Coherent effects in synchrotron radiation (SR) have been observed for the first time from 180-MeV short electron bunches of 2.5 mm using the Tohoku 300-MeV Linac. The intensity of the coherent SR was about 10 5 times as strong as that of incoherent SR in the wavelengths of 0.4-2.2 mm. This enhancement factor corresponds to the number of electrons in a bunch. The SR intensity showed a quadratic dependence on the electron beam current. The radiation was mainly polarized in the orbital plane. The possibility of induced rf in a vacuum chamber was excluded experimentally.

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The quenched probability distribution in the captions of Figs. 2 and 3 should read PUij) =[S(J-a-Jij) -hS(J-ha-Jij)]/2, cj=//4.

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