Measurements of the linac coherent light source laser heater and its impact on the x-ray free-electron laser performance

Abstract

The very bright electron beam required for an x-ray free-electron laser (FEL), such as the linac coherent light source (LCLS), is susceptible to a microbunching instability in the magnetic bunch compressors, prior to the FEL undulator. The uncorrelated electron energy spread in the LCLS can be increased by an order of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. To this end, a ''laser-heater'' system has been installed in the LCLS injector, which modulates the energy of a 135-MeV electron bunch with an IR-laser beam in a short undulator, enclosed within a four-dipole chicane. In this paper, we report detailed measurements of laser-heater-induced energy spread, including the unexpected self-heating phenomenon when the laser energy is very low. We discuss the suppression of the microbunching instability with the laser heater and its impact on the x-ray FEL performance. We also present the analysis of these experimental results and develop a threedimensional longitudinal space charge model to explain the self-heating effect.

Much Ado about Microbunching: Coherent Bunching in High Brightness Electron Beams

“…A.2 and A. 4, we see that the 1D and 3D expressions are similar; the only difference is the δ term, which is generally small.…”

“…The simulations are consistent with measurements for both soft and hard x-rays. √ P L , with the laser heater power, P L , given in µJ [4]. Note that at low laser power trickle heating causes a noticeable bump in the energy spread.…”

“…5.9). (At low laser power, a trickle heating causes a noticeable bump in the energy spread above the expected level [4]. The trickle heating is due to an instability resulting from surviving energy modulation downstream from the heater, a process analogous to the microbunching instability of chapter 3).…”

“…The laser heater interacts with the beam in an undulator. By placing the undulator in the middle of a chicane, the sinusoidal energy modulation washes out into an approximately uniform increase in energy spread [4]. The energy spread increases as the square root of the laser power, allowing us to triple the energy spread in the beam (Fig.…”

“…The laser heater produces an energy spread in keV of ∆E ≈ 8 √ P L , with the laser heater energy, P L , in µJ (Fig. 5.9) [4]. The final energy spread is then multiplied by the bunch compression factor, ∼ 90 at 3 kA.…”