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High power gyrotron testing at long pulses is a necessary step in the development of mm-wave systems for fusion research. For this purpose a compact calorimetric load is being developed, with low overall reflectivity (<5%) and aiming at 1 MW-CW capabilities, after the good performances of the loads (0.5 MW, 0.5 s per pulse) installed on the 140 GHz ECRH plant of the FTU Tokamak in Frascati. Tests at the same frequency but at higher power (0.7 MW, 1 s) and longer pulses (up to 2 s at 0.5 MW) on the ASDEX-Upgrade ECRH plant showed, after repeated exposures, damages on the absorbing layer, recognized as thermal effects due to the local enhancement of incident power: the effects visible near the entrance port are explained studying the superposition (with interference) of waves reflected inside the sphere. Samples of degraded and non-degraded coating have been analyzed with various techniques, revealing changes of the substrate''s physical characteristics. Measurements of absorber temperature during and after the pulse, performed with an infrared detector looking directly into the load, allowed estimates of the peak temperature and thermal properties of the coating in the real working conditions. Improvements on power deposition, geometry and cooling are proposed, to be integrated in a new load design. [Copyright &y& Elsevier]

Abstract: In the electron cyclotron frequency range the handling of high power is critical. In some cases an unpredictable amount of stray radiation can reach some components or accumulate in localized regions, with risk of damages caused by thermal overloads, and any uncontrolled reflection represents a danger for the sources. A possibility to mitigate the problem consists in covering some regions exposed to radiation with absorbers. Enhanced absorption of stray radiation lowers requirements on active protection systems in microwave diagnostics. The released heat can be extracted by dedicated cooling systems. The chromium oxide (Cr2O3), largely tested at IFP-CNR, has been routinely used as internal coating for matched loads. The performances of a variable thickness coating has been tested at high power at Karlsruhe Institute of Technology (KIT), with a 140GHz gyrotron of the W7-X ECRH system and an averaged power density absorbed at the coating surface higher than 1MW/m2 for 3min. Also boron carbide (B4C) has been tested at low power and patented as a millimeter-wave absorber. In the paper, the results of some tests performed on these coatings are given, together with some simulations of the absorption capability based on low power measurements on samples. Finally, some calculations are presented for a coating obtained combining together Cr2O3 and B4C. [Copyright &y& Elsevier]

Abstract: The development of high power gyrotrons for plasma physics research needs properly matched calorimetric loads able to absorb and measure the power, which nowadays is foreseen to be as high as 2MW during CW operations. To this end IFP/CNR has developed a family of matched loads useful in the mm-wave frequency band for applications ranging from a few ms to CW in pulse length. The different loads in the family, made of an integrating sphere with a partially reflecting coating on the inner wall, are characterized by having the same absorbing geometry for the incoming beam and a different heat removal system for the specific application. Some important advances have been recently achieved from the point of view of the uniformity of power distribution on the absorbing wall and of the load construction. With the high precision achieved in the coating thickness a better control of the heating power distribution is possible by proper shaping of the local reflectivity, in addition to the shaping of the mirror dispersing the input beam. A more sophisticated model describing the power distribution has been developed, taking into account a variable thickness of the absorbing coating, the proper shape of the spreading mirror, the frequency of the incoming radiation and the shape of the input beam. Lower coating thickness is shown to be preferable, for a given local reflectivity, from the point of view of a lower peak temperature and thermal stress. The paper describes a load with variable coating thickness along the meridian of the sphere, showing a uniform power deposition on the inner walls. The cooling pipe is completely electroformed on the spherical copper shell, ensuring the maintenance of the correct curvature of the inner surface and fast heat conduction from the absorbing coating to the water through the thin copper body. For CW use all heated parts of the load must be cooled and this is achieved with 16 electroformed spiral channels. Both short pulse loads (0.1–1s) and the CW version at 2MW, 170GHz, are described in the paper. High power tests on short-pulse loads have been done using a double frequency gyrotron, 105GHz/600kW for 0.5s and 140GHz/800kW for 1s. Also a method for emulating 2MW conditions while using 1MW gyrotron has been applied to test the load to be used for the European 2MW coaxial cavity gyrotron development programme. [Copyright &y& Elsevier]

Abstract: An increased power handling of rf-beam loads is required by the development of high power gyrotrons for fusion research, as well as the achievement of full capability to work under vacuum conditions, which is imposed by the use of evacuated transmission lines. New techniques were developed at IFP/CNR, leading to two prototypes. One is capable of 1MW CW operation, while the second one is designed for precise measurements at short-pulses (0.1s) at higher peak power (2MW). Preliminary tests at 140GHz, high power and long pulse (500kW, 10s) were performed. [Copyright &y& Elsevier]