Method for operating a microwave device , the microwave device comprising a cavity and multiple microwave channels (CH1 - CH4) for providing microwaves within said cavity , the method comprising the steps of: - operating one or more microwave channels (CH1 - CH4) at one or more first power levels and with varying phases in a data acquisition mode; - gathering information regarding channel reverse power (RP) at the one or more microwave channels (CH1 - CH4) during said data acquisition mode; - establishing a mathematical model for each microwave channel (CH1 - CH4) based on said gathered information, said mathematical model providing information regarding channel reverse power (RP) for the respective microwave channel (CH1 - CH4); - determining operating parameters based on the established mathematical models; and - operating the microwave channels (CH1 - CH4) of the microwave device at one or more second power levels based on the determined operation parameters, the power of the second power levels being higher than the power of the first power levels characterized in that - said multiple microwave channels (CH1 - CH4) are divided into multiple groups; - said groups comprise one master microwave channel and at least one slave microwave channel; - the mathematical model is established based on a set of curves or a 3D-plot indicating the dependency of the channel reverse power (RP) and/or the total reverse power on the phases of the microwaves provided by two or more master microwave channels; and in that - the mathematical model is established by determining the mean channel reverse power, maximum channel reverse power and information regarding the phase relation between the phases of two or more master microwave channels. - operating one or more microwave channels (CH1 - CH4) at one or more first power levels and with varying phases in a data acquisition mode; - gathering information regarding channel reverse power (RP) at the one or more microwave channels (CH1 - CH4) during said data acquisition mode; - establishing a mathematical model for each microwave channel (CH1 - CH4) based on said gathered information, said mathematical model providing information regarding channel reverse power (RP) for the respective microwave channel (CH1 - CH4); - determining operating parameters based on the established mathematical models; and - operating the microwave channels (CH1 - CH4) of the microwave device at one or more second power levels based on the determined operation parameters, the power of the second power levels being higher than the power of the first power levels - said multiple microwave channels (CH1 - CH4) are divided into multiple groups; - said groups comprise one master microwave channel and at least one slave microwave channel; - the mathematical model is established based on a set of curves or a 3D-plot indicating the dependency of the channel reverse power (RP) and/or the total reverse power on the phases of the microwaves provided by two or more master microwave channels; - the mathematical model is established by determining the mean channel reverse power, maximum channel reverse power and information regarding the phase relation between the phases of two or more master microwave channels. <EOS>