15 February 2000 Ž . Optics Communications 175 2000 139–146 www.elsevier.comrlocateroptcom Experimental observation of Cherenkov superradiance from an intense electron bunch N.S. Ginzburg a, ) , A.S. Sergeev a , Yu.V. Novozhilova a , I.V. Zotova a , R.M. Rosenthal a , A.D.R. Phelps b , A.W. Cross b , P. Aitken b , V.G. Shpak c , M.I. Yalandin c , S.A. Shunailov c , M.R. Ulmaskulov c a Institute of Applied Physics, Russian Academy of Science, 603600, Nizhny NoÕgorod, Russia b Department of Physics and Applied Physics, UniÕersity of Strathclyde, Glasgow, G4 0NG, UK c Institute of Electrophysics, Russian Academy of Science, 620049, Ekaterinburg, Russia Received 21 January 1999; received in revised form 14 September 1999; accepted 7 December 1999 Abstract Experimental results are presented of the observation of stimulated Cherenkov radiation when intense subnanosecond Ž electron bunches were moving through a dielectric-loaded waveguide. The ultrashort duration microwave pulses up to 400 . ps which is equivalent to ;10 periods of the radiation in combination with specific dependencies of peak microwave Ž . power on interaction length demonstrate that this emission can be interpreted as coherent superradiant SR emission. The peak power associated with the SR emission was ;1–2 MW. The spectrum of radiation belongs to the frequency range 38–48 GHz. The experimental results are compared with theoretical analyses. q 2000 Published by Elsevier Science B.V. All rights reserved. 1. Introduction Coherent emission from short electron bunches with lengths much smaller than the wavelength is well known wx and was first observed in the Smith–Purcell experiment 1 . Another type of conventional coherent emission, w x which is used extensively in microwave electronics 2–8 , arises from continuous or quasi-continuous electron beams with length much greater than the wavelength. In this case coherent emission is related with stimulated processes resulting in self-bunching of the electrons. For the intermediate case, i.e. for electron bunches with a size of several wavelengths it was traditionally assumed that radiation can be related only with density Ž . fluctuations inside the electron bunch incoherent component or with coherent emission from the sharp edges of the bunch. Only recently it was recognised that much more intense coherent emission can occur from the entire volume of such bunches due to self-bunching arising from the mutual influence of different fractions of the ) Corresponding author. Fax: q7-8312-384-552; e-mail: ginzburg@appl.sci-nnov.ru 0030-4018r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 99 00758-0