Beam Energy Measurements With a Optical Transition Radiation for the ELI-NP Compton Gamma Source

A high brightness electron LINAC is being built in the Compton Gamma Source at the ELI Nuclear Physics facility in Romania. To achieve the design luminosity, a train of 32 bunches, 16 ns spaced, with a nominal charge of 250 pC will collide with the laser beam in the interaction point at two electron beam energies, namely 280 MeV and 720 MeV. Electron beam spot size is measured with optical transition radiation (OTR) profile monitors. The paper deals with the possibility of using the OTR monitors to measure also beam energy along the machine; such measurements may help monitoring the accelerating sections performances, especially when the whole bunch train is being accelerated. We discuss the measurement principle, the expected accuracy and the main characteristic of the optical line to retrieve the angular distribution of the emitted radiation.