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We study the efficiency limitation affecting laser-induced coherent synchrotron radiation (CSR) at high laser power. Experiments are made on the UVSOR-II storage ring in conditions of narrowband terahertz CSR emission. While, at moderate power, CSR power increases quadratically with laser power, a noticeable decrease in efficiency and eventually a decrease in CSR power is observed experimentally at high power. Details of the underlying process are analyzed numerically. As the saturation effect depends almost instantaneously on the laser intensity, the saturation occurs locally in longitudinal space. This has important consequences on the modulation pattern induced on the electron bunch.

We analyze the narrowband terahertz emission process occurring from electron bunches passing in a bending magnet, after a laser-induced sinusoidal modulation has been performed. In particular, we focus on experimental tunability curves, and power scalings with current and laser power. Theoretically, we simplify the problem formulation using the slowly varying envelope approximation. At low powers, the scaling with laser power appears to be quadratic, and analytical expressions for the tuning curves are obtained. Emission at first passage in the bending magnet, and after one full turn in the storage ring, are considered both experimentally and theoretically. The experiments are performed on the UVSOR-II storage ring.

The new trend is to operate storage ring based light sources in a “chromatic mode” with a distributed dispersive function in the straight sections for low emittance. The electron bunch heating induced by a storage ring free electron laser (FEL) has been investigated for such optics, and exhibits a more complex saturation process as compared to a usual achromatic mode of operation without dispersion in the straight sections. The correlated measured FEL power is then interpreted in terms of the electron bunch heating and compared to theoretical expectations. Experiments performed at UVSOR-II are here reported. The theoretical interpretation of the new saturation phenomenon is then discussed.