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Department of Engineering, ICT and Technologies for Energy and Transport Patent Title Bolometric device with receiving cavity for high power microwave beam measurements at high frequency and procedure to coat the inner surface of the load. Ref. CNR 1657 Assignee(s): CNR CNR Institute: IFP Main Inventor: Nicolò Spinicchia Countries: US Priority date: 24/02/2005 Abstract The development of powerful microwave sources requires the use of 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, usually in concentrated beams, are injected in the load through an appropriate opening but a load can reflect up to 10% of the injected power. An elevated reflection, beyond to being harmful for the source, in the bolometric load case does not allow an accurate measure. For the development of these loads, advanced design and construction techniques are required, particularly in the definition of the thickness and the deposition of the ceramics layer, that has to be both microwave- absorbing and very resistant to thermal shocks. The coating, object of this invention and selected after a number of tests, allows a reduction in reflection to less than 1%. Background The use of the bolometric loads concerns all sectors where high power microwave sources (gyrotrons) are employed. In the framework of thermonuclear fusion research the gyrotrons are employed in experiments of Plasma Heating with frequencies in the range 28 GHz - 170 GHz. The expected increase of the power calls for new technological solutions in manufacturing bolometric loads, particularly in making use of layer of microwave-absorbing ceramics very resistant to thermal shocks. Technology The invention consists in a load of a hollow copper sphere (bolometric load) with a spreading mirror in front of the beam entrance. The millimeter-wave absorption occurs in a thin ceramic coating deposited by Plasma Spray technology. An external cooling circuit removes the absorbed thermal energy. The power measurement is performed acquiring the difference between input and output temperature of the cooling fluid and the flow rate. Advantages and Applications The main advantage is due to the coating of Boron Carbide (B C) deposited in the inner wall of the load. The 4 B C has better resistance to the temperature and shows an improved microwave absorption compared with 4 other coatings. The bolometric load with better performances allows power measurements of gyrotrons of the new generation (2 MW continuous power). Development stage The device has been designed in different versions and prototypes, coated with a traditional absorber and tested in European Laboratories, thus demonstrating the suitability of the design. The new coating has been deposited on small samples and tested in Laboratory, showing superior performances both for absorbance 35 and for resistance to thermal shocks. It’s licensed to LTCalcoli srl.