A new gyrotron operation technique to increase oscillation efficiency was developed using the JT-60 electron cyclotron range of frequency (ECRF) system. In order to increase the efficiency without a significant increase in the anode leakage current by trapped electrons that limits the efficiency of gyrotrons with a collector potential depression technology, we actively optimized the electron pitch factor by controlling only the anode voltage within ∼0.1 s after the start of the gyrotron operation, and a high-efficiency oscillation in the so-called hard-self-excitation region was achieved from the start of the oscillation. As an application of this technique in the JT-60 ECRF system, the gyrotron output power of 1.5 MW for 4 s, which is the longest pulse length in the world at an output power of 1.5 MW, was recorded with a successful reduction in the collector heat load by 20% as compared with the conventional operation. The reduced collector heat load at the 1.5 MW operation was acceptable for a steady-state operation. Further progress was made with respect to the expansion of the long-pulse capability of the ECRF system. A new gyrotron with an improved mode converter was developed in order to demonstrate a reduction in the stray radiation in the gyrotron; such radiation has thus far hindered long-pulse operations by causing an unacceptable heat load. We confirmed that the stray radiation was reduced to 1/3 of that of the original gyrotron; this reduced heat load is acceptable for steady-state operation. A conditioning operation of the improved gyrotron proceeds up to 31 s at 1 MW. This progress significantly contributes to an enhancement in the high-power and long-pulse capability of the ECRF system used in JT-60SA, where a total output power of 9 MW for 100 s is planned.

Air breakdown plasma induced by a strong electromagnetic (EM) wave field with a millimeter wavelength has a self-formed filamentary structure. The scale of the breakdown plasma is much larger than the wavelength scale, and the power density is lower than the self-ignition threshold of atmospheric breakdown. We observed a breakdown plasma that consisted of small plasma spots, and their trajectory was identical to that of the filamentary structure observed macroscopically. The numerical analysis showed that the interaction of the ionization front, EM field, and neutral-gas heating determines the self-organized structure of the branching plasma. The plasma trajectory identified in the analysis was identical to that of the experimental results. This novel physical process was entirely different from the discrete filament formation observed in the breakdown plasma formed under a focused beam, and the physical process of the breakdown strongly depended on the scale and power density.

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.