure 1

Simplified schematic of start-to-end experimental configuration, from the photoinjector to the SXRSS diagnostic end station (not drawn to scale).

Figure 2

Slice energy spread of the $e$ beam after the laser heater as a function of ${\mathrm{LG}}_{01}$ laser energy. Total energy spread measured at 135 MeV spectrometer is fitted by the black line. The initial energy spread (with laser heater off) is removed in quadrature to obtain the red dashed line and the circles. (a)–(d) Four examples of energy distributions with (a) 25.1, (b) 30.3, (c) 36.8, and (d) 55.7 keV rms energy spread and their corresponding Gaussian fits.

Figure 3

Gaussian fitting coefficient of determination $R^2$ as a function of induced energy spread at laser heater for ${\mathrm{LG}}_{01}$ and Gaussian modes. (a)–(d) Four examples of energy distributions from transverse Gaussian mode laser heater with (a) 20.5, (b) 26.7, (c) 30.1, and (d) 37.2 keV rms energy spread and their corresponding Gaussian fits.

Figure 4

(a) 2D MIR spectrograph as a function of induced energy spread by ${\mathrm{LG}}_{01}$ transverse mode LH and (b) integrated MIR spectral intensity for $k\in (3000,5000){\mathrm{cm}}^{-1}$ as a function of induced energy spread by both the ${\mathrm{LG}}_{01}$ and Gaussian mode LHs.

Figure 5

(a),(b) SXRSS experiment results with ${\mathrm{LG}}_{01}$ mode LH. (a) Averaged (blue line) and single shot (red line) SXRSS spectra centered at 750 eV photon energy and (b) fraction of SXRSS spectral power as a function of bandwidth for varying ${\mathrm{LG}}_{01}$ mode energy in the LH. (c) Start-to-end simulation of SXRSS spectral bandwidth with Gaussian and ${\mathrm{LG}}_{01}$ mode laser heater at undulator length 23.1 m.

Figure 6

Influence of transverse laser jittering for ${\mathrm{LG}}_{01}$ mode laser heater. (a) Positions of laser center on the camera of different shots with color representing $R^2$ of the corresponding laser-induced $e$-beam energy distribution. (b) Correlation between the Gaussian fitting $R^2$ of the energy distribution and radial distance laser jittering away from the optimal position.