Development of a Feasible High-Power mm Microwave Pulse Source Utilizing Superconducting Cavity

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It is the purpose of the present project to develop a feasible mm microwave pulse sourse, utilizing the storage and quick extraction of energy from a superconducting cavity, in which the time constants can be of the order of <10^(-2)> sec. In the first year, we designed and constructed a niobium cavity, a mm microwave AFC circuit and a FRP cryostat etc. Moreover, small semi-circular discharge tubes with 20-50 Torr helium gas at room temperature were designed to use for the output discharge antenna. Niobium cavity was designed to be <TE_(0,1,17)> cylindrical mode with resonant frequency 24.5 GHz, and the inner diameter 44 mm, the length 111 mm. In the next year, i.e., the final year of the project, we first measured precisely the frequency characteristic of the niobium cavity at room temperature. Various parameters of the cavity were obtained. The resonant frequency and the loaded Q value were, respectively, 23.77 GHz and 2x <10^3> . To increase the loaded Q value of niobium cavity, we electro-polished The inner surface of the niobium cavity, in which the roughness of the surface was estimated to be less than 0.1 <micro> m. In the electro-polishing solution, the DC current with 60 mA/ <cm^2> was given between a niobium anode and the part of the niobium cavity, i.e., the two disks and the cylinder, respectively. Electro-polishing took 4 hours. Then, the parts were fabricated quickly in a clean room. After evacuating and baking, vacuum inside the cavity was attained to be 3x <10^(-8)> Torr. The electro-polished cavity was considered to have loaded Q value of the <10^8> at helium temperature 4.2 K. At this moment, we are carrying out the final experiment to give the voltage 20 kV, current 2000 A, time width 1 <micro> sec to the discharge antenna. We are expecting to have the final purpose of the present project, i.e., pulse power gain of <10^4> .