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Department of Engineering, ICT and Technologies for Energy and Transport Patent Title Device for the reduction of back scattered rays from a load subjected to microwave radiation by a source (pre-load). Ref. CNR 1629 Assignee(s): CNR CNR Institute: IFP Main Inventor: Alessandro Bruschi Countries: IT Priority date: 30/08/2004 Abstract It is common practice, while developing and using powerful microwave sources, to use a cavity (“load”) in order to absorb the generated power and in some cases to provide a measure of the absorbed power (“bolometric load”). The microwaves, in shape of concentrated beam, are injected in the load through an appropriate opening. An elevated reflection, beyond to being harmful for the source, in the bolometric load case does not allow an accurate measure. The developed device is constituted from a metallic body with passing cavity and convenient inner geometry (opposite cone sections), such not to block a microwave beam incoming in a load but to suppress the reflected radiation towards the source. That is possible because the microwave radiation inside the load reflects with a great angular dispersion at the mouth of the load. Background In the development of microwave absorbing loads, due to the geometry of the system, a certain fraction of the bouncing radiation can escape from the load entrance: depending on the characteristics of the load, up to 10% of the power can be reflected at angles greater than 40° from the axis of symmetry. Safe operation of high power sources asks for overall reflections from the load lower than about 1%. In order to reduce this amount, an external preload is added in front of the load. Technology The preload inner surface is constituted by a corner-cube arrangement capable to reflect back into the load all the rays coming out at a large angle while not interfering with the incoming beam. The developed device can individually be obtained by mechanical working from a single block of metal and inner turning or assembling single worked sections: in the first case the device is vacuum compatible and can also be used in order to pump down the system. Advantages and Applications Without preload, a load can reflect up to 10% of the injected power. The developed accessory can reduce the reflection down to 1%, improving the operation of a load for the absorption and the measure of powerful microwave beams. It is useful in all the fields that demand use and/or development of powerful microwave sources, with frequency of 100÷200 GHz and power of 1÷2 megawatt. Development stage Devices for use under vacuum and in air have been developed and applied to loads built for European laboratories working on the development of high-power microwave sources and on microwave heating of 33 plasmas for application to Nuclear Fusion research.