ELETTRA is a multidisciplinary Synchrotron Light Laboratory in AREA Science Park, open to researchers in diverse basic and applied fields. The laboratory is equipped with ultra-bright light sources in the spectral range from UV to X-rays and offers a stimulating and competitive environment to researchers from all over the world.

FERMI@elettra is the Italian linac-based free electron laser (FEL) presently under commissioning at Sincrotrone Trieste. The extremely high coherence and instantaneous power of this 4th generation light source will be achieved by means of a very high charge density electron beam. The beam is shortened with a ballistic principle from 10ps to sub-ps duration in two magnetic chicanes, in order to achieve a peak current near the kilo-Ampere. Unfortunately, the compression procedure enhances particle energy and density modulations that affect the FEL performance; such a perturbing dynamics is called microbunching instability development.

We studied a new scheme of compression in which, after removing the linear energy chirp required for the first stage of compression, an additional and properly tuned magnetic chicane is able to dilute the initial energy modulation and to suppress the current spikes created by the micro-bunching instability without affecting the bunch length. A by-product of the study is the observation that a single compressor is more effective than the two-compressors scheme in reducing the unwanted modulations caused by the microbunching instability.

The pictures show the predicted particle energy (top line) and current (bottom line) distribution before (left) and after (right) the application of the new scheme of compression.