Multi-frequency ECRH system at ASDEX upgrade

A new ECRH system is currently under construction at the ASDEX Upgrade tokamak. It employs for the first time depressed collector gyrotrons, step-tunable in the range 105-140 GHz. In its final stage it will consist of 4 gyrotrons with a total output power of 4 MW and a pulse length of 10 s. In this paper we describe recent extensions of the system and some experimental results.