FERMI is a single-pass linac-based FEL user-facility covering the wavelength range from 80 nm to 4 nm and is located next to the third generation synchrotron radiation facility Elettra in Trieste, Italy. The first FEL line in operation (FEL-1) has been opened to users at the end of 2012, while the second FEL line (FEL-2) covering the shorter wavelength range down to 4 nm is in commissioning. The linac is composed of 16 S-band accelerating structures. Seven of them are backward structures equipped with SLED system. Following the preliminary tests on one plant, in order to increase the operational accelerating gradient, phase modulation of the SLED drive power has been implemented in all these structures. The shape of the phase modulation is applied via the LLRF system firmware and can be modified if needed. This paper discusses the optimization of the phase modulation parameters and provides a summary of the experimental results.
