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Plasma heating at upcoming thermonuclear fusion experiment facilities will be partially performed by large Electron Cyclotron Resonance Heating (ECRH) installations. The required pulse length is 1000 seconds (ITER specification) and more in order to meet the physical aims at Stellarators and Tokamaks as well. The conditioning and testing of the ECRH sources, high power gyrotrons, is an important task of these installations. CW operation under full power conditions requires a microwave load with a power level in the range of 1 MW. In the ECRH system at W7-X several types of loads are in operation but their long term stability needs to be improved. A new approach of load design will be discussed which is currently under test.

A 10 MW, 140 GHz ECRH system with a pulse duration of 30 minutes is currently under construction for the stellarator W7-X at Greifswald. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent progress with the series gyrotrons.

High power gyrotrons for plasma heating utilize a single stage depressed collector potential in order to reduce the collector load and to recover a large fraction of the remaining energy of the spent electron beam. This operation mode requires two high voltage power supplies which usually possess different characteristics. In the case of RF modulation, both power supplies must nevertheless operate synchronously and with a certain timing. In addition, the power must be shut down instantaneously when the collector load exceeds a certain limit, e.g. due to a dropout of the RF output. This paper discusses the realization of the above functionality in a single FPGA and describes the implementation at the W7-X stellarator.