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Abstract
A new 28 GHz - 1 MW gyrotron has been developed for ECRH system of GAMMA 10 tandem mirror. To achieve the design target of the new gyrotron, which is 1 MW output for 1 s, the magnetron injection gun (MIG) and cavity were designed in detail with calculation codes. A first tube has been fabricated, and performance test is in progress. The maximum output power of 1.04 MW and the maximum output efficiency of 40.6% were obtained with short pulse operation. However, at the same time, it was confirmed that the output efficiency had decreased with the increase of beam current. This is explained with α decrease, which is a one of issues to develop a higher efficiency gyrotron.