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The existing ECRH system at W7-X consists of 10 gyrotrons, with output power levels ranging from 0.6 MW up to 1.0 MW each at a frequency of 140 GHz, quasi-optical transmission lines and microwave launchers at the plasma vessel. Compared to other large fusion experiments, W7-X has a relatively low power-to-volume ratio. However high heating power is particularly necessary for achieving high plasma beta values, where the improved confinement of fast ions, one of the optimization criteria of W7-X, can be examined. It is therefore necessary to expand the ECRH systems in several consecutive steps. It is planned to increase the number of gyrotron positions from 10 to 12 and at the same time to evolve the gyrotron output power in several development steps from 1 MW to nominal 1.5 MW and, finally, up to 2 MW. At the same time, the transmission lines will also be upgraded for 2 MW operation. A special effort is also made to improve the reliability of the system by the fast control system.

In its second operation phase (OP1.2a) W7-X was equipped with full 3d island divertor and an upgraded ECRH-system of 10 gyrotrons with a total port through power of 7 MW. The combination of pellet fueling and high density ECRH enabled to explore density above 1 1020 m-3. In particular with the O2-polarisation combined with a multi-pass reflector tile system a density of up to 1.4 1020 m-3 was achieved. At slightly lower densities high core beta values and record values of the fusion tripple product of 0.66 1020 m-3 keVs for stellarators were reached. In addition routine plasma start-up and ECRH wall conditioning were performed. The island divertor enables to demonstrate the intrinsic steady state capabilities of W7-X, where stationary discharges of up to 30s were demonstrated being only limited by the heat capacity of the uncooled divertor. With the flexible ECRH launch system current density profile variations were used for MHD stability investigations. Here by fine-tuning of the ECCD profile different MHD activity could be triggered.

At the stellarator Wendelstein 7-X (W7-X) Electron Cyclotron Resonance Heating (ECRH) is the main heating system for steady-state operation and was the only available heating system during the operation phase OP1.2a in 2017. The ECRH, equipped with 10 operational 140 GHz gyrotrons [1], was used for different tasks at W7-X such as wall conditioning, controlled plasma start-up from the neutral gas up to steady state plasma control and different heating scenarios such as X2-mode and O2-mode heating as well as current drive (ECCD). The operational experiences and improvement opportunities of the ECRH plant will be discussed.