Development of a High-Power Millimeter-Wave Pulse Source Using High-T_c Superconductors

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A high-power millimeter-wave pulse source using high-T_c superconductors was studied. Y-Ba-Cu-O (YBCO) superconducting thin films were prepared by the laser ablation technique and the critical temperature T_c of the filim was about 90 K.The YBCO film was installed in a cylindrical TE_<011> mode cavity made of copper. The diameter and the length of the cavity were 15.8 mm and 23.8 mm, respectively. The YBCO film was attached to the circular window with 8 mm in diameter at the center of the end wall of the cavity. The resonant frequency was 24.0 GHz. The unloaded Q values of the cavity were 1.3 X 10^3 at room temperature and 2.6 X 10^3 at 20 K.The CW transmitted power through the film gradually increases with temperature above T_c and was not observed below 50 K.The characteristic of the CW transmission was not better than that result using 2.8 GHz cavity, since the surface resistance of the YBCO film increased with frequency. The switching from super- to normal-conducting state in the YBCO film was made using a 1.06-mum pulsed Q-switched Nd : YAG laser with a duration of 10 ns. Millimeter-wave pulses were extracted from the cavity. The peak power of the output pulses at 30 K was 1.1 times as large as the CW input power. If a cavity with a unloaded Q value of 10^6 is used, the peak power of the output pulse is thought to be one thousand times greater than the CW input power theoretically. A millimeter-wave pulse source utlizing a dielectric resonator were also studied theoretically. It was found that the dielectric resonator was useful.