Runaway electrons generation in FTU during EC assisted breakdown Discharges

The necessity to use EC (Electron Cyclotron) power to assist plasma start-up in future large devices (JT60-SA, ITER, DEMO) is a matter of fact, considering the low electric field (0.3 V/m) foreseen. An effort has been done both in experimental [1] and in simulation [2] activities to define requirements of EC power, configuration and prefilling pressure for a reliable start-up. In ITER up to 4 MW of additional power are considered to initiate and sustain plasma start-up. EC power in a low density plasma can be a concern for the possibility of fast electrons formation that potentially can be accelerated by the toroidal electric field up to the run-away condition. The studies on Runaway Electrons (RE) formation in tokamak are usually carried out during current flat top or at disruption, [3]. Even if the electric field in EC assisted break-down is generally low and well below the Dreicer limit [4], it is necessary to complement these studies considering the start-up phase of the current in presence of strong RF wave heating. The consolidated theory based on toroidal electric field, as source for RE acceleration, must be extended in presence of high frequency waves resonating with thermal electrons and transferring them perpendicular momentum that can modify the electric field threshold. In this paper the FTU data base on EC assisted start-up, extended over 10 years of different experiments, is analysed, with the aim to give a starting point for future comparative inter-machine studies on RE generation in the initial phase of the plasma current formation.