Validation Experiments on the 2-MW CW 170-GHz Load for the European ITER Gyrotron

The development of high-power gyrotrons needs matched loads with good performance capable of absorbing and measuring powers as high as 1 MW in the International Thermonuclear Experimental Reactor (ITER) and higher (up to two times) in the future devices, such as the DEMOnstrating fusion power reactor. At Istituto di Fisica del Plasma - Consiglio Nazionale delle Ricerche IFP-CNR (Milan), several spherical loads were developed, all with identical absorbing geometry but different heat removal systems for operations with pulselengths (PLs) from a few milliseconds to continuous wave (CW). A dummy load designed for 2-MW CW 170-GHz operations has been constructed in the frame of the development program of the European gyrotron for ITER. The first prototype hemisphere of this load, joined with an uncoated one, has been successfully tested with long pulses and under vacuum using the gyrotron for ITER of the National Institutes for Quantum and Radiological Science and Technology in Naka. The symmetry of the system is such that the equivalent power (EP) load of the absorbing (coated) hemisphere is doubled when the second one is reflective (uncoated), with an almost unchanged distribution with respect to a fully coated load. The tests demonstrate the capability of the IFP-CNR load to withstand and measure radiation at an EP in an excess of 1.5MW. Experiments involving gyrotron pulses with increasing power and PL have been performed in the load and are reported. The maximum EP load reached in the tests was
$\approx 1.8$
MW for 15 s while the longest pulse was 300 s, at
$\approx 1$
MW. During the experiments, the temperatures of either the load or the preload were monitored using thermocouples and multiple infrared cameras, while a pressure sensor and a hydrophone have been used for the monitoring of cavitation phenomena and water boiling in the cooling circuits. The results achieved in the tests are described in this paper.