Commisioning of the FERMI@elettra laser heater

The linac of the FERMI seeded free electron laser includes a laser heater to control the longitudinal microbunching instability, which otherwise is expected to degrade the quality of high brightness electron beam sufficiently to reduce the FEL power. The laser heater consists of a short undulator located in a small magnetic chicane through which an external laser pulse enters to modulate the electron beam energy both temporally and spatially. This modulation, which varies on the scale of the laser wavelength, together with the effective R52 transport term of the chicane increases the incoherent energy spread (i.e., e-beam heating). We present the first commissioning results of this system, and its impact both upon the electron beam phase space, and upon the FEL output intensity and quality.