Matched calorimetric loads for high power, long pulse millimiter-wave gyrotrons

A compact matched load for high power gyrotron testing in ECRH plants for fusion research applications is in development, with the main goal of providing reflection-free absorption and fast calorimetric measurement of the millimeter-wave power, also at long pulse duration. It is based on the results of the tests at full power and pulse length (140 GHz, 0.5 Mw, 0.5 s) on similar loads installed on the ECRH plant of the FTU Tokamak in Frascati. Basically, the load is made of an integrating sphere in copper, with the inner wall covered by plasma spray with a mixture of lossy ceramics, providing an average wall reflectivity in the order of 40%. Absorbing material degradation, small-scale and large-scale damage patterns, caused by fatigue and long-term exposure to high power mm-wave, have been analyzed in detail, with the aim of improving the performances in terms of power handling capability and energy extraction efficiency. Physical-chemical analyses, performed on the original and on the degraded absorbing material, showed the effects of the mm-wave exposure. Damage patterns were recognized as interference of multiple reflections inside the load, with radiation accumulation close to the entrance port, as confirmed by a detailed numerical analysis. Increase in power and energy deposition capabilities require improvements of the present design in different areas:. - strategies for the mitigation of the different causes of non-homogeneous deposition;. - use of different plasma-sprayed materials as mm-wave absorbers. - improved cooling channel geometry with increased heat transfer rate to the cooling medium, for achieving 1MW-CW power capability.